Electrical Machines - Reluctance Motor

There are very few single-phase motors which run at true synchronous speed and they do not require any external DC excitation for the rotor. Therefore, these motors are also called as unexcited single-phase synchronous motors.

The efficiency and the torque developing capability of the single-phase synchronous motors is low and the output power of the commercial single-phase synchronous motors is only a few Watts.

The reluctance motor is a 1-phase synchronous motor which does not require DC excitation to the rotor.

The operation of the reluctance motor is based on the following principle −

"When a piece of ferromagnetic material is located in a magnetic field, a force is acted upon the material that tending to align the material to the magnetic field so that the reluctance of the magnetic path is minimum.

Construction of Reluctance Motor

The reluctance motor consists of a stator and a squirrel-cage rotor as shown in the figure.

The reluctance motor consists of a stator and a squirrel-cage rotor as shown in the figure.

The stator is carrying a 1-phase main winding along with an auxiliary winding to produce a synchronously revolving magnetic field.

The rotor has an unsymmetrical magnetic construction. The unsymmetrical construction of the rotor is achieved by removing some of the teeth from the squirrel-cage rotor to produce salient poles on the rotor.

The number of salient poles produced on the rotor must be equal to the poles on the stator. The salient poles of the rotor offer low reluctance to the stator flux and hence become strongly magnetised.

Operation of Reluctance Motor

When a single-phase AC supply is fed to the stator winding, a synchronously revolving magnetic field is produced and the motor starts as a standard squirrel-cage induction motor and will accelerate to near the synchronous speed.

When the rotor approaches the synchronous speed, the rotating magnetic field of the stator will exert reluctance torque on the salient poles of the rotor which tends to align the salient-pole axis with the axis of the rotating magnetic field. At some position, the salient poles of the rotor lock with the poles of the revolving magnetic field. As a result, the motor will continue to run at the synchronous speed.

When a mechanical load is applied to the motor, the poles of the rotor fall slightly behind the poles of the stator, while continuing to run at synchronous speed. With the increase in the load on the motor, the mechanical angle between the rotor and stator poles increases progressively. Nevertheless, the magnetic attraction keeps the rotor locked with the rotating magnetic field of the stator.

If the load on the motor shaft is increased beyond the amount under which the reluctance torque can maintain synchronous speed, then the rotor drops out of the step with revolving magnetic field and hence, the speed of the motor drops to some value at which the slip is sufficient to develop the necessary torque to drive the load by the induction motor action.

Characteristics of Reluctance Motor

A reluctance motor possesses the following characteristics −

• Reluctance motors have poor efficiency and torque.
• Reluctance motors have low power factor.
• Reluctance motors cannot accelerate high inertia loads to the synchronous speed.
• Reluctance motors are cheaper than any other kind of synchronous motors.

Applications of Reluctance Motor

Reluctance motors are widely used in applications where constant-speed is required such as timing and signalling devices.