Reluctance motor construction and principle

A reluctance motor is a single-phase synchronous motor whose rotor has no DC excitation; it runs at synchronous speed because the rotor tends to align itself in the position of minimum reluctance to the stator field.

Construction

• Stator: the same as a single-phase induction motor - a main winding and an auxiliary winding (split-phase or capacitor type) to produce a rotating field.
• Rotor: a modified squirrel-cage rotor. Some teeth are removed at symmetrical points to form salient poles; the number of salient poles equals the number of stator poles. The cage bars and end rings remain, so it can start as an induction motor.

        ___________
      /  __     __  \     removed teeth (high
     |  |  |   |  |  |    reluctance gaps)
     |  |__|   |__|  |    remaining parts act as
      \_____________/     salient poles
   rotor with cage bars and cut-outs

Principle of operation

1. Starting: the stator rotating field induces currents in the cage bars, so the motor starts and accelerates as an induction motor.
2. Pull-in: near synchronous speed (about 75-80 %), the reluctance torque acts. A piece of magnetic material in a field experiences a force that moves it to the position of minimum reluctance (shortest air path). The salient poles snap into alignment with the stator field poles.
3. Synchronous running: the rotor locks with the rotating field and runs at $N_s = 120f/P$. The auxiliary winding may be disconnected by a centrifugal switch.
4. The reluctance torque is $T \propto \left(\frac{1}{X_q} - \frac{1}{X_d}\right)\sin 2\delta$; it is zero if $X_d = X_q$, which is why saliency is essential.

Features

• Constant speed, no slip rings, brushes or DC supply.
• Low power factor and efficiency; output is about 1/3 of an induction motor of the same frame size.
• Torque-speed curve shows induction motor region up to pull-in, then a vertical line at $N_s$.

Applications

Electric clocks, timers, signalling devices, recording instruments, teleprinters and other constant-speed drives.