Energy Systems Engineering Technology
College of Technology
Motors and Controls
Module # 4 Motor Controls
Document Intent:
The intent of this document is to provide an example of how a subject matter expert might teach
Motor Controls. This approach is what Idaho State University College of Technology is using to
teach its Energy Systems Instrumentation and Control curriculum for Motor Controls. The
approach is based on a Systematic Approach to Training where training is developed and
delivered in a two step process. This document depicts the two step approach with knowledge
objectives being presented first followed by skill objectives. Step one teaches essential
knowledge objectives to prepare students for the application of that knowledge. Step two is to
let students apply what they have learned with actual hands on experiences in a controlled
laboratory setting.
Examples used are equivalent to equipment and resources available to instructional staff
members at Idaho State University.
Fundamentals of Motor Controls Introduction:
This module covers fundamental aspects of Motor Controls as essential knowledge necessary to
perform work safely according to national and local standards on or around electrical power
sources that are associated with motors and controls. Students will be taught the fundamentals of
Motor Controls using classroom instruction, demonstration, and laboratory exercises to
demonstrate knowledge and skill mastery of Motor Controls. Completion of this module will
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allow students to demonstrate mastery of knowledge and skill objectives by completing a series
of tasks demonstrating safe work practices on or around electrical power sources.
References
This document includes knowledge and skill sections with objectives, information, and examples
of how Motors and Control could be taught in a vocational or industry setting. This document
has been developed by Idaho State University’s College of Technology. Reference material used
includes information from:
American Technical Publication – Electrical Motor Controls for Integrated Systems, Third
Edition, by Gary J. Rockis and Glen A. Mazur, ISBN 0-8269-1207-9 (Chapters 8, 10, 17, &
18)
National Electrical Code® International Electrical Code® Series, NFPA 70TM, NEC 2008,
ISBN-13: 978-087765790-3
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STEP ONE
Motor Controls Course Knowledge Objectives
Knowledge Terminal Objective (KTO)
KTO 3. 1.
ANALYZE Motor Controls to compare advantages and disadvantages to ensure
they are correctly selected for applications according to manufacturing
specifications and electrical requirements
Knowledge Enabling Objectives (KEO)
Knowledge Enabling Objectives (Chapter 8 Contactors and Motor Starters):
KEO 4.1.
DESCRIBE the differences between Manual and Automatic Motor Control.
KEO 4.2.
DESCRIBE the differences between Manual Contactors and Motor Starters
KEO 4.3.
DESCRIBE what Manual Contactors are and how they are used in the control of
electrical loads to include: Double-Break Contacts, Three Phase Manual
Contactors, and Contact Construction.
KEO 4.4.
DESCRIBE what a Manual Starter consists of and how they provide motor
control and what protection they provide motors.
KEO 4.5.
DESCRIBE the three stages a motor goes through during normal operation and
how they are protected against a potential overload condition to include: Motor
Overload Conditions, Melting Alloy Overloads, Heating Coils, and Resetting
Overload Devices.
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KEO 4.6.
EXPLAIN how Electronic Overloads provide protection against changes in
current and temperature.
KEO 4.7.
EXPALIN the criteria for Selecting AC Manual Motor Starters to include:
Phasing, Enclosures, and Manual Starter Applications.
KEO 4.8.
DESCRIBE the principles of operation of Magnetic Contactors to include:
Magnetic Contactor Construction, Magnetic Contactor Wiring, Control Circuit
Wiring, and Control Circuit Voltage.
KEO 4.9.
DESCRIBE the principles of operation of AC and DC Controllers to include: AC
Arc Suppression (AC and DC), Arc Chutes, and DC Magnetic Blowout Coils.
KEO 4.10.
DESCRIBE the basic Contact Construction criteria associated with Contacts to
include: Single and Double Breaking Contacts, and General-Purpose AC/DC
Contactor Sizes and Ratings.
KEO 4.11.
DESCRIBE principles of operation for a Magnetic Motor Starter to include how
it provides Overload Protection including: Melting Alloy Overload Relays,
Magnetic Overload Relays, Bimetallic Overload Relays, Trip Indicators, and
Overload Current Transformers.
KEO 4.12.
DESCRIBE how Overload Heater Sizing and Selection is accomplished to
include: Full-Load Current, Service Factor, Ambient Temperature,
Manufacture Heater Selection Charts, Checking Selections, and Ambient
Temperature Compensation.
KEO 4.13.
DESCRIBE what Inherent Motor Protection consist of and their principles of
operation to include: Bimetallic Thermo-Discs, and Thermistor Overload
Devices.
KEO 4.14.
EXPLAIN how Electronic Overload Protection is incorporated into Contactor
and Magnetic Motor Modifications to expand their capabilities to include:
Additional Electrical Contacts, Power Poles, Pneumatic Timers, Transient
Suppression Modules, and Control Circuit Fuse Holders.
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KEO 4.15.
DESCRIBE Troubleshooting Contactors and Motor Starter techniques and
procedures.
KEO 4.16.
DESCRIBE how a Contactor and Motor Starter Troubleshooting Guide can be a
useful tool for a technician.
KEO 4.17.
DESCRIBE role Motor Drives provide in the control of Contactors and Motor
Starters to include: Motor Drive Programming and Programming Overload
Protection.
Knowledge Enabling Objectives (Chapter 10 Reversing Motor Circuits):
KEO 4.18.
EXPLAIN the concepts associated with how to reverse the direction of motors to
include: AC Three Phase, AC Single Phase / Capacitor Start, and DC Motors.
KEO 4.19.
EXPLAIN the concepts and needs associated with mechanical interlocking
devices used to protect motors.
Knowledge Enabling Objectives (Chapter 17 Reducing-Voltage Starting):
KEO 4.20.
DESCRIBE the role Reduced Voltage Starting and Silicon-Controlled Rectifiers
provide in motor controls.
Knowledge Enabling Objectives (Chapter 18 Accelerating and Decelerating
Methods:
KEO 4.21.
EXPLAIN the concepts associated Braking and Plugging as they apply to motors.
KEO 4.22.
EXPLAIN the concepts associated Motor Drive Stopping to include: Stopping
without Applied Force from the drive, and Stopping using Reduced Voltage as
the motor Decelerates.
KEO 4.23.
EXPLAIN the concepts of multiple poles in the acceleration, deceleration control
of AC Motors.
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Motor Controls
Knowledge Enabling Objectives (Chapter 8 Contactors and Motor Starters):
KEO 4.1. DESCRIBE the differences between Manual and Automatic Motor Control.

Manual Starting involves a mechanical device (like a switch) that is used to start and
stop a motor. This is performed by a human given the responsibility to ensure that a
motor has either started or stopped in order to control the process or product output
driven by a motor. The motor is essentially controlled by someone telling someone else
the action to be taken.

Automatic Starting involves mechanical controls that receive a feedback signal from
electrical devices that not only start and stop a motor, they also provide a host of control
options such as safety of the equipment or personnel, control of the speed at which the
motor is to run, and the direction it is to rotate. The monitor load and torque to provide
safe and efficient use of the motor to control the process or product output load for a
given motor.
KEO 4.2. DESCRIBE the differences between Manual Contactors and Motor Starters

Manual Contactors are devices that utilize a pushbutton operated by hand to energize or
de-energize the load connected to it. Manual Contactors manually opens or closes
contacts in an electrical circuit. Manual Contactors cannot be used to start or stop motors
as they do not provide overload protection as it is not built into the Manual Contactor.
Manual Contactors are generally used with lighting circuits, and resistive loads such as
heaters or large lamp loads. A fuse or circuit breaker may be provided in the same
enclosure with a Manual Contactor. Manual Contactors typically are energized by a
separate electrical signal with a magnetic coil that closes a set of electrical contacts
providing power to the intended electrical load.
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o The below picture depicts a typical Manual Contactor showing where three
phase power is utilized for a load via a coil activated by a smaller control voltage:
Figure 8-3 page 174
o In the picture above, the load power comes in the top three connections of the
contactor and the coil (two small screws in the lower portion of the contactor)
closes the contactor, which then applies the load power o the load from the
bottom three connections of the contactor.

