Reluctance motors operate on the following principle :
"Whenever a piece of ferromagnetic material is located in a magnetic field, a force is exerted upon the material, lending to bring it into the position of the densest portion of the field. The force tends to align the Specimen of material so that the reluctance of the magnetic path passing through the material will be minimum."
Construction and operation:
The stator winding is supplied by single-phase a.c. supply and it produces a revolving magnetic field according to the split-phase principle.
The rotor is a squirrel cage type having projected poles as shown.
When stator winding is switched on, a revolving field is established, revolving at Ns.
Due to the presence of a squirrel cage, the motor starts as an induction motor and reaches a speed, very near to synchronous speed.
Due to projecting poles, the air gap between stator and rotor is nonuniform i.e. reluctance to the flow of flux is less where the air gap is small and it is more where the air gap is large.
The revolving flux of the stator tries to follow a path of least reluctance, as a result, the rotor gets pulled into synchronism and continues to run at synchronous speed.
The torque due to which the rotor gets pulled in synchronism is called reluctance torque.
The magnitude of starting torque depends upon the position of the rotor.
Also, the torque changes when switching action from induction motor to synchronous motor takes place. (Refer Fig.).
Characteristics:
Such motors have poor power factors, low efficiency, and poor torque. They cannot accelerate high inertia loads to synchronous speed. The pull-in and pull-out torques of such motors are small.
Such motors are subject to "cogging" because the locked rotor torque varies with the rotor position, but the effect may be minimized by skewing the rotor bars and not having the number of rotor slots equal to an exact multiple of the number of poles.
Reluctance motors are generally made in the fractional kilowatt ratings and employ the conventional split-phase stator and the centrifugal switch to open the auxiliary winding.
When they are built for heavier loads, sometimes in integral kilowatt ratings, the permanent split capacitor construction with no centrifugal switch is preferred.
When the number of salient poles on the rotor is greater, by some multiple than the number of electrical poles on the stator, the motor will operate at a constant average speed that is a submultiple of apparent synchronous speed; it is then called a sub-synchronous reluctance motor.
Reversal of direction of rotation is obtained as in any single-phase induction motor.
Advantages:
1. This motor has constant speed like three-phase synchronous motor,
2. But it does not require d.c, supply for excitation purposes. Hence it is less complicated and needs less maintenance.
Disadvantages :
1. It has limitations due to the low value of efficiency.
2. Also rotor must be light in weight to start rotating.
3. Power factor of the motor is also low.
Applications :
Due to the constant speed of reluctance motors, they are preferred in the following applications:
1. Timing devices (clocks)
2. Gramophones (to rotate the disc)
3. Recording instruments
4. signaling devices
5. phonograph turntables
6. control apparatus