ecet 211 electric machines & controls lecture 8 …lin/ecet211/spring2016/1-lectures/...1 1 ecet...

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ECET 211 Electric Machines & Controls

Lecture 8 Motor Control Circuits

Text Book: Electric Motors and Control Systems, by Frank D.

Petruzella, published by McGraw Hill, 2015.

Paul I-Hai Lin, Professor

Electrical and Computer Engineering Technology

P.E. States of Indiana & California

Dept. of Computer, Electrical and Information Technology

Purdue University Fort Wayne Campus

Prof. Paul Lin


Part 1. NEC Motor Installation

Requirements

• Sixing Motor Branch Circuit

Conductor

• Branch Circuit Motor

Protection

• Selecting a Motor Controller

• Disconnecting Means for

Motor ad Controller

• Providing a Control Circuit

Part 2. Motor Starting

• Full-Voltage Starting of AC

Induction Motors

• Reduced-Voltage Starting

of Induction Motors

• DC Motor Starting Prof. Paul Lin 2

Part 3. Motor Reversing and

Jogging

• Reversing of AC Induction

Motors

• Reversing of DC Motors

• Jogging

Part 4. Motor Stopping

• Plugging and Anti-plugging

• Dynamic Breaking

• DC Injection Breaking

• Electromechanical Friction

Brakes

Part 5. Motor Speed

• Multispeed Motors

• Wound-Rotor Motors

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Part 1. NEC Motor Installation

Requirements

• NEC Article 430 covers

application and installation of

motor circuits including

conductors, short-circuit, and

ground fault protection, starters,

disconnects, and overload

protection.

• Motor Brach Circuits – include

the final overcurrent device

(disconnect switch and fuses or

circuit breaker), the motor

starter and associated control

circuits, circuit conductors, and

the motor.

Prof. Paul Lin 3

Figure 8-1 Basic elements of a motor

branch circuit that the NEC addresses

Motor Control Circuits – NEC Motor Installation

Requirements

Sizing Motor Branch Circuit

Conductor

NEC Article 430, Part II

Article 430.6

• Installation requirements for motor

branch circuit conductor

• A single motor used in a

continuous-duty application must

have an ampacity of not less than

125 percent of the motor’s Full-

Load Current (FLC)

Article 430.247 through 430.250

• Conductor ampacity must be

determined by NEC Tables

430.247 through 430.250 and is

based on the motor nameplate

horsepower rating and voltageProf. Paul Lin 4

Full-Load Current (FLC) –

indicates the use of NEC table

rating

Full-Load Amperes (FLA) –

indicates the actual nameplate

rating

Article 430.247 through

430.250

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Lecture 8 Motor Control CircuitsPart 1. NEC Motor Installation Requirements:

Sizing Motor Branch Circuit Conductor

Example 8-1

Problem: using your edition of the NEC, determine

the minimum branch circuit conductor ampacity

required for each of the following motors:

(a) 2 hp, 230V single-phase motor

(b) 30 hp, 230V, three-phase motor with a

nameplate FLA rating of 70A

Solution:

(a) NEC Table 430-248 shows the FLC as 12 A.

Conductor ampacity required is 12 x 125% = 15A

(b) NEC Table 430.250 shows the FLC as 80A.

Conductor ampacity required is 80 x 125% = 100A

Prof. Paul Lin 5

http://www.automationdirect.com/

adc/Shopping/Catalog/Motors

Lecture 8 Motor Control CircuitsPart 1. NEC Motor Installation Requirements: Sizing Motor Branch

Circuit Conductor

Feeder Conductors supplying two or more motors must have:

An ampacity not less than 125 percent of the FLC rating of the

highest-rated motor, plus,

The sum of the FLC ratings of the other motor supplies.

Ampacity of the conductor => NEC Table 310.15(B)(16) => American

Wire Gauge (AWG)

Prof. Paul Lin 6

http://www.automationdirect.com/adc/Shopping/Catalog/Motors

4

Lecture 8 Motor Control CircuitsExample 8-2. Problem: Three 460V, 3Φ motors rated at 50, 30, and 10 hp

share the same feeder (Figure 8-2). Using your edition of the NEC,

determine the ampacity required for size the feeder conductors.

Solution:

50hp motor – NEC Table 430.250 shows the FLC as 65A.



Required ampacity of the feeder conductor is

(1.25)(65) + 40 + 14 = 135.25 A

Prof. Paul Lin 7

Part 1. NEC Motor Installation Requirements

Branch Circuit Motor Protection

Nonmotor loads – use circuit breaker that combines overcurrent

protection with short-circuit and ground fault protection.

Motor loads

• Draws up to 6 times of normal FLC of the motor.

• Best method of protection for motors – separate the overload

protection devices from the short circuit and ground fault protection

• Figure 8-3 Motor branch circuit protection

Prof. Paul Lin 8

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NEC Article 430, Part IV

• Explains the requirements for branch circuit short-circuit and ground

fault protection.

• The NEC requires that branch circuit protection for motor circuits

must protect the circuit conductors, the control apparatus, and the

motor against over current due to “short circuit” or “ground faults.”

