principles of medium & large ac motors - easa.com

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IntroductionPrinciples of Medium and Large AC Motors–IEC

i

INTRODUCTION

SCOPEThis book covers horizontal and vertical induction and

synchronous motors rated 37 to 3700 kW (50 to 5000 hp). Many of the principles of operation and construction also apply to motors of all sizes and types. The focus is on squirrel-cage induction motors built to International Electrotechnical Commission (IEC) designs.

PURPOSEThis book is intended as the source document for EASA’s

seminar Principles of Medium and Large AC Motors, and as a reference manual for service centers. The book is organized into 15 sections, divided by subject or motor component, with some redundancy to facilitate its role as a reference manual. It is especially useful as an entry-level training tool but will be valuable to technicians, sales personnel and engineers with experience levels ranging to 30+ years. Material may be excerpted to educate end users and support reports, within copyright guidelines.

EASA, Inc.1331 Baur Boulevard • St. Louis, Missouri 63132 • USA

+1 314 993 2220 • Engineering Fax +1 314 993 2998 • www.easa.com

The information in this book was carefully prepared and is believed to be correct,but EASA makes no warranties respecting it and disclaims any responsibility or liability of any kindfor any loss or damage as a consequence of anyone’s use of or reliance upon such information.

Principles of Medium and Large AC Motors–IEC


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Introduction

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Principles of Medium and Large AC Motors–IEC Table of Contents

Table of ContentsSection Outline Section

Motor Nomenclature and Definitions .................................................................................................................. 1Motor Enclosures ............................................................................................................................................... 2Typical Motor Applications .................................................................................................................................. 3Safety and Handling Considerations .................................................................................................................. 4Basic Motor Theory ............................................................................................................................................ 5Motor Standards ................................................................................................................................................. 6Stators ................................................................................................................................................................ 7Squirrel Cage Rotors .......................................................................................................................................... 8Shafts ................................................................................................................................................................. 9Bearings and Lubrication ................................................................................................................................. 10Motor Accessories and Terminal Boxes ............................................................................................................11Test and Inspection Procedures ....................................................................................................................... 12Motor Alignment, Vibration and Noise .............................................................................................................. 13Storage Procedures ......................................................................................................................................... 14Synchronous Machines .................................................................................................................................... 15

Principles of Medium and Large AC Motors–IECTable of Contents

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Principles of Medium and Large AC Motors – IEC Section 1 — Motor Nomenclature and Definitions

1 Motor Nomenclature and DefinitionsSection Outline Page

Medium and large horizontal and vertical motor terminology .......................................................................... 1-2IP23 dripproof motor terminology .................................................................................................................... 1-4Large IP23 dripproof motor terminology .......................................................................................................... 1-6Large horizontal motor terminology (additional) .............................................................................................. 1-8Synchronous machine terminology ............................................................................................................... 1-10Rolling-element and sleeve bearing terminology .......................................................................................... 1-12Vertical hollow-shaft terminology ................................................................................................................... 1-14AC stator form and random winding cross-section terminology .................................................................... 1-16AC stator form and random winding end turn terminology ............................................................................ 1-18Notes ............................................................................................................................................................. 1-20

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Principles of Medium and Large AC Motors – IEC Section 2 — Motor Enclosures

2Motor EnclosuresSection Outline Page

IEC standards for motor enclosures with various examples ........................................................................... 2-2Medium and large motor enclosures ............................................................................................................... 2-2

Examples of IEC motor enclosures .............................................................................................................. 2-3Cooling methods and airflow ........................................................................................................................ 2-5

Degree of protection (IP) and methods of cooling codes for rotating electrical machines .............................. 2-7Degree of protection (IP) code ..................................................................................................................... 2-7Methods of cooling (IC) code ....................................................................................................................... 2-7Simplified IC code designation ..................................................................................................................... 2-8

Classification according to degree of protection and methods of cooling ....................................................... 2-91.1 Open machine (IP00, IC01) ................................................................................................................ 2-91.2 Dripproof machine (IP23, IC01) .......................................................................................................... 2-91.3 Totally enclosed machine (IP44, IC01) ............................................................................................... 2-91.3.1 Totally enclosed machine (IP54, IC01) ............................................................................................ 2-91.3.2 Totally enclosed machine (IP55, IC01) ............................................................................................ 2-91.3.3 Totally enclosed machine (IP65, IC01) ............................................................................................ 2-91.3.4 Totally enclosed machine (IP55, IC611) ........................................................................................... 2-91.3.5 Totally enclosed machine (IP55, IC81W) ......................................................................................... 2-91.4.1 Flameproof machine Ex “d” .............................................................................................................. 2-91.4.2 Increased safety machine Ex “e” ................................................................................................... 2-101.4.3 Flameproof machine Ex “p” ............................................................................................................ 2-101.4.4 Dust ignition-proof machine Ex “t” .................................................................................................. 2-10

Test finger for degree of protection IP 2x ................................................................................................... 2-10Referenced standards ................................................................................................................................... 2-10

Principles of Medium and Large AC Motors – IEC

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Typical Motor Applications — Section 3

