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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
Lecture 8 Motor Control Circuits
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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Lecture 8 Motor Control Circuits
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)
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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.
30hp motor – NEC Table 430.250 shows the FLC as 40A.
10hp motor – NEC Table 430.250 shows the FLC as 14A.
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
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Part 1. NEC Motor Installation Requirements
Branch Circuit Motor Protection
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
Part 1. NEC Motor Installation Requirements
Branch Circuit Motor Protection
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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Part 1. NEC Motor Installation Requirements
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
Part 1. NEC Motor Installation Requirements
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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Part 1. NEC Motor Installation Requirements
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
Part 1. NEC Motor Installation Requirements
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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Part 1. NEC Motor Installation Requirements
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
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Part 2. Motor Starting
Full-Voltage Starting of AC
Induction Motors: Manual Starters
Figure 8-12 Typical magnetic
across-the-line starter
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Part 2. Motor Starting
Full-Voltage Starting of AC Induction Motors: Manual Starters
Figure 8-13 Connection diagram for motor pushbutton stations
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Part 2. Motor Starting
Full-Voltage Starting of AC Induction Motors: Manual Starters
Figure 8-14 Timed starting of two motors
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Part 2. Motor Starting
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
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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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Lecture 8 Motor Control Circuits
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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Part 3. Motor Reversing and Jogging
Prof. Paul Lin 27
Reversing of AC Induction Motors
• Figure 8-32 Reversing starter circuit implemented using IEC
symbols
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 28
Reversing of AC Induction Motors
• Figure 8-33 Pushbutton interlocking
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Part 3. Motor Reversing and Jogging
Prof. Paul Lin 29
Reversing of AC Induction Motors
• Figure 8-34 Limit switches incorporated into a reversing
starter circuit to limit travel
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 30
Reversing of AC Induction Motors
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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Part 3. Motor Reversing and Jogging
Prof. Paul Lin 31
Reversing of AC Induction
Motors
Figure 8-36 Reciprocating
machine process
a repeated forward and
reverse action
Part 3. Motor Reversing and Jogging
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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Part 3. Motor Reversing and Jogging
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
Part 3. Motor Reversing and Jogging
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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Part 4. Motor Stopping
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
Part 4. Motor Stopping
Prof. Paul Lin 36
Plugging and Antiplugging
Figure 8-43 Antiplugging protection circuit
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Part 4. Motor Stopping
Prof. Paul Lin 37
Dynamic Braking
Figure 8-44 Dynamic braking applied to a DC motor
Part 4. Motor Stopping
Prof. Paul Lin 38
DC Injection Braking
Figure 8-45 DC injection braking applied to an DC
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Part 4. Motor Stopping
Prof. Paul Lin 39
Electromechanical Friction Brake
Figure 8-46 Electromechanical drum and shoe-type friction brake
used on DC series motor drives
Part 4. Motor Stopping
Prof. Paul Lin 40
Electromechanical Friction Brake
Figure 8-47 AC electromagnetic brake
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Lecture 8 Motor Control Circuits
Prof. Paul Lin 41
Part 5. Motor Speed
• Multispeed Motors
Figure 8-48 Two-speed separate winding across-the-line
motor starter
Lecture 8 Motor Control Circuits
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