Introduction to

AC Motors

ChE 462

AC Motors

• Synchronous motor – constant speed independent of load; compressors

• Induction motor is a common form of asynchronous motor and is basically an ac transformer with a rotating secondary. Rotational speed a function of load

Induction motor

• Induction motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator).

• The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.

An Industrial Induction Motor

Allen-Bradley (Rockwell)

Internals of an Induction Motor

3 Phase Stator Connection of an Induction Motor

http://www.sea.siemens.com/step/templates/lesson.mason?motors:2:6:6#

Induction motor

• Magnetic field rotates is the synchronous speed of the motor and is determined by the number of poles in the

stator and the frequency of the power supply:

p

fns

120

ns = synchronous speed f = frequencyp = the number of poles

s

as

n

nns

)(100

s = slipna = actual

speed

Induction motorSynchronous speed is the absolute upper limit of motor speed. If the rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero.

When running, the rotor always rotates slower than the magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed:

Therefore, the full load speed is reduced from the synchronous speed.

Example: 5 HP motor TEFC (totally enclosed fan cooled) 460 volt 3 phase motor: Synchronous RPM Full Load RPM Slip in Percent 3600 3500 2.78 1800 1760 2.22 1200 1175 2.08 900 869 3.44 Induction motors for agitators or positive displacement pumps frequently will use gear reducers to reduce the shaft speed. Most industrial motors in the processing industries are induction motors. Some are multi speed, for such applications as cooling tower fans, or dual service pumps.

Induction Motor Performance Curves