repair book

8/8/2019 Repair Book

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REPAIR SPECIFICATION FOR LOW VOLTAGE

POLYPHASE INDUCTION MOTORS

INTENDED FOR PWM INVERTER APPLICATION2nd Edition

Book 2 of the MotorDoc Series

Originally:

A Final Project

Presented to the

Faculty of the

School of Engineering

Kennedy-Western University

by

Howard W. Penrose, Ph.D.

Old Saybrook, CT


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Disclaimer

Use of this Specification or the information contained in this study does not imply

or infer warranty or guaranties in any form.

MotorDoc E-Book

2nd Book of the MotorDoc Series

1997, 2001

Howard W. Penrose, Ph.D.SUCCESS by DESIGN

5 Dogwood LaneOld Saybrook, CT 06475

e-mail: [email protected]

All Rights Reserved

$99.95


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Repair Specification i

Abstract

REPAIR SPECIFICATION FOR LOW VOLTAGE

POLYPHASE INDUCTION MOTORS

INTENDED FOR PWM INVERTER APPLICATIONS

byHoward W. Penrose, Ph.D.

SUCCESS by DESIGN

Purpose of the Study

With the increasing use of new Variable Frequency Drive (VFD) technologies new types of electric

motor failures have been discovered. These failures are the result of reflective wave, harmonics, motor

cooling, shaft currents, and others, which come from the types of outputs from Pulse Width Modulated

(PWM) inverters. While electric motor and drive manufacturers have been researching and modifying

new motors and drives to reduce these challenges in new applications, very little has been done in the

area of electric motor repair to withstand some VFD applications.

Repairing low voltage polyphase induction motors for potential VFD applications while maintaining

the original characteristics and efficiency of the motor is a possibility. Traditional electric motor


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Repair Specification ii

repair practices have a tendency to damage the motor, reducing its original characteristics and

efficiency. In addition, the damage reduces the motor's ability to withstand VFD applications.

The purpose of this study is to determine the best electric motor repair practices to improve induction

motor survivability in potential inverter applications while maintaining the original efficiency rating of

the motor. The result will be a written electric motor repair specification to be used by electric motor

repair centers and end users for the repair of low voltage induction motors for VFD applications. The

specification is to help reduce the billions of dollars lost, per year, in unnecessary energy costs,

downtime, and repairs. The specification should also be utilized for the repair of all low voltage

induction motors to set the standard for the highest of quality repairs.

Method

Research data was collected for the purposes of the study. The format for the collection of the data

was to separate it into four stages. These four stages included a Previous Research Review, an Electric

Motor Repair Study, an Inverter Effect Study, and a Modified Repair Specification. The research has

determined that a specification, which has additional benefits, is both cost effective and justifiable. It

also has determined that there are additional areas for research which will have significant impacts in

the industry.

The study consisted of reviews of existing and ongoing research in the areas of inverter effects and

motor repair as well as an experiment with a motor and inverter. Research revealed that not only is a

repair standard feasible, but that existing equipment is available to meet a motor repair specification of


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Repair Specification iii

this type. Additional work will continue in this area to improve repair methods and determine new

ways to reduce motor - VFD electro-mechanical challenges.

2 nd Edition

The second edition includes additional information developed from research since its original

publication in 1996. These areas include the mechanical impacts of repair practices, the impact on

newer electric motor designs, and new test methods.


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Repair Specification 1

Chapter 1

INTRODUCTION

Present Situation

Traditional electric motor repair practices are insufficient for electric motors in modern electrical

environments. This situation has been further aggravated by the use of electronic variable frequency

drives (VFD's) in commercial and industrial settings for energy conservation and manufacturing

processes. It is necessary to investigate and present a standard repair specification for low voltage

induction motor repair to reduce premature breakdown following the repair of an electric motor for

VFD environments.

Since 1990 the population of VFD's in industrial and commercial motor systems has

increased due to reduced cost, energy consciousness, and process improvement.

With the introduction of Pulse Width Modulated (PWM) technology using Insulated

Gate Bipolar Transistors (IGBT), the cost and size of modern VFD's have decreased

dramatically. This is compounded with the increased understanding by end users that

modern VFD's can usually pay for themselves, with energy savings, in a very short

period of time, in most cases under two to three years. Many industrial firms have

discovered that modern VFD's can help increase production while reducing


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has a minimum operating speed for cooling purposes which, if the speed falls below this value, will

cause it to overload.

Figure 1-2: VFD Basics

Traditional induction electric motor repair leaves much to be desired for the harsh electrical

environment imposed by modern VFD's. In most cases, electric motor repair centers reduce costs,

wherever they can, in order to be competitive. Some of the short cuts include reducing wire size,

reducing insulation, machining short cuts, and shortening oven curing times in dip and bake processes.

Equipment costs are reduced through the use of burnout ovens, manual coil winding machines, and

varnish tanks. Traditional practices tend to:

1 Increase various motor losses resulting in higher operating temperatures and hot spots which

reduce insulation life.

2 Increase winding impedances resulting in greater susceptibility to reflective wave.

3 Poor machining practices which result in a greater chance that shaft currents will affect motor

bearings.


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4 Reduced insulation resulting in a greater chance that the motor will fail from reflective wave,

increased operating temperatures, or insulation imperfections.

5 Reduce motor operating efficiencies due to increased losses.

Purpose of Study

The purpose of this study is to determine the best electric motor repair practices to improve induction

motor survivability in potential inverter applications while maintaining the original efficiency rating of

the motor. The result will be a written electric motor repair specification to be used by electric motor

repair centers and end users for the repair of low voltage induction motors for VFD applications. The

specification is to help reduce the billions of dollars lost, per year, in unnecessary energy costs,

downtime, and repairs. The specification should also be utilized for the repair of all low voltage

induction motors to set the standard for the highest of quality repairs.

Definitions

Following are a series of definitions necessary for the purposes of this project:

1 End user - A term used for the owner of the electric motor and equipment or theirrepresentative.

2 Repair Center - A business whose purpose is to repair and rewind electric motors.

3 Electric Motor - A device for converting electrical energy to mechanical torque. A basic

electric motor consists of a stator, endshields, rotor, bearings, and, depending on the

enclosure, a fan and fancover (Figure 1-3). The stator houses the stator core and windings

and acts as a shield from foreign material. The endshields house the bearings and center the


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rotor within the stator. The rotor consists of the rotor windings and core. Bearings are used

to allow for the low friction turning of the rotor.

Figure 1-3: Electric Motor Cutaway (GE Motor)

4 Stator and Rotor Core - The core of the stator and rotor consists of many layers of laminated

low carbon annealed, or silicon in newer motors, steel which is used as the medium for

magnetic fields. Laminations are often 0.019 to 0.045 inches thick to reduce eddy currents

(0.019 to 0.039 in energy efficient electric motors). The chemical makeup of the material is

such that it reduces the effects of hysterisis.

5 Stator Windings - Are made of many turns of insulated copper wire which are laid in slots

within the stator core. The basic winding styles are concentric and lap. Concentric windings

are often used for machine insertion at the factory or repair center. Lap windings are often

used when hand winding for mechanical and electrical strength.


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6 Rotor Windings - Are mounted within slots in the rotor laminations. They are often made up

of a copper or aluminum alloy with aluminum being the most common in motors under 600

VAC. The windings are in the form of solid bars through the rotor with shorting rings at either

end. The windings are energized as the result of magnetic fields cutting through the rotor

windings from the stator. A current flows through the rotor windings and interacts with the

magnetic fields in the airgap of the motor (The space between the rotor and stator

laminations), and a torque is developed.

7 Motor Losses - The energy that is consumed by electric motors is accounted for in heat losses

(Watts). These losses are broken down as core losses (eddy current and hysterisis), friction

and windage losses, stator losses, rotor losses, and stray load losses.

Figure 1-4: Motor Losses

0

0.5

1

1.5

2

2.5

3

3.5

4

25% 50% 75% 100% 125%

StrayRotorStatorFrictionCore

8 Core Losses - A combination of eddy current and hysterisis losses within the stator core.

Accounts for 15 to 25 percent of the overall losses.

9 Friction and Windage - Mechanical losses which occur due to air movement and bearing

friction. Accounts for 5 to 15 percent of the overall losses.


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10 Stator Losses - The I 2R losses within the stator windings. Accounts for 25 to 40 percent of

the overall losses.

11 Rotor Losses - The I 2R losses within the rotor windings. Accounts for 15 to 25 percent of the

overall losses.

