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Cutaway View of Alternating Current Induction Motor |
AC induction motors are commonly used in industrial applications. The following motor discussion will center around three-phase, 460 VAC, asynchronous, induction motors. An asynchronous motor is a type of motor where the speed of the rotor is other than the speed of the rotating magnetic field. This type of motor is illustrated. Electromagnetic stator windings are mounted in a housing.
Power connections, attached to the stator windings, are brought out to be attached to a three-phase power supply. On three-phase, dual-voltage motors nine leads are supplied for power connections. Three power connection leads are shown in the following illustration for simplicity. A rotor is mounted on a shaft and supported by bearings. On self-cooled motors, like the one shown, a fan is mounted on the shaft to force cooling air over the motor.
The nameplate of a motor provides important information
necessary when applying a motor to an AC drive. The following
drawing illustrates the nameplate of a sample 25 horsepower
AC motor.
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Nameplate of an AC Induction Motor Photo: Siemens |
Motor Connections
This motor can be used on 230 VAC or 460 VAC systems. A wiring diagram indicates the proper connection for the input power leads. The low voltage connection is intended for use on 230 VAC with a maximum full load current of 56.8 Amps. The high voltage connection is intended for use on 460 VAC with a maximum full load current of 28.4 Amps.
Motor Speed
Base speed is the nameplate speed, given in RPM, where
the motor develops rated horsepower at rated voltage and
frequency. It is an indication of how fast the output shaft will
turn the connected equipment when fully loaded and proper
voltage is applied at 60 hertz. The base speed of this motor is
1750 RPM at 60 Hz. If the connected equipment is operating at
less than full load, the output speed will be slightly greater than the base speed.
Service Factor
A motor designed to operate at its nameplate horsepower
rating has a service factor of 1.0. Some applications may require
a motor to exceed the rated horsepower. In these cases a
motor with a service factor of 1.15 can be specified. The service
factor is a multiplier that may be applied to the rated power. A
1.15 service factor motor can be operated 15% higher than the
motor’s nameplate horsepower. Motors with a service factor of
1.15 are recommended for use with AC drives. It is important
to note, however, that even though a motor has a service factor
of 1.15 the values for current and horsepower at the 1.0 service
factor are used to program a variable speed drive.
Insulation Class
The National Electrical Manufacturers Association (NEMA)
has established insulation classes to meet motor temperature
requirements found in different operating environments. The
four insulation classes are A, B, F, and H. Class F is commonly
used. Class A is seldom used. Before a motor is started, its
windings are at the temperature of the surrounding air. This is
known as ambient temperature. NEMA has standardized on an
ambient temperature of 40° C, or 104° F for all motor classes. The temperature rises in the motor as soon as it is started. The
combination of ambient temperature and allowed temperature
rise equals the maximum winding temperature in a motor. A
motor with Class F insulation, for example, has a maximum
temperature rise of 105° C. The maximum winding temperature
is 145° C (40° ambient plus 105° rise). A margin is allowed for a
point at the center of the motor’s windings where the temperature
is higher. This is referred to as the motor’s hot spot.
NEMA Design
The National Electrical Manufacturers Association (NEMA) has
established standards for motor construction and performance.
The nameplate on page 20 is for a motor designed to NEMA B
specifications. NEMA B motors are commonly used with AC
drives. Any NEMA design (A, B, C, or D) AC motor will work
perfectly well with a properly sized variable speed drive.
Efficiency
AC motor efficiency is expressed as a percentage. It is an
indication of how much input electrical energy is converted to
output mechanical energy. The nominal efficiency of this motor
is 93.0%.
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