3Typical Motor ApplicationsSection Outline PageUse of electrical motors ................................................................................................................................... 3-2The motor system ........................................................................................................................................... 3-3Motor application selection chart ..................................................................................................................... 3-4Examples of motor applications ...................................................................................................................... 3-8Referenced standards ....................................................................................................................................3-11Notes ............................................................................................................................................................. 3-12


Section 4 — Safety and Handling ConsiderationsPrinciples of Medium and Large AC Motors – IEC

4Safety and HandlingConsiderationsSection Outline Page

Safety considerations for the application, installation, operation and maintenance of AC motors .................. 4-3Motor selection ............................................................................................................................................. 4-3

Environmental ........................................................................................................................................... 4-3Mechanical ................................................................................................................................................ 4-3Electrical ................................................................................................................................................... 4-3Other ......................................................................................................................................................... 4-3

Hazardous locations ..................................................................................................................................... 4-3Arctic duty motors ........................................................................................................................................ 4-3Motors operated from variable-frequency drives (VFDs) ............................................................................. 4-4

Electrical safety considerations ................................................................................................................. 4-4Objectives of earthing ............................................................................................................................... 4-4Earthing cables ......................................................................................................................................... 4-4Bonding of electrical machines ................................................................................................................. 4-4Physical safety considerations .................................................................................................................. 4-5Additional considerations for permanent magnet (PM) converter motor operation ................................... 4-5

Lifting motors safely ........................................................................................................................................ 4-5Lifting equipment & accessories .................................................................................................................. 4-6Lifting standards ........................................................................................................................................... 4-6Operating personnel ..................................................................................................................................... 4-6Special handling of large motor frames to prevent damage ......................................................................... 4-6

Mechanical guarding ....................................................................................................................................... 4-8Securing and guarding the shaft during testing ............................................................................................... 4-9Earthing ........................................................................................................................................................... 4-9High-potential testing .................................................................................................................................... 4-10Motor controls and electrical accessories ..................................................................................................... 4-10Motor operation safety considerations ...........................................................................................................4-11

Motor starting ..............................................................................................................................................4-11Noise ...........................................................................................................................................................4-11

Safety and Handling Considerations — Section 4

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Section Outline Page

Overspeeding ............................................................................................................................................. 4-12Shaft loading .............................................................................................................................................. 4-12Torsional vibration and stress ..................................................................................................................... 4-12Transient torques ....................................................................................................................................... 4-12Transient voltages and currents ................................................................................................................. 4-12Surface temperatures ................................................................................................................................. 4-12

Referenced standards ................................................................................................................................... 4-13Notes ............................................................................................................................................................. 4-14



Section 5 — Basic Motor Theory

5Basic Motor TheorySection Outline Page

Induction motor fundamentals ......................................................................................................................... 5-3How does the motor work? ............................................................................................................................. 5-4Basic induction motor equations ..................................................................................................................... 5-7AC squirrel-cage induction motor temperature considerations ....................................................................... 5-7

Factors that affect motor temperature .......................................................................................................... 5-7The thermal aging process ........................................................................................................................... 5-7

Machine Selection ........................................................................................................................................... 5-9Duty types .................................................................................................................................................... 5-9

Rating class selection ............................................................................................................................... 5-9Rated loads ............................................................................................................................................... 5-9

S1 Continuous running duty ................................................................................................................. 5-9S2 Short time duty ................................................................................................................................. 5-9S3 Intermittent periodic duty ................................................................................................................ 5-10S4 Intermittent periodic duty with starting ............................................................................................ 5-10S5 Intermittent periodic duty with electric breaking ............................................................................. 5-10S6 Continuous operation periodic duty .................................................................................................5-11S7 Continuous operation periodic duty with electric braking ................................................................5-11S8 Continuous operation periodic duty with related load/speed changes ............................................5-11S9 Duty with non-periodic load/speed variation ................................................................................... 5-12S10 Duty with discrete constant loads and speeds ............................................................................. 5-12

Site operating conditions ............................................................................................................................... 5-13Normal operating conditions ...................................................................................................................... 5-13Unusual operating conditions ..................................................................................................................... 5-13

Operation at non-standard altitude ......................................................................................................... 5-13Special insulation considerations at altitude >1,000 ASL ........................................................................ 5-13Special ambient considerations .............................................................................................................. 5-14

Effects of phase voltage variation ................................................................................................................. 5-14Effects of voltage unbalance ......................................................................................................................... 5-15

Graphical method to determine Un & Up .................................................................................................... 5-16An approximation for voltage unbalance factor .......................................................................................... 5-16Effects of unbalanced voltages on motor performance .............................................................................. 5-16

Heating .................................................................................................................................................... 5-16



Basic Motor Theory — Section 5


Torques ................................................................................................................................................... 5-16Derating of motors to prevent overheating .............................................................................................. 5-17

Effects of harmonics, including variable voltage and variable-frequency controls ........................................ 5-17Frequency .................................................................................................................................................. 5-17Voltage stress ............................................................................................................................................. 5-17

Speed-torque characteristics .................................................................................................................. 5-19Resonant speed bands ........................................................................................................................... 5-19High impact loads ................................................................................................................................... 5-20Electric machine requirements ................................................................................................................ 5-20Additional losses due to converter supply and ways to reduce them ..................................................... 5-22