12 Stray Load Losses - All the other losses not otherwise accounted for. Accounts for 10 to 20

percent of the overall losses.

13 Full Load Torque - The full load torque of a motor is the torque necessary to produce its

rated horsepower at full load speed. In pounds at foot radius, it is equal to the horsepower

times 5250 divided by full load speed.

14 Locked Rotor Torque - The locked rotor torque of a motor is the minimum torque which will

develop at rest for all angular positions of the rotor, with rated voltage applied at rated

frequency.

15 Pull-Up Torque - The pull-up torque of an alternating current motor is the minimum torque

developed by the motor during the period of acceleration from rest to the speed at which

breakdown torque occurs. For motors which do not have a defined breakdown torque, the

pull-up torque is the minimum torque developed up to rated speed.

16 Breakdown Torque - The breakdown torque of a motor is the maximum torque that will

develop with rated voltage applied at rated frequency, without an abrupt change in speed.

Figure 1-5: Motor Design and Torque


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17 Variable Frequency Drive (VFD) - A variable frequency drive is a device used to vary

voltage and frequency to a polyphase induction motor to vary the speed of the motor. The

purposes vary from energy efficiency applications to precise control of machinery.

18 VFD Input Section - Consists of a rectifier and filter. The purpose is to transform the

incoming AC voltage to DC voltage, also known as the bus voltage.

Figure 1-6: Input Section of a Variable Frequency Drive


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19 VFD Control Section - Consists of a control board and firing circuits. The control section

accepts real world input and performs the required operations through the use of a

microprocessor.

20 VFD Output Section - The output section includes the base drive circuits and the inverter.

The base drive sends signals which tell the inverter which IGBT's (or other types of power

transistors) to turn on and in which order. There is a diode in parallel with the IGBT, known

as a free wheeling diode, which allows energy stored in the collapsing motor fields to return

to the circuit when the IGBT turns off.

Figure 1-7: Inverter Section

21 Harmonics - Harmonics, generated by variable frequency drives, generally are fed back into

the power system and towards the induction motor. Voltage and current harmonics tend to

create alternate fields within motors and rotors, cause transformers to overheat, and interfere

with other electronic systems. Odd harmonics of the fundamental frequency are generally

found in power electronic systems. VFD's and three phase electronic systems generate 5th

and 7th harmonic distortion. Multiples of the 5th harmonic cause counter rotating fields in an


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electric motor, while multiples of the 7th harmonic cause positive rotating fields in an electric

motor. Combined 5th and 7th harmonics generate a 6th rotor harmonic. Harmonics generate

heat within the motor core, increase losses, and, if large enough, can cause the motor to

overload.

22 Reflective Wave Phenomenon - An interesting phenomenon which occurs in PWM drives is

known as reflective wave. This is a case where the fast rise time (dV/dt) of each pulse causes

a steep voltage wavefront that is directed towards the electric motor. As a result of a

combination of voltage overshoot on the switching portion of the IGBT, the switching

frequency, dV/dt, and the impedance of the motor cables and windings, voltage levels at the

motor terminals can exceed 1600 VAC in under 0.2 microseconds in a 480 VAC application.

This means that, at certain critical cable distances and high carrier frequencies, motor

winding conductors, which are generally rated for 500 Volts per micro-second withstand, will

short causing catastrophic motor failure.

23 Partial Discharge Occurs in small voids in insulation between conductors and conductors

and ground. These discharges result in ozone, which damages the insulation system until the

winding faults.

24 Shaft Voltages - The type of voltage and current entering an electric motor from a VFD cause

imperfect currents to flow within the motor rotor. In some cases, this will cause eddy

currents to flow within the rotor shaft. These currents will want to flow out one motor

bearing into the motor housing and back through the other bearing. The bearings act like

capacitors, with energy collecting on the inner race of the bearing until it discharges through

the balls and outer race. The grease within the bearing acts as an electrolyte, and discharges

tend to occur where bearing surface imperfections cause high spots on the bearing housings


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and balls to come in close contact with each other. These discharges cause minute pits to

form on the machined surfaces causing early bearing failure.


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Chapter 2

REVIEW OF RELATED LITERATURE

Most Discussed Topics

Current literature surrounding the topics of electric motors and VFD's cover both energy efficiency and

motor failure due to inverter operation. Energy efficiency is a major topic as motors used in variable

torque applications can have paybacks well under one year. The topic of VFD effects on electric

motors tend to focus on five major areas:

1 Harmonic Distortion and how it effects the motor and other electrical equipment.

2 Reflective wave and steep fronted wave phenomenon and reduced insulation life.

3 Shaft voltages and reduced bearing life.

4 Increased temperature rise.

5 Minimum operating speeds and motor cooling.

Traditional electric motor repair practices are inadequate for electric motors which are being applied

with VFD's. If repaired electric motors are to be used with modern VFD inverters, the types of failures

must be addressed and corrected for during the repair.

Motors, Drives, and Energy Efficiency

With the advent of the Energy Policy Act of 1992 (EPACT), the US Department of Energy (US DOE)

set the Office of Industrial Technologies (OIT) to educate end users and distributors on the benefits of


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energy efficient motors. OIT assigned members of the US DOE Motor Challenge Group to pursue this

task and create cooperation between the government, industry, distributors, and manufacturers in order

to reduce energy consumption in the area of electric motors. While it was first determined that electric

motors were consumers of electrical energy, it was later discovered that electric motors were

converters of energy, rivaled only by transformers in efficiency. A group of individuals, representing

all aspects of the motor industry, met to put together Motor Challenge '95 in Chicago and determined

that electric motors were only part of an "electric motor system."

Electric motor systems account for 20% of all energy consumed in North America, with 57% of all

electrical energy generated. It was also found that roughly 70% of all electrical energy consumed by

manufacturers and industry, 48% of commercial electrical consumption, and 43% of home energy

consumption. When studied, it was found that a motor system is broken down into several

components: Incoming power; Motor control; Electric Motor; Coupling; Load; and Process. It was

also found that each component had an average opportunity for energy improvement (USDOE, 1994):

1 Power Quality Improvement - 8%

2 Motor Control (including VFD's) - 44%

3 Retrofitting with energy efficient electric motors - 18%

4 Coupling through Process - 33%

For example, a 250 horsepower (hp), 1800 Revolutions Per Minute (RPM), 92% efficient electric

motor versus a 250 hp, 1800 RPM, 94% motor would have a payback of $1,035/year (Formula 2-1) at

4000 hours, full load, and $0.06/kWh. A VFD in a variable torque application operating for 4000

hours per year, the energy efficient electric motor, 1000 hrs/yr at 100% speed, 1500 hrs/yr at 75%


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speed, and 1500 hrs/yr at 50% speed, would yield a payback of $25,999/yr (Formula 2-2). As it can

be seen, VFD's can yield a much larger payback due to pump and fan affinity laws (hp varies by the

cube of the operating speed).

Formula 2-1: Energy Payback of Energy Efficient Motors

250hp x .746kW/hp x 1.0 Load Factor x $0.06/kWh x 4000hrs x

[(100/92) - (100/94)] = $1,035/ year

Formula 2-2: Energy Payback for VFD's

100% Speed = 250 hp; 75% Speed = 105 hp; 50% Speed = 31 hp

[4000hrs x .746 kW/hp x (250hp/.94)] - {[1000 hrs x .746 kW/hp x (250hp/.94)] +

[1500 hrs x .746 kW/hp x (105hp/.94)] + [1500 hrs x .746 kW/hp x (31hp/.94)]}

= 433,314 kWh less energy/yr x $0.06/kWh = $25,999 / yr

As it can be seen, in the above example, the application of VFD's carry a tremendous financial benefit

in Variable Torque applications. However, with the benefits come a potential problem, how modern

VFD's affect the life of the electric motor and motor system.

Variable Frequency Drive Effects On Electric Motors Variable Frequency Drives directly effect the electrical, mechanical, and thermal capacity of an

electric motor (EMCW '96, 1996, p. 109). These effects are in the form of harmonic distortion,

reflective wave, shaft currents, temperature rise, and minimum operating speeds.


