A reluctance-start induction motor is shown in Fig. 11. Its characteristics are similar to that of shaded pole motor. In this motor too the magnetic field shifts across the pole, but the effect is obtained by the non-uniform air gap of salient poles. Where there is a greater air gap, the flux in that portion is more nearly in phase with the current. There is a greater lag between flux and current where there is a lower reluctance or where the air gap is smaller. Since both fluxes are produced by the same current, the flux across the larger air gap leads the flux across the smaller one. The two fluxes are obviously displaced in time, and so the magnetic field shifts across the poles from larger air gap to the shorter gap. Thus the direction of rotation is firmly fixed by the construction, and the motor cannot be reversed at all.
Fig. 11. Reluctance start motor.
Uses. For most small power applications, the shaded-pole motor is preferred, and the reluctance-start motor has limited use, usually only where starting torque requirements are low.
Note. This motor is an induction motor and should not be confused with reluctance motor which is actually a non-excited synchronous motor.