1. State the purpose and general principles of motor control.
Discuss the differences between manual and automatic motor control.
Discuss considerations when installing motors or control equipment.
Discuss the basic functions of a control system.
Discuss surge protection for control systems.
The term "motor control" can have very broad meanings.
It can mean anything from a simple toggle switch in tended to turn a
motor on or off to an extremely complex system intended to control
several motors, with literally hundreds of sensing devices that govern
the operation of the circuit. The electrician working in industry should be
able to install different types of motors and the controls necessary to
control and protect them and also to troubleshoot systems when they
2. Motor control was a simple problem when motors were
used to drive a common line shaft to which several
machines were connected. It was simply necessary to
start and stop the motor a few times a day. However,
with individual drive, the motor is now almost an
integral part of the machine and it's necessary to de
sign the motor controller to fit the needs of the machine
to which it's connected.
Motor control is a broad term that means any thing from
a simple toggle switch to a complex system with
components such as relays, timers, and switches. The
common function of all controls, however, is to control
the operation of an electric motor. As a result, when
motor control equipment is selected and installed, many
factors must be considered to ensure that the control
will function properly for the motor and the machine for
which it's selected.
12. When installing electric motors and equipment, several factors
should be considered. When a machine is in stalled, the motor,
machine, and controls are all inter related and must be considered
as a unit. Some machines will have the motor or motors and control
equipment mounted on the machine itself when it's delivered from
the manufacturer, and the electrician's job in this case is generally to
make a simple power connection to the machine. A machine of this
type is shown in ill. 2.
Other types of machines require separately mounted motors that are
connected by belts, gears, or chains. Some machines also require
the connection of pilot sensing devices such as photo switches, limit
switches, pressure switches, and so on. Regardless of how easy or
complex the connection is, several factors must be considered.
ill. 1 Motor controlled by a simple toggle switch.
ill. 2 This machine was delivered with self-contained motors and
13. One of the main considerations when installing a machine is the power
source. Does the machine re quire single-phase or three-phase power
What is the horsepower of the motor or motors to be connected? What
is the amount of in-rush current that can be expected when the motor
starts? Will the motor require some type of reduced voltage starter to
limit in-rush current? Is the existing power supply capable of handling
the power requirement of the ma chine or will it be necessary to install a
new power system?
The availability of power can vary greatly from one area of the country
to another. Power companies that supply power to heavily industrialized
areas can generally permit larger motors to be started across-the line
than companies that supply power to areas that have light industrial
needs. In some areas, the power company may permit a motor of
several thousand horse power to be started across-the-line, but in other
areas the power company may require a reduced voltage starter for
motors rated no more than one hundred horsepower.
14. When connecting motors, several factors should be considered,
such as: horsepower, service factor (SF), marked temperature rise,
voltage, full load current rating, and National Electrical
Manufacturers Association (NEMA) Code letter. This information is
found on the motor nameplate. The conductor size, fuse or circuit
breaker size, and overload size are generally determined using the
National Electrical Code (NEC) and /or local codes. It should be
noted that local codes generally supersede the National Electrical
Code and should be followed when they apply. Motor installation
based on the NEC® will be covered in this text.
15. The type of motor best suited to operate a particular piece of equipment
can be different for different types of machines. Machines that employ
gears generally require a motor that can start at reduced speed and
increase speed gradually. Wound rotor induction motors or squirrel cage
motors controlled by variable frequency drives are generally excellent
choices for this requirement. Machines that require a long starting
period, such as machines that operate large inertia loads such as
flywheels or centrifuges, require a motor with high starting torque and
relatively low starting current.
Squirrel cage motors with a type ‘A’ rotor or synchronous motors are a
good choice for these types of loads.
Synchronous motors have an advantage in that they can provide power
factor correction for themselves or other inductive loads connected to
the same power line.
