Switched reluctance motor construction and principle

A switched reluctance motor (SRM) is a doubly salient motor in which torque is produced by the rotor's tendency to move to the position of minimum reluctance. The phases are switched on in sequence by an electronic converter using rotor-position feedback.

Construction

• Stator: laminated, salient poles with concentrated coils; diametrically opposite coils form one phase.
• Rotor: laminated steel with salient poles, no winding, no magnet, no brushes.
• Stator and rotor pole numbers differ, e.g. 6/4 (3-phase) or 8/6 (4-phase).
• A position sensor and an asymmetric half-bridge converter feed each phase.

   6/4 SRM          phase A on:
     A               rotor poles
   /   \             pulled into line
  C'     B'          with A-A'
  |  [R]  |          then B on -> next step
  B       C
   \   /
     A'

Working principle

1. Phase A is energised when a pair of rotor poles is approaching the A poles (unaligned position).
2. The rotor turns to the aligned (minimum reluctance, maximum inductance) position.
3. Phase A is switched off and phase B switched on; the rotor keeps moving. Repeating the sequence gives continuous rotation.

$$T = \frac{1}{2}i^2\frac{dL}{d\theta}$$

Torque does not depend on current direction, so unipolar current is enough; reversing the phase sequence reverses rotation.

Features/uses: simple, rugged, cheap, high speed and fault tolerant, but noisy with torque ripple. Used in fans, washing machines, vacuum cleaners, pumps, electric vehicles and aerospace drives.