Effect on efficiency ..................................................................................................................................... 5-22Power factor correction .............................................................................................................................. 5-22Volts/hertz ratio (base) ............................................................................................................................... 5-22Resonances, sound, vibration .................................................................................................................... 5-22Leakage currents ....................................................................................................................................... 5-23Shaft voltages and bearing insulation ........................................................................................................ 5-23

Winding temperature under load ................................................................................................................... 5-24Methods for measuring winding temperature ............................................................................................ 5-24Correction for stopping time ....................................................................................................................... 5-24

Starting considerations for AC induction motors ........................................................................................... 5-25Application considerations ......................................................................................................................... 5-25Electrical energy use by application ........................................................................................................... 5-25Comparison of IEC design N and H motors ............................................................................................... 5-26Load and starting characteristics ............................................................................................................... 5-26Starting performance .................................................................................................................................. 5-26Basic starting requirements ........................................................................................................................ 5-27

Design NY and HY starting requirements (star/delta starting) ............................................................. 5-27Design H starting requirements ........................................................................................................... 5-28Design N starting requirements ........................................................................................................... 5-28

Breakdown torque (Tb) ........................................................................................................................... 5-28Pull-up torque (Tu) ................................................................................................................................... 5-28Locked rotor torque (Tl) ........................................................................................................................... 5-28Rated torque (TN) .................................................................................................................................... 5-28Rated output (PN) .................................................................................................................................... 5-28Locked rotor apparent power (Si) ............................................................................................................ 5-28Starting current and torque values .......................................................................................................... 5-28Starting torque characteristics ................................................................................................................ 5-29Locked rotor apparent power .................................................................................................................. 5-29

Electrical apparatus for explosive atmospheres ............................................................................................ 5-30Definition of load inertia .............................................................................................................................. 5-32IEC 60034-5 degrees of ingress protection (IP Code) ............................................................................... 5-32Application considerations ......................................................................................................................... 5-34Methods of controlling the starting current ................................................................................................. 5-34

Measurement units and conversions ............................................................................................................ 5-35Appendix: Transient current .......................................................................................................................... 5-39Referenced standards ................................................................................................................................... 5-40



Motor Standards — Section 6

6 Motor StandardsSection Outline Page

Understanding motor temperature rise limits .................................................................................................. 6-3Other factors to consider .............................................................................................................................. 6-3Measuring resistance ................................................................................................................................... 6-3Embedded detectors monitor temperature ................................................................................................... 6-4Hot operating temperature ........................................................................................................................... 6-4

Motor ratings and operating parameters ......................................................................................................... 6-4Torque .......................................................................................................................................................... 6-4

Standard torque ........................................................................................................................................ 6-4High torque Design H ................................................................................................................................ 6-4

Number of starts ........................................................................................................................................... 6-4Safe operating speeds ................................................................................................................................. 6-4Overspeeds .................................................................................................................................................. 6-5Variations from rated voltage and frequency ................................................................................................ 6-5Operation from variable-frequency or variable-voltage power supplies or both ........................................... 6-5Performance tests ........................................................................................................................................ 6-5Voltage withstand test .................................................................................................................................. 6-7

Rewound machines .................................................................................................................................. 6-7Overhauled and repaired machines .......................................................................................................... 6-7

Degrees of ingress protection (IP Code) ...................................................................................................... 6-7Methods of cooling (IC Code) ...................................................................................................................... 6-7

Characteristic letter for coolant ................................................................................................................. 6-8Simplified coolant code designation .......................................................................................................... 6-9

Methods of mounting (IM Code) ................................................................................................................. 6-13Mechanical vibration .................................................................................................................................. 6-18Noise levels ................................................................................................................................................ 6-18

Unbalanced voltages–causes and consequences ........................................................................................ 6-18Variations in the load .................................................................................................................................. 6-18Voltage unbalance defined ......................................................................................................................... 6-18Common causes of unbalanced voltages .................................................................................................. 6-18



Section 6 — Motor Standards

Section Outline PageEffects of single-phase operation ............................................................................................................... 6-19

Effective protection against single-phasing ............................................................................................. 6-19Multiple motors on a single-phased circuit .............................................................................................. 6-19

Tests for unbalanced voltage ..................................................................................................................... 6-19Devices to detect voltage unbalance ......................................................................................................... 6-20

Unbalance relays .................................................................................................................................... 6-20Negative sequence voltage relays .......................................................................................................... 6-20Phase sequence undervoltage relays ..................................................................................................... 6-20Phase voltage relays ............................................................................................................................... 6-20

Voltage unbalance vs. voltage variation ..................................................................................................... 6-20Motor markings and additional design parameters (excerpts from IEC 60034-1) ......................................... 6-21

Nameplate marking ................................................................................................................................. 6-21Embedded temperature detectors .......................................................................................................... 6-21Usual service conditions ......................................................................................................................... 6-21Bus transfer and reclosing ...................................................................................................................... 6-21Power factor correction ........................................................................................................................... 6-21Surge capabilities of AC windings with form-wound coils ....................................................................... 6-21Starting torque and locked rotor apparent power .................................................................................... 6-22

