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There
are literally thousands of different motors available from
many different manufacturers. Making the best choice can be
confusing. Not only must a user make efficiency choices but
he or she must be sure that the motor being selected is appropriate
to the application. The National Electrical Manufacturers
Association (NEMA) has developed standards for the design, operating speed, and enclosure type of three-phase "squirrel cage" induction
motors. This standardization allows for ready interchangeability
of motors from different manufacturers in common applications.
Design
NEMA
standard designs for polyphase induction motors fall into
four categories: A, B, C, and D.
- Design
B motors are the dominant type of motor, commonly referred
to as general purpose motors, and are used in most applications,
including fans and pumps.
- Design
A motors are similar to Design B but have slightly greater
torque.
- Design
C motor are intended for applications that require a high
starting torque, such as conveyors or compressors.
- Design
D motors have both high torque and high slip, allowing them
to handle shock-loads seen in some manufacturing operations
such as punch presses.
Operating
Speed
Motors are available
at rated synchronous speeds that are determined by their number
of poles (2 poles = 3,600 revolutions per minute [rpm], 4 poles
= 1,800, 6 poles = 1,200, 8 poles = 900, etc.). Actual speed
will be lower than the synchronous speed because of slip. The
actual operating speed will decrease with increasing load, and
the rate of this change and the full-load speed can vary significantly
among different motors. In general, the cost of the motor increases
as the speed decreases due to smaller market demand and increased
complexity required in the design. It is common to use a 2,
4, or 6 pole motor for most applications and, where necessary,
reduce the speed of the equipment using either belts or gears.
Enclosure
Type
NEMA
also defines 20 types of motor enclosures, which fall into
two broad categories: open and totally enclosed.
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Open motors have ventilation holes allowing for air-cooling
of the windings. The most common design are open drip-proof
(ODP) in which ventilation openings are positioned to
keep particles and water from falling in from overhead.
Most motors found in commercial buildings are ODP motors.
Splash-proof motors add protection from material coming
from underneath, while guarded motors use screens or baffles
to protect the motor from particle entry.
- Totally
enclosed motors are designed to prevent free exchange
of air between the inside and the outside of the motor.
The most common design is the totally enclosed fan-cooled
(TEFC) motor in which a fan on the opposite end of the
motor from the load draws air over the case to provide cooling.
Explosion-proof motors are designed to prevent the ignition
of external gas or vapor by motor sparks and heat, and to
withstand an inadvertent internal explosion of gas or vapor.
Motors
are also available in different temperature ratings, which
are identified by different insulation classes. The most common
insulation is Class B, which is used for general purpose applications.
Class F and H are used in motors intended for high ambient-temperature
applications, or where high operating temperatures are anticipated,
as may occur from frequent overloading of the motor or the
use of variable frequency drives.
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