Motor Starters utilize Contacts that when they are closed either by a manual pushbutton
to the contactor coil or by a control signal from a device. Motor Starters will start and
stop a motor providing overload protection.
SUMMARY:




Manual Starting involves a mechanical device (like a switch) that is used to start and
stop a motor.
Automat Starting involves mechanical controls that receive a feedback signal from
electrical devices to start and stop a motor.
Manual Contactors are devices that utilize a pushbutton operated by hand to energize or
de-energize the load connected to it and cannot be used to start or stop motors as they do
not provide overload protection.
Motor Starters will start and stop a motor and include overload protection.
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KEO 4.3.

DESCRIBE what Manual Contactors are and how they are used in the control of
electrical loads to include: Double-Break Contacts, Three Phase Manual
Contactors, and Contact Construction.
Double-Break Contacts can act a direct controller for an electrical load. Double-Break
Contacts break and electrical circuit in two power lines. The below picture illustrates
how this is done:
Figure 8-4 page 174
Double-Break Contacts allow devices to be designed that have a higher contact rating
(current rating) in a smaller space than devices with a single-break contact. The can be so
constructed to provide Normally Open (NO) or Normally Closed (NC) contact. They are
manually opened or closed by the push button device.
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
Three Phase Manual Contactors are similar to a Double-Break Contacts except that they
open and close three sets of contacts for a three phase system. Three Phase Manual
Contactors are similar to a disconnect device where the mechanical linkage consistently
and quickly makes or breaks the three phase load circuit.
o The below picture illustrates a Three Phase Manual Contactors:
Figure 8-5 page 175
The illustration above is for Three Phase Power loads. These devices can also purchased
to open and close single or dual voltage sources by design. The advantage of these
manual contactors is that the movable contacts have no physical connection to external
electrical wires.
The movable contacts move into arc hoods and bridge the gap between as set of fixed
contacts to make or break the circuit. All physical electrical connections are made
indirectly to the fixed contacts, normally through saddle clamps.

Contact Construction is improved today from the knife switches made from soft copper.
Today most contacts are made of a Low-Resistance Silver Alloy. Silver is alloyed
(mixed) with cadmium or cadmium oxide to make an arc-resistant material which has
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good conductivity (low resistance). In addition, the Silver Alloy has good mechanical
strength, enabling it to endure the continual wear encountered by many openings and
closings. Another advantage is that the oxide forming on the metal is an excellent
conductor of electricity, even when the contacts appear dull or tarnished, they are still
cable of normal operation.
Manual Contactors are used to directly control power circuits and an understanding of
wiring diagrams is required to make changes in power circuits. The below two diagrams
illustrates a wiring diagram showing the connection on an installation or is component
devices and parts used in a single control of a heating element and dual control for a high
and lower heat control option:
Figure 8-7 page 176
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Figure 8-8 page 176
In the second diagram showing dual heat level controls, Mechanical Interlocks are
utilized such that only one set of set of contacts are forced open to allow closing of a
second set of contacts. This is provided by a separate relay activating a set of NO and NC
contacts so that the load is distributed to the correct resistive circuit to provide either low
or high heat demand. This separate relay uses a coil that when it is energized it allows the
normal status of internal contacts to open or close when energized.
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KEO 4.4.
DESCRIBE what a Manual Starter consists of and how they provide motor
control and what protection they provide motors.
Manual Starters consist of a Contactor with an added Overload Protection Device. Manual
Starters are used only for Motor Starters for Electrical Motor Circuits. The primary difference
between a Manual Contactor and a Manual Starter is the addition of overload devices as depicted
in the below picture:
Figure 8-10 page 176

The overload protection MUST be added because the National Electrical Code®
(NEC®) requires that a control device shall not only turn ON or OFF a motor, it
should also protect the motor from destroying itself under an overloaded situation, such
as a lock rotor.

A Locked Rotor is a condition created when a motor is loaded so heavily that the motor
shaft cannot rotate. This condition draws excessive current and heat that can cause a
motor to burn up if not disconnected from the applied line voltage. Overload devices
sense the excessive current and open the circuit to protect the motor.
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SUMMARY:
 Double-Break Contacts break and electrical circuit in two power lines.
 Three Phase Manual Contactors are similar to a Double-Break Contacts except that they
open and close three sets of contacts for a three phase system.
 Manual Contactors are used to directly control power circuits and not for motor cicuits.
 Manual Starters consist of a Contactor with an added Overload Protection Device and
are used for motors.
 Motor Overload protection MUST be added because the National Electrical Code®
(NEC®) requires that a motor control device shall not only turn ON or OFF a motor, it
should also protect the motor from destroying itself under an overloaded situation.
 Overload Devices sense the excessive current and open the circuit to protect the motor.
KEO 4.5.
DESCRIBE the three stages a motor goes through during normal operation and
how they are protected against a potential overload condition to include: Motor
Overload Conditions, Melting Alloy Overloads, Heating Coils, and Resetting
Overload Devices.
The Three Stages A Motor Goes Through are: Resting, Starting, and Operating Under Load as
illustrated in the below picture:
Figure 8-11 page 177
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
A motor at Rest requires no current as the motor circuit is open. A motor that is Starting
draws a tremendous amount of Inrush Current (normally 6 – 8 times the motor running
current as the motor ramps up to its speed. A motor Operating Under Load is operating
at its expected full load current. Fuses, Breakers, or Heaters as Overload Protective
Devices must be rated at a sufficiently high ampere rating to avoid the immediate
opening of the circuit caused by Inrush Current. Breaker and Fuses Overload Devices
are designed to allow the Inrush Current to pass through them momentarily while the
motor is starting and reaching its operating (under load) current limit. Fuses Overload
Devices are termed as Slow Blow, whereas breakers are designed internally to accept
excessive current before it heats up enough to trip the breaker. The National Electrical
Code® addresses the types and specifications that MUST be used for motors.

Motor Overload Conditions may occur while a motor is running after it has reached its
designed Operating Under Load condition. This may occur as the motor is:
o
o
o
o
o
o
o
Forced to do more work than it was intended to do.
Motor bearings failing.
Lack of air flow through the motor.
The insulation of its internal windings starts to age.
Increased ambient temperatures.
Lose electrical connections (electricity’s worst night-mare)
Etc.
These conditions may not draw enough excess current to trip the protective devices;
however it can increase the temperature enough to cause motor damage sufficient to burn
the motor up. The most common motor overload devices are sensitive enough to trip on
excessive heat and are interlocked into the motor control circuit to open the voltage
source to the motor. To meet overload motor protection needs, overload relays are
designed to have a time delay to allow harmless, temporary overloads without disrupting
the circuit.
Overload relays must also have a trip capability to open the circuit if mildly dangerous
currents that could result in motor damage over a period of time. All overload relays have
some means of resetting the current once the overload condition is resolved.
Heat is the end product that can destroy a motor if not monitored. There are several
devices used to detect a combination of heat and overload:
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
Melting Alloy Overloads consist of a device to measure both overload and the
temperature of the motor based on the current being drawn. Sometimes this temperature
is a product of lose connections close to the overloads or at the motor junctions or the
increase of current in the motor windings. Melting Alloy Overloads indirectly monitor
conditions of the motor because the overload relay is normally located as some distance
from the motor. The alloy used provides continuity for current to flow to the motor and
when current flows, it gives off heat. This heat under normal conditions does not affect
the alloy, however when over current conditions exist, excessive heat also exists. This
excessive heat actually melts the alloy and breaks the current path to stop the motor.
When the heat dissipates, the alloy actually sets back up to a solid state to be able to
provide a current path back to the motor.

