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.
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:
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
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.