Electrical Revolution

Single Phase unexcited Synchronous Motor

The characteristic of single phase unexcited synchronous motor in which excitation does not necessary is given below.

( 1 ) It operates on single phase supply.

( 2 ) The stator rotating magnetic field rotates at synchronous speed.

( 3 ) It does not require DC Excitation.

( 4 ) It is self starting.

When a magnetic material is placed in the magnetic material, it always aligns in the minimum reluctance path.

The stator of the reluctance motor is similar to that stator of the single phase induction motor.
It consists of starting and running winding in the stator slots.
This type of motor is also called as Split phase reluctance motor.

The rotor of the reluctance motor is of salient or projecting poles.
Let us consider that the rotor of the squirrel cage induction motor consists of 24 copper bars.
If the rotor bar 5, 6, 11, 12,17,18,23 and 24 are cut, it is similar to 4 salient poles.

When a single phase supply is given to the stator winding, a rotating magnetic field is produced in the stator winding.
When a salient poles rotor cut this magnetic field, rotor aligns in the minimum reluctance path due to reluctance torque.
The reluctance depends upon air – gap between stator and rotor.
Figure A shows 4 pole salient pole rotor in which direction of four high Permeance and four low Permeance is shown.
High Permeance means higher magnetic conductivity and higher inductance. Similarly low Permeance means lower magnetic conductivity and lower inductance.
The reluctance is inverse of Permeance. Low reluctance means higher inductance and vice versa.

L α N² / S

Where L = Inductance and

S = Reluctance of magnetic path

S = L / μ₀μ_ra

Where L = Length of air – gap

μ₀ = Absolute permeability = 4π × 10 ^{– 7} Henry / meter

μ_r= Absolute permeability

a = Area

There is low reluctance path between stator and salient poles due to small air – gap whereas high reluctance path between stator and inter – polar axis due to large air – gap.
The reluctance motor starts as an induction motor.
When the rotor rotates at its maximum speed, it aligns with the stator synchronous magnetic field due to reluctance torque.
The angle between stator poles and rotor poles of opposite polarity is called as torque angle.
As the torque angle increases, the reluctance torque also increases.
The maximum reluctance torque attains at torque angle of 45⁰.
The load taken by the reluctance motor is only fraction of the load taken by the three phase inductance motor.