Mechanical vibration (excerpts from IEC 60079-14) ..................................................................................... 6-22Mounting machines for vibration test ......................................................................................................... 6-22Measurement positions for vibration .......................................................................................................... 6-23Bearing housing vibration ........................................................................................................................... 6-24Vibration test conditions ............................................................................................................................. 6-24Bearing housing vibration limits ................................................................................................................. 6-24Relative shaft vibration ............................................................................................................................... 6-25Measurement positions for relative shaft vibration ..................................................................................... 6-25Reed frequency of vertical motors ............................................................................................................. 6-25Noise limits ................................................................................................................................................. 6-25

Performance standards for constant speed motors used on sinusoidal bus with harmonic content............. 6-26Motor application data (excerpts from NEMA Stds. MG 1, Part 14) .............................................................. 6-26

Sleeve bearing motor coupling ................................................................................................................... 6-26Drawings and shaft markings ..................................................................................................................... 6-26Determination of power factor from nameplate data .................................................................................. 6-26

Determination of capacitor rating for correcting power factor to desired value ....................................... 6-27Determination of corrected power factor for specified capacitor rating ................................................... 6-27

Importance of power factor correction ........................................................................................................ 6-27Corrections pay off .................................................................................................................................. 6-27Locations are important .......................................................................................................................... 6-28Considerations for electronic equipment ................................................................................................. 6-30

Standards for new motors and for the repair of existing motors ................................................................... 6-31Standards organizations and other resources .............................................................................................. 6-31Helpful references/bibliography ..................................................................................................................... 6-31Referenced standards ................................................................................................................................... 6-32Notes ............................................................................................................................................................. 6-34

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Principles of Medium and Large AC Motors – IEC Stators — Section 7

7StatorsSection Outline Page

Introduction ..................................................................................................................................................... 7-4Stator core ....................................................................................................................................................... 7-4Stator winding ................................................................................................................................................. 7-5

Components of stator windings (groups ➭ coils ➭ turns ➭ conductor strands) ............................................ 7-6Group ........................................................................................................................................................ 7-6Coil ............................................................................................................................................................ 7-7Turn ........................................................................................................................................................... 7-7Conductor strand ...................................................................................................................................... 7-7

Types of stator windings ............................................................................................................................... 7-7Lap winding ............................................................................................................................................... 7-7Concentric winding .................................................................................................................................... 7-9Random-wound and form-wound stators .................................................................................................. 7-9

Stator winding insulation systems ...............................................................................................................7-11Insulation system components .................................................................................................................7-11Conductor strand insulation .................................................................................................................... 7-12Turn insulation ......................................................................................................................................... 7-12Ground wall insulation ............................................................................................................................. 7-14Formed coil finishing tape ....................................................................................................................... 7-15Mechanical support in the slot ................................................................................................................ 7-16Mechanical support in the end turns ....................................................................................................... 7-17

Winding impregnation ................................................................................................................................ 7-18Terminal connections .................................................................................................................................... 7-20

Multi-torque connections ............................................................................................................................ 7-20Bus connections ......................................................................................................................................... 7-20Corrosion .................................................................................................................................................... 7-20Taping terminal connections ....................................................................................................................... 7-20Cautions ..................................................................................................................................................... 7-20

IEC standard connection diagrams ............................................................................................................... 7-21Symbols ..................................................................................................................................................... 7-21AC machines without commutators ............................................................................................................ 7-21Auxiliary devices ........................................................................................................................................ 7-21


Principles of Medium and Large AC Motors – IECSection 7 — Stators


Prefixes ...................................................................................................................................................... 7-21Direction of rotation .................................................................................................................................... 7-21

IEC terminal markings and connections ........................................................................................................ 7-22Part winding start connections ................................................................................................................... 7-22

Single voltage–6 terminals ...................................................................................................................... 7-22Dual voltage (1:2)–9 terminals ................................................................................................................ 7-22

Single speed connections .......................................................................................................................... 7-22Single voltage–3 terminals ...................................................................................................................... 7-22Single voltage–6 terminals ...................................................................................................................... 7-22Single and dual voltage–6 terminals ....................................................................................................... 7-22Dual voltage (1:2)–9 terminals ................................................................................................................ 7-23Dual voltage (1:2)–12 terminals .............................................................................................................. 7-23

Multi-speed–more than one connection ..................................................................................................... 7-24Two speed, one winding–6 terminals ...................................................................................................... 7-24Two speed, single voltage–6 terminals ................................................................................................... 7-24Two speed, dual voltage (1:√3), one winding–9 terminals ...................................................................... 7-24Two speed, two winding–6 terminals ...................................................................................................... 7-24Three speed, two winding–10 terminals ................................................................................................. 7-25Three speed, three winding–10 terminals ............................................................................................... 7-25Four speed, two winding–12 terminals ................................................................................................... 7-25

Identifying unmarked leads of 6-lead motors with 1 or 2 windings ............................................................. 7-26Safety first ............................................................................................................................................... 7-26Determining continuity ............................................................................................................................ 7-262 circuits, 3 leads each ........................................................................................................................... 7-26

