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Reluctance Motor - principle, Working, Advantages

Reluctance Motor

The stator has the main winding and an auxiliary (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. 

Figure 9.1 shows the 4- pole reluctance-type synchronous motor. Her 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.

Reluctance Motor

Working principle Of Reluctance Motor

When the stator is connected to a single-phase supply, the motor starts as a single-phase induction motor. At a speed, of about 75 percent 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 close to the synchronous speed, a reluctance torque is produced due to the tendency of the rotor to align itself in the minimum reluctance position with respect to the synchronously 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 synchronism, the induction torque disappears but the rotor remains in synchronism due to the synchronous reluctance torque alone. 

Torque Speed characteristic of a reluctance motor

Figure 9.2 shows the typical torque-speed characteristic 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 to run with the main winding only. 

When the speed is close to synchronous speed, the reluctance torque developed as a synchronous motor pulls the rotor into synchronism and the rotor continues to rotate at synchronous speed.
Torque Speed characteristic of a reluctance motor

The motor operates at a constant speed of up to 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 up to over 500 percent 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 minimized 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 the absence of de-field excitation. Therefore the size of a reluctance motor is larger than that of an equivalent synchronous motor. 

Advantages Reluctance motor

The main advantages of a reluctance motor are its simple construction (no slip rings, no brushes, node field winding), low cost, and easy maintenance. 

In spite of its shortcomings, the reluctance motor is widely used for many constant-speed applications such as electric clock timers, signaling devices, recording instruments and photographs, etc.

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