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"A major effect of harmonic voltages and currents in rotating machinery is increased heating due to

iron and copper losses at the harmonic frequencies. The harmonic components thus effect the machine

efficiency, and can also affect the torque developed." (IEEE 519-1992, p. 35) The 5th and 7th

harmonics present in the inverter side of the VFD create a 6th rotor harmonic, which causes excessive

rotor heating, especially in areas where the laminations are smeared or shorted. The 5th and 7th also

cause excessive heating in the stator core where laminations are shorted or smeared. "Harmonics

create excessive heat in motors. Most motors are cooled by an internal fan driven by the motor. As

speed is decreased, the cooling capacity of the motor is decreased. The amount of heat the motor can

withstand depends also on the insulation of the windings." (Bonneville Power Administration, 1990,

p. 20) Harmonic distortion also causes uneven flux distribution in the air gap causing a situation know

as cogging (IEEE 519-1992, p.35). Combined, harmonic distortion problems from VFD's cause

additional heating and other electro-mechanical effects in electric motors.

"Fast rising voltage pulses cause non-linear voltage distribution in the winding (ie: uneven voltages

between coils and between turns). The shorter the rise time, the higher are the voltages between coils

and turns." (Voltage Reflection, 1994, p.7) This phenomenon is known as voltage reflection, which is

a case where the terminal voltage at the electric motor can exceed 1.9 times the VFD's DC bus voltage

(ie: 480 VAC application = 1250 V DC in less than 0.2 microseconds). The interturn wire insulation is

designed to withstand between 1000 and 1500 VDC in 2 microseconds, or greater. Fortunately, this

phenomenon occurs only at critical distances based upon the frequency of PWM pulses (carrier

frequency) and the impedance of the cable and motor. "... the electrical stress is limited to 60 Hz and

600 V rms AC in accordance with IEEE Standards. Unfortunately, these standards do not specify the


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maximum repetitive voltage transients or rate of rise (dV/dt) that the winding can safely withstand and

still meet the life expectations." (Industry Applications, 1996, p. 386).

"Recent experience suggests that PWM voltage sources with steep wavefronts especially increase the

magnitude of the above electrical problems, leading to motor bearing material erosion and early

mechanical failure." (Industry Applications, 1996, p. 250) Penrose (EMCW '96, 1996, p. 111) states

that bearing life is affected by electrical current flowing through the shaft, bearings, and motor

housings causing pitting in the bearing balls and surfaces. This has the effect of drastically reducing

the mechanical life of the electric motor. Values greater than 0.4 V may develop harmful bearing

currents while values over 2.0 V are considered catastrophic. (Industry Applications, 1996, p.252)

Traditional Motor Repair and VFD's

Penrose (Electrical Insulation, 1997) explains that peening and metalizing can cause mechanical

stresses in motor bearings, reducing bearing life. This stress is primarily caused by reduced clearances

within the bearing between the ball surfaces and the races. This increases the opportunities for shaft

currents to discharge as well as increasing friction, reducing efficiency.

"Fan replacement should also be considered, when the original fan has been damaged. The

replacement fan should be original, as well. If a fan is replaced by a larger fan, or one with more fins,

the motor efficiency will be reduced. If a fan is replaced by a smaller fan, or one with fewer fins,

cooling will be reduced, reducing the life of the motor." (Electrical Insulation, 1997) With the

increased heating inherent with VFD's, reduced fan size will reduce the life of the motor further.


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"The magnetic properties of the stator cores are damaged during burnout. The high temperatures

break down the lamination insulation along with the winding insulation creating paths for eddy

currents and hot spots... Often the stator core warps, causing irregular magnetic field paths and in turn

causing odd rotor currents." (E/EIC / EMCW '95, 1995, p. 458) These problems are aggravated by

the type of output of a variable frequency drive increasing shaft voltages and increasing hot spot

temperatures.

"Each coil's length may be different [in traditional coil winding] due to operator controlled tensioning,

causing unbalanced impedance values... If the turns are not layered correctly, coil insertion times will

increase and the chances of damaging the wire insulation are greater." (E/EIC / EMCW '95, 1995, p.

458) In addition, there may be a great number of wire crossovers, creating high potential points where

voltage can flash across, shorting the wire and failing the motor.

"... the varnish coating in the stator is wasted and acts as a thermal insulator, increasing internal

temperatures and stator and rotor I 2R losses, and may clog cooling paths through the core. Dip and

bake provides moderate resistance to humidity due to voids in the winding insulation, particularly in

the stator slots." (E/EIC / EMCW '95, 1995, p. 458) In addition to increased heat, the voids in the

insulation system create weak spots which can flash over in VFD environments.

Summary

As it can be seen, there are a number of areas within traditional electric motor repair which can be

improved through alternate motor repair practices and motor repair standards for inverter application.


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These methods would maintain motor efficiency, reduce repair turnaround, and maintain acceptable

motor life in inverter applications.


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Chapter 3

METHOD

Project Approach

This research project shall consist of four stages with the final conclusion consisting of a repair

specification for three phase induction motors under 600 V AC. The four stages consist of the

following:

1 Previous Research Review

2 Electric Motor Repair Study

3 Inverter Effect Study

4 Modified Repair Standard

Stage 1: Previous Research Review

The first portion of the project shall consist of a review of previous research studies of inverter

applications and electric motor repair. The purpose is to further review research in these areas in order

to determine the necessary procedural changes to existing electric motor repair standards and practices.

Sources include the US Department of Energy (US DOE), Canadian Electrical Association (CEA),

Trade Journals, and The Institute of Electrical and Electronics Engineers, Inc.'s (IEEE) publications.

In addition, ongoing research by the Dreisilker Electric Motors, Inc.'s Research and Development

Department are reviewed. Notes, articles, and research papers are kept on file for the duration of the

project.


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Stage 2: Electric Motor Repair Study

Motor repair processes shall be analyzed through observation and research. The types of insulation,

stripping processes, machining and machine tolerances, winding methods, testing, and varnishing

processes shall be reviewed. A review of existing repair standards published by the Electrical

Apparatus Service Association (EASA) and IEEE will be compared to the existing practices. The

effect of repair on induction motors, which may be used in inverter applications, shall be determined

by reviewing collected data and the materials and practices observed.

Stage 3: Inverter Effect Study

A Pulse Width Modulated (PWM) Variable Frequency Drive (VFD) was set up to allow possible

maximum reflection pulses. The drive is to be connected for 480 V AC with a carrier frequency of

approximately 16 kHz or better then to a 480 V AC, 1800 RPM motor set at about twice the

recommended distance, per the maintenance manual. Both the motor and drive are to be matched.

The following is to be collected using a Fluke 97 Digital Storage Oscilloscope and a Fluke 41B Digital

Storage Harmonics Analyzer.

1 PWM Voltage output at the inverter (1)

2 PWM Current output at the inverter (1)

3 Harmonic Content (Voltage and Current) at the inverter output (2)

4 PWM Voltage input to the motor measured at the motor terminals (3)

5 PWM Current input to the motor measured at the motor terminals (3)


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The data is to be collected and stored on a Toshiba, Pentium, 120 MHz, Laptop for inclusion into

Chapter 4 of this project. The output of both instruments can be captured and the output imported into

WordPerfect 6.0 for Windows(tm) files.

Motor Nameplate:

1 General Electric

2 Model 5K6184BX205B

3 5 horsepower

4 1745 RPM

5 230 / 460 VAC

6 3 Phase

7 60 Hz

8 12.8 / 6.4 Amps

9 83.5 % efficiency

10 40 oC Ambient

11 Insulation Class B

12 184T Frame

13 Type KS

Variable Frequency Drive:

1 General Electric

2 Model 6KAF343005E$-A1

3 5 horsepower

4 460 VAC

5 3 Phase


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6 Serial Number 615236R078

The critical distance for the VFD is 60 feet, the drive to motor cable distance is to be 110 feet and the

carrier frequency set at 16 kHz. The calculated distance would be as figured in Formula 3-1, 2, 3 and

4.

Formula 3-1: Speed of Pulse Propagation (v)

v = 1 / _(I * C)

I = Inductance per Meter; C = Capacitance per Meter

Formula 3-2: Cable Characteristic Impedance (Z C)

ZC = _(I / C)

The Motor Characteristic Impedance (Z M)

Formula 3-3: Reflection Factor at Motor (R M)RM = (ZM - ZC) / (Z M + ZC)

Formula 3-4: Critical Cable LengthLCR = (v * t r) / 2

tr = the rise time of the pulse

Figure 3-1: Test Points


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Stage 4: Modified Repair Standard

Through observation and experiment, the summary of this study consists of a Repair Specification for

Low Voltage Polyphase Induction Motors Intended for PWM Inverter Application. The Specification

is meant to reduce the effects of inverter application on repaired motors while maintaining relatively

low repair costs, improving repair quality, and maintaining the efficiency value of the motor. The

standard also includes quality test sheets for maintaining records of motor testing associated with the

standard.


