Hunting in Synchronous Machines

What is Hunting in Synchronous Machines?

Hunting in Synchronous Machines

What is Hunting in Synchronous Machines?

When the load applied to the synchronous motor is suddenly increased or decreased, the rotor oscillates about its synchronous position with respect to the stator field. This action is called Hunting in Synchronous Machines.


Hunting in Synchronous Machines

Sudden changes of load on synchronous motors sometimes set up oscillations that are superimposed upon the normal rotation, giving rise to periodic variations of a very low frequency in speed. This effect is known as Hunting in Synchronous Machines or phase-swinging. Occasionally, the trouble is aggravated by the motor having a natural period of oscillation approximating to the hunting period, when it is possible for the motor to phase-swing into the unstable region, thus causing it to fall out of synchronism.

What causes hunting in synchronous machines?

  • When a synchronous motor is used for driving a varying load, then a condition known as hunting is produced. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines). We know that when a synchronous motor is loaded (such as punch presses, shears, compressors and pumps etc.), its rotor falls back in phase by the coupling angle α. As load is progressively increased, this angle also increases so as to produce more torque for coping with the increased load.
  •  If now, there is sudden decrease in the motor load, the motor is immediately pulled up or advanced to a new value of α corresponding to the new load. But in this process, the rotor overshoots and hence is again pulled back. In this way, the rotor starts oscillating (like a pendulum) about its new position of equilibrium corresponding to the new load. If the time period of these oscillations happens to be equal to the natural time period of the machine then mechanical resonance is set up.
  • The amplitude of these oscillations is built up to a large value and may eventually become so great as to throw the machine out of synchronism.

Minimizing Hunting in Synchronous Machines using Damper Winding

The tendency to hunt can be minimized by the addition of a mechanical flywheel, but this practice is rarely adopted, the use of a damper winding being preferred.

What is Damper Winding?

Damper Winding
  • To stop the build-up of these oscillations, dampers or damping grids (also known as squirrel-cage winding) are employed. These dampers consist of short-circuited Cu bars embedded in the faces of the field poles of the motor. The oscillatory motion of the rotor sets up eddy currents in the dampers which flow in such a way as to suppress these oscillations. But it should be clearly understood that dampers do not completely prevent hunting because their operation depends upon the presence of some oscillatory motion. Howover, they serve the additional purpose of making the synchronous motor self-starting.
  • It is mentioned earlier that in the slots provided in the pole faces, a short circuited winding is placed. This is called damper winding.

Effect of Damper Winding on Hunting

  • When rotor starts oscillating i.e. when hunting starts a relative motion between damper winding and the rotating magnetic field is created. Due to this relative motion, e.m.f. gets induced in the damper winding. According to Lenz’s law, the direction of induced e.m.f. is always so as to oppose the cause producing it. The cause is the hunting. So such induced e.m.f. oppose the hunting. The induced e.m.f. tries to damp the oscillations as quickly as possible.
  • Thus Hunting in Synchronous Machines is minimized due to damper winding. The time required by the rotor to take its final equilibrium position after hunting is called as setting time of the rotor. If the load angle is plotted against time, the schematic representation of hunting can be obtained as shown in the fig. It is shown in the diagram that due to damper winding the setting time of the rotor reduces considerably.


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