Single-phase salient-pole synchronous-induction motors, are generally called reluctance motors. If the rotor of any uniformly distributed single-phase induction motor is altered so that the laminations tend to produce salient rotor poles, as shown in Fig. 24, the reluctance of the air-gap flux path will be greater where there are no conductors embedded in slots. Such a motor, coming up to speed as an induction motor, will be pulled into synchronism with the pulsating A.C. single-phase field by the reluctance torque developer, at the salient iron poles which have lower-reluctance air gaps.
Working of a reluctance motor. In order to understand the working of such a motor the basic fact which must be kept in mind is that when a piece of magnetic material is located in a magnetic field, a force acts on the material, tending to bring it into the densest portion of the field.
The force tends to align the specimen of materials in such a way that the reluctance of the magnetic path that passes through the material will be minimum.
When supply is given to the stator winding, the revolving magnetic field will exert reluctance torque on the unsymmetrical rotor tending to align the salient pole axis of the rotor with the axis of the revolving magnetic field (because in this position, the reluctance of the magnetic path would be minimum).
If the reluctance torque is sufficient to start the motor and its load, the rotor will pull into step with the revolving field and continue to run at the speed of the revolving field.
(Actually the motor starts as an induction motor and after it has reached its maximum speed as an induction motor, the reluctance torque pulls its rotor into step with the revolving field so that the motor now runs as synchronous motor by virtue of its saliency).
Reluctance motors have approximately one-third the Fig. 24. Reluctance-motor lamination horsepower rating they would have as induction motors with cylindrical rotors, although the ratio may be increased to one-half by proper design of the field
windings. Power factor and efficiency are poorer than for the equivalent induction motor. Reluctance motors are subject to ‘cogging’, since, the locked-rotor torque varies with the rotor position, but the effect may be minimized by skewing the rotor bars and by not having the number of rotor slots exactly equal to an exact multiple of the number of poles.
Uses. Despite its short-comings, the reluctance motor is widely used for many constant, applications such as recording instruments, time devices, control apparatus, regulators, and phonograph turntables.
- Reversing is obtained as in any single··phase induction motor.
Speed-torque characteristics. Fig. 25 shows speed-torque characteristics of a
typical single-phase reluctance motor.
- The motor starts at anywhere from 300 to 400 per cent of its full-load torque (depending on the rotor position of the unsymmetrical rotor with respect to the field windings) as a two-phase motor as a result of the magnetic rotating field created by a starting and running winding (displaced) 900 in both space and time.
- At about 3/4th of the synchronous speed, a centrifugal switch opens the starting winding, and the motor continues to develop a single-phase torque produced by its running winding only.
As it approaches synchronous speed, the reluctance torque (developed as a synchronous motor) is sufficient to pull the rotor into synchronism with the pulsating single-phase field.
- The motor operates at a constant speed up-to a-little over 200% or its full-load torque. If it is loaded beyond the value of pull-out torque, it will continue to operate as a single-phase induction motor up to 500% of its rated output.