• Table 430.52 – maximum values on the ratings or setting of these

devices

• NEC Article 240.6 – lists the standard sizes of fuses and breakers

Instantaneous trip circuit breakers

Inverse time circuit breaker – the higher the overcurrent, the shorter the

time required for the breaker to trip and open the circuit

Prof. Paul Lin 9



Example 8-3. Problem:

• Determine the size of inverse time circuit breaker permitted to be

used to provide motor branch circuit short circuit and ground fault

protection for a 10 hp, 208V, 3Φ squirrel-cage motor.

Solution:

• NEC Table 430.250 => the motor FLC = 30.8A.

• NEC Table 430.52 => maximum ratings for an inverse time breaker

as 250 percent of the FLC.

30.8 x 2.5 = 77A

Use 80A inverse time circuit breaker if a 70A’s is not adequate.

Prof. Paul Lin 10

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Selecting a Motor Controller

Motor controller

• Any device that is used to directly

start and stop an electric motor by

closing and opening the main

power current to the motor.

• It can be a switch, starter, or other

similar type of control device.

• Figure 8-4 Examples of motor

controllers

• NEC Article 430, Part VII – details

the requirements for motor

controllers – see page 204 for

some of the highlights.

Prof. Paul Lin 11


Disconnecting Means for Motor and

Controller

NEC Article, Part IX – covers the

requirements for the motor

disconnecting means.

The Code requires that a means (a

motor circuit switch rated in horsepower

or a circuit breaker) must be provided in

each motor circuit to disconnect both the

motor and its controller from all

ungrounded supply conductors.

Separate disconnects and controllers

may be mounted on the same panel or

contained in the same enclosure, such

as Figure 8-5 Combination fused-switch,

magnetic starter unit

Prof. Paul Lin 12

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Disconnecting Means for Motor and Controller

If a person is working on the motor, the disconnect will be where he

or she can see.

It protects the person from a motor accidentally starting.

The NEC defines “within sight” as being visible and not more than

50 ft (15 m) distant from the other.

Figure 8-6 The disconnecting means must be located within sight

from the controller, and the driven machine location

Prof. Paul Lin 13


Disconnecting Means for Motor and Controller

For stationary motors rated more than 40 hp DC or 100 hp AC, a

general-use or isolating switch can be used but should be plainly

marked “DO NOT OPERAE UNDER LOAD.”

An isolating switch

• Intended to isolate an electric circuit from its source of power

• No interrupting rating

• Intended to be operated only after the circuit has been opened

by some other means.

Example 8-4 Problem: determine the current rating of the motor

disconnect switch required for a 460V, three-phase, 125 hp motor.

Solution: NEC Table 430.250 => Motor FLC = 156 A

NEC 430.110 => motor disconnecting means to have an ampere

rating of at least 115 percent of the FLC rating of the motor

156A x 1.15 = 179A

A 200 A disconnect switch is requiredProf. Paul Lin 14

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Providing a Control Circuit

Has its load devices: coils of magnetic contactor, magnetic starter,

relay, etc

NEC Article 430 covers the requirements for motor control circuits

• The elements of control circuit all the equipment and devices

concerned with the function of the circuit:

Conductors, Raceways, Contactor coils, Source of energy

supply to the circuit, Overcurrent protection devices, and all

switching devices that govern energization of the operating

coil

• Control circuit voltages and control transformers: 120V, 460V,

600V

• Ground fault

NEC Article 430.75 requires that motor control circuits be arranged

so that they will be disconnected from all source of supply when

the disconnecting means in the open position.

Prof. Paul Lin 15

Part 1. NEC Motor Installation RequirementsProviding a Control Circuit

Figure 8-7 The design of the control circuit must prevent the motor from

being started by a ground fault in the control circuit wiring

Figure 8-7a. A ground fault on the coil side of the start button can

short-circuit the start circuit and start the motor

Prof. Paul Lin 16

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Full-Voltage Starting of AC

Induction Motors: Manual Starters

Figure 8-12 Typical magnetic

across-the-line starter

Prof. Paul Lin 17


Full-Voltage Starting of AC Induction Motors: Manual Starters

Figure 8-13 Connection diagram for motor pushbutton stations

Prof. Paul Lin 18

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Full-Voltage Starting of AC Induction Motors: Manual Starters

Figure 8-14 Timed starting of two motors

Prof. Paul Lin 19


Reduced-Voltage Starting of AC Induction Motors:

Two reasons:

1) Limits line disturbances

2) Reduces excessive torque to the driven equipment

When a motor is started at full voltage, the current drawn from

the power line is typically 600 percent of normal full-load

current

The large starting inrush current of a big motor could cause

line voltage dips and brown-out.

Higher than full-load torque can cause mechanical damage

such as belt, chain, or coupling breakage.

Electric utility current restrictions, as well as in-plant bus

capacity, may require motors above a certain horsepower to

be started with reduced voltage.