Squirrel cage motors controlled by variable frequency drives or direct
current motors can be employed to power machines that require
variable speed. Squirrel cage induction motors are used to power most
of the machines throughout industry. These motors are rugged and
have a proven record of service unsurpassed by any other type of
16. The type of controller can vary depending on the requirements of the
motor. Motor starters can be divided into two major classifications:
NEMA (National Electrical Manufacturers Association) and IEC
(International Electro technical Commission).
NEMA is an American organization that rates electrical components.
NEMA starter sizes range from 00 through 8. A NEMA size 00 starter is
rated to control a 2 horsepower motor connected to a 460 volt three
phase power supply. A size 8 starter will control a 900 horsepower
motor connected to a 460 volt three phase power source. IEC starter
sizes range from size A through size Z. Size A starters are rated to
control a 3 horsepower motor connected to a 460 volt three phase
source. Size Z starters are rated to control a 900 horsepower motor
connected to a 460 volt source.
It should be noted that the contact size for an IEC starter is smaller than
for a NEMA starter of the same rating. It is common practice when using
IEC starters to increase the listed size by one or two sizes to
compensate for the difference in contact size.
17. Another consideration is the type of environment in which the motor and
control system operates. Can the controls be housed in a general
purpose enclosure similar to the one shown in ill. 3, or is the system
subject to moisture or dust? Are the motor and controls to be operated
in a hazardous area that requires explosion proof enclosures similar to
that shown in ill. 4? Some locations may contain corrosive vapor or
liquid, or extremes of temperature. All of these conditions should be
considered when selecting motors and control components. Another
type of starter commonly found in industry is the combination starter (ill.
5). The combination starter contains the disconnecting means, fuses or
circuit breaker, starter, and control transformer. They may also have a
set of push buttons or switches mounted on the front panel to control
ill. 4 Explosion proof enclosure (NEMA 7).
ill. 5 Combination motor starter with circuit breaker, disconnect switch,
starter, and control transformer.
18. Another important consideration is the safety of the operator or persons that
work around the machine.
In 1970, the Occupational Safety and Health Act (OSHA) was established. In
general, OSHA requires employers to provide an environment free of recognized
hazards that are likely to cause serious injury.
Another organization that exhibits much influence on the electrical field is
Underwriters Laboratories (UL). Underwriters Laboratories was established by
insurance companies in an effort to reduce the number of fires caused by
electrical equipment. They test equipment to determine if it's safe under different
Approved equipment is listed in an annual publication that's kept current with
Another previously mentioned organization is the National Electrical Code. The
NEC is actually part of the National Fire Protection Association. They establish
rules and specifications for the installation of electrical equipment. The National
Electrical Code isn't a law unless it's made law by a local authority.
Two other organizations that have great influence on control equipment are
NEMA and IEC. Both of these organizations will be discussed later in the text.
19. When choosing a specific device for a particular application, it's
important to remember that the motor, machine, and motor controller
are interrelated and need to be considered as a package. In general,
five basic factors influence the selection and installation of a
20. Establish whether the service is direct (DC) or
alternating current (AC). If AC, deter mine the frequency
(hertz) and number of phases in addition to the voltage.
21. The motor should be matched to the electrical service and correctly
sized for the ma chine load in horsepower rating (hp). Other
considerations include motor speed and torque. To select proper
protection for the motor, its full-load current rating (FLC), service
factor (SF), time rating (duty), and other pertinent data-as shown on
the motor nameplate-must be used.
ill. 1 Five 2000 hp, 1800 rpm induction motors driving water pumps
for a Texas oil/water operation. Pumps are used to force water into
the ground and "float" oil upward. ill. 2 Horizontal 4000 hp
synchronous motor driving a large centrifugal air compressor.
22. The fundamental tasks of a motor controller are to start and stop the
motor and to protect the motor, machine, product, and operator. The
controller may also be called upon to provide supplementary functions
such as reversing, jogging or inching, plugging, or operating at several
speeds or at reduced levels of current and motor torque (see Glossary).