Heating Coils are sensing devices that sense heat generated by excessive current. This
heat is detected by an interlock device like the Melting Alloy Overloads to stop a motor
circuit that has overheated due to ambient temperature changes to the heating coil. Most
manufactures rely on Eutectic Alloy in conjunction with a mechanical mechanism to
activate a tripping device when an overload occurs. The Eutectic Alloy is a metal that has
a fixed temperature at which it changes directly from a solid to a liquid state. The fixed
state at which this changes is not affected by repeated melting and resetting. Most
manufactures use a Ratchet Wheel and the Eutectic Alloy combination to activate a trip
mechanism when the overload occurs.
o A heater coil device is depicted in the below picture:
Figure 8-13 page 178
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
The Ratchet Wheel is held firmly in the tube by the solid Eutectic Alloy. The inner
shaft and ratchet wheel are locked into position by a Pawl (locking mechanism) so
that the wheel cannot turn when the alloy is cool and in solid form. This Ratchet
Wheel and the Eutectic Alloy combination is illustrated with how this action takes
place in the picture below:
Figure 8-14 page 179

Excessive current applied to the heater coil (current running the motor) melts the
Eutectic Alloy, which allows the Ratchet Wheel to turn freely to activate a trip
mechanism as illustrated above. The main device in an Overload Relay is the
Eutectic Alloy Tube.
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o The following picture illustrates how a typical Manual Starter Overload Relay
uses a compressed spring to push the normally closed contacts open under
normal operating conditions:
Figure 8-15 page 179

The heater coil heats the Eutectic Alloy Tube when an overload occurs. The heat then
melts the alloy, which allows the ratchet wheel to turn. The spring pushes the reset
button up, which open the contacts to the voltage coil of the contactor and stops the
motor. When the Eutectic Alloy cools and returns to its normal solid state, this
contact can be reset until an overload conditions exists again.
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
Resetting Overload Devices requires determining the cause of the overload and when this
condition is determined and corrected, the device(s) can be reset as illustrated in the below
picture:
Figure 8-16 page 180
o This same basic relay is used with all sizes of motors. The only difference is that the
heater coil size is changed. For a single phase motor, one or two will be used and for
a three phase motor, three will be used. The NEC® should be checked for the correct
selection of the appropriate overload heater sizes.
o Resetting Overload Devices requires that the technician must determine the cause of
an overload before resetting an overload relay. If the cause is not determined and the
overload devices have cooled enough to be reset, the motor circuit will trip again
causing increased risk of extending damage to the motor. Nothing in the Overload
Device Relay requires replacement because the heaters do not open like a fuse would
unless they are defective and will not reset. When the overload condition is removed
and the reset button is pressed after the eutectic alloy has cooled and become a solid
again.
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SUMMARY:











The Three Stages A Motor Goes Through are: Resting, Starting, and Operating
Under Load
A motor at Rest requires no current as the motor circuit is open.
A motor that is Starting draws a tremendous amount of Inrush Current (normally 6
– 8 times the motor running current as the motor ramps up to its speed.
A motor Operating Under Load is operating at its expected full load current.
Motor Overload Conditions may occur while a motor is running after it has reached
its designed Operating Under Load condition.
To meet overload motor protection needs, overload relays are designed to have a
time delay to allow harmless, temporary overloads without disrupting the circuit.
Heat is the end product that can destroy a motor if not monitored.
Melting Alloy Overloads consist of a device to measure both overload and the
temperature of the motor based on the current being drawn.
Heating Coils are sensing devices that sense heat generated by excessive current.
Excessive current applied to the heater coil (current running the motor) melts the
Eutectic Alloy, which allows the Ratchet Wheel to turn freely to activate a trip
mechanism.
Resetting Overload Devices requires determining the cause of the overload and
when this condition is determined and corrected, the device(s) can be reset.
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KEO 4.6.
EXPLAIN how Electronic Overloads provide protection against changes in
current and temperature.
Electronic Overloads provide protection against changes in current and temperature by a device
that has built in circuitry to sense changes (increases) in motor circuit current. Today’s newer
Manual Starters include Electronic Overload Protection instead of heaters. This is
accomplished by the built-in circuitry sensing the motor current increase from its normal full
load condition. The following picture illustrates how Electronic Overloads provide protection:
Figure 8-17 page 180
o Electronic Overloads measure the strength of the magnetic field around a wire
instead of measuring current that causes heat. As current flows in a conductor, the
magnetic field provides a signal that is electronically is equal to that current flowing
in a conductor. The Solid State Current Monitor then provides the signal necessary to
open a motor circuit when the current exceeds the motor requirements. The higher the
current flowing in the wire, the stronger the magnetic field produced to activate
disconnecting means opening the starter power circuit contacts.
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KEO 4.7.
EXPALIN the criteria for Selecting AC Manual Motor Starters to include:
Phasing, Enclosures, and Manual Starter Applications.
The Criteria for Selecting AC Manual Motor Starters includes identification of specific
characteristics a starter must be able to provide for the proper operation of a motor, such as what
phasing requirements are, the type of enclosure needed, and the application in which the starter is
to provide.
The following picture illustrates typical characteristics for determining starter selection and its
size:
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Figure 8-18 page 181

Phasing and Enclosures are the determining factors in selecting an enclosure for single
or three phase motor manual starters. The smallest size starter for a single phase motor
(single pole or double pole) without overload protection is “00” and smallest size of
enclosure for a three phase three pole starter with overload protection is a size “0” to a
size “1”.
NEC® Requires that each Ungrounded Conductor in a motor circuit (meaning the hot
conductors) MUST be opened when disconnecting those conductors from the motor. For
example, a Single Pole Motor has one ungrounded conductor and one grounded (neutral)
conductor and so only the ungrounded (hot) conductor needs to have a disconnecting
means. For a Double Pole Motors and Three Pole motors ( Single Phase and Three Phase
Motors) the conductors are all ungrounded (hot) and MUST have a disconnecting means
provided.
The following picture illustrates how this is accomplished for a Single Pole motor with a
Neutral (grounded) Conductor, A Double Pole with two (ungrounded) Hot Conductors,
and a Three Pole with three (ungrounded) Hot Conductors that are protected by a fuse, or
breaker device as a disconnecting means:
Figure 8-19 page 182
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
Enclosures provide mechanical and electrical protection for personnel and for the starter
itself. Enclosures are designed to Provide Protection in a variety of situations: Water,
Dust, Oil, and Hazardous Locations. Inside the enclosure, starter wiring and physical
construction are the same for any location. The NEC® defines a NEMA Type 1
Enclosure as intended for indoor use primarily to provide a degree of protection against
human contact with the enclosed equipment in locations where unusual conditions do not
exist.

Manual Starter Applications usually include applications such as Conveyor Systems,
and drill presses similar to those depicted in the below figure:
Figure 8-20 page 182

In these types of applications, the manual starter provides the disconnecting means for ON
and OFF while providing motor overload protection. Manual Starter Applications in most
all cases involve the starter to be at or near the equipment being started, whereas Automatic
Starter Applications are remotely located.
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SUMMARY:









Electronic Overloads provide protection against changes in current and
temperature by a device that has built in circuitry to sense changes (increases) in
motor circuit current.
Today’s newer Manual Starters include Electronic Overload Protection instead
of heaters.
Electronic Overloads measure the strength of the magnetic field around a wire
instead of measuring current that causes heat.
The Criteria for Selecting AC Manual Motor Starters includes identification of
specific characteristics a starter must be able to provide for the proper operation of
a motor, such as what phasing requirements are, the type of enclosure needed, and
the application in which the starter is to provide.
Phasing and Enclosures are the determining factors in selecting an enclosure for
single or three phase motor manual starters.
NEC® Requires that each Ungrounded Conductor in a motor circuit (meaning
the hot conductors) MUST be opened when disconnecting those conductors
from the motor.
Enclosures provide mechanical and electrical protection for personnel and for the
starter itself. Enclosures are designed to Provide Protection in a variety of
situations: Water, Dust, Oil, and Hazardous Locations.
Manual Starter Applications usually include applications such as Conveyor
Systems, and drill presses.
Manual Starter Applications in most all cases involve the starter to be at or near
the equipment being started, whereas Automatic Starter Applications are remotely
located.
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KEO 4.8.

DESCRIBE the principles of operation of Magnetic Contactors to include:
Magnetic Contactor Construction, Magnetic Contactor Wiring, Control Circuit
Wiring, and Control Circuit Voltage.
Magnetic Contactors include devices that may be operated manually or magnetically.
They are devices for repeatedly establishing and interrupting an electrical power
(ungrounded) circuit. Contactor Construction consists of the principle operating
mechanism of a Solenoid Action as indicated in the picture below:
Figure 8-22 page 183
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o Control Circuit Wiring is associated with the number of conductors used in the
control circuit such as two and three wire control. In a Two Wire Control Circuit,
two wires lead from the control device to the contactor or starter as illustrated in
the below Two-Wire Control Schematic:
Figure 8-23 page 184
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o A Two-Wire Control Circuit provides low voltage release, but not low voltage
protection. Caution must be exercised in the use and service of Two-Wire Control
Circuits.
o A Three-Wire Control Circuit has three wires leading from the control device to the
starter or contactor as illustrated in the below Three-Wire Control Schematic:
Figure 8-24 page 185
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This Three-Wire Control Circuit uses a momentarily closed ON pushbutton (NC) wired
in series with a momentary contact OFF pushbutton (NO) wired in parallel to a set of
contacts which form a holding circuit interlock (memory).
A Three-Wire Control Circuit provides low-voltage release and low-voltage protection.
The coil drops out at low or no voltage and cannot be reset unless the voltage returns and
the operator presses the start button.