Alternating-pole part-winding start ....................................................................................................... 7-27Consequent-pole part-winding start ..................................................................................................... 7-272-speed, 2-winding .............................................................................................................................. 7-27

3 circuits, 2 leads each ........................................................................................................................... 7-27All 6 leads have continuity ................................................................................................................... 7-272-speed, 1-winding .............................................................................................................................. 7-27Constant torque or power .................................................................................................................... 7-28Variable torque ..................................................................................................................................... 7-28

Identifying unmarked leads of 9-lead motors ............................................................................................ 7-29Identify the circuits and connections ....................................................................................................... 7-29Star-connected motor ............................................................................................................................. 7-29Delta-connected motor ............................................................................................................................ 7-29

Converting IEC to NEMA lead markings .................................................................................................... 7-30Motor starting considerations ........................................................................................................................ 7-31

Common methods for starting motors ........................................................................................................ 7-32Direct on-line starting .............................................................................................................................. 7-32Series reactance reduced voltage starting .............................................................................................. 7-32Solid-state reduced voltage starting ........................................................................................................ 7-32Autotransformer reduced voltage starting ............................................................................................... 7-33Star-start, delta-run starting .................................................................................................................... 7-33Double delta starting ............................................................................................................................... 7-34Part winding starting ............................................................................................................................... 7-34


Principles of Medium and Large AC Motors – IEC Stators — Section 7


Variable-frequency drive starting............................................................................................................. 7-34Variable-speed coupling starting ............................................................................................................. 7-35

Examples of motor starting problems ......................................................................................................... 7-35Remote starting of open-valve pump motors .......................................................................................... 7-35Starting an open-valve pump motor with an emergency generator ........................................................ 7-35Starting a high-inertia load under low inrush restrictions ........................................................................ 7-35

Motor control circuit .................................................................................................................................... 7-36IEC 60364-1 Clause 30: Assessment of general characteristics ............................................................ 7-36IEC 60364-1 Clause 313: Characteristics of the supply ......................................................................... 7-36IEC 60364-1 Clause 314: Division of installations .................................................................................. 7-36IEC 60364-1 Clause 34: Maintainability .................................................................................................. 7-36BS7671:2008 Amendment 3:2015 ......................................................................................................... 7-36

Referenced standards ................................................................................................................................... 7-38


Principles of Medium and Large AC Motors – IEC Section 8 — Squirrel Cage Rotors

8Squirrel Cage RotorsSection Outline PageThe squirrel cage rotor .................................................................................................................................... 8-2

How to determine rotor bar current .............................................................................................................. 8-3The effect of open bars ................................................................................................................................ 8-3Progressive bar failures ............................................................................................................................... 8-4Likely outcome of operating a motor with a broken rotor bar ....................................................................... 8-4

Rotor forces ..................................................................................................................................................... 8-4Rotor construction ........................................................................................................................................... 8-5

Construction of cast rotors ........................................................................................................................... 8-5Construction of fabricated rotors .................................................................................................................. 8-6Rotor bar material ........................................................................................................................................ 8-7The air gap ................................................................................................................................................... 8-9

Effect of air gap on motor characteristics .................................................................................................. 8-9Unbalanced magnetic pull and rotor rub ................................................................................................. 8-10

Rotor air ducts ............................................................................................................................................ 8-10Number of ducts and spacing ................................................................................................................. 8-10Siren effect .............................................................................................................................................. 8-10

Rotor bar shape ..........................................................................................................................................8-11Rotor bar dynamics .................................................................................................................................... 8-13Rotor cooling fans ...................................................................................................................................... 8-13

Rotor testing .................................................................................................................................................. 8-14Single phase rotor test ............................................................................................................................... 8-14Testing for loose rotor bars ......................................................................................................................... 8-14Vibration analysis ....................................................................................................................................... 8-14Rotor sparking ............................................................................................................................................ 8-16

Non-destructive rotor sparking ................................................................................................................ 8-16Destructive rotor sparking ....................................................................................................................... 8-17

Phase angle displacement test .................................................................................................................. 8-17Electromagnetic noise and vibration .......................................................................................................... 8-17Skew .......................................................................................................................................................... 8-17Durability considerations ............................................................................................................................ 8-18


Principles of Medium and Large AC Motors – IEC Section 9 — Shafts

9ShaftsSection Outline Page

Motor shaft materials ....................................................................................................................................... 9-2Stress systems acting on shafts ................................................................................................................... 9-2Stress/strain curves ...................................................................................................................................... 9-2

Tools of shaft failure analysis .......................................................................................................................... 9-3Failure analysis sequence ............................................................................................................................ 9-3Methodology for analysis ............................................................................................................................. 9-4

Causes of failure ............................................................................................................................................. 9-4Impact of stress raisers on fatigue strength ................................................................................................. 9-4Areas of highest concentration ..................................................................................................................... 9-5Shaft keyways .............................................................................................................................................. 9-6Effect of radius ............................................................................................................................................. 9-6

Failure mode ................................................................................................................................................... 9-6Defining the fatigue process ......................................................................................................................... 9-7Appearance of fatigue fractures ................................................................................................................... 9-7

Failure patterns ......................................................................................................................................... 9-7Surface finish effects ................................................................................................................................. 9-8