Section 430 of the National Electrical Code (NEC) provides
requirements concerning the installation of motor circuits. This section
is employed to deter mine the proper conductor size, overload size, and
short circuit protection rating for motor installations. In some industries
electrical engineers are responsible for determining the requirements
for installing a motor or motors. In other industries the electrician is
expected to perform this task.
23. Controller enclosures serve to provide safety protection for operating
personnel by preventing accidental contact with live parts. In certain
applications, the controller itself must be protected from a variety of
environmental conditions, which might include:
++ Water, rain, snow, or sleet
++ Dirt or noncombustible dust
++ Cutting oils, coolants, or lubricants
Both personnel and property require protection in environments
made hazardous by the presence of explosive gases or combustible
24. Motor control equipment is designed to meet the provisions of the National
Electrical Code (NEC). Also, local code requirements must be considered and met
when installing motors and control de vices. Presently, code sections applying to
motors, motor circuits, and controllers and industrial control devices are found in
Article 430 on motors and motor controllers, Article 440 on air-conditioning and
refrigeration equipment, and Article 500 on hazardous locations of the NEC.
The 1970 Occupational Safety and Health Act (OSHA), as amended, requires that
each employer furnish employment in an environment free from recognized
hazards likely to cause serious harm.
Standards established by the National Electrical Manufacturers Association (NEMA)
assist users in the proper selection of control equipment. NEMA standards provide
practical information concerning the construction, testing, performance, and
manufacture of motor control devices such as starters, relays, and contactors. One
of the organizations that actually tests for conformity to national codes and
standards is Underwriters' Laboratories (UL). Equipment that's tested and approved
by UL is listed in an annual publication, which is kept current by means of bimonthly
supplements to reflect the latest additions and deletions. A UL listing does not mean
that a product is approved by the NEC. It must be acceptable to the local authority
25. Some of the complicated and precise automatic
applications of electrical control are illustrated in ill. 3 and
ills 1-7A and B.
Factors to be considered when selecting and in stalling
motor control components for use with particular machines
or systems are described in the following paragraphs.
26. The motor may be started by connecting it directly across the source
of voltage. Slow and gradual starting may be required, not only to
protect the machine, but also to ensure that the line current inrush
on starting isn't too great for the power company's system. Some
driven machines may be damaged if they are started with a sudden
turning effort. The frequency of starting a motor is another factor
affecting the controller. A combination fused disconnect switch and
motor starter is shown in ill. 4.
27. Most controllers allow motors to coast to a standstill. Some impose
braking action when the machine must stop quickly. Quick stopping
is a vital function of the controller for emergency stops. Controllers
assist the stopping action by retarding centrifugal motion of
machines and lowering operations of crane hoists.
28. Controllers are required to change the direction of
rotation of machines automatically or at the command of
an operator at a control station. The reversing action of
a controller is a continual process in many industrial
29. The maintaining of desired operational speeds and characteristics is
a prime purpose and function of controllers. They protect motors,
operators, machines, and materials while running.
There are many different types of safety circuits and devices to
protect people, equipment, and industrial production and processes
against possible injury that may occur while the machines are
30. Some controllers can maintain very precise speeds for industrial
processes. Other controllers can change the speeds of motors either
in steps or gradually through a continuous range of speeds.
31. Many mechanical safeguards have been re placed or
aided by electrical means of protection.
Electrical control pilot devices in controllers pro vide a
direct means of protecting machine operators from
32. Part of the operation of an automatic machine is to protect
the machine itself and the manufactured or processed
materials it handles. E.g., a certain machine control
function may be the prevention of conveyor pileups. A
machine control can reverse, stop, slow, or do whatever is
necessary to protect the machine or processed materials.
33. Requirements Once properly installed and adjusted, motor starters
will provide reliable operation of starting time, voltages, current, and
torques for the benefit of the driven machine and the power system.
The NEC, supplemented by local codes, governs the selection of the
proper sizes of conductors, starting fuses, circuit breakers, and
disconnect switches for specific system requirements.