Electrical Machines - Hunting Effect Alternator

What is Hunting in Alternator?

When an alternator or synchronous generator is running under normal condition, the armature field and the rotor magnetic field will be rotating in the same direction at synchronous speed.

When either a sudden change in the load of the alternator or a change in the torque of turbine occurs, then the speed of the rotor decreases with respect to that of armature (or stator) field by some angle called as load angle. Once this happens, the alternator rotor starts to hunt, i.e., search for a new equilibrium position. Although the rotor has high inertia, it cannot achieve its final position instantaneously and hence the rotor starts oscillating. This process of oscillating of the rotor to obtain its new equilibrium position is called as hunting.

In other words, when the driving torque applied to an alternator is pulsating such that produced by a diesel engine, then the rotor of the alternator may be pulled periodically ahead of or behind the normal position as it rotates. This oscillating action of the alternator rotor is known as hunting.

Effect of Hunting in Alternator

An alternator will not operate satisfactorily in parallel with the others due to hunting. The effect of hunting causes the alternators to shift the load from one to another. In some cases, the oscillation of power becomes cumulative and violent enough to cause the alternator to pull out of synchronism.

In salient-pole type alternator, the effect of hunting can be reduced by providing damper winding. The damper winding consists of short-circuited copper bars which are embedded in the pole faces as shown in the figure.

When hunting occurs in the alternator, the shifting of armature flux takes place across the pole faces, thereby inducing currents in the damper winding. According to Lenz's law, the induced current in the winding opposes the cause that produces it. Therefore, the hunting action is opposed and reduced by the flow of induced currents in the damper winding.

Important points about hunting in alternators

Generally, hunting occurs in the alternators driven by diesel engines since the driving torque of diesel engines is not uniform.

Hunting does not occur in the alternators that are driven by steam turbines since the driving torque of steam turbine does not pulsate.

In cylindrical rotor type alternators, the damper windings are not used because the solid rotor itself provides considerable damping.

Under normal operating conditions, the damper winding does not carry any current since the rotor runs at synchronous speed.