Hunting in a Synchronous Motor
Hunting in a Synchronous Motor
The phenomenon of oscillation of the rotor about its final equilibrium
position is called Hunting. On the sudden application of load,
the rotor search for its new equilibrium position and this process is known
as Hunting. The Hunting process occurs in a synchronous motor as
well as in synchronous generators if an abrupt change in load occurs.
The steady state or stable operation of a synchronous
motor is a condition of equilibrium. In it, the load torque is equal as well as
opposite to the electromagnetic torque. The rotor of the motor runs at
synchronous speed in the steady state condition, maintain a constant value of
the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in
the load torque. Thus, a resulting torque takes place which changes the speed
of the motor. It is given by the equation shown below.
Where J is the moment of inertia
Where J is the moment of inertia
ωM is the angular velocity of the rotor
in mechanical units.
The speed of the motor slows down temporarily, and the
torque angle δ is sufficiently increased. This is done to restore the torque
equilibrium and the synchronous speed when there is a sudden increase if the
load torque.
The electromagnetic torque is given by the equation shown
below.
continues to increase beyond the synchronous speed.
As the rotor accelerates above synchronous speed, the
torque angle δ decreases. The point where the motor torque becomes equal to the
load torque, the equilibrium is not restored because now the rotor speed is
greater than the synchronous speed. Therefore, the rotor continues to swing
When the load angle δ becomes less
than the required value, the mechanical load becomes greater than the developed
power. Therefore, the motor starts to slow down. The load angle starts
increasing again. Thus, the rotor starts to swing or oscillates around the
synchronous speed.
The motor responds to a decreasing
load torque by a temporary increase in speed and a reduction of the torque
angle δ. Thus, the rotor swings and rotate around the synchronous speed. Thus,
this process of rotation of the rotor speed equal or around the synchronous
speed is known as Hunting. Since, during the rotor oscillation, the phase of
the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.
Causes of Hunting
The various causes of hunting are as
follows:-
- Sudden changes of load.
- Faults were occurring in the system which the generator supplies.
- Sudden change in the field current.
- Cyclic variations of the load torque.
Effect of Hunting
The various effects of hunting are
as follows:-
1. It can lead to loss of synchronism.
2. It can cause variations of the supply voltage producing undesirable lamp flicker.
3. The possibility of Resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place.
4. Large mechanical stresses may develop in the rotor shaft.
5. The machine losses increases and the temperature of the machine rises.
1. It can lead to loss of synchronism.
2. It can cause variations of the supply voltage producing undesirable lamp flicker.
3. The possibility of Resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place.
4. Large mechanical stresses may develop in the rotor shaft.
5. The machine losses increases and the temperature of the machine rises.
Reduction of Hunting
The following technique given below
is used to reduce the phenomenon of hunting.
1. Use of
damper windings
2. Uses of
flywheels
The prime mover is provided with a
large and heavy flywheel. This increases the inertia of the prime mover and
helps in maintaining the rotor speed constant.
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