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Motors, Motor Circuit & Controller
Module 5 – Motors, Motor Circuit & Controller
Learning After finishing this module, you are expected to:
Objectives
1. Explain what is motor, motor branch-circuit and feeder conductors
and protection
2. Determine the full load current of motors and sized of overload
device
Topics
1.0.
1.1.
1.2.
1.3.
1.4.
1.5.
Overview
Definition
Table of FLC versus Motor Nameplate current
Sizing a branch circuit conductor
Branch - Circuit, Short - Circuit and Ground - Fault Protection.
Sizing Contactor, Overload device and Disconnect switch.
Industrial Electricity & Motor Control, Rex Miller & Mark Miller, C 2013
th
Industrial Motor Control, 6 Edition, Stephen L. Herman, C 2010
NEC 2008 and PEC 2017
Warm Up
Give Three things that you expect you will learn in this module.
1. _________________________________________________________
2. _________________________________________________________
3. _________________________________________________________
Pre-Test:
1. Explain the importance of proper sizing of the branch circuit conductor
for a single or group of motors.
2. Explain the importance of proper sizing of the feeder short circuit and
ground fault protective device.
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1.0
Motors, Motor Circuit & Controller
Overview
Most electrical equipment is rated in volt-amperes (VA) or watt input.
Basic understanding of Article 4.30 is the fact that motors traditionally have been
rated in horsepower output. Circuits supplying motors are sized according to the
input power of the motor. The input power includes the motor losses and the
power factor of the motor. The losses are not the type of information found on the
nameplate of a motor. Tables 4.30.14.2 and Table 4.30.14.4 contain accurate
industry-wide input ampere ratings for motor.
However, some motor are available with their output ratings expressed in
watts and kilowatts. (One horsepower equals approximately 746 watts.) It is
important to understand that circuits that supply motors not rated in horsepower
still must be sized according to the input of the motor, rated in amperes. Sizing
circuits based solely on kilowatts output results in seriously undersized conductors
(because the current requirements of the losses and the power factor are
neglected) and the improper application of overcurrent devices. See Article
4.30.1.6 for ampacity and motor rating determination. (NEC 2008, Article 430 and PEC
2017, Article 4.30)
1.1.
Definitions
Adjustable Speed Drive. A combination of the power converter, motor and
motor-mounted auxiliary devices such as encoder, tachometer, thermal switches
and detectors, air blowers, heaters, and vibration sensor.
Adjustable Speed Drive System. An interconnected combination of equipment
that provides a means of adjusting the speed of a mechanical loads coupled to a
motor. A drive system typically consists of an adjustable speed drive and auxiliary
electrical apparatus.
Controller. For the purpose of this module, a controller is any switch or devices
that is normally used to start and stop a motor by making and breaking the motor
circuit current.
Motor Control Circuit. The circuit of a control apparatus or system that carries
the electric signals directing the performance of the controller but does not carry
the main power current.
1.2.
Table of FLC versus Motor Nameplate current
Full Load Current (FLC). The motor full load current is the current indicated in
the nameplate of the motor or as specified in the tables or from manufacturers
data. The full load current ratings listed in Table 4.30.14.2 for Single Phase and
Table 4.30.14.4 for Three phase motor.
Motor Nameplate Current Rating. Overload devices must be sized based on
motor nameplate current rating in accordance with section 4.30.
Note: The motor nameplate full-load ampere rating is identified as full-load
amperes (FLA). The FLA rating is the current in amperes the motor draws while
producing its rated horsepower load at its rated voltage, based on its efficiency
and power factor.
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Motors, Motor Circuit & Controller
Table of Full Load Current of AC Motors.