Control Circuit Voltage involves the use of Pushbuttons, Limit Switches, Pressure
Switches, Temperature Switches, etc., to control the flow of power to the contactor/motor
starter magnetic coil in control of the circuit. In most cases dealing with motor controls,
the control circuit is operated at a lower voltage level that the load. This is for safety as
well as efficiency. Step-Down transformers are used to reduce the line voltage to a
control voltage. The control voltage can be AC or DC, and us usually a reduced AC. The
following schematic illustrates how line voltage is reduced (stepped-down) to provide a
separate control voltage:
Figure 8-25 page 186
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SUMMARY:





Magnetic Contactors include devices that may be operated manually or
magnetically. They are devices for repeatedly establishing and interrupting an
electrical power (ungrounded) circuit.
Control Circuit Wiring is associated with the number of conductors used in the
control circuit such as two and three wire control.
A Two-Wire Control Circuit provides low voltage release, but not low voltage
protection. Caution must be exercised in the use and service of Two-Wire Control
Circuits.
A Three-Wire Control Circuit provides low-voltage release and low-voltage
protection.
Control Circuit Voltage involves the use of Pushbuttons, Limit Switches, Pressure
Switches, Temperature Switches, etc., to control the flow of power to the
contactor/motor starter magnetic coil in control of the circuit.
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KEO 4.9.

DESCRIBE the principles of operation of AC and DC Controllers to include: AC
Arc Suppression (AC and DC), Arc Chutes, and DC Magnetic Blowout Coils.
Arc Suppression is required on contactors and motor starters. An Arc Suppressor is a
device that dissipates the energy present across opening contacts. Without Arc
Suppression contactors and motors would require maintenance prematurely resulting in
excessive down time. A period of time (a few thousandths of a second) exist when a set
of contacts is open under load which the contacts are neither fully in touch with each
other, nor completely separated as illustrated in the below picture:
Figure 8-27 page 187
o As contacts continue to separate, the contact surface area decreases, increasing the
electrical resistance. With full load current passing through this increased
resistance, temperature rises on the contact surface and cause the contact surface
to become molten and emit ions of vaporized metal into the gap. An arc is then
created, which can damage the contact surface. The sooner this arc can be
extinguished, the longer the life expectancy of the contacts. There are AC and DC
Arcs. DC is the most difficult to extinguish because DC causes current to flow
constantly across a much wider gap. DC Contactors are larger and need to
function faster than AC Contactors.
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o An Arc Chute is a device that confines, divides, and extinguishes arcs drawn
between contacts opened up under load as depicted in the below picture:
Figure 8-29 page 188
o Arc Chutes employ the De-Ion Principle to extinguish the arc for each contact.
Arc chutes and traps are used to confine, divide, and extinguish arcs down
between contacts opened under load.
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o DC Magnetic Blowout Coils are an Electromagnetic Blowout Coil and is referred
to as a Puffer because of its blowout ability as illustrated in the below picture:
Figure 8-31 page 189
With DC Arcing, an action must be taken to quickly limit the damaging effects of the
heavy circuit current arc as a sustained arc may melt the contacts, weld them together, or
severely damage them The Blowout Coil (Puffer) is used to reduce the distance required
to and yet quenches the arc quickly. Magnetic Blowout Coils provide this action by
providing a magnetic field that blows out the arc similarly to blowing out a lit match.
SUMMARY:
 Arc Suppression is required on contactors and motor starters.
 An Arc Suppressor is a device that dissipates the energy present across opening contacts.
 Without Arc Suppression contactors and motors would require maintenance prematurely
resulting in excessive down time.
 DC Arcs are the most difficult to extinguish because DC causes current to flow
constantly across a much wider gap. DC Contactors are larger and need to function
faster than AC Contactors.
 Arc Chutes employ the De-Ion Principle to extinguish the arc for each contact.
 Arc chutes and traps are used to confine, divide, and extinguish arcs down between
contacts opened under load.
 DC Magnetic Blowout Coils are an Electromagnetic Blowout Coil and is referred to as a
Puffer because of its blowout ability.
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KEO 4.10.


DESCRIBE the basic Contact Construction criteria associated with Contacts to
include: Single and Double Breaking Contacts, and General-Purpose AC/DC
Contactor Sizes and Ratings.
Single and Double Breaking Contacts design depends on the size, current rating, and
application of the contactor being utilized. Double Breaking Contacts are usually made
of Silver-Cadmium Alloy. Single Breaking Contacts in large contactors are frequently
made of copper because of the low cost.
o Single Breaking Contacts are designed with a wiping action to remove copper
oxide film that forms on the contacts. The Wiping Action is necessary because
copper oxide formed on the contacts when not in use is an insulator and must be
eliminated for good circuit continuity. In most cases, the slight rubbing action
and burring that occur in normal operation keeps the contact surfaces clean.
Copper contacts seldom open or closed should be cleaned to reduce contact
resistance as high contact resistance causes serious heating of the contacts.
General-Purpose AC/DC Contactor Sizes and Ratings are according to the size and type
of load by NEMA (National Electrical Manufacturing Association) and are specified in
the NEC® Requirements. An example of NEMA Standard Ratings for AC and DC
Contactors are shown in the below picture:
Figure 8-32 page 190
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These ratings are for each contact and not the entire contactor. Contactor dimensions vary
greatly from inches to feet in length and are based on type, size and voltage available as
depicted in the picture below:
Figure 8-33 page 191
Contactors vary from inches to several feet in height.
SUMMARY:


Double Breaking Contacts are usually made of Silver-Cadmium Alloy.
Single Breaking Contacts in large contactors are frequently made of copper because of
the low cost.
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

Single Breaking Contacts are designed with a wiping action to remove copper oxide film
that forms on the contacts. The Wiping Action is necessary because copper oxide formed
on the contacts when not in use is an insulator and must be eliminated for good circuit
continuity.
General-Purpose AC/DC Contactor Sizes and Ratings are according to the size and type
of load by NEMA (National Electrical Manufacturing Association) and are specified in
the NEC® Requirements.
KEO 4.11.
DESCRIBE principles of operation for a Magnetic Motor Starter to include how
it provides Overload Protection including: Melting Alloy Overload Relays,
Magnetic Overload Relays, Bimetallic Overload Relays, Trip Indicators, and
Overload Current Transformers.
Magnetic Motor Starters are available in sizes that can switch loads of a few amperes to several
hundred amperes. An example of a magnetic motor starter with electronic overload protection is
depicted below:
Figure 8-34 page 192
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A magnetic motor starter is a contactor with overload protection added. The difference from a
Manual Starter and Magnetic Motor Starter is that a Motor Starter is equipped with motor
overload protection. A Magnetic Motor Starter is an electrically operated switch (contactor) that
includes motor overload protection. They include overload relays that detect excessive current
passing through a motor and are used to switch all types and sizes of motors.
The following pictures illustrate the principles of operation for: Magnetic Overload Relays,
Bimetallic Overload relays, Trip Indicators, and Overload Current Transformers:

Magnetic Overload Relays
Figure 8-35 page 192
Magnetic overload relays use a current coil which, at a specific over-current
value, acts like a solenoid and causes a set of normally closed contacts to
open.
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
Bimetallic Overload relays
Figure 8-36 page 193
The warping effect of a bimetallic strip is used as a means for separating
contacts.
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
Trip Indicators
Figure 8-37 page 193
Trip indicators indicate that an overload has taken place for within the
device.
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
Overload Current Transformers
Figure 8-38 page 194
Standard overload relays may be used on very large starters by using current
transformers with specific reduction ratios.
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SUMMARY:






