Basic of all industrially used AC motors in one place.

Basic principle of Synchronous AC Motor

The synchronous motor is similar to the induction motor with one important difference: The rotor in the synchronous motor rotates at exactly the speed of the rotating field—there is no slip. Put another way, the speed of the synchronous motor is always an exact multiple of the line frequency.

This feature is extremely desirable in industrial applications, for example, when several motors along a conveyer belt must all be going exactly the same speed. Although many synchronous motors are large, the concept is also used extensively in small clock or timing motors where an exact relationship must exist between frequency and speed.

The rotor in an induction motor receives its power through induction, which requires a difference (slip) between the speed of the rotor and the rotating field. To make a synchronous motor work, the power to form a magnetic field in the rotor must come from another source. Traditionally, this is done by supplying DC power into the rotor via slip rings and brushes.

Basic principle of AC Servo Motor:

AC servomotor torque–speed curve. / Diagram of an AC servomotor.
AC servomotor torque–speed curve. / Diagram of an AC servomotor.

A special case of the two-phase motor is the AC servomotor. This is a high-slip, high torque motor, designed specifically for control systems, and it has a relatively linear torque-speed curve. The maximum torque occurs when the speed is zero. When the motor is running, the speed is inversely proportional to the load torque; put another way: the lighter the load, the faster the motor runs. This is very similar to the way a DC motor behaves.
The two windings are called the main winding and the control winding. The main winding is connected to an AC source, usually 120 Vac. The control winding is driven by an electronic circuit. Electronic circuit has two functions- (1) Causes the phase to be either leading or lagging the main winding (thereby controlling the motor direction) and (2) sets the magnitude of the control-winding voltage, which determines the speed.

Typically, the maximum control winding voltage is about 35 Vac. If the control winding has 0 V, the motor will coast to a stop, even though the main winding is still connected to the line voltage. This is different from a normal induction motor that will continue to run on a single phase.

Split-Phase Control Motors

A split-phase motor (two-phase) driven with single-phase AC.
A split-phase motor (two-phase) driven with  single-phase AC.

The split-phase control motor is technically a two-phase motor because it has two sets of windings . These motors have application in control systems. The operating parameters that make them desirable are (1) they are self-starting and (2) they can easily be controlled to turn in either direction.

Typically, they are small (less than 1 hp) and are used to move something back and forth, such as opening and closing a valve or raising and lowering a garage door. The problem is that two-phase AC is not available directly from the power company; it must be created, usually from single-phase AC.
The required two-phase power is created from single-phase AC by placing a capacitor in series with one of the windings. The capacitor causes the current in winding 2 to lead the line current in winding 1 by almost 90° (it doesn’t have to be exact). To change the direction of rotation, the capacitor must be able to switch so that it is in series with the other winding. Notice that the switch is in the down position, which causes the line voltage to be applied directly to winding 1, while winding 2 is fed through the capacitor. In Figure 9.23(b), the switch is in the up position, which allows winding 2 to get the line voltage, while winding 1 is fed through the capacitor. This causes the motor to rotate in the opposite direction.

Controlling the direction of a splitphase control motor (two-phase motor).
Controlling the direction of a split phase control motor (two-phase motor).

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