Source: PEC 2017
Source: PEC 2017
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Motors, Motor Circuit & Controller
The nameplate rating of motor:
The nameplate data of AC Motor as specified if the Figure shown, compose of the
following:
a. Full-Load Ampere (FLA)
b. Temperature Rise
c. Power Factor
d. Phase (1-Phase or 3-Phase)
e. Volts
f. Service Factor
g. Cycle
h. RPM
i. HP
j. Code letter
k. Thermal Protection
l. Impedance Protection
m. Serial number
n. Frame
o. Duty
There are three methods on how to determine the Full-Load Current of AC Motor.
1. By nameplate data (as illustrated above)
2. By Tables (PEC Table 4.30.14.2 & Table 4.30.14.4)
3. By Formula.
By Formula:
For Single Phase motor:
𝑁𝑎𝑚𝑒 𝑃𝑙𝑎𝑡𝑒 (𝑁𝑃) =
746𝑊 𝑥 𝐻𝑃
𝐸 𝑥 𝐸𝑓𝑓 𝑥 𝑃𝐹
Where:
HP
E
Eff
PF
– Horse Power rating
– voltage rating of the machine
– Efficiency
– Power Factor
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For Three Phase motor:
𝑁𝑎𝑚𝑒 𝑃𝑙𝑎𝑡𝑒 (𝑁𝑃) =
746𝑊 𝑥 𝐻𝑃
√3𝑥𝐸 𝑥 𝐸𝑓𝑓 𝑥 𝑃𝐹
Where:
HP
E
Eff
PF
√3
– Horse Power rating
– Voltage rating of the machine
– Efficiency
– Power Factor
- constant for Three phase
Example 1:
What is the full load current of 5 HP, 230 volts Single Phase motor?
Solution:
a. As per Table 4.30.14.2, the Full-Load Current is equals to FLC = 28
amperes. See the table below for specifying the full-load current.
Source: PEC 2017
b. By Formula: Assume pf = 80% and Eff = 72.3%
𝐹𝑢𝑙𝑙 − 𝐿𝑜𝑎𝑑 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 =
746𝑊 𝑥 𝐻𝑃
746 𝑥 5
=
= 27.693 𝐴
𝐸 𝑥 𝐸𝑓𝑓 𝑥 𝑃𝐹 (230)(0.732)(0.80)
Approximately 28 amperes.
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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Motors, Motor Circuit & Controller
Example 2:
What is the full load current of 7.5 HP, 230 volts Three Phase induction type
squirrel cage motor?
Solution:
1. As per Table 4.30.14.4, the Full-Load Current is equals to FLC = 22
amperes. See the table below for specifying the full-load current.
Source: PEC 2017
2. By Formula: Assume pf = 80% and Eff = 79.8%
𝐹𝑢𝑙𝑙 − 𝐿𝑜𝑎𝑑 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 =
746𝑊 𝑥 𝐻𝑃
√3 (𝐸)(𝐸𝑓𝑓)(𝑃𝐹)
=
746 𝑥 7.5
(230)(0.798)(0.80)
= 22.0 𝐴
Approximately 22 amperes.
1.3.
Size of Branch Conductor serving a single motor:
Conductor size. Conductor to a single motor must be sized not smaller than
125% of the motor FLC rating as listed in Table 4.30.14.2 and Table 4.30.14.4. To
specify the motor branch circuit conductor, Table 3.10.2.6(B)(16) of the PEC 2017
edition will be used. This table shows the Ampacity of the conductors.
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Motors, Motor Circuit & Controller
Source: PEC 2017
Example 3:
What size of branch circuit conductor is required for 5 HP, 230 volts three phase
induction type squirrel cage motor?
Solution:
Base from Table 4.30.14.4, the Full-Load Current (FLC) of 5HP, 230V, 3-phase
motor is 15.2 amperes.
Conductor size
= FLC x 125%
= 15.2 x 125%
= 15.2 x 1.25
= 19 amperes.
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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Base from Table 3.10.2.6(B)(16), the conductor size is 3.5 mm2, TW rated 20
amperes at 60C. if THW, the conductor size is 2.0 mm2, rated 20 amperes at 75C. If
THHN, the conductor size is 2.0 mm2, rated 25 amperes at 90C.