Magnetic Motor Starters are available in sizes that can switch loads of
a few amperes to several hundred amperes.
A Magnetic Motor Starter is an electrically operated switch
(contactor) that includes motor overload protection. They include
overload relays that detect excessive current passing through a motor
and are used to switch all types and sizes of motors.
Magnetic overload relays use a current coil which, at a specific overcurrent value, acts like a solenoid and causes a set of normally closed
contacts to open.
The warping effect of a bimetallic strip is used as a means for
separating contacts.
Trip indicators indicate that an overload has taken place for within
the device.
Standard overload relays may be used on very large starters by using
current transformers with specific reduction ratios.
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KEO 4.12.
DESCRIBE how Overload Heater Sizes and Selection is accomplished to
include: Full-Load Current, Service Factor, Ambient Temperature,
Manufacture Heater Selection Charts, Checking Selections, and Ambient
Temperature Compensation.
Overload Heater Sizes – Each motor must be sized according to its own unique operating
characteristics and applications. Thermal over-load heaters are selected based on the Full-Load
Current rating (FLC), Service Factor (SF), and Ambient Temperature (surrounding air
temperature) on the motor when it is operating. Each motor has its own nameplate which
provides the motor Class, Type, and Size of the starter as illustrated in the below picture:
Figure 8-39 page 195
Selection of Motor Overload Heaters Coils for Continuous-Duty Motors are selected from
manufactures tables based on the motor nameplate full-load current for maximum protection and
compliance with Section 430.32 of the NEC®.
Common applications use 40oC as the ambient temperature. If the temperature is different, it
must be determined in order to make the correct heater selection. A motor’s Phase, Service
Factor, and Full-Load Current are determined from the motor’s nameplate information.
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It is important to Always Refer To Manufactures Instructions on thermal overload relay
selection to see if any restrictions are placed on the class of the motor starter required. The below
picture illustrates an example of manufactures instructions on thermal overload relay selection
detailing restrictions of classes of starters:
Figure 8-40 page 195

Full-Load Current is based on information found on the motor nameplate or in
manufacturing specification sheets. Heater manufactures develop charts indicating which
heaters should be used with each full-load current.

Service Factor is a number (SF) designation that represents the percentage of extra
demand that can be placed on a motor for short intervals with causing motor damage.
This is a multiplier that can be used to determine the SF current rating. If a motor name
plate indicated an SF 1.15 and the motor was a 10 amp motor. 10 x 1.15 = 11.5 amps this
motor could operate for a short time interval without causing motor damage.

Ambient Temperature is associated with thermal overload devices operating on the
principle of heat. Excessive current will melt a metal alloy, produce movement in a
current coil or warp a bimetallic strip to allow the device to trip. Temperature (ambient)
surrounding a thermal overload relay must be considered as the relay device is sensitive
to heat from any source. Overload relays usually have a rating of 400 C or 1040 F. This is
a standard acceptable range for most devices and temperature above or below these
ranges need to be compensated for.
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
Manufacture Heater Selection Charts are provided to use in selecting proper thermal
overload heaters. This information is also found within the enclosure of many motor
starters. An example of a Manufacture Heater Selection Chart is shown below:
Figure 8-41 page 196
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
Ambient Temperature Compensation is required when ambient temperature increases or
decreases. As ambient temperature decreases, more current is needed to trip the overload
devices. An example of a Thermal Unit Selection criteria is shown below:
Figure 8-42 page 196
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
Manufactures also provide charts for approximating Full-Load Current when motor
nameplate information is not available as illustrated below:
Figure 8-43 page 197
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SUMMARY:













Each motor must be sized according to its own unique operating characteristics and
applications.
Thermal over-load heaters are selected based on the Full-Load Current rating (FLC),
Service Factor (SF), and Ambient Temperature (surrounding air temperature) on the
motor when it is operating.
Each motor has its own nameplate which provides the motor Class, Type, and Size of the
starter.
Selection of Motor Overload Heaters Coils for Continuous-Duty Motors are selected
from manufactures tables based on the motor nameplate full-load current for maximum
protection and compliance with Section 430.32 of the NEC®.
Common applications use 40oC as the ambient temperature. If the temperature is
different, it must be determined in order to make the correct heater selection.
A motor’s Phase, Service Factor, and Full-Load Current are determined from the
motor’s nameplate information.
It is important to Always Refer To Manufactures Instructions on thermal overload relay
selection to see if any restrictions are placed on the class of the motor starter required.
Full-Load Current is based on information found on the motor nameplate or in
manufacturing specification sheets.
Service Factor is a number (SF) designation that represents the percentage of extra
demand that can be placed on a motor for short intervals with causing motor damage.
Ambient Temperature is associated with thermal overload devices operating on the
principle of heat.
Manufacture Heater Selection Charts are provided to use in selecting proper thermal
overload heaters. This information is also found within the enclosure of many motor
starters.
Ambient Temperature Compensation is required when ambient temperature increases or
decreases.
Manufactures also provide charts for approximating Full-Load Current when motor
nameplate information is not available.
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KEO 4.13.
DESCRIBE what Inherent Motor Protection consist of and their principles of
operation to include: Bimetallic Thermo-Discs, and Thermistor Overload
Devices.
Inherent Motor Protection consists of a device (or devices) located locally on the motor to
provide overload protection. When motors are remotely located away from the motor control
center, they may be subject to ambient conditions that can cause motor failure before the
overload protection will be able to open the circuit.

Bimetallic Thermo-Discs are normally used on small horsepower motors to directly
disconnect the motor from the power circuit. When these devices reach a level of heat,
they warp and open a circuit. They operate on a principle associated with how different
metals when joined together will react differently as temperature is applied. This twisting
or warping motion will open a contact to prevent the motor from becoming damaged. An
example of a Bimetallic Thermo-Disc (usually used on small HP motors) is shown
below:
[ Insert Figure 8-44 page 198 ]
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
Thermistor Overload Devices are an overload device combined with a thermistor, solidstate relay, and contactor into a custom-built overload protector as illustrated below:
Figure 8-45 page 198
SUMMARY:



Inherent Motor Protection consists of a device (or devices) located locally on the motor
to provide overload protection.
Bimetallic Thermo-Discs are normally used on small horsepower motors to directly
disconnect the motor from the power circuit.
Thermistor Overload Devices are an overload device combined with a thermistor, solidstate relay, and contactor into a custom-built overload protection device.
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KEO 4.14.
EXPLAIN how Electronic Overload Protection is incorporated into Contactor
and Magnetic Motor Modifications to expand their capabilities to include:
Additional Electrical Contacts, Power Poles, Pneumatic Timers, Transient
Suppression Modules, and Control Circuit Fuse Holders.
Contactor and Magnetic Motor Modifications expand a motor contactor starter by incorporating
certain devices to the basic contactors or motor starters. These designed by the contactor or
starter manufacture to be optional equipment that can be electrically and mechanically attached
to a contactor or starter assemble inside of its NEMA enclosure.

The following picture shows 5 such devices that can be utilized for added motor
protection:
Figure 8-46 page 199
SUMMARY:
 Contactor and Magnetic Motor Modifications expand a motor contactor
starter by incorporating certain devices to the basic contactors or motor
starters.
 Five devices that are typically installed on motor starters are
1. Additional Electrical Contacts
2. Power Poles
3. Pneumatic Timers
4. Transient Suppression Modules
5. Control Circuit Fuse Holders
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KEO 4.15.
DESCRIBE Troubleshooting Contactors and Motor Starter techniques and
procedures.
SAFETY NOTE REGARDING TROUBLSHOOTING MOTOR CONTROLS:

Safety precautions must be followed when troubleshooting motor control circuits.
This is because to check control circuits effectively, power must be energized and
the appropriate safety personal protective equipment and procedures must be
utilized by the technician at all times when working on energized equipment.

Contactors and Motor Starters are the first devices checked by a technician when
troubleshooting a circuit that does not work, or has a problem because they are the point
where incoming power, load, and control circuit are connected. Basic voltage readings
are taken at a contactor or motor starter to determine where a problem may be. The same
basic procedure used to troubleshoot a motor starter also works for contactors because a
motor starter is a contactor with added overload protection.

