The reluctance motor
The reluctance motor is arguably the simplest synchronous motor of all, the rotor consisting of a set of iron laminations shaped so that it tends to align itself with the field produced by the stator.
The stator winding is identical to that of a three-phase induction motor. The rotor is different, containing saliency which provides a preferred path for the flux. This is the feature which tends to align the rotor with the rotating magnetic field, making it a form of synchronous machine. In order to start the motor a form of cage needs to be incorporated into the rotor design, and the motor can then start as an induction motor. Once a higher speed is reached, the reluctance torque ‘pulls in’ the rotor to run synchronously in much the same way as a permanent magnet rotor.
Reluctance motors may be used on both fixed frequency supplies and inverter supplies. These motors tend to be one frame size larger than a similarly rated induction motor and have a low power factor (perhaps as low as 0.4) and poor pull-in performance. As a result, their industrial use has not been widespread except for special applications such as textile machines where large numbers of reluctance motors may be connected to a single ‘bulk’ inverter and may maintain synchronism. Even in this application, as the cost of inverters has reduced, bulk inverters are infrequently used and the reluctance motor is now rarely seen.