Source: PEC 2017
Example 4:
What size of branch circuit conductor is required for 15 HP, 230 volts three phase
induction type squirrel cage motor?
Solution:
Base from Table 4.30.14.4, the FLC of 15HP, 230V, 3-phase motor is 42
amperes.
Conductor size
= FLC x 125%
= 42 x 125%
= 42 x 1.25
= 52.5 amperes.
Base from Table 3.10.2.6(B)(16), the conductor size is 14 mm2, TW rated 55
amperes at 60C. if THW, the conductor size is 14 mm2, rated 65 amperes at 75C. If
THHN, the conductor size is 8.0 mm2, rated 55 amperes at 90C. See Table 3.10.2.6(B).
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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Motors, Motor Circuit & Controller
Source: PEC 2017
1.4.
Several Motors – Conductor size.
Circuit conductors that supply several motors (typically called as feeders)
must not be sized smaller than 125% of the largest motor FLC, plus the sum of
the Full-Load Current (FLCs) of the other motors in the group.
Example 5:
What size feeder conductor is required for a 5 HP & 10 HP, 230 volts three
phase induction type squirrel cage.
Solution:
Base from Table 4.30.14.4, the FLC of 5HP & 10 HP, 230V, 3-phase motor
is 15.2 and amperes and 28 amperes respectively.
Motor feeder Conductor
= FLC of largest motor x 125% + FLC of other motor
= (28)(125%) + 15.2
= (28)(1.25) + 15.2
= 50.2 amperes.
Referring to Table 3.10.2.6(B)(16), the conductor size is 14 mm2, TW rated
55 amperes at 60C. if THW, the conductor size is 14 mm2, rated 65 amperes at
75C. If THHN, the conductor size is 8.0 mm2, rated 55 amperes at 90C.
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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Motors, Motor Circuit & Controller
Example 6:
What size feeder conductor is required for a 10 HP, 15 HP and 25 HP, 230
volts three phase motors, if the terminal rated for 75C?
Solution:
Base from Table 4.30.14.4, the FLC of 10HP, 15 HP and 25 HP, 230V, 3phase motor is 28, 42 and 68 amperes respectively.
Motor feeder Conductor
= FLC of largest motor x 125% + FLC of other motor
= (68)(125%) + 42 + 28
= (68)(1.25) + 42 + 28
= 155 amperes.
Referring to Table 3.10.2.6(B)(16), feeder conductor size is 60 mm2, THW
rated 160 amperes at 75C.
Example 7:
What size feeder conductor is required for a 5 HP, 15 HP and 30 HP, 460
volts three phase motors, if the terminal rated for 75C?
Solution:
Base from Table 4.30.14.4, the FLC of 5HP, 15 HP and 30 HP, 460V, 3phase motor is 7.6, 21 and 40 amperes respectively.
Motor feeder Conductor
= FLC of largest motor x 125% + FLC of other motor
= (40)(125%) + 21 + 7.6
= (40)(1.25) + 21+ 7.6
= 78.6 amperes.
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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Referring to Table 3.10.2.6(B)(16), feeder conductor size is 22 mm2, THW
rated 85 amperes at 75C.
Source: PEC 2017
1.5.
Branch - Circuit, Short - Circuit and Ground - Fault Protection.
1. General. The motor branch-circuit short-circuit and ground fault protective
device must comply with Section 4.30.4.2(b) and Section 4.30.4.2(c).
2. All Motors. A motor branch-circuit, short-circuit and ground-fault protective
device capable of carrying the starting motor.
3. Rating or Setting. In accordance with Table 4.30.4.2. A protective device
that has a rating or setting not exceeding the value calculated according to
the values given in Table 4.30.4.2. shall be used.
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Motors, Motor Circuit & Controller
Source PEC 2017
1.6.
Feeder Short-Circuit and Ground Fault Protection
Feeder Protection.
