Induction motors are common forms of asynchronous motors. They are called induction motors because currents flowing in their rotors are induced by alternating currents.

There are two principal types of induction motor—single-phase and polyphase. Polyphase induction motors are classed as squirrel-cage or wound-rotor.

Polyphase induction motors have wound stators and either squirrel-cage or wound rotors. They operate in accordance with the same physical principles, and their stators are made the same way, but rotor construction differs. Polyphase motors are the simplest and most robust electric motors now being built.

They act like AC transformers but have stationary primary windings and rotating secondary windings. The stator primary windings are connected to power sources, and their short-circuited rotor windings produce mechanical torque in response to the induced secondary current.

This motor torque is produced by the interaction of the secondary rotor currents with the electromagnetic flux or field that exists in the air gap between the stator and rotor.

Single-phase motors are classified into three general classes: (1) commutator motors, (2) induction motors, and (3) synchronous motors. Most single-phase AC motors are fractional-horsepower motors. Induction motors require additional components and special methods for starting, so they are further classified according to starting method: (1) split-phase, (2) repulsion-start, and (3) shaded-pole.

Synchronous motors are constant-speed motors that operate in absolute synchronism with AC line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles, and is always a multiple of the line frequency. Synchronous motor sizes range from subfractional self-excited units for driving clocks to large integral-horsepower DC-excited units for industrial applications.

Integral-horsepower synchronous motors are generally more efficient than induction motors with the same horsepower and speed ratings. The normal value of field current in a synchronous motor provides a unity power factor and minimum stator current.

For a given load, varying the field excitation can alter the power factor from a low lagging to a low leading condition, providing a convenient means for correcting power factor. The two major types of synchronous motors are nonexcited and DC-excited.

Nonexcited motors, made in reluctance and hysteresis designs, have self-starting circuits and do not require external excitation. DC-excited motors, generally made in integral-horsepower sizes, require DC supplied either through slip rings from a separate source or a DC generator connected directly to their shafts.

A synchronous motor rotor is first brought up to synchronous speed by one of the two methods. The excited rotor poles are then attracted by the rotating stator’s magnetic field, and the rotor continues to turn at synchronous speed.

In this way, the rotor is locked into step magnetically with the rotating magnetic field. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles, and is always a multiple of line frequency.

If for some reason the rotor is forced out of step with the rotating stator magnetic field, the attraction is lost, no torque is developed, and the motor will stop. Thus a synchronous motor develops torque only when it is running at synchronous speed.

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