Hunting of Synchronous Motors
This is a problem associated with synchronous motors. As the load on the motor increases, the angle between the stator pole and the locked rotor pole gradually increases. The rotor of the motor falls back by a certain angle, behind the poles of the forward rotating field, in order to produce the necessary torque. The stator current will also increase. While the motor speed slows down, it will remain synchronized, unless the load causes synchronization or the locking (of the rotor pole) breaks.
An increase in phase difference causes the motor to draw more current from the mains and increase power flow in the armature. Some of the kinetic energy of the rotating parts is transferred to the load during a speed slow down.
The motor cannot decelerate exactly at the required torque angle of the increased load. It passes beyond this, develops more torque, and increases the speed. This is followed by a reduction in speed and the cycle is repeated. If the load is suddenly thrown off, the rotor poles are pulled into almost opposition to the poles of the forward field, but due to the rotor inertia, the rotor poles travel too far, and are pulled back again. This results in oscillations about the position of equilibrium corresponding to the load conditions on the motor. This periodic change in speed is known as hunting.
The phasor diagram for sudden increase in the load of a synchronous motor is shown in Figure 4.5.
The armature current increases from Ia to Ia1, Ia2, Ia3, etc., and the torque angle increases from γ, to γ1, γ2, γ3, etc. The excitation voltage (E ) is assumed to be a constant. If the variations in the load are periodic and are synchronized with the natural frequency of the oscillation of the rotor, the amplitude of these oscillations increases cumulatively, and becomes so great, that the motor may fall out of step.
Hunting is also known as phase swinging.
In order to sort it out, it is required to damp the oscillations and prevent an increase in the amplitude of swinging.
This is achieved by providing a damper winding in the pole shoe of the synchronous motor. When hunting occurs, there is a shift of flux across the faces of the pole shoe, due to the effect of the armature reaction on the field flux. The shifting flux induces circulating currents in the damper winding. The kinetic energy of oscillations is dampened by being converted into heat energy. The induced current opposes the change in the relative positions of the armature flux and the field flux, and thus acts as an effective damper.
Thus, this problem of hunting in synchronous motors can be avoided.