1. Motors Only. Feeder conductors must be protected against short
circuit and ground faults by a protective device sized not more than the
largest rating of the branch circuit short circuit and ground fault
protective device for any motor plus the sum of the full-load currents of
the other motors in the group. (Source: Understanding Basic Motor Control by
Mike Holt)
Example 8:
What size of conductor and inverse time circuit breaker are required for a 5
HP, 230V, three phase motor?
Solution:
Step 1: Determine the FLC, refer to Table 4.30.14.4 of PEC
Full-Load Current (FLC) = 15.2 amperes
Step 2: Determine the branch circuit conductor:
= FLC x 125% = 15.2 x 1.25 = 19 Amperes
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Motors, Motor Circuit & Controller
Based from Table 3.10.2.6(B)(16), use 3.5 mm2 ,TW rated 20 amperes
Step 3: Determine the branch circuit protection [Table 2.40.1.6(A)], [Table
4.30.4.2].
=FLC x 250% = 15.2 x 2.50 = 38A, next size up = 40A
= Use 40 AT, 240 V, 3-Pole, Inverse Time Circuit Breaker.
Source: PEC 2017
Example 9:
What size feeder conductor is required for a 5 HP, 15 HP and 30 HP, 460
volts three phase motors, if the terminal rated for 75C?
Solution:
Base from Table 4.30.14.4, the FLC of 5HP, 15 HP and 30 HP, 460V, 3phase motor is 7.6, 21 and 40 amperes respectively.
Motor feeder Conductor
= FLC of largest motor x 125% + FLC of other motor
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Motors, Motor Circuit & Controller
= (40)(125%) + 21 + 7.6
= (40)(1.25) + 21+ 7.6
= 78.6 amperes.
Referring to Table 3.10.2.6(B)(16), feeder conductor size is 22 mm2, THW
rated 85 amperes at 75C.
Size of protective device (Individual Motor)
For 5 HP, 460V 3-Phase:
= FLC x 250% = 7.6 x 2.5 = 19A, next size up = 20A
USE: 20AT, 3P, 460V
For 15 HP, 460V, 3-Phase
= FLC x 250% = 21 x 2.5 = 52.5A, next size down = 50A
USE: 50AT, 3P, 460V
For 30 HP, 460V, 3-Phase
= FLC x 250% = 40 x 2.5 = 100A, next size up = 100A
USE: 100AT, 3P, 460V
Size of main protective device (for group of Motors)
= Largest rating of the branch circuit short circuit and ground fault
protective device for any motor plus the sum of the full-load currents
of the other motors in the group.
= 100A + 21 + 7.6 = 128.6 A, next size up = 150A
USE 150AT, 3P, 460V.
Example 10:
What size feeder conductor is required for a 5 HP, 15 HP and 30 HP, 230
volts three phase motors, if the terminal rated for 75C?
Solution:
Base from Table 4.30.14.4, the FLC of 5HP, 15 HP and 30 HP, 460V, 3phase motor is 15.2, 42 and 80 amperes respectively.
Motor feeder Conductor
= FLC of largest motor x 125% + FLC of other motor
= (80)(125%) + 42 + 15.2
= (40)(1.25) + 21+ 7.6
= 157.2 amperes.
Referring to Table 3.10.2.6(B)(16), feeder conductor size is 80 mm2, THW
rated 190 amperes at 75C.
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Motors, Motor Circuit & Controller
Size of protective device (Individual Motor)
For 5 HP, 230V 3-Phase:
= FLC x 250% = 15.2 x 2.5 = 38A, next size up = 40A
USE: 40AT, 3P, 240V
For 15 HP, 230V, 3-Phase
= FLC x 250% = 42 x 2.5 = 105A, next size down = 100A
USE: 100AT, 3P, 240V
For 30 HP, 230V, 3-Phase
= FLC x 250% = 80 x 2.5 = 200A, next size up = 200A
USE: 200AT, 3P, 240V
Size of main protective device (for group of Motors)
= largest rating of the branch circuit short circuit and ground fault
protective device for any motor plus the sum of the full-load currents
of the other motors in the group.