The Number 1 overall problem with any electrical circuit is caused by poor or loose
connections. This condition generates heat that can not only destroy wiring, but also
components upstream of the load and out to the load. The tightness of all terminals and
bus-bar connections needs to be checked. Loose connections causes:
o Overheating in power circuits, contactors, and starters
o Leads to equipment failure
o Creates control circuit failures
o Lead to Shock Hazards
o Cause Electromagnetic-Generated Interference
o Etc.
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
To troubleshoot a motor starter, apply the following procedure (Below is a pictorial
presentation of this procedure to help emphasize this 6-Step approach to troubleshooting
a motor starter):
Figure 8-48 page 202
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1. Inspect the motor starter and overload assembly.
 Service or replace motor starters that show heat damage, arcing, or wear.
 Replace motor starters that show signs of burning.
 Check the motor and driven load for signs of an overload or other
problems.
2. Reset the overload relay if there is not sign of visual damage.
 Replace the overload relay if there is visual indication of damage.
3. Observe the motor starter for several minutes if the motor starts and after resetting the
overload relay (observe if the overload relay continues to open if an overload problem
continues to exist.
4. Check the voltage into the starter if resetting the overload relay does not start the motor.
 Check circuit voltage ahead of the starter if zero voltage is at the starter.
 The voltage is acceptable if within a + or – 10% of the normal voltage
necessary for the circuit.
5. Energize the starter and check the starter contacts if the voltage into the starter is present
at the correct value.
 Verify contacts are good.
 Open the starter, turn off and lockout-tagout power to replace contacts as
necessary.
6. Check the overload relay if voltage is coming out of the starter contacts.
 Lockout and Tagout power if the problem is downstream of the starter and
continue troubleshooting downstream.
KEO 4.16.
DESCRIBE how a Contactor and Motor Starter Troubleshooting Guide can be a
useful tool for a technician.
A Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a technician in
that it states problems, possible causes, and corrective actions that may be taken.
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An example of a troubleshooting guide is shown below:
Figure 8-49 page 203
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SUMMARY:




Safety precautions/procedures MUST be followed when troubleshooting motor
control circuits because to check control circuits effectively, power must be energized.
Contactors and Motor Starters are the first devices checked by a technician when
troubleshooting a circuit that does not work, or has a problem because they are the
point where incoming power, load, and control circuit are connected.
The Number 1 overall problem with any electrical circuit is caused by poor or loose
connections. These conditions generate heat that can not only destroy wiring, but also
components upstream of the load and out to the load.
A Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a
technician in that it states problems, possible causes, and corrective actions that may be
taken.
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KEO 4.17.
DESCRIBE role Motor Drives provide in the control of Contactors and Motor
Starters to include: Motor Drive Programming and Programming Overload
Protection.
MOTOR DRIVES are being incorporated more and more and replacing many applications for
motor control. A Motor Drive is an electronic device designed to control the speed of a motor.
This is accomplished by using solid state components. Motor Drives may control AC or DC
Motors. AC motors Drives are more common than DC Motor Drives.
Other terms for AC Motor Drives are:
1. Adjustable Speed Drives
2. Variable Frequency Drives
3. Inverters
The following picture illustrates both an AC and DC Motor drive showing solid-state devices
used to control motor speed:
Figure 8-50 page 204
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
Motor Drives are designed to control the speed of motors using solid-state components
and may be AC or DC drives. They perform the same function as a Motor Starter, but can
also:
o
o
o
o
o
o
Vary motor speed.
Reverse motor direction.
Provide additional protection features.
Displays operating information.
Interfaces with other electrical equipment
Provides motor control for motors from a fractional to hundreds of HP

AC Motor Drives control motor speed by converting the incoming AC to DC and then
converting the DC back to a Variable Frequency AC. A typical AC motor at 60 Hz runs
at full speed, at 30 Hz runs at half speed, and at 15 Hz runs at one-quarter speed.

DC Motor Drives control motor speed by controlling and monitoring the DC Output
Voltage to the motor and the Current on the motor field windings and armature windings.
AC Power can also be the input power source that is then converted to DC.

AC Motor Drives have revolutionized motor control making it more efficient and cost
effective. They do this by providing a pulsating DC to drive the motor and control its
speed by varying the frequency to the motor. Using a microprocessor circuit located
inside the drive device provides fast and reliable electronic switching.

Motor Drive Programming is essential to ensure proper motor operation. A properly
programmed motor drive provides maximum system performance. An improperly
programmed motor drive can cause damage to the motor, other system components and
create safety hazards to the area and personnel.
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The following picture depicts the use of SCR to convert AC to DC and controlling the
level of DC voltage, and the use of IGBT (Insulated Gate Bipolar Transistors) to provide
PWM (Pulse Width Modulated Inverter):
Figure 8-51 page 204
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AC drives include a converter, DC link, and an inverter as illustrated above.
o Programming is done via a Human Interface Module, which is a manually
operated input control unit that includes programming keys. Programming is
performed by trained technicians. The following two pictures depict a typical
Human Interface Module, and levels utilized for system information and
programming:
Figure 8-52 page 205
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Figure 8-53 page 206
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

Overload Protection is needed when a motor starter turns on a motor to protect it while it
is operating using heaters or electronic overload devices included in a typical motor
starter per NEC® Requirements.
Programming Overload Protection with an AC Motor Drive is programmed into the
Drive using the Human Interface Module to meet NEC® Requirements as well. These
requirements are specified in Article 430 of the NEC®. Part III of Article 430 covers
motor overload (running) protection requirement. Motor Drives meet these requirements
if they are programmed properly. Over Current and Ground-Fault protection is covered in
Part IV of Article 430 as well.
o Overload Protection needs to be set to open a circuit at a maximum of 115% to
1125% of the motor full-load current and is addressed in NEC Article 432.32.
The percentage depends on the motor temperature rise and its service factor.
o Motors marked with a service factor (SF) not less than 1.15, require maximum
overload protection of 125% times the motor full-load current.
o Motors with a marked temperature rise not over 400 C also require a maximum
overload protection of 125% times the motor full-load current.
o All Other Motors are required to be to have a maximum overload protection of
115% times the motor full-load current.
The following picture illustrates how a motor name plate is read to determine its Service
Factor (SF) in order to program the overload protection according to NEC®
Requirements:
Figure 8-55 page 208
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SUMMARY:











MOTOR DRIVES are being incorporated more and more and replacing many
applications for motor control as they cost less and are more efficient.
A Motor Drive is an electronic device designed to control the speed of a motor by using
solid state components.
Motor Drives may control AC or DC Motors. AC motors Drives are more common than
DC Motor Drives.
AC Motor Drives are called Adjustable Speed Drives, Variable Frequency Drives, or
Inverters.
Motor Drives are designed to control the speed of motors using solid-state components
and may be AC or DC drives. They perform the same function as a Motor Starter and can
also be programmed to do the following:
o Vary motor speed.
o Reverse motor direction.
o Provide additional protection features.
o Displays operating information.
o Interfaces with other electrical equipment
o Provides motor control for motors from a fractional to hundreds of HP
AC Motor Drives control motor speed by converting the incoming AC to DC and then
converting the DC back to a Variable Frequency AC.
DC Motor Drives control motor speed by controlling and monitoring the DC Output
Voltage to the motor and the Current on the motor field windings and armature windings.
AC Motor Drives have revolutionized motor control making it more efficient and cost
effective.
AC Motor Drives use SCRs to convert AC to DC and controlling the level of DC
voltage, and the use of IGBT (Insulated Gate Bipolar Transistors) to provide PWM (Pulse
Width Modulated Inverter).
Programming is done via a Human Interface Module, which is a manually operated
input control unit that includes programming keys and is programmed by trained
technicians.
Programming Overload Protection with an AC Motor Drive is programmed into the
Drive using the Human Interface Module to meet NEC® Requirements:
o Overload Protection needs to be set to open a circuit at a maximum of 115% to
1125% of the motor full-load current and is addressed in NEC Article 432.32.
The percentage depends on the motor temperature rise and its service factor.
o Motors marked with a service factor (SF) not less than 1.15, require maximum
overload protection of 125% times the motor full-load current.
o Motors with a marked temperature rise not over 400 C also require a maximum
overload protection of 125% times the motor full-load current.
o All Other Motors are required to be to have a maximum overload protection
of 115% times the motor full-load current.
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Knowledge Enabling Objectives (Chapter 10 Reversing Motor Circuits):
KEO 4.18.
EXPLAIN the concepts associated with how to reverse the direction of motors to
include: AC Three Phase, AC Single Phase / Capacitor Start, and DC Motors.
Reversing the direction of motors is accomplished with: Manual Reversing Starters, Drum
Switches, Magnetic Reversing Starters, Programmable Logic Controllers (PLCs) or Motor
Drives. The below picture depicts a typical Three Phase Manual Starter having the capability of
forward or reverse starting:
Figure 10-1 page 259
Manual Starters are used in pairs to reverse DC, Single Phase AC, and Three Phase AC motors.
Manual Motor Starters in pairs used to provide forward or reverse rotation uses a mechanical
interlock to separate the contactors starter contacts so that the motor will only run in one
direction (forward or reverse).
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The following picture illustrates how two starters are interlocked to accomplish this Three Phase
AC Motor Action:
Figure 10-2 page 260

Reversing Three Phase AC Motors is a simple concept. It is accomplished by
interchanging any two of the three phase main power sources to the motor. The industry
standard that is most often performed so that there is consistency in wiring
configurations is to interchange L1 and L3 for all three phase motors to include 3, 6,
and 9 lead Wye (Y), and Delta (Δ) connected motors. This can be accomplished at the
power source to the starter or at the motor interchanging T1 with T3.
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o The picture below illustrates a wiring diagram for electrical connections necessary
to reverse a Three Phase Motor using a manual reversing starter:
Figure 10-3 page 260
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o The following picture illustrates a schematic that shows how a manual reversing
starter is connected to accomplish a forward or reverse direction for a three phase
AC motor:
Figure 10-4 page 261
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o The basic concept associated with reversing a three phase motor is simply
interchanging two of the three phase power leads to the motor (at starter or at
motor) with changing at the starter being generally the easiest location to make
the change.