Residual stress failures ................................................................................................................................ 9-8Shaft fretting ................................................................................................................................................. 9-8Miscellaneous non-fracture types of shaft failures ....................................................................................... 9-8


Principles of Medium and Large AC Motors–IEC Bearings and Lubrication — Section 10

10Bearings and LubricationSection Outline Page

Rolling element bearing fundamentals .......................................................................................................... 10-4Rolling element bearing design options ..................................................................................................... 10-4Factors influencing the selection of bearing enclosures ............................................................................ 10-4

Open bearings ........................................................................................................................................ 10-4Shielded bearings ................................................................................................................................... 10-4Sealed bearings ...................................................................................................................................... 10-5

Rolling element bearing dimensions, fits and tolerances ........................................................................... 10-6Boundary dimensions ............................................................................................................................. 10-6IEC Bearing classes ................................................................................................................................ 10-7Tolerance classes ................................................................................................................................... 10-7IEC parameters ....................................................................................................................................... 10-7Internal clearance ................................................................................................................................... 10-7Bearing fits .............................................................................................................................................. 10-7

Bearing locating or preloading with wavy washer .................................................................................... 10-10Typical bearing lubrication systems for general purpose motors ................................................................ 10-10

Lubrication of rolling element bearings .................................................................................................... 10-12Lubricant compatibility .......................................................................................................................... 10-12

Identifying lubricant compatibility problems ....................................................................................... 10-13Possible lubricant compatibility solutions ........................................................................................... 10-13Use the same grease ......................................................................................................................... 10-13Helpful references for lubricant compatibility issues .......................................................................... 10-13

Grease fill and relubrication intervals .................................................................................................... 10-14Rolling element bearing relubrication procedures and cautions ........................................................... 10-16

Bearing temperature ................................................................................................................................ 10-17Operating temperature range ................................................................................................................ 10-17Bearing and winding temperature relationships .................................................................................... 10-17Why bearing temperature protection is offered ..................................................................................... 10-17Types of temperature protection devices .............................................................................................. 10-18

Switch or relay type ........................................................................................................................... 10-18Dial thermometers ............................................................................................................................. 10-18Dial thermometer with switch ............................................................................................................. 10-18Thermocouples .................................................................................................................................. 10-19


Principles of Medium and Large AC Motors–IECBearings and Lubrication — Section 10


Resistance temperature detectors ..................................................................................................... 10-19Thermistors ........................................................................................................................................ 10-19

Bearing loading and life ............................................................................................................................ 10-20Loading ................................................................................................................................................. 10-20Limiting speed ....................................................................................................................................... 10-20

External motor shaft seals ........................................................................................................................ 10-21Factors affecting seal selection ............................................................................................................. 10-21Bearing isolator–non-contact for severe duty ....................................................................................... 10-21

Rotor assembly end play .......................................................................................................................... 10-22Shaft currents ........................................................................................................................................... 10-22

Causes of shaft currents ....................................................................................................................... 10-22Damage caused by shaft currents ........................................................................................................ 10-22Possible methods to reduce bearing currents for motors on PWM drives ............................................ 10-23

Bearing load capability: How to determine it; and what to do if it is excessive ........................................ 10-25Calculating bearing load and life ........................................................................................................... 10-25

Bearing load calculation ..................................................................................................................... 10-25Standard bearing life calculation ........................................................................................................ 10-25ISO 281 bearing life calculation ......................................................................................................... 10-26

Ball to roller bearing conversion ............................................................................................................ 10-27Installing and tensioning V-belt drives ...................................................................................................... 10-28

Over-tensioning of belts ........................................................................................................................ 10-28Steps for proper installation of V-belts .................................................................................................. 10-28

Sleeve bearings .......................................................................................................................................... 10-31Principles of sleeve bearings ................................................................................................................... 10-31Sleeve bearing design considerations ..................................................................................................... 10-31Lubrication of sleeve bearings ................................................................................................................. 10-33

Circulating oil systems .......................................................................................................................... 10-33Lubricating oil wedge ............................................................................................................................ 10-34

Sleeve bearing velocity and pressure ................................................................................................ 10-35Thrust bearing velocity and pressure ................................................................................................. 10-36

Lubricating oil viscosity ......................................................................................................................... 10-36Dynamic viscosity .............................................................................................................................. 10-36Kinematic viscosity ............................................................................................................................ 10-37

Synthetic oil lube application ................................................................................................................. 10-37New and rebuilt sleeve bearings .............................................................................................................. 10-38

Whitemetal (babbitt) linings ................................................................................................................... 10-38Sleeve bearing clearances .................................................................................................................... 10-40

Bearing-to-shaft diametric clearance ................................................................................................. 10-40Bearing-to-housing clearance ............................................................................................................ 10-41Labyrinth seal-to-shaft clearance ....................................................................................................... 10-41Summary of sleeve bearing clearances ............................................................................................. 10-42

Protection from shaft currents ............................................................................................................... 10-42Start-up ................................................................................................................................................. 10-42Oil level, leaks and related issues ......................................................................................................... 10-43Bearing temperature ............................................................................................................................. 10-44End float and magnetic centre .............................................................................................................. 10-44Alignment of sleeve bearing machines ................................................................................................. 10-45


