When a synchronous motor is operated on a continuously varying load, the speed of the motor also fluctuates continuously with it. Now we will discuss hunting or phase swinging of synchronous motor.
What is hunting or phase swinging?
When the synchronous motor is operated with continuously variable excitation or the supply line frequency is pulsating or changing, the speed of the motor also fluctuates continuously with it. This state of the motor is called hunting or phase swinging.
Explanation of Hunting or Phase swinging
When the mechanical load on a synchronous motor is constant, its rotor turns at constant speed at a fixed torque angle according to the supplied horsepower. If the load changes, the rotor speed changes momentarily until the torque angle automatically adjusts to the new horsepower. If the load increases then the rotor goes into backward slip increasing the torque angle.
Again when the load is reduced the rotor is leading and the torque angle is reduced. But due to mechanical inertia in various rotating parts, the rotor overshoots, slows, or overshoots without moving to the correct final position.
That is, when the load is continuously decreased or increased, new torque angles are generated according to the load. This causes oscillation in the rotor. The rotor then cannot settle to synchronous speed at all. As a result, hunting or phase swinging occurs in the motor.
The rotor tries to oscillate continuously like a clock. If the amount of hunting in the oscillation period increases further, the swingings increase to a point where the torque angle is greater than the pull-out value. In this case, the hunting is so intense that the machine cannot bear it. Then the circuit breaker opens and the machine stops.
- Also Read: Types of Torque in a Synchronous Motor
Causes of hunting in synchronous motor
Hunting can occur in a synchronous motor due to the following reasons:
1. When the load on the shaft of a synchronous motor is suddenly increased or removed or a continuously variable load is applied,
2. If the speed of the prime mover is variable,
3. If the supply frequency is variable,
4. If the synchronous motor is connected to a very long transmission line.
5. Hunting causes unwanted noise in the motor.
Disadvantages of hunting of synchronous motor
Hunting in synchronous motors has the following disadvantages:
1. The current and power consumption of the motor varies greatly.
2. Motors are subjected to a lot of stress and damage to mechanical parts, such as bearings, shafts, poles, etc. which can corrode, rub or break.
3. Motor windings may burn.
When a synchronous motor runs at synchronous speed, there is magnetic coupling between the rotor and stator poles. The resulting stator field keeps the rotor in a fixed position. As a result of changing frequency or load, the rotor moves backwards and forwards relative to the position, resulting in vibration or oscillation. The level of this vibration or solon is so high that the motor deviates from synchronism and stops.
Undesirable vibration or oscillation of the rotor due to hunting causes excessive flux cutting in the rotor core resulting in core loss. Vibration or oscillation of the rotor puts a lot of stress on the mechanical parts of the motor. This results in frictional wear and can often lead to breakage. Hunting cause huge changes in current and power consumption in the motor. Sometimes excessive current flow can cause the motor winding to overheat and burn out. If the level of hunting or phase swinging is too high. Then the motor may stop.
Remedies for Hunting in a Synchronous Motors
In synchronous motor hunting is mainly eliminated by using damper winding or amortisseur. Besides, all the other methods used are:
(a) Using the well “dashpot” with the governor,
(b) using heavy flywheels,
(c) Using thick copper foil around the field pole.
Hunting mitigation using damper winding or amortisseur
In synchronous motors it is common practice to use damper windings or amortisseur to reduce hunting. For this purpose a thick copper bar is grooved transversely on the surface of the salient pole of the synchronous motor. Their two ends are shorted by copper ring, i.e. squirrel cage winding. This is the damper winding. It is also called amortisseur.
Oscillations such as hunting are mitigated by the eddy currents generated in the vamper windings when the motor is running. The eddy current generated prevents the relative position of armature and field flux from changing according to Len’z law and acts as effective damping.
Damper Winding Damping efficiency depends on the resistance of the damping winding and the length of the air gap. A low-resistance damping winding produces strong damping.
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Frequently Asked Questions
What is hunting of synchronous motor?
When the load of a synchronous motor is continuously variable or the supply line frequency is continuously varying, the speed of the motor fluctuates continuously. This state of the motor is called hunting or phase swinging.
How to eliminate hunting in synchronous motor?
Hunting can be eliminated mainly by using damper winding or amortisseur in synchronous motor.
What is the function of damper winding?
Oscillations such as hunting are damped by the eddy currents generated in the damper windings when the synchronous motor is running. This eddy current generated prevents the relative position of the armature and field flux from changing according to Lenz’s law and acts as effective damping.