SOLUTION

Reluctance Motor

The three-phase as well as single-phase motors can be built with rotors without DC excitation but having non-uniform air-gap reluctance. The stator construction of such motors is similar to 3-phase or 1-phase induction motors but the shape of the squirrel cage rotor of these motors is changed such that it has variable magnetic reluctance along the air gap. The 3-phase induction motors with such rotors are usually called synchronous induction motors. However, the single-phase induction motors built with a variable air-gap reluctance rotor without DC excitation are called reluctance motors.

Working of reluctance motor

• When supply is given to the stator of a single-phase (split-phase) reluctance motor, a revolving field is set up in the stator.
• The rotor starts rotating as an induction motor rotor and picks up the speed near to synchronous speed.
• Then the rotor is pulled into synchronous speed with the stator field since reluctance torque is developed at the salient poles which have smaller air-gap reluctance.
• Thus, the rotor starts rotating at synchronous speed once the reluctance torque is developed.
• If it is loaded beyond the value of synchronous pull-out torque, it will continue to operate as a single-phase induction motor up to over 500 percent of its rated torque.
• If the load is increased beyond the given value then the reluctance motor becomes out of synchronism.
• The speed is dropped to a value where slip is sufficient to develop necessary torque to drive the load and at that condition, the reluctance motor runs as an induction motor.