Reversing Single Phase AC Motors including a Capacitor Start is not as easy as
reversing the direction of a three phase motor. To reverse the direction of a Single Phase
AC Motor requires interchanging the leads of the starting or the running windings. To
best way to accomplish this is to refer to the manufactures wiring diagram to determine
the exact wires to interchange for reversing the single phase motor. The picture below
illustrates how a manual reversing motor starter is used to change the direction of a single
phase motor:
Figure 10-5 page 261
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
If manufacture information is not available, an electrician/technician can measure the
resistance of the start and run windings to determine which leads are which. The
running winding is made of a larger (heaver) gauge wire than the starting winding,
thus the running winding has a lower resistance than the start winding.

The below picture illustrates how a manual reversing motor starter is wired to allow
changing of forward or reverse direction of a single phase motor:
[ Insert Figure 10-6 page 262 ]
The preferred method to change the rotation of a single phase motor (or capacitor start
motor) is to reverse the leads of the starting winding.
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
Reversing DC Motors is accomplished by reversing the direction of DC Current Flow
through the Armature of all DC Motors. The following schematics illustrate how this is
accomplished using a reversing starter for: DC Series Motor, DC Shunt Motor, DC
Compound Motor, and a DC Permanent-Magnet Motor:
a. DC Series Motor
Figure 10-7 page 263
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b. DC Shunt Motor
Figure 10-8 page 263
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c. DC Compound Motor
Figure 10-9 page 264
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d. DC Permanent-Magnet Motor
Figure 10-10 page 264
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SUMMARY:
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Reversing the direction of motors is accomplished with: Manual Reversing Starters,
Drum Switches, Magnetic Reversing Starters, Programmable Logic Controllers (PLCs)
or Motor Drives.
Manual Starters are used in pairs to reverse DC, Single Phase AC, and Three Phase AC
motors. Manual Motor Starters in pairs used to provide forward or reverse rotation, uses a
mechanical interlock to separate the contactors starter contacts so that the motor will
only run in one direction (forward or reverse).
Reversing Three Phase AC Motors is a simple concept. It is accomplished by
interchanging any two of the three phase main power sources to the motor.
o The industry standard that is most often performed so that there is consistency
in wiring configurations is to interchange L1 and L3 for all three phase motors
to include 3, 6, and 9 lead Wye (Y), and Delta (Δ) connected motors.
Reversing Single Phase AC Motors including a Capacitor Start is not as easy as
reversing the direction of a three phase motor.
o To reverse the direction of a Single Phase AC Motor requires interchanging the
leads of the starting or the running windings.
o The preferred method to change the rotation of a single phase motor (or
capacitor start motor) is to reverse the leads of the starting winding.
Reversing DC Motors is accomplished by reversing the direction of DC Current Flow
through the Armature of all DC Motors.
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KEO 4.19.
EXPLAIN the concepts and needs associated with mechanical interlocking
devices used to protect motors.
Mechanical Interlocking includes devices that mechanically prevent the control circuit to only
function in the control mode it has been requested to function in by an operator or by devices
using automatic control logic circuitry. The following schematics illustrate how this is done:
1. Magnetic Reversing Starter controlled by forward and reverse pushbuttons:
Figure 10-16 page 267
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2. Magnetic Reversing Starter with secondary backup using auxiliary contacts to provide
electrical interlocking:
Figure 10-17 page 267
3. Pushbutton Interlocking using both NO and NC contacts mechanically connected on each
pushbutton:
Figure 10-18 page 268
4. Start/Stop/Forward/Reverse circuit with indicator lights showing the direction of rotation
for a motor at a given moment:
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Figure 10-20 page 269
SUMMARY:
Instructor Note: This summary addresses schematics and in preparation for this summary, the
instructor could generate a few questions for each of the schematics listed below to assist in
determining mastery of objective KEO 4.19: EXPLAIN the concepts and needs associated with
mechanical interlocking devices used to protect motors.





Mechanical Interlocking includes devices that mechanically prevent the control circuit
to only function in the control mode it has been requested to function in by an operator or
by devices using automatic control logic circuitry.
Magnetic Reversing Starter controlled by forward and reverse push-buttons is an
example of mechanical interlocks (Figure 10-16 page 267).
Magnetic Reversing Starter with secondary backup using auxiliary contacts to provide
electrical interlocking is also used to ensure mechanical interlocks (Figure 10-17 page
267).
Pushbutton Interlocking using both NO and NC contacts mechanically connected on
each pushbutton is another example (Figure 10-18 page 268).
Start/Stop/Forward/Reverse circuit with indicator lights showing the direction of
rotation for a motor at a given moment is another example (Figure 10-20 page 269).
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Knowledge Enabling Objectives (Chapter 17 Reducing-Voltage Starting):
KEO 4.20.
DESCRIBE the role Reduced Voltage Starting and Silicon-Controlled Rectifiers
provide in motor controls.
Silicon-Controlled Rectifiers provide the ability for both AC and DC motors to start with
Reduced-Voltage. Full-Voltage is the least expensive and most efficient means of starting small
horsepower (HP) motors. Applications where large horsepower (HP) are started require
Reduced-Voltage Starting to reduce interference in the power system, the load, and the
electrical environment surrounding the motor. Silicon-Controlled Rectifiers provide this
ability.
Reduced-Voltage Starting reduces starting current as illustrated below:
Figure 17-1 page 486
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Reduced-Voltage Starting reduces the amount of motor torque produced on a load as illustrated
below:
Figure 17-2 page 486
Silicon-Controlled Rectifiers have the ability to rapidly switch heavy currents. As compared to a
diode, they have the ability of added control from the gate that is not possible with a diode. The
following figure depicts how solid state starters reduce inrush current, minimizes starting torque,
and smoothes acceleration of a motor:
Figure 17-19 page 502
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When the signal is applied to the gate of an SCR DC Voltage Control Circuit, the SCR is
triggered ON and the anode resistance decreases sharply, such that the resulting current flow
through the SCR is only limited by the resistance of the load as illustrated in the picture below:
Figure 17-21 page 503
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SCRs may be used alone in a circuit to provide one-way current control, or may be wired in
reverse-parallel circuits to control AC Line current in both directions as illustrated below:
Figure 17-22 page 503
An SCR circuit with Reverse-Parallel Wiring of SCRs provides maximum control of an AC
Load 3 Phase Motor as illustrated below:
Figure 17-23 page 503
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SUMMARY:








Silicon-Controlled Rectifiers provide the ability for both AC and DC motors to start
with Reduced-Voltage.
Full-Voltage is the least expensive and most efficient means of starting small horsepower
(HP) motors.
Applications where large horsepower (HP) are started require Reduced-Voltage Starting
to reduce interference in the power system, the load, and the electrical environment
surrounding the motor.
Reduced-Voltage Starting reduces starting current.
Reduced-Voltage Starting reduces the amount of motor torque produced on a load.
Silicon-Controlled Rectifiers have the ability to rapidly switch heavy currents and when
used in solid state starters, they not only reduce inrush current, they also minimizes
starting torque, and smoothes acceleration of a motor.
SCRs may be used alone in a circuit to provide one-way current control, or may be wired
in reverse-parallel circuits to control AC Line current in both directions
An SCR circuit with Reverse-Parallel Wiring of SCRs provides maximum control of
an AC Load 3 Phase Motor.
Knowledge Enabling Objectives (Chapter 18 Accelerating and Decelerating
Methods):
KEO 4.21.
EXPLAIN the concepts associated Braking and Plugging as they apply to motors.
Braking is used when it is necessary to stop a motor more quickly than coasting allows. Braking
is accomplished by different methods, each having advantages and disadvantages. The method
for Braking used depends on the application, available power, circuit requirements, cost, and
desired results. Examples of Braking include braking every time a motor is stopped, in an
emergency, or to slow the motor down.
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
Friction Brakes are the oldest motor stopping method and are similar to automobile
breaks. They are normally controlled by a solenoid device that activates the brake shoes
to stop a motor. An example of Friction Brake is shown in the below picture:
Figure 18-2 page 512
Advantages of Friction Brakes are they are low initial cost and easy to maintain. The
disadvantage is that they require more maintenance than other braking methods.
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
Plugging is a method of Braking in which the motor connections are reversed so that the
motor develops a counter-torque that acts as a braking force. The counter-torque is
accomplished by reversing the motor at full speed. This Plugging method allows for a
very rapid stopping via a Plugging Switch as illustrated below:
Figure 18-4 page 514
The use of the Plugging Switch allows the motor to stop and not run in the opposite
direction. A Continuous Plugging uses a Plugging Switch that allows the motor plug to a
stop each time the motor is stopped as illustrated in the below schematic:
Figure 18-5 page 514
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Limitations of Plugging are that it may not be applied to all motors and or applications.
Braking a motor to stop using plugging requires that the motor be a revisable motor and
that it can be reversed at full speed. Even though a motor can be reversed at full speed,
the damage that plugging may do may outweigh it advantages.
A Single Phase Shaded Pole motor cannot be reversed at full speed and cannot have
plugging used to stop it and Single Phase capacitor start motors cannot be plugged as
the centrifugal switch removes the start winding when it accelerates and cannot be
reversed without the start winding.
Heat from plugging can be created by high current to motors and for this reason, only
motors with a high service factor (SF) should be used in all cases except for emergency
situations. The SP needs to be at 1.35 or greater for plugging applications. The following
schematic illustrates how Plugging can be used in an emergency situation:
Figure 18-6 page 515 ]

Electric Braking is a method where DC voltage is applied to the stationary windings of a
motor after the AC voltage is removed. Electric Braking is also known as DC Injection
Braking. Electric Braking can be applied to bring a motor to an immediate stop if the
coasting time is unacceptable, particularly in an emergency situation.
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o An example of how Electric Braking can be provided to a Shaded-Pole, SplitPhase, and Three Phase motors is illustrated below by applying the DC voltage to
the stationary windings when AC has been removed:
Figure 18-9 page 517
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The following schematic illustrates how Electric Braking can be applied to a Three Phase
with AC removed so the motor can come to a complete stop quickly:
Figure 18-11 page 518
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
Dynamic Braking of DC Motors is a method in which a motor is reconnected to act as a
generator immediately after it is turned OFF. Connecting a DC motor in this way makes
the motor act as a loaded generator that develops a Retarding Torque, which rapidly
stops the motor. This example is illustrated in the below:
Figure 18-13 page 518
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SUMMARY:










Braking is used when it is necessary to stop a motor more quickly than coasting allows
and is used to slow a motor or to stop a motor in an emergency situation.
Friction Brakes are the oldest motor stopping method and are similar to automobile
breaks. They are normally controlled by a solenoid device that activates the brake shoes
to stop a motor.
Advantages of Friction Brakes are they are low initial cost and easy to maintain.
Disadvantage of Friction Brakes is that they require more maintenance than other
braking methods.
Plugging is a method of Braking in which the motor connections are reversed so that the
motor develops a counter-torque that acts as a braking force.
Limitations of Plugging are that it may not be applied to all motors and or applications.
o Braking a motor to stop using plugging requires that the motor be a revisable
motor and that it can be reversed at full speed.
o Even though a motor can be reversed at full speed, the damage that plugging may
do may outweigh it advantages.
A Single Phase Shaded Pole motor cannot be reversed at full speed and cannot have
plugging used to stop it and Single Phase capacitor start motors cannot be plugged as
the centrifugal switch removes the start winding when it accelerates and cannot be
reversed without the start winding.
Heat from plugging can be created by high current to motors and for this reason, only
motors with a high service factor (SF) should be used in all cases except for emergency
situations.
o The SP needs to be at 1.35 or greater for plugging applications.
Electric Braking is a method where DC voltage is applied to the stationary windings of a
motor after the AC voltage is removed.
o Electric Braking is also known as DC Injection Braking.
o Electric Braking can be applied to bring a motor to an immediate stop if the
coasting time is unacceptable, particularly in an emergency situation.
Dynamic Braking of DC Motors is a method in which a motor is reconnected to act as a
generator immediately after it is turned OFF.
o Connecting a DC motor in this way makes the motor act as a loaded generator
that develops a Retarding Torque, which rapidly stops the motor.
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KEO 4.22.
EXPLAIN the concepts associated Motor Drive Stopping to include: Stopping
without Applied Force from the drive, and Stopping using Reduced Voltage as
the motor Decelerates.
Motor Drive Stopping can be accomplished with a Motor Drive. The stopping time is
programmed by setting the deceleration parameter for 1 second or less to several minutes. For
fast stops, (especially with high inertia loads), a braking resistor can be added. The below picture
illustrates how this can be configured:
Figure 18-14 page 519
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Motor Drive Stopping can also be accomplished by reducing the voltage to slow down or stop
the motor.
KEO 4.23.
EXPLAIN the concepts of multiple poles in the acceleration, deceleration control
of AC Motors.
AC Motors are considered constant speed motors. This is because of the synchronous speed of
an induction motor is based on the power supply frequency and the number of poles in the
motor winding. Motors designed for 60 Hz use have synchronous speeds of 3600, 1200, 900,
514, and 450 rpm. Following formula is used for calculating the Synchronous Speed of an
Induction Motor:
AC Induction Motor Speed Calculation Formula
RPM syn
RPM syn
f
NP
=
=
=
=
120 x
NP
f
Where
Synchronous Speed (in rpm)
Supply Frequency (in cycles/sec)
Number of Poles (in motor winding)
Example: What is the synchronous speed of a fourpole motor operating at 50 Hz?
RPM syn
=
RPM syn
=
RPM syn
=
RPM syn
=
120 x f
NP
120 x 50
4
6000_
4
1500 rpm
Pages 535-536
NOTE: Supply frequency and number of poles are the only variables that determine the
speed of an AC Motor. Unlike the speed of a DC Motor, the speed of an AC Motor should
not be changed by varying the applied voltage as damage may occur to an AC Motor if the
supply voltage is varied more than 10% above or below the rated nameplate voltage. This
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is because in an Induction Motor, the starting torque and breakdown torque vary as the
square of the applied voltage. Example: With 90% of rated voltage, the torque is 81% (.92
= .81 or 81% of its rated torque).
SUMMARY:

Motor Drive Stopping can be accomplished with a Motor Drive. The stopping time is
programmed by setting the deceleration parameter for 1 second or less to several minutes.
For fast stops, (especially with high inertia loads), a braking resistor can be added.

Motor Drive Stopping can also be accomplished by reducing the voltage to slow down or
stop the motor.

AC Motors are considered constant speed motors.
o This is because of the synchronous speed of an induction motor is based on the
power supply frequency and the number of poles in the motor winding.

The formula used for calculating the Synchronous Speed of an Induction Motor:
RPM syn = 120 x
NP
f
NOTE: Supply frequency and number of poles are the only variables that determine
the speed of an AC Motor. Unlike the speed of a DC Motor, the speed of an AC
Motor should not be changed by varying the applied voltage as damage may occur
to an AC Motor if the supply voltage is varied more than 10% above or below the
rated nameplate voltage.
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STEP TWO
Motor Controls
Skill/Performance Objectives
Skill Knowledge Introduction:
Below are the skill knowledge objectives. How these objectives are performed depend on
equipment and laboratory resources available. With each skill objective it is assumed that a set
of standard test equipment and tools be provided and safety procedures be implemented during
each tasked being performed.

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
Design, install and test a standard three wire motor control system
Demonstrate the following means of Motor Starting
Across the Line Starting
Reduced Voltage Starting
Demonstrate the installation and testing of the following protections
Overcurrent Protection
Overload Protection
Over voltage
Under current
Phase differential
Demonstrate the following means of stopping a Motor
Coasting
Electrical Braking
Mechanical Braking
Demonstrate the ability to design, install and test these motor controls
Speed Control
Reversing
Jogging
Using a PLC or relays design and functionally test motor sequence control
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