Which of the following is an unexcited single-phase synchronous motor?
Right Answer is:
Single-Phase Synchronous Motors
Very small single-phase motors have been developed which run at true synchronous speed. They do not require d.c. excitation for the rotor. Because of these characteristics, they are called unexcited single-phase synchronous motors.
The most commonly used types are :
(i) Reluctance motors
(ii) Hysteresis motors
The efficiency and torque-developing ability of these motors are low. The output of most of the commercial motors is only a few watts.
It is a single-phase synchronous motor that does not require d.c. excitation to the rotor. Its operation is based upon the following principle:
Whenever a piece of ferromagnetic material is located in a magnetic field, a force is exerted on the material, tending to align the material so that reluctance of the magnetic path that passes through the material is minimized.
The stator consists of a single winding called the main winding. But single winding cannot produce a rotating magnetic field. So for the production of a rotating magnetic field, the must be at least two windings separated by a certain phase angle. Hence staler consists of an additional winding called auxiliary winding which consists of the capacitor in series with it. Thus there exists a phase difference between the currents carried by the two winding and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field. The technique is called the split phase technique of the production of a rotating magnetic field. The speed of this field is the synchronous speed which is decided by the number of poles for which stator winding is wound.
The rotor carries the short-circuited copper or aluminum bars and acts as the squirrel cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically. Similarly, in the reluctance motor, the rotor tries to align itself with the axis of the rotating magnetic field in a minimum reluctance position. But due to rotor inertia, it is not possible when the rotor is at standstill. So the rotor starts rotating near synchronous speed as a squirrel cage induction motor. When the rotor speed is about synchronous, the stator magnetic field pulls the rotor into synchronism i.e. minimum reluctance position, and keeps it magnetically locked. Then the rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very small and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor.
The reluctance motor has the following advantages
- No d.c. supply is necessary for the rotor
- Constant speed characteristics
- Robust construction
- Less maintenance
The reluctance motor has the following limitation
- Less efficiency.
- Poor power factor.
- The Need for a Very low inertia rotor.
- Less capacity to drive the loads.