Typical reduced voltage starters: Primary-resistance,

Autotransformers, Wye-Delta, Part-winding, solid-state

startersProf. Paul Lin 20

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Part 2. Motor StartingTable 8-1 Typical voltage, Current, and torque characteristics for NEMA Design B

Motors

Prof. Paul Lin 21

Motor starting

current as a

percent of:

Line current as a

percent of:

Motor starting

torque as a percent

of:

Starting

Method

%

voltage at

motor

terminals

Locked-

rotor

current

Full-

load

current

Locked-rotor

current

Full-load

current

Locked-rotor

current

Full-load

current

Full voltage 100 100 600 100 600 100 180

Autotransfo

rmer

80% tap

65% tap

50% tap

80

65

50

80

65

50

480

390

300

64

42

25

64

42

25

307

164

25

115

76

45

Part-

winding

100 65 390 65 390 50 90

Wye-delta 100 33 198 33 198 33 60

Solid-state 0-100 0-100 0-600 0-100 0-600 0-100 0-180

Reduced Voltage Starting of AC

Induction Motors

Figure 8-20 Wye and delta motor

winding connections

Figure 8-21 Wye-delta starter

Prof. Paul Lin 22

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Reduced Voltage Starting of AC

Induction Motors

Figure 8-22 Part-winding starting

Prof. Paul Lin 23

Part 2. Motor StartingReduced Voltage Starting of AC

Induction Motors

Figure 8-24 Soft start ramped-up

voltage and current limiting

Figure 8-25 Typical soft start starter

Starting Modes

• Soft start

• Selectable kick start

• Current limit start

• Dual-ramp start

• Full-voltage start

• Liner speed acceleration

• Preset slow speed

• Soft stopProf. Paul Lin 24

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Prof. Paul Lin 25

Part 3. Motor Reversing and

Jogging

Reversing of AC Induction Motors

• Reversing three-phase

Induction Motor Starter

Figure 8-29 Magnetic full-voltage

three-phase reversing

Part 3. Motor Reversing and Jogging

Prof. Paul Lin 26

Reversing of AC Induction

Motors

• Figure 8-30 Mechanical

interlocking of forward

and reverse contactors

Figure 8-31 Magnetic

reversing starter with

electrical interlock in the

motor starter

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Prof. Paul Lin 27


• Figure 8-32 Reversing starter circuit implemented using IEC

symbols


Prof. Paul Lin 28


• Figure 8-33 Pushbutton interlocking

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Prof. Paul Lin 29


• Figure 8-34 Limit switches incorporated into a reversing

starter circuit to limit travel


Prof. Paul Lin 30


Figure 8-35 Reversing a single-phase

motor

The direction of rotation is changed

by interchanging the start winding

leads, while those of the run

winding remain the same

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Prof. Paul Lin 31

Reversing of AC Induction

Motors

Figure 8-36 Reciprocating

machine process

a repeated forward and

reverse action


Prof. Paul Lin 32

Reversing of DC Motors

The reversal of a DC motor can be

accomplished in two ways:

• Reversing the direction of the

armature current (IA); leaving the

field current the same

• Reversing the direction of the field

current (IF) and leaving the

armature current the same

Figure 8-37 DC motor reversing

power circuits

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Prof. Paul Lin 33

Jogging

Jogging (sometimes called Inching) is momentary operation of a

motor for the purpose of accomplishing small movements of the

driven machine.

Figure 8-38 Push button job circuit


Prof. Paul Lin 34

Jogging

Figure 8-39 Jog circuit with

control relay

Figure 8-40 Start/stop/selector

jog control circuit

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Prof. Paul Lin 35

Motor Stopping

Remove the power supply plus

electric braking

Plugging and Antiplugging

Plugging – stops a polyphase

motor quickly by momentarily

connecting the motor for revere

rotation while the motor is still

running in the forward direction.

Figure 8-41 Plugging switch

Figure 8-42 Plugging a motor to

stop it


Prof. Paul Lin 36

Plugging and Antiplugging

Figure 8-43 Antiplugging protection circuit

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Prof. Paul Lin 37

Dynamic Braking

Figure 8-44 Dynamic braking applied to a DC motor


Prof. Paul Lin 38

DC Injection Braking

Figure 8-45 DC injection braking applied to an DC

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Prof. Paul Lin 39

Electromechanical Friction Brake

Figure 8-46 Electromechanical drum and shoe-type friction brake

used on DC series motor drives


Prof. Paul Lin 40

Electromechanical Friction Brake

Figure 8-47 AC electromagnetic brake

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Prof. Paul Lin 41

Part 5. Motor Speed

• Multispeed Motors

Figure 8-48 Two-speed separate winding across-the-line

motor starter


Prof. Paul Lin 42

Part 5. Motor Speed

• Wound-Rotor Motors

Figure 8-49 Wound-rotor magnetic

motor controller

Low speed (full resistance) – both S

& H are open

Medium speed – S closed

Maximum speed – H closed

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Summary & Conclusion

Questions?Contact Prof. Lin through:

Email: [email protected]

Prof. Paul Lin 43

mailto:[email protected]

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