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Chapter 4

RESEARCH ANALYSIS

Stage 1: Previous Research Review

Continued study of the effects of PWM waveforms from IGBT inverters on induction motors is

outlined at this stage. Harmonic distortion, reflective wave, shaft currents, temperature rise, and

minimum operating speeds are covered through the review of previous research. Not surprisingly,

there is a great deal of information on the problems associated with this type of application, but few

published papers discuss solutions.

It is clear that the output waveforms of modern Variable Frequency Drives have a direct bearing on

electric motor life and premature failure. While the failures tend to be electro-mechanical in nature, it

is readily apparent that most engineers focus on either the electrical or mechanical effects depending

on their particular discipline. However, to truly understand the implications of VFD's on modern

induction motors, both the electrical and mechanical causes of failure must be understood.

Steep fronted surges and pulses created through the use of IGBT's are the primary culprits of electro-

mechanical failure. These are usually combined with improper application of the motor and drive (ie:

the installer did not read the instructions). Factors influencing the PWM effects are (Persson, 1992, p.

1095):

1 RMS input voltage to the inverter.


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2 The rise time of the inverter output voltage.

3 Cable length from the inverter to the motor.

4 Motor insulation system, including wire, slot, and varnish insulation.

5 Stator winding design.

After reviewing many of the new studies, it is apparent that most of them focus on the interturn failures

of the winding conductors. In a few cases, it is noted that the entire insulation system must be

considered when viewing winding insulation failures. As noted in "Failure Mechanism of the Interturn

Insulation of Low Voltage Electric Machines Fed by Pulse Controlled Inverters," (Electrical

Insulation, 1996, p. 9) the causes of failure are due to partial discharges between conductors of

different potentials. This is further supported by Les Manz in "Motor Insulation System Quality for

IGBT Drives." (Industry Applications Magazine, 1997, p. 51) It should also be noted that the

switching frequency of the inverter plays a major part as it affects the distribution of the surges and

standing waves in the electric motor (Persson, 1992, p. 1095). The greater the frequency, the less the

effects are able to penetrate deep into the motor windings. This led to the original belief that standing

wave and steep fronted surge failures would be found as shorted or burned insulation in the first few

turns of each phase.

Interturn conductor failures were soon found to occur in other areas of electric motors. Phase to phase,

coil to coil, and deep turn to turn shorts had been found. However, these failures were not identified as

inverter related failures as few individuals understood how to identify them. It was later determined

that the winding design played an important part in the survival of electric motors in Variable

Frequency Drive applications. Concentric wound motors were found to be more likely to fail due to

high potential between the conductors in the coils of each phase. When the conductors are allowed to


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touch between coils of the same phase, the high potential voltage between the conductors and the

repetitive voltage pulses from the VFD, cause partial discharges across the insulation, resulting in

failure. Lap wound stators have a lower potential between conductors in each phase, coil to coil, and

have a higher resistance to coil to coil failures. Phase to phase failures occur where the phases are not

properly insulated from each other. This will be the result when a manufacturer will cut costs by

reducing the amount of insulation in the motor, creating a very high potential between conductors of

two separate phases. If the coils are not properly rewound and inserted in the stator, there is the

potential of a first and last turn touching or one conductor crossing several others. This creates an

opportunity for partial discharge between the conductors ending with a turn to turn short in a coil. It is

also noted that these failures occur primarily in motors operating on 460 + voltage systems, at critical

distances, and without added inductance between the motor and drive.

Motor speed is another consideration when operating an induction motor with a Variable Frequency

Drive. The concept behind the use of a VFD is to vary the speed of the electric motor. A new barrier

is introduced: What is the minimum operating speed of the motor at which point it will no longer

effectively cool itself and overload? This is compounded by the fact that VFD's cause the electric

motor to operate much warmer than it would normally. This is the result of a waveform which appears

"dirty", harmonic distortion, and electrical stresses from the voltage pulses. On average, most motors

can drop to around 50% speed without requiring additional cooling from an externally operated fan.

Many manufacturers now use Class F (Table 4-1) insulation, as opposed to class B, in order to allow

the electric motors to operate at higher temperatures.


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Table 4-1: Insulation Class Rating

System Class Temperature, oC Temperature, oF

A 105 221

B 130 266

F 155 311

H 180 356

Another result of the output of a Variable Frequency Drive on an electric motor is the potential for

increased shaft currents in the electric motor. The erratic currents within the stator create

imperfections within the magnetic fields in the air gap. This impresses currents within the rotor shaft

which travel through the bearings, reducing the life of the bearings dramatically (Transactions on

Industry Applications, 1996, p.250). The currents also occur as a direct result of capacitive coupling

between the conductors, stator frame, and rotor.

As noted in "Those Pesky Inverter Drives," (Kevin Jones, 1996, p. 25) Kevin Jones notes that most

electric motor repair shops are just realizing the necessity to improve electrical insulation in repaired

motors to avoid inverter failure. Wire manufacturers are taking advantage of the present publicity by

developing wire insulation meant for inverter duty. This, however, takes away from the view of the

total motor insulation system and its importance over just one component (wire insulation). In effect,


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the total insulation system from wire to slot and phase insulation, not to mention varnishing methods,

must be observed.

Summary of Inverter Effect and Varnishing Systems

It has been suggested that motors repaired for inverter applications would withstand Variable

Frequency Drive effects through the use of Vacuum Pressure Impregnation (See Stage 2) versus dip

and bake or trickle varnish processes. In order to review this area, the effects of inverter output on

induction motors shall be summarized as well as the results of the three varnishing processes.

Modern Variable Frequency Drives affect motor insulation systems in a very specific manner. The

newer VFD's use a technology known as "Pulse Width Modulation (PWM)." The input of the VFD

uses a diode rectifier to create a constant DC Bus voltage filtered with a capacitor bank and inductors.

The inverter section consists of six (normally) power transistors known as Insulated-Gate Bipolar

Transistors (IGBT's). These are fired in a specific sequence through internal drive logic (main and

driver boards) too create a series of pulses at DC Bus levels at varying duration in either the positive or

negative direction. The result is a nearly sinusoidal current to the electric motor, although it will

actually appear kind of ragged. The greater the number of pulses (generated by the control, or

switching, frequency) and how they are fired, the smoother the current waveform. The "dirtier" the

current waveform, the more heat is generated in the electric motor due to increased heat loss.

The pulses in a PWM inverter will operate at switching frequencies from 2 kHz to 20 kHz. The lower

the frequency, the more "noise" the motor makes. This noise resembles that of bearing failure, but is

actually caused by expanding and collapsing magnetic fields in the airgap of the motor, but is not


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harmful. So, especially in commercial applications, the switching frequency may be set towards the

higher end in order to reduce noise (the motor rotor bar design also has an effect). IGBT's also have a

very fast turn on rate, as little as 0.1 microseconds.

When the IGBT turns on, there is a little overshoot, due to the drive capacitors discharging, and

ringing at the edge of each pulse. This creates a high, steep, travelling wavefront approaching the

motor, which, at short distances, has limited effects. At greater distances, line transmission effects

come into play, as well as voltage reflection as the wavefront approaches the high impedance of the

motor through low impedance cable. These can cause a voltage doubling at the motor terminals with a

very steep wavefront which can penetrate deep into the motor windings, depending on the frequency of

the pulses (the lower the frequency, the deeper into the windings). The weakest point in the insulation

system will usually fail.

The failures normally occur in 460 V + applications which are installed incorrectly. This is usually a

motor and drive which are installed at a critical distance without installing line reactors to reduce the

voltage and rise time of the pulses. The other cause is the motor construction. Areas within the coils

where a first and last turn touch (sloppy winding), concentric coils in a delta wound motor that has the

coils touching, reduced motor insulation (including varnish voids), and sharp turns in the coil ends. A

number of things have been done, or claimed, in order to reduce "inverter failures," which are actually

quite rare, to include VPI, "inverter duty wire," etc. Each, alone, will not eliminate these challenges,

but have to be considered as a group.


