416 Electrical Power Systems Technology
an excessive amount of current.
Another type of motor starter is the magnetic starter, which relies
upon an electromagnetic effect to open or close the power-source circuit
of the motor. Often, motor starters are grouped together for the control of
adjacent equipment in an industrial plant. Such groupings of motor start-
ers and associated control equipment are called control centers. Control
centers provide a relatively easy access to the power distribution system,
since they are relatively compact, and the control equipment is not scat-
tered throughout a large area. Motor-starting systems will be discussed in
greater detail in Chapter 16.
Function of Motor Starters
Various types of motor starters are used for control of motor power.
The functions of a starter vary in complexity; however, motor starters usu-
ally perform one or more of the following functions:
- On and off control.
- Acceleration.
- Overload protection.
- Reversing the direction of rotation.
Some starters control a motor by being connected directly across the
power input lines. Other starters reduce the level of input voltage that is
applied to the motor when it is started, so as to reduce the value of the
starting current. Ordinarily, motor overload protection is contained in the
same enclosure as the magnetic contactor.
Sizes of Motor Contactors
The contactors used with motor starters are rated according to
their current capacity. The National Electrical Manufacturers Association
(NEMA) has developed standard sizes for magnetic contactors according
to their current capacity. Table 15-1 lists the NEMA standard sizes for mag-
netic contactors. By looking at this chart, you can see that a NEMA size 1
contactor has a 30-ampere current capacity if it is open (not mounted in
a metal enclosure), and a 27-ampere capacity if it is enclosed. The corre-
sponding maximum horsepower ratings of loads for each of the NEMA
contactor sizes are also shown in Table 15-1.
Manual Starters
Some motors use manual starters to control their operation. This type