What is Reluctance motor ?
A single-phase synchronous reluctance motor is basically the same as the single phase cage type induction motor. The stator has the main winding and an auxiliary winding or starting winding. In general the stator of a single-phase reluctance motor is similar to that of any one of the single-phase induction motors.
The rotor of a reluctance motor is basically a squirrel cage with some rotor teeth removed at the appropriate places such as to provide the desired number of salient rotor poles.
The 4-pole reluctance type synchronous motor. Here teeth have been removed in four locations to produce a 4-pole salient-pole structure. The rotor bars are kept intact even in the spaces from where teeth are removed. The two end rings short-circuit these bars as in a cage rotor.
Working principle of Reluctance Motors :
When the stator is connected to a single-phase power supply, the motor starts as a single-phase induction motor. At a speed of about 75% of the synchronous speed, a centrifugal switch disconnects the auxiliary winding and the motor continues to speed up as a single-phase motor with the main winding in operation.
When the speed is closed to the synchronous speed, a reluctance torque is produced due to tendency of the rotor to align itself in the minimum reluctance position with respect to the synchronous rotating flux of the forward field. The rotor pulls into synchronism. For this to happen effectively, the load inertia must be within limits. After pulling into synchronous, the induction torque disappears but the rotor remains in synchronous reluctance torque alone.
The typical torque-speed characteristics of the single-phase reluctance motor. The starting torque is dependent upon the rotor position because of the salient pole rotor. The value of the starting torque is between 300 to 400 percent of its full-load torque. At about 75% of the synchronous speed, a centrifugal switch disconnects the auxiliary winding and the motor continues speed, the reluctance torque developed as a synchronous motor pulls the rotor into synchronism and the rotor continues to rotate at synchronous speed.
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| Torque-speed characteristics of reluctance motor |
The motor operates at a constant speed upto a little over 200% of its full-load torque. If the loading is increased beyond the value of the pull-out torque, the motor loses synchronism but continues to run as a single-phase induction motor upto over 500% of its rated torque. Reluctance motors are subject to cogging at the time of starting. This is due to the saliency of the rotor. The cogging is minimised by skewing the rotor bars and by having the rotor slots not exact multiples of the number of poles.
In reluctance motors, since the rotor is unexcited and has saliency, the power factor is lower than that of the equivalent induction motor. The maximum output of a reluctance motor is greatly reduced due to absence of dc field excitation. Therefore the size of a reluctance motor is larger than that of an equivalent synchronous motor. The main advantages of a reluctance motor are its simple construction, low cost and easy maintenance.
In spite of its shortcomings, the reluctance motor is widely used for many constant-speed applications such as electric clocks timers, signalling devices, recording instruments and phonographs etc.
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