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There are three types of varnish application systems which have their pro's and con's for this type of

application:

1 Dip and Bake (Usually epoxy varnish): An inexpensive and common varnishing system.

Most motor shops and manufacturers can afford the equipment. When done correctly, two

dips and bakes without shortcuts, and with correct application of the rest of the insulation

system, Dip and Bake can be acceptable for inverter use. However, another type of failure

may result - Partial Discharge (PD). At 60 Hz PD will normally occur in systems over 6000

VAC, at higher frequencies PD may occur at much lower voltages. PD occurs where voids

between conductors exist, ie: bubbles in the dip varnish within a coil. Charges, much like

those in a capacitor, occur in the gasses within the voids and discharge, reducing the

insulation life. These voids may also capture moisture which, when the inception voltage of

the wire insulation is overcome, will cause a short.

2 Vacuum Pressure Impregnation (VPI): Very expensive equipment and varnish. The

difference between this system and dip and bake is the cost, it does not eliminate voids in

windings which increase the chance for failure due to Partial Discharge. While the concept is

reasonable, it is not realistic. The Medium Voltage motors which go through the VPI process

have taped windings which holds the varnish in, low voltage induction motors do not. The

low voltage motor is allowed to drain before it is placed in a curing oven. Voids occur in the

windings as the motor drains as the varnish does not cure at this point.

3 Trickle Varnishing (Normally Polyester): A relatively inexpensive system, falling

somewhere between dip and bake and VPI. Curing occurs as the process is being applied and

produces a relatively low amount of waste varnish. The varnish flows through the windings


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due to gravity and capillary action, removing any voids. The final result is equivalent to

three dips and bakes, minus voids.

Rewind Studies

In 1991, a study was published by Ontario Hydro on electric motor rewind. The purpose was to

determine the effects of rewind repair on electric motor efficiency by failing nine of ten twenty

horsepower which were then randomly sent to electric motor rewind shops. The motors were of a

standard efficiency design and used an older annealed steel core. The average loss of efficiency was in

the area of 1.1% with the greatest reduction around 3.4%, the average was 2.2%. Compounded by the

loss in efficiency, the motors were found to have had conductor cross sectional area reduction, core

damage resulting in hot spots, and reduced insulation systems. The stators were all burned out in

burnout ovens and the stators dipped and baked. In inverter applications, the cost saving shortcuts and

stripping methods will reduce the life of the motor further.

In 1993, a similar study was performed by BC Hydro on ten of eleven energy efficient motors. The

results were slightly different than the Ontario Hydro study as the average loss was 0.5%. The core

steels were of silicone steel manufacture and had little or no increase in core losses. The greatest

losses were found to be in increased bearing losses due to friction. The one motor which had the least

reduction in efficiency failed in a later inverter test due to improper coil insertion and scratches on the

wire insulation due to improper insertion. The losses due to bearing replacement indicate that lower

quality bearings were used. The lower quality bearings would have a much reduced life due to shaft

currents.


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The Bonneville Power Administration's US Department of Energy sponsored "Industrial Motor Repair

in the United States," while studying the effects of repair on energy efficiency, did not focus on motor

repair. Additionally, the study focused on surveys and not on site analysis of motor repair. The

resulting "Electric Motor Model Repair Specifications" utilized the repair specifications of one repair

shop and overlooked many accepted repair methods. The repair methods, however, represent the

majority of repair methods of an average medium to high quality repair shop.

In 1995, the Canadian Electrical Association (CEA) performed a controlled electric motor repair

study. Demand Side Energy of Vancouver, BC, in conjunction with Hydro Quebec, a motor repair

industry organization, and Dreisilker Electric Motors, Inc. repaired several motors using a burnout

oven and the Dreisilker Thumm method. All of the motors in the program underwent the same repair

process, including dip and bake. It was found that in controlled repair processes there is little or no

reduction in efficiency after three rewinds.

In 1998, Dreisilker Electric Motors, Inc. and a Senior Research Engineer from the University of

Illinois at Chicagos Energy Resources Center performed a study on the mechanical impact of

stripping temperatures on the electric motor. It was found that temperatures above 600 oF would cause

the stator frames to distort, generating soft-foot conditions (0.002 to 0.050 inches) in materials ranging

from cast iron to aluminum (aluminum had greatest impact, cast iron had the least).


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Stage 2: Electric Motor Repair Study

Electric motor repair is normally not considered by most until a motor actually fails and causes process

or environmental inconvenience. The failed motor would be pulled from the customer location and

sent to a motor repair shop for evaluation. Past operating histories are not normally made available to

the motor repair shop or even considered by either the shop or customer. The motor may end up being

repaired repeatedly before it is noticed that a cause of failure needs to be investigated, including

inverter related failures.

Following outlines the various traditional repair methods utilized for electric motor repair (Electrical

Insulation, 1997, p. 14):

Upon receipt of an electric motor by an electric motor shop, a number of tests are normally performed

including a Megger test, phase to phase continuity test, and no-load test run. The Megger test

measures leakage to ground by applying 500 V DC or 1000 V DC (for up to 575 Volt motors) in order to

determine if the motor is grounded. A reading of 1.5 Megohms is the absolute minimum with several

hundred Megohms recommended for safe operation. Either the Megger or a multimeter is used to

check for continuity between phases in order to detect opens. If both sets of readings are acceptable,

most repair shops will test run the motor at full voltage and no-load. The voltage, speed, and current

are measured and recorded for future reference. At this stage, many of the motor defects may be

detected and noted.


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The motor is disassembled after marking the endshields and attachments for location and position. All

of the parts are removed, bearings pulled, and rotor removed from the stator. Visible internal defects

are observed and noted. The parts are cleaned and all windings are baked at 290 oF for at least eight

hours.

Winding and mechanical tests are performed in order to evaluate the condition of the parts for

quotation purposes. A Megger test is performed at 500 or 1000 V DC with a minimum of (1 Megohm +

1 Megohm / Volt) for safety and several hundred Megohms recommended. An AC or DC Hi-Potential

test is performed at a voltage as found in formula 4-1. The AC test is a pass / fail test as, if the

winding fails the test

Formula 4-1: Hi-Potential Voltage Calculation

AC Potential = 0.65 * (2E m + 1000 V)

DC Potential = 0.65 * (2E m + 1000 V) * 1.7

Em = Motor Voltage Rating

a path to ground is formed through the motor insulation. A DC test is more forgiving, as it involves a

series of voltage increases (steps) up to the calculated level.


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Table 4-2: Bearing Shaft Fits (k5)

Bearing Number Average Bore (mm) Bearing Fit Tolerance (inches)

Minimum Maximum

03 17 0.6693 0.6697

04 20 0.7875 0.7878

05 25 0.9844 0.9847

06 30 1.1812 1.1815

07 35 1.3781 1.3785

08 40 1.5749 1.5753

09 45 1.7718 1.7722

10 50 1.9686 1.9690

11 55 2.1655 2.1660

12 60 2.3623 2.3628

13 65 2.5592 2.5597

14 70 2.7560 2.7565

15 75 2.9529 2.9534


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16 80 3.1497 3.1502

If there is a sudden increase in the measured leakage, then the winding has failed. A surge

comparison test is performed at a level calculated as the Hi-Potential test.

Table 4-3: Bearing Housing Fits (H6)

Bearing No. 200 Series Housing Bore 300 Series Housing Bore

ODmm Min inch Max inch ODmm Min inch Max inch

03 40 1.5748 1.5754 47 1.8504 1.8510

04 47 1.8504 1.8510 52 2.0472 2.0479

05 52 2.0472 2.0479 62 2.4409 2.4416

06 62 2.4409 2.4416 72 2.8346 2.8353

07 72 2.8346 2.8353 80 3.1496 3.1503

08 80 3.1496 3.1503 90 3.5433 3.5442

09 85 3.3465 3.3474 100 3.9370 3.9379

10 90 3.5433 3.5442 110 4.3307 4.3316

11 100 3.9370 3.9379 120 4.7244 4.7253

12 110 4.3307 4.3316 130 5.1181 5.1191


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13 120 4.7244 4.7253 140 5.5118 5.5128

14 125 4.9213 4.9223 150 5.9055 5.9065

15 130 5.1181 5.1191 160 6.2992 6.3002

16 140 5.5118 5.5128 170 6.6929 6.6939

The winding waveforms determine the phase to phase, coil to coil, or turn to turn condition of the

windings. Mechanical tests are performed with inside and outside micrometers. Acceptable

measurements are found in Tables 4-2 and 3.

Assuming machining and rewinding is required, certain steps are taken in traditional electric motor

repair. Through the rest of this stage of the study many types of low voltage repair processes shall be

reviewed.