= 200A + 42 + 15.2 = 257.2 A, next size down = 250A
USE 250AT, 3P, 240V.
or
= 200A + 42 + 15.2 = 257.2 A, next size up = 300A
USE 300AT, 3P, 240V.
Either of these two rating of protective device will used the motors will be
protected during the starting period.
Size of Contactor:
To size up the contactor, the formula will apply.
=
𝐻𝑃 𝑥
1000
Example 11:
What is the equivalent size of the contactor for a 10 HP, 230V, 3
Phase motor?
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Motors, Motor Circuit & Controller
=
10 𝐻𝑃 𝑥
= 7.46 𝑘𝑊
1000
𝑈𝑠𝑒 ∶ 8 ~10 𝑘𝑊, 240𝑉, 3 − 𝑃ℎ𝑎𝑠𝑒 𝑐𝑜𝑛𝑡𝑎𝑐𝑡𝑜𝑟
Example 12:
What is the equivalent size of the contactor for a 30 HP, 460V, 3
Phase motor?
=
30 𝐻𝑃 𝑥
= 22.38 𝑘𝑊
1000
𝑈𝑠𝑒 ∶ 23 ~25 𝑘𝑊, 460𝑉, 3𝑃ℎ𝑎𝑠𝑒 𝑐𝑜𝑛𝑡𝑎𝑐𝑡𝑜𝑟
OVERLOAD PROTECTION:
Overload devices are intended to protect motors, motor control apparatus
and motor branch circuit conductors against excessive heating due to motor
overloads.
Overload is the operation of equipment in excess of the normal, ful-load
current rating, which, if it persist for a sufficient time, will cause damage or
dangerous overheating of the apparatus.
Overload Sizing for Continuous duty.
1. Motors Rated More than One Horsepower. Motors rated more than 1HP,
used in a continuous duty application without integral thermal protection
must have an overload device as follows:
a. Separate Overload Device. A separate overload device must be
selected to open at no more than the following percent of the motor
nameplate full-load current rating:
Service Factor. Motor with a marked service factor (SF) of 1.15 or
more on the nameplate must have the overload device sized no more
than 125 percent of the motor nameplate current rating.
Temperature Rise. Motors with a nameplate temperature rise of 40C
or less must have the overload device sized no more than 125
percent of the motor nameplate current rating.
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Motors, Motor Circuit & Controller
All other motors. No more than 115 percent of the motor’s
nameplate current rating. (Source: Understanding Basic Motor Controls,
Mike Holt and PEC 2017, section 4.30.3.2)
Example 13:
What size of an overload must be used to protect a 10 HP, 230V, 3
Phase motor with a service factor of 115%?
Solution:
From Table 4.30.14.4, the full-load current of 10 HP, 230V 3-Phase
is 28 amperes.
FLC = 28A
Size of Overload Device
= FLC x 125%
= 28 x 1.25
= 35 amperes
Overload settings should not be less than 35 amperes say 40
amperes.
Engr. Eddie L. Cabaltera | Camarines Sur Polytechnic Colleges
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DO – IT – YOURSELF:
Determine the size of the feeder conductors, protective devices and size of
contactors for single or group of motors. (Use inverse time breaker for protective
device)
1. What size feeder conductor is required for a 10 HP, 230 volts three phase
induction type squirrel cage.
2. What size feeder conductor is required for a 15 HP & 10 HP, 230 volts three
phase induction type squirrel cage.
3. What size feeder conductor is required for a 7.5 HP & 25 HP, 230 volts three
phase induction type squirrel cage.
4. What size feeder conductor is required for a 7.5 HP, 20 HP, and 50 HP, 460
volts three phase induction type squirrel cage.
5. What size feeder conductor is required for a 25 HP, 30 HP, and 40HP, 230
volts three phase induction type squirrel cage.
Note: Write the solution on a separate sheet of papers.
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