Quantcast Chapter 5 Motor Controls

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
5-1. Functions of motor controls.
ceptions are noted in NEC 430. Starters without
The terms, controls, controllers, and starters are
overloads are called contractors. The holding coil of a
magnetic starter (or contactor) is designed to drop
used interchangeably. The most common name for
out whenever line voltage drops below about 60
the device that controls the operation of the motor is
percent of its normal value, thereby providing
starter. This name is not the best description of the
undervoltage protection to the motor or load.
device as the starter does much more than start the
c. Combination starters. All motors, motor cir-
motor. It also stops the motor, it provides overload
cuits and controllers require short-circuit and
and short circuit protection, and it disconnects the
ground-fault protection. This may be located with
motor from the line after a period of overcurrent. It
the starter as in a combination starter or may be
may also contain auxiliary devices that limit the
the branch-circuit short-circuit and ground-fault
motor inrush current, torque, and/or speed. Addi-
protective device as in a manual motor starter.
tional protection features may include undervolt-
(NEC 430, part D). Starters connected to a power
age, phase reversal, and/or field loss.
distribution system with an available fault current
in excess of the starter short circuit interrupting
5-2. Types of motor controls.
capacity must be protected from that fault current.
Some of the more common motor starters are de-
Combining a contactor with a thermal overload re-
scribed in this chapter beginning with the elemen-
lay is called a magnetic motor starter and combin-
ing a magnetic motor starter with a circuit breaker
tary document starter and ending with the more
or fuses in a common enclosure is called a combina-
complex adjustable speed frequency starter.
tion starter. These starters carry an interrupting
a. Document across-the-line starters. Document
rating that indicates the ability of all components in
starters are most often used on small single phase
the integrated combination starter to withstand mo-
fractional horsepower motors. They usually consist
mentary overcurrent and thermal effects. Depend-
of a push button-type or a toggle-type mechanism
ing upon the type of short-circuit protective device
(fig 5-1) that actuates a set of quick-make/quick-
employed, combination starters (fig 5-3) may be
break contacts that connect the motor directly to
classified as breaker-protected starters, fuse-
the line. Document starters have provisions for
protected starters or fused breaker-protected start-
overload protection and their low cost provides eco-
nomical starter selection for applications where no
(1) Breaker-protected starters. B r e a k e r -
undervoltage protection is required.
protected starters use almost exclusively molded-
b. Magnetic across-the-line starters. Magnetic
case breakers. Low voltage power circuit breakers
starters are suitable for application over a wide
have sometimes been applied, especially for use on
range of horsepower and voltage for both single and
larger motors- Breakers, as compared to fuses, are
three phase motors. Magnetic starters are full volt-
slower in fault clearing for higher magnitudes of
age starters designed to provide thermal overload
and undervoltage protection for squirrel cage mo-
short circuit currents. Consequently, three pole
breakers afford the least protection against thermal
tors and can be operated remotely from push button
overload relay and contactor damage. However,
stations or automatically, for example, through a
they offer positive protection against single phas-
float switch. They differ from document starters in
ing. Breakers are usually designed for both thermal
that they contain a contactor which, when its elec-
and magnetic protection even though the overloads
tromagnetic coil is energized, closes its line contacts
are the best thermal protection because overload
to connect the motor directly to the line (fig 5-2).
relay heaters can be very closely selected to cause
The primary purpose of a motor starter is to provide
tripping at precise values of current flow. Motor
thermal overload protection, it is not designed to
circuit protectors (MCPs) used in combination start-
interrupt fault current. A short circuit study must
ers are magnetic trip only and have no thermal trip
always be performed to determine if protection is
necessary from fault currents and, if so, short cir-
(2) Fuse-protected starters. Fuse-protected
cuit protection must be provided. A circuit breaker,
starters provide the best degree of starter and ther-
or fuses, upline of the contactor gives fault current
mal overload relay protection particularly for severe
protection to the starter and the motor. Starters
short circuits (fig 5-4). The disadvantages of fused-
must always include thermal overload relays. Ex-


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