There are a number of ways to machine bearing fits, including (Electrical Insulation, 1997, p. 15):

1 Peening: Is the practice of punching or marring mechanical fits to create a tighter fit. This

practice is not recommended for repair as it is uncontrolled. The marring of the metal

deformed the surface creating high spots meant to hold the bearing solidly in place. The

force used to mar the surface determines the tightness of the fits and the number of marks in a

given spot determines the surface area of the new fit. Wear on the bearing surface, the

amount and concentricity, will affect the internal forces within the bearing, usually increased

bearing friction and reduced internal clearances. This has the multiple effect of reducing


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motor efficiency, bearing life, and increasing the opportunity for bearing currents to damage

the bearings.

2 Metallizing: Consists of a one- or two-part spray process that requires metal to be removed

first. This process is susceptible to separation from the material to which it is attached in

instances of non-symmetrical pressure or when the surfaces have not been properly prepared.

When the material does separate, it creates uneven pressure on the bearing, having the same

effect as peening.

3 Welding: Similar to metallizing; However, it creates a stronger metal to metal bond, when

properly applied. If a repair requires adding metal, this is the preferred method. Machining

does create excessive wear and tear on the cutting tools, however.

4 Sleeving: The process of returning fits by machining and sleeving a motor shaft or housing.

This is the recommended method of motor repair, as it is more controlled. The bearing

housing is machined open concentrically to allow for insertion of a new sleeve, or the shaft is

turned down to allow a sleeve to be sweated on. Both methods are press fits and the sleeves

are normally .250 inches thick per side, allowing the part to be recentered on the lathe and

enough material to be removed to ensure a proper fit and concentricity.

5 Refabrication: While expensive, this method is the best for machining severely worn motor

parts, shafts in particular.

In order to rewind the electric motor, the windings must be removed. All processes begin with

removing one coil end. Following are the traditional methods for coil removal (Electrical Insulation,

1997, p. 16):


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1 Direct Flame: A flame from a torch or other source is directed into the core of the motor,

also includes placing the stator in a bonfire. The temperature is uncontrolled and severe

damage to the core will occur. Damage to the frame occurs causing warping, uneven stator

air gap, and soft foot. The winding is reduced to ash, and the windings removed.

2 Chemical Stripping: The core is lowered into a chlorinated solvent bath and kept submerged

until the varnish is dissolved enough for coil removal. Chemical stripping is ineffective in

many cases, such as overloaded stators. The chlorinated solvent presents potential health,

environmental, and disposal problems. In some cases, the solvent is not completely removed

when the stator is rewound, and the solvent works against the new motor insulation.

3 Burnout: The stator is placed into a burnout oven that is set for a recommended temperature

of 650 oF (345 oC). It is kept at this temperature until all the varnish and insulating materials

are turned to ash (eight hours or more). If the temperature exceeds this level, and often does,

damage to the stator core and frame will result, reducing motor efficiency and mechanical

reliability. Gasses and other byproducts are exhausted through a smoke stack into the

atmosphere.

4 Mechanical Stripping (Dreisilker / Thumm Method): Using a heat source, such as gas jets, a

distance away from the core, the back iron and insulation is warmed until the windings

become soft and pliable (approximately 10 oC above the insulation class of the varnish

insulation). The coils and insulation are removed using a slow steady hydraulic pull.

Temperatures remain low, stripping times extremely fast (ie: 2.5 hours for a 350 hp motor),

and there are no airborne byproducts or disposal problems. Attempts at duplicating this

process using pneumatic pulling methods have resulted in core laminations being pulled

apart. Therefore, pneumatic machines of this type should be avoided.


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5 Mechanical Stripping (Water Blasting): A high-pressure stream of water is used to blast the

coils out of the stator slots. This is a fast method of coil removal. Personal injury due to

high water pressure and mechanical damage can be avoided by experienced personnel and

safety devices.

6 Mechanical Stripping (Hot vapor process chemical stripping): A stator is submerged in a

bath of non-chlorinated petroleum based solvent at a temperature of 370 oF for a short period

of time. It is then removed and the coils are removed with high pressure air. The solvent has

an oily smell, which must be masked, and is difficult to dispose of. Personal injury and

mechanical damage can be avoided by experienced personnel and safety devices.

Direct Flame and burnout ovens cause hot spots and warping of the stator core. The concern over

these types of damage include reduced operating efficiency and reduced insulation life in normal

operating conditions. In VFD operation, the hot spot temperature and core losses increase

dramatically reducing the motor's ability to withstand VFD operation dramatically. Warping causes

uneven magnetic fields within the stator airgap, which increases the possibility of harmful shaft

currents.

Once the stator has been stripped and cleaned, the coils must be replaced. The first step is to insulate

the stator slots with Class F or H insulating paper. Some electric motor shops will not fully insulate

windings, will reduce wire sizes for easier installation, and / or change winding design (Industrial

Motor Repair In the United States, BPA, 1995). The insulation rating of the motor is determined by

the lowest insulation class any portion of the electric Motor (ie: Class B lead wire insulation will


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result in a Class B insulation rating for the motor). The wire insulation is normally of a double bonded

type with a voltage withstand of 500V per microsecond (Table 4-4).

Table 4-4: Properties of Conventional Wires and Survival at

2 kV, 20 kHz, 90 oC, 83 kV/ microsecond

Std Wire 1 Std Wire 2 Std Wire 3

18 H APTz 18 T APTz 18 H APTz-DDg

Wire Diameter 0.0434-0.0436" 0.0443-0.0444" 0.0539-0.0543"

Bare Wire Dia. 0.0404 inch 0.0401 inch 0.0398-0.0400"

Insulation Build 0.0030-0.0032" 0.0042-0.0043" 0.0141-0.0143"

Dielectric

Breakdown

13607 volts

4389 volts/mil

16012 volts

3766 volts/mil

10246 volts

722 volts/mil

Dissipation Factor @

1kHz

0.09 0.08 N/A

Time to failure 580 seconds 1198 seconds N/A

There are several types of winding methods (Electrical Insulation, 1997, p. 17):


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1 Hand Winding: Is performed by a tower-type coil winding machine and mechanical counter.

The winding technician must try to maintain correct tension and layering of the coils, or the

coils will be difficult to lay in the stator slots. In the worst case, there will be wires crossing,

which will increase the turn to turn potential in the wire, creating an area that may short under

certain operating conditions, including VFD's.

2 Automatic Coil Winding Machines: Maintain constant tension and proper count of the coils.

This process still requires a technician to observe operation, but still succeed in reducing

labor time.

3 Computerized Coil Winding Machines: The process of winding is fully automated, leaving

the technician free to perform other tasks while the machine winds the stator coils. Proper

tension, layering, and turn counts are maintained. Layering of the coils allows for nick-free

insertion of the coils, and lower potential between conductors.

The coils are then inserted by hand or machine. Once the coils have been inserted, the coil ends are

insulated and connected. The coil ends are tied down for mechanical strength. Care must be taken not

to pull up the phase insulation, if any.

Winding tests are performed before the motor is varnished. A 500V DC Megger test is performed with

1.5 Megohms as a minimum, with 500 Megohms recommended. A hi-potential test is performed at

values calculated at formula 4-2, as well as a surge comparison test at the same level.

Formula 4-2: Hi-Potential Calculation

VAC = (2E m + 1000)

VDC = (2E m + 1000) * 1.7


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The final step is to insulate the windings with varnish. As with the slot insulation, it is common

practice to use Class F or H varnish on the windings. There are several different varnish methods

(Electrical Insulation, 1997, p. 17):

1 Dip and Bake (Usually Epoxy): An inexpensive and common system. Most motor shops and

manufactures can afford the equipment. When done correctly, two dips and bakes without

shortcuts, and with correct application of the rest of the insulation system, can be acceptable

for inverter duty use. However, another type of failure may result - Partial Discharge. At 60

Hz, PD will normally occur in systems over 6000 V AC, at higher frequencies PD will occur at

much lower voltages. PD occurs where voids between conductors exist, ie: bubbles in the

dip varnish within a coil. Charges, much like those in a capacitor, occur in the gasses within

the voids and discharge, reducing the insulation life. Requires approximately 20 hours for

the complete the dip and bake process.

2 VPI: Very expensive equipment and varnish. The difference between this system and dip

and bake is the cost, it does not eliminate voids in windings which increase the chance for

failure due to PD. While the concept is reasonable, it is not realistic. The Medium Voltage

motors which go through the VPI process have taped windings which holds the varnish in,

low voltage induction motors do not. The low voltage motor is allowed to drain before

putting it in the oven. Voids occur in the windings as the motor drains as the varnish does

not cure at this point. Not much different in time from Dip and Bake and the Varnish is very

expensive.