Principles of Medium and Large AC Motors–IEC Bearings and Lubrication — Section 10


Determining the cause of sleeve bearing failure ...................................................................................... 10-45Initial inspection .................................................................................................................................... 10-45Root causes of sleeve bearing failures ................................................................................................. 10-46

Static overload ................................................................................................................................... 10-46Dynamic overload .............................................................................................................................. 10-47Overheating ....................................................................................................................................... 10-47Friction wear ...................................................................................................................................... 10-48Inadequate lubrication ....................................................................................................................... 10-48Contamination .................................................................................................................................... 10-49Cavitation erosion .............................................................................................................................. 10-50Electrical discharge/arc erosion ......................................................................................................... 10-50Hydrogen diffusion ............................................................................................................................. 10-51Wear patterns within the bearing ....................................................................................................... 10-52Failure of whitemetal lining bond ....................................................................................................... 10-52

Damage matrix for sleeve bearings ...................................................................................................... 10-53Damage matrix Example 1 ................................................................................................................ 10-54Damage matrix Example 2 ................................................................................................................ 10-55

Medium and large vertical motor bearing systems ...................................................................................... 10-56Development of the vertical hollow shaft .............................................................................................. 10-56Vertical motor thrust ratings .................................................................................................................. 10-57Thrust loading ....................................................................................................................................... 10-57Lubricating oil ........................................................................................................................................ 10-57Types of bearings .................................................................................................................................. 10-58Assembly tips for vertical motors .......................................................................................................... 10-68

Hollow shaft motor initial installation and adjustments ............................................................................. 10-69Pump shaft alignment and adjustment .................................................................................................. 10-69Coupling adjustment ............................................................................................................................. 10-69Water cooling for bearing oil reservoir .................................................................................................. 10-70Thrust bearing end play ........................................................................................................................ 10-70

Spherical roller thrust bearings and angular contact bearings (with springs) .................................... 10-70Angular contact ball bearings (without springs) ................................................................................. 10-71

End play adjustment methods ............................................................................................................... 10-71Method 1 ............................................................................................................................................ 10-71Method 2: Spring-loaded bearings only ............................................................................................. 10-71Method 3 ............................................................................................................................................ 10-72Cautions for all three methods ........................................................................................................... 10-72

Coupling adjustment and steady bushings ........................................................................................... 10-73Purpose of steady bushing ................................................................................................................ 10-73Installation of steady bushing ............................................................................................................ 10-73

Non-reverse ratchet feature .................................................................................................................. 10-73Referenced standards ................................................................................................................................. 10-75Notes ........................................................................................................................................................... 10-76



Motor Accessories and Terminal Boxes — Section 11

11Motor Accessories and Terminal BoxesSection Outline Page

Motor accessories ..........................................................................................................................................11-2Areas of protection .....................................................................................................................................11-2Vendor data sheets .....................................................................................................................................11-2

Winding temperature protection .....................................................................................................................11-2Why protect? ...............................................................................................................................................11-2Methods of winding temperature protection ................................................................................................11-3

Winding temperature detectors ................................................................................................................11-3Temperature settings ...............................................................................................................................11-3Thermocouples ........................................................................................................................................11-3Thermistors ..............................................................................................................................................11-4Resistance temperature detectors ...........................................................................................................11-4

Field checks of winding protection system ..................................................................................................11-53-lead resistance cancelling bridge–RTD ....................................................................................................11-6

Bearing temperature protection ......................................................................................................................11-7Space heating to prevent damage from condensation ...................................................................................11-9

Types of electric heating elements ..............................................................................................................11-9Cartridge and strip steel space heaters ...................................................................................................11-9Silicone rubber strip heaters ....................................................................................................................11-9

How to size space heaters ........................................................................................................................11-10Space heater testing .................................................................................................................................11-10Trickle voltage heating .............................................................................................................................11-10

Advantages and disadvantages .............................................................................................................11-10How to determine the proper current level .............................................................................................11-11

Other accessories ........................................................................................................................................11-12Current transformers ..............................................................................................................................11-12Power factor correction capacitors .........................................................................................................11-12Lightning arrestors / surge protection capacitors ...................................................................................11-13Differential pressure switches ................................................................................................................11-13Vibration detection .................................................................................................................................11-14


Principles of Medium and Large AC Motors – IEC Test and Inspection Procedures — Section 12

12Test and InspectionProceduresSection Outline Page

Introduction to motor testing and inspection ................................................................................................. 12-2Test and inspection procedures for medium and large motors ..................................................................... 12-4

Incoming motor inspection and test form ................................................................................................... 12-5AC three-phase motor service order .......................................................................................................... 12-7Final tests of electric motors .....................................................................................................................12-11

Cost of observer-only vs. witnessed tests ..............................................................................................12-11Types of tests .........................................................................................................................................12-11

