reluctance motor

reluctance motor [ri′lək·təns ‚mōd·ər]

(electricity)

A synchronous motor, similar in construction to an induction motor, in which the member carrying the secondary circuit has salient poles but no direct-current excitation; it starts as an induction motor but operates normally at synchronous speed.

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Reluctance motor

An alternating current motor with a stator winding like that of an induction motor, and a rotor that has projecting or salient poles of ferromagnetic material. When connected to an alternating-current source, the stator winding produces a rotating magnetic field, with a speed of 4&pgr;f/p radians per second (120f/p revolutions per minute), where f is the frequency of the source and p the number of magnetic poles produced by the winding. When the rotor is running at the same speed as the stator field, its iron poles tend to align themselves with the poles of that field, producing torque. If a mechanical load is applied to the shaft of the motor, the rotor poles lag farther behind the stator-field poles, and increased torque is developed to match that of the mechanical load. This torque is given by the equation below,

where &phgr; is the flux per pole, determined largely by the applied voltage. The quantity dR/dδ is the rate of change of magnetic reluctance per pole with respect to δ, the angle of lag in mechanical radians. This quantity typically varies as sin pδ. Here, pδ = 2δ_e, where δ_e is the lag angle in electrical radians. Therefore, at constant torque load the rotor runs in synchronism with the stator field, with the rotor poles lagging the field poles by a constant angle. See Electrical degree

This phenomenon develops torque only at synchronous speed, and thus no starting torque is produced. For that reason, induction-motor rotor bars are usually built into the pole faces, and the motor starts as an induction motor. When the rotor speed approaches that of the magnetic field, the pole pieces lock in step with the magnetic poles of the field, and the rotor runs at synchronous speed.

Single-phase reluctance motors may be started by the methods used for single-phase induction motors, such as capacitor, split-phase, or shaded-pole starting. See Alternating-current motor, Electric rotating machinery, Induction motor, Motor, Synchronous motor