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3 Trickle Varnishing (Usually Polyester): A relatively inexpensive system (falls somewhere

between dip and bake and VPI). Curing occurs as the process is being applied (ie: 2 hours

for a 350hp motor for the complete process, including curing) and produces a relatively low

amount of waste varnish. The varnish flows through the windings due to gravity and

capillary action, removing any voids. The final result is an equivalent of 3 dips and bakes,

minus voids.

In most cases, the rotor is balanced, before assembly, with all of the rotating components mounted.

The rotor is mounted on centers and spun at a multiple of the running speed. The amount and angle of

vibration is determined by the balancing machine and technician. Weight is added or removed in order

to reduce vibration in the rotor at running speed. Reducing vibration in the electric motor is important

as vibration causes reduced bearing life and can damage structures.

The motor bearings are mounted on the shaft bearing journals. Bearings are mounted using the

following methods:

1 Arbor Press: The bearings are placed on the shaft, then a sleeve is placed against the inner

race of the bearing. The assembly is placed in an arbor press and the bearings are pressed on.

If the bearing or sleeve is not mounted correctly, or if the bearing journal is too tight, this

type of installation may mar the shaft surface. If the surface becomes marred, it will cause

uneven pressure in the bearing as if the shaft surface was peened.

2 Induction Heater: The bearings are place on a laminated bar which is placed on a coil. The

coil is energized and induction causes the bearing to get hot. The bearing is allowed to heat


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to approximately 203 oF which allows the inner race of the bearings to expand. The bearing is

then slid onto the journals and cooled down. If the bearings are allowed to overheat, the

inner race may deform the balls within the bearing and the bearing races themselves. If the

bearing is allowed to get hot enough, the metal may become brittle or crack.

3 Convection Oven: The bearings are placed in a convection oven set for 203 oF long enough

for the bearings to reach temperature. The bearings are removed from the oven and mounted

on the shaft. This is a time consuming process as it may take several hours to get to

temperature.

4 Hot Oil Bath: The bearings are placed in a temperature controlled oil bath until they reach

temperature. They are then removed from the oil bath and mounted on the shaft. This

process does not take as long as the convection oven, but is messy.

Once the bearings have cooled the motor is assembled. The shaft is placed into the stator in a manner

not to damage the windings or laminations. The endshields are then placed onto the stator over the

shaft and bearings.

The motor is test run before placing other components, such as fans and fan covers, to determine that

there are no defects. This test run is normally performed unloaded for ten or fifteen minutes to

determine if there are unusual noises or if the bearing housings are overheating. Amp and voltage

readings are taken and recorded, the amperage should be approximately 30 to 50% of full load at rated

voltage for 1800 or 3600 RPM motors.


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All components are remounted on the motor depending on the marks placed on the motor during

disassembly. The motor is then run at all voltages and speeds. The motor is run for 30 minutes at the

customer operating voltage and speed. Voltage and Amperage readings are taken and recorded. In

some cases, bearing temperature and vibration readings are recorded.

The motor is painted and returned to the customer. While painting has the least effect on the operation

of the electric motor, it is what the end user sees. Therefore, the poorest repair job can appear to the

end user to be the best, based upon the outer appearance of the motor. Shipping an electric motor is

also a concern, as the motor must be shipped in the correct manner to avoid damage to bearings. For

example, a vertical pump motor should be shipped and handled vertically.


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Stage 3: Inverter Effect Study

As outlined in the Chapter 3, Stage 3 outline, an inverter effect study was performed as part of this

research paper. The study was designed to determine the electrical environment of an electric motor

and Variable Frequency Drive System.

The inverter effect study was performed on April 1, 1997, during the course of an 8-hour day. The

location, personnel, and equipment was provided by Dreisilker Electric Motors, Inc., 352 Roosevelt

Rd., Glen Ellyn, IL 60137, as part of the Field Service Division's Research and Development budget.

The personnel involved were:

1 Howard W. Penrose; Director, Field Service / R&D - Provided experiment outline, direction,

and verification.

2 Hao Zhong; Electronic Engineer, Field Service - Experiment design and performance as well

as data collection.

3 Jay Malo; Electronic Technician, Field Service - Assisted with the design and data collection

for the experiment.

The provided location was the Field Service / Electronics Department. All appropriate safety

measures were followed to ensure the safety of personnel both working and not working on the

experiment, as well as protective measures for the equipment. This included barriers and signs where

conductors crossed exit / entrances.


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The electric motor and Variable Frequency Drive were procured as outlined in Chapter 3. All

adjustments were made as outlined. It was determined, through review of the owners manual, that a

distance of five feet would be selected for the close readings, and 110 feet for the critical distance

readings. Three separate conductors of #16 gage, stranded wire, were selected and run around the

perimeter of the Electronic Department space completing one loop. The three conductors were kept

separate and at a height of approximately two feet from the ground. The motor and VFD were located

three feet from each other and were connected to a common ground. The conductors were connected

by twisting strands together to allow for data collection points. The motor was not loaded during

operation.

Motor and VFD at Five Feet The motor, drive, Fluke 41B Power Analyzer, and Fluke 97 Digital Storage Oscilloscope were set up

for the first part of the experiment. The applied voltage was 480 VAC RMS. The drive was started at 16

kHz switching frequency and a 20 second ramp-up time. The data collected were as shown in the

following Figures:

Figure 4-1: Inverter Output Voltage


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Figure 4-2: Current Waveform

As it can be seen, the captured waveforms are not "neat," which is an unfortunate idiosyncrasy of the

Fluke 97. The same points were double checked with a TekTronics 50 MHz oscilloscope with a CRT.

The data is interpreted from the TekTronics scope and the Fluke 41B Analyzer.


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Motor and VFD at 110 Feet

The motor and Variable Frequency Drive were reconnected with the 110 feet of wire. They were then

restarted as before. One of the most significant changes in operation was the sudden increase in

electrical noise from the electric motor. Following is the data collected by the Fluke 97:

Figure 4-3: Inverter Output Voltage



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Current

mSec

Amps1 0.0

2.55.0

-2.5-5.0

. 2.08 4.17 6.25 8.34 10.42 12.51 14.59

Figure 4-7: Power Waveform

Power

mSec

Watts1 0

25005000

-2500-5000

. 2.08 4.17 6.25 8.34 10.42 12.51 14.59

Figure 4-8: Voltage Harmonics

Voltage

Harmonic

Volts rms1

0

100

200

300

400

500

DC 2 4 6 8 10 12 14 16 18 20 22 24 26 28 301 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Figure 4-9: Current Harmonics

Current

Harmonic

Ampsrms 1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

DC 2 4 6 8 10 12 14 16 18 20 22 24 26 28 301 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Figure 4-10: Power HarmonicsPower

Harmonic

KW 1

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

DC 2 4 6 8 10 12 14 16 18 20 22 24 26 28 301 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31


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The Fluke 41B Analysis was taken at the output of the VFD. However, it is observed that the

instrument converts the pulses back to a waveform. The harmonic content is fairly high as IEEE 519

only allows voltage Total Harmonic Distortion (THD) to be 5% and current THD to be 3%. In this

case it can be observed that the THD of the drive and motor system would contribute to motor heating.

Table 4-5: Power Analysis Summary

Summary Voltage Current

Frequency 59.96 RMS 487.2 2.96

Power 1 phase Peak 678.5 4.17

KW -0.88 DC Offset 0.1 -0.03

KVA 1.44 Crest 1.39 1.41

KVAR 1.14 THD Rms 1.72 2.41

Peak KW -2.39 THD Fund 1.72 2.41

Phase 128 o lead HRMS 8.4 0.07

Total PF -0.61 KFactor 1.04

DPF -0.62

Stage 3 Conclusion


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Although the levels of reflective wave and steep wavefronts were not as significant as was expected,

the level of harmonic content was. When reviewing the difference between the experiment and an

actual application several distinct differences show:

1 Wire - In a true application, the wire would have been run through conduit. This would have

allowed capacitive coupling between the cable and ground. In many cases, the wire would

have been solid, which would have allowed for increased distortion. Also, the wires would

have also been immediately adjacent to each other, allowing for some additional noise at long

distances.

2 Motor Load - A motor load would have significantly changed the power quality and voltage

reflection. Most motor applications will be 50% loaded or better.

3 Frequency - Varying the frequency and speed would have had some effect on the motor and

drive in this application.

