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This chapter introduced you to the basic principles concerning ac motors.

While many variations of types exist, the three types presented provide you with background for further study if you require more extensive knowledge of the subject. The following information provides a summary of the major subjects of this chapter for your review.

The three AC MOTOR TYPES presented are the series, synchronous, and induction ac motors.

AC SERIES MOTORS are nearly identical to the dc series motors. Special construction techniques allow ac series motors to be used as UNIVERSAL MOTORS, operating on either ac or dc power.

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ROTATING FIELDS are developed by applying multiphase voltages to stator windings, which consist of multiple field coils.

This rotating magnetic field causes the rotor to be pushed and pulled because of interaction between it and the rotor's own field.

TWO-PHASE ROTATING FIELDS require two pairs of field coils displaced by 90. They must be energized by voltages that also have a phase displacement of 90.

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THREE-PHASE ROTATING FIELDS require three pairs of windings 120 apart, energized by voltages that also have a 120-degree phase displacement.

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SYNCHRONOUS MOTORS are specifically designed to maintain constant speed, with the rotor synchronous to the rotating field. Synchronous motors require modification (such as squirrel-cage windings) to be self-starting.

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INDUCTION MOTORS are the most commonly used of all electric motors due to their simplicity and low cost. Induction motors may be single-phase or multiphase. They do not require electrical rotor connection. Split-phase motors with special starting windings, and shaded-pole motors, are types of single-phase induction motors.

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SYNCHRONOUS SPEED is the speed of stator field rotation. It is determined by the number of poles and the frequency of the input voltage. Thus, for a given motor, synchronous speed is constant.

SLIP is the difference between actual rotor speed and the synchronous speed in induction motors. Slip must exist for there to be torque at the rotor shaft.

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