Heat run test .......................................................................................................................................12-11Performance test ............................................................................................................................... 12-12Locked-rotor test ................................................................................................................................ 12-12Speed-torque test .............................................................................................................................. 12-13No-load saturation test ....................................................................................................................... 12-14Insulation resistance test ................................................................................................................... 12-15Polarization index test ........................................................................................................................ 12-15Voltage withstand (or high-potential) test ........................................................................................... 12-16Surge test .......................................................................................................................................... 12-17Single phase rotor test ....................................................................................................................... 12-18Reed critical frequency test ............................................................................................................... 12-19Coast-down vibration (resonance) test .............................................................................................. 12-19Bearing tests ...................................................................................................................................... 12-20Noise test ........................................................................................................................................... 12-20

Test recommendations ............................................................................................................................. 12-21Referenced standards ................................................................................................................................. 12-21Notes ........................................................................................................................................................... 12-22


Principles of Medium and Large AC Motors – IEC Motor Alignment, Vibration and Noise — Section 13

13Motor Alignment, Vibration and NoiseAlignment and misalignment ......................................................................................................................... 13-3

Types of misalignment ............................................................................................................................... 13-3How misalignment is assessed .................................................................................................................. 13-3Shaft alignment methods ........................................................................................................................... 13-4Soft foot ...................................................................................................................................................... 13-5

Vibration ........................................................................................................................................................ 13-6Vibration conversion formulas .................................................................................................................... 13-7

Sinusoidal vibration ................................................................................................................................. 13-7Converting unbalance to centrifugal force .............................................................................................. 13-7

IEC standard for mechanical vibration ....................................................................................................... 13-7Vibration test conditions .......................................................................................................................... 13-7Bearing housing vibration limits ............................................................................................................. 13-7

Frequency domain ..................................................................................................................................... 13-8Synchronous components ...................................................................................................................... 13-8Subsynchronous components ................................................................................................................. 13-8Non-synchronous components ............................................................................................................... 13-8

Why balance to tight limits? ..................................................................................................................... 13-12Balance standards ................................................................................................................................ 13-12Excitation–forcing phenomena .............................................................................................................. 13-12Critical speeds ...................................................................................................................................... 13-12Reed critical frequency of vertical pump motors ................................................................................... 13-13

Mounting .................................................................................................................................................. 13-14Structural resonance and mounting ...................................................................................................... 13-14Vibration testing on resilient mountings ................................................................................................ 13-14

Motor noise and its measurement ............................................................................................................... 13-17Sound ....................................................................................................................................................... 13-17Noise ........................................................................................................................................................ 13-17

Terms and definitions ............................................................................................................................ 13-17Combining sound levels ........................................................................................................................ 13-17



Principles of Medium and Large AC Motors – IECSection 13 — Motor Alignment, Vibration and Noise

Section Outline PageEnvironment .......................................................................................................................................... 13-17Thresholds ............................................................................................................................................ 13-20

Noise classifications ................................................................................................................................. 13-21Mechanical noise .................................................................................................................................. 13-21Ventilation noise .................................................................................................................................... 13-22Electrical noise ...................................................................................................................................... 13-23Excitation frequencies ........................................................................................................................... 13-23

Sound testing ........................................................................................................................................... 13-26Pressure vs. power ............................................................................................................................... 13-27

IEC noise limits ........................................................................................................................................ 13-27Noise measurement .............................................................................................................................. 13-28Sound power level of polyphase squirrel-cage induction machines ..................................................... 13-28

Referenced standards ................................................................................................................................. 13-29Notes ........................................................................................................................................................... 13-30


Principles of Medium and Large AC Motors–IEC Storage Procedures — Section 14

14Storage ProceduresSection Outline Page

Motor storage procedures ............................................................................................................................. 14-2Record keeping .......................................................................................................................................... 14-2Storage conditions ..................................................................................................................................... 14-2

Short-term storage .................................................................................................................................. 14-2Long-term storage of larger machines .................................................................................................... 14-2

Shafts and machined surfaces ............................................................................................................ 14-2Bearings–rust and corrosion prevention .............................................................................................. 14-2Bearings–ambient vibration ................................................................................................................. 14-2Shaft rotation ....................................................................................................................................... 14-3Special care for windings ..................................................................................................................... 14-3Carbon brushes ................................................................................................................................... 14-3

Placing the motor in service ...................................................................................................................... 14-3Referenced standards ................................................................................................................................... 14-5Notes ............................................................................................................................................................. 14-6



Synchronous Machines — Section 15

15Synchronous MachinesSection Outline Page

Synchronous machine terminology ............................................................................................................... 15-3Synchronous machines ................................................................................................................................. 15-4

Cylindrical-body rotors ............................................................................................................................... 15-5Salient-pole rotors ...................................................................................................................................... 15-8Test methods for synchronous motors ..................................................................................................... 15-10

Brush-type synchronous motors ........................................................................................................... 15-10Brushless synchronous motors .............................................................................................................15-11

Slip rings .................................................................................................................................................. 15-13Testing field coils ...................................................................................................................................... 15-14

Drop test: AC or DC? ............................................................................................................................ 15-14Interpretation of drop test ..................................................................................................................... 15-14

Reducing high operating temperature ...................................................................................................... 15-15Special handling of large motor frames to prevent damage ........................................................................ 15-15Helpful references ....................................................................................................................................... 15-16

principles of medium & large ac motors - easa.com

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