The harmonic distortion and voltage reflection would impact the motor application. Even unloaded

there was an audible change in the motor operation, in the form of electrical noise. The level of

harmonic distortion in this experiment was enough to create some additional heating in the motor, even

at full speed. Therefore, it is prudent to apply a specification to the repair of motors for inverter duty

applications.


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maintenance history, recurring problems and other correctable situations

may be identified.

2. Proactive Management: By remaining proactive with the repair facility

and providing authorized contact names, confusion can be avoided

through excellent communication. Also ensure that the name(s) of

contacts at the repair facility are available to the authorized contacts.

3. Pre-Evaluate the Repair Facility (Attachment 1):

(1) Quality System: Ensure the repair facility has a recognized

quality control program equal to or exceeding ISO 9002

requirements. Also ensure that this program is available for end

user review and audit.

(2) Pricing Structure: Agreement as to costs, inspection,

rewinding, or reconditioning motors should be established in

advance. It should be clear that any changes to any agreements

are agreed to in writing before or during the repair process.

(3) Warranty: All warranty and conditions should be in writing and

clearly understood by both parties before repairs are begun.

(4) Customer Service: The repair facility should have a dedicated

customer service staff. The end user should also ensure that a

customer service representative is assigned to their account.

(5) Field Servicing: The repair facility should have personnel

trained and in place to conduct field investigation and repair of


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the electric motor and associated controls. Services such as field

balancing, vibration analysis, and infrared inspection are a plus.

(6) Pickup and Delivery: The repair facility should have a system

or vehicles in place to provide pickup and delivery services.

(7) Lifting Equipment Capacity: The facility should have

appropriate equipment of the correct capacity and condition to

adequately handle the motors and move them throughout the

repair shop.

(8) Cleanliness: Facilities should be clean and orderly, and tools

and equipment in good repair. The area, containers, and

equipment used to apply the insulation systems should be in

excellent condition.

(9) Insulation Requirements: The necessary equipment to install

and test the insulation system should be available. The minimum

insulation system capability for rewound electric motors should

be Class F.

(10) Winding Removal: Appropriate coil removal practices should

be in place. Mechanical stripping methods (ie: Dreisilker /

Thumm Method) should be used due to speed, environmental

cleanliness, and they result in no reduction in efficiency. Most

mechanical methods do not damage the electric motor. Burnout

and Chemical stripping methods are environmentally unfriendly


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and are proven to reduce electric motor efficiency as well as

damaging the motor stator.

(11) Equipment (All equipment to be used for testing must be

calibrated):

(1) Machining equipment capable of handling any

machining requirements of the electric motor.

(2) Machine measuring tools: Inside and outside

micrometers, dial indicators, depth gages, etc.

(3) Balancing equipment capable of handling all provided

rotating parts of the equipment.

(4) Vibration Analysis equipment and certified vibration

analysts. The equipment should be capable of

producing vibration spectra which can be used for

reporting and troubleshooting.

(5) Infrared Analysis capabilities for core testing stators.

(6) Core test capabilities for before and after Watts per

Pound loss records.

(7) Computer controlled or automatic rewind machines.

(8) Equipment to perform dip and bake, trickle

impregnation, and Vacuum Pressure Impregnation

(VPI) of insulating varnish.

(9) Growler.

(10) Bearing Heaters: Induction, oven, oil bath, or arbor

press.


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(11) Test voltage and power capabilities to test the motors

under full voltage and no load. Also an inverter test

stand designed to operate motors used in inverter

applications.

(12) Ohmmeters and Milli-Ohmmeters.

(13) High Voltage Meggers (up to 1000 VDC)

(14) High Potential Testers (AC and DC)

(15) Surge Comparison Testers.

(12) Repair vs. Replace: A repair vs. replace policy should be

agreed between the end user and repair facility to ensure

unnecessary disassembly and inspection time is avoided.

2. Repair Facility Responsibility:

1. Traceability: A system for tracking repairs and paperwork must be in

place. All parts must be individually marked or in marked bins and

located. Job numbers should associate parts and paperwork and must be

located on the repaired motor.

2. Record Keeping: All records maintained through the repair including Job

Tickets, Test Sheets, and Winding Data, should be kept for a determined

period of time.

3. Personnel: The repair center shall maintain sufficient personnel of

suitable training and experience to perform repairs. Training records shall

be maintained. All training shall meet the repair center's quality program

requirements and all appropriate OSHA and other safety standards.


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3. INCOMING INSPECTIONS

1. Job Tickets (Attachment 2): Should have space for customer information,

nameplate information, and any additional notes and information.

1. Record all nameplate information available. If the motor nameplate is

missing, the end user should be contacted to determine if the information

is available.

2. The motor should be visually inspected upon receipt of the motor. Any

discrepancies should be noted on the Job Ticket and communicated to the

customer. For instances where unusual damage or conditions cannot be

described, pictures should be taken.

3. The motor is evaluated for possible repair vs. replace. This may include

an evaluation determining that a certain amount of repair would merit

replacement (ie: OK for bearings, replace if rewind or machining).

2. Initial Inspection (A Test Sheet (Attachment 3) is initiated):

1. Physical Inspection: The shaft is inspected, including the keyway. The

shaft is then checked for freedom of rotation and looseness in the

endshields.

2. Insulation Resistance Test: A megger test of 500 or 1000 VDC must be

performed for one minute. The minimum safe insulation level is 1

Megohm + 1 Megohm per kV of the motor with several hundred

Megohms being recommended. The measurement should be adjusted for

temperature (IEEE 43-1974, p. 9).


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3. Motor Circuit Analysis: A Motor Circuit Analysis test should be

performed prior to test running the electric motor. Readings of resistance,

impedance, phase angle, inductance, and current/frequency should be

performed and compared, phase to phase, compensating for rotor position.

An ALL-TEST or equivalent instruments should be used.

4. Test run the motor, noting any operating faults and noting voltage and

current readings.

4. DISASSEMBLY AND INSPECTION

1. Mechanical Disassembly:

1. Punch mark all parts, including endshields and connection box, for

position identification. The standard is two punchmarks on the drive end

and one on the opposite drive end. Each set of punchmarks should be

unique from all others.

2. Remove all bolts and parts. Look closely for cracks or physical defects in

the parts. Remove the rotor in a way that the laminations are not smeared

(some devices are available to assist with this process).

3. Remove the bearings with bearing pullers. Observe the condition of the

bearing journal as the bearing is being removed. High shiny spots, rust,

flaking, or other obvious defects indicate potential machining.

4. Inspect the windings for excessive grease, dirt, or moisture and indicate

any findings on the test sheet. Also observe for obvious signs of failure

such as overloaded, single phased, or shorted windings and record any

findings.


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5. All parts should be cleaned with solvent or soap and water. The stator and

rotor should be placed in an oven set for 295 oF for six to eight hours until

dry.

2. Mechanical Tests and Measures:

1. The bearing housings and journals are measured with inside and outside

micrometers. The results are compared to Tables 4-2 and 3 for

acceptance.

2. The shaft is placed on centers and the output shaft checked for straightness

with a dial indicator.

3. Electrical Tests and Measures:

1. The rotor windings are to be tested with a growler and iron filings. The

filings are placed on a sheet of paper and run across the surface of the

rotor. Any breaks in the line of iron filings indicate broken rotor bars.

The surface should be viewed for holes or pits.

2. Stator Winding Tests:

(1) Visually inspect stator windings looking for loose ties, broken

conductors, loose stator, discolored or brittle insulation, or other

visual defects.

(2) Insulation resistance test at 500 V DC with the results to be 200

Megohms, or better.

(3) An AC or DC hi potential test should be performed at a voltage

calculated in Formula 4-1. The AC hi-pot is a pass / fail test with

the fail damaging the insulation beyond repair. The DC hi-pot is


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more forgiving with a sudden increase in DC leakage indicates

failure.

(4) A motor circuit analysis test is performed.

4. Repair Evaluation:

1. All test results are reviewed and repair recommendations and pricing are

determined, including availability of parts. Repair versus replace is again

reviewed.

2. The end user is contacted and a purchase order is obtained.

3. The scope of work is outlined on the job ticket for reference.

5. Mechanical Repairs: The following methods are used to bring mechanical fits back to

original, or better. Reference the appropriate journal and bearing housing tables for correct

sizes.

1. Sleeving the shaft or housing is preferred. The housing is opened up or the shaft is

turned down to accept the sleeve. The sleeve is pressed into the housing or heated

up and slipped onto the shaft. The part is then recentered ba

repair book

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