In this article, we will discuss hunting in a synchronous motor. But before that let us first know a bit about a synchronous motor.
A synchronous motor is a type of AC (Alternating Current) motor, i.e. an electric motor powered by an AC supply. It is called a synchronous motor because its rotor runs at a speed that is equal to the speed of the rotating magnetic field (RMF). The speed of the RMF is called synchronous speed, denoted by NS.
Therefore, a synchronous motor is a type of constant-speed AC motor that always runs at the synchronous speed (NS).
Now, let us discuss hunting in the synchronous motor, its causes, and remedies.
Hunting in Synchronous Motor
Sometimes, a synchronous motor does not operate satisfactorily due to hunting. The hunting in a synchronous motor is an oscillating action of the rotor in which the rotor of the synchronous motor is pulled periodically ahead of or behind its normal position as it rotates.
When the load is applied to the shaft of the synchronous motor, its rotor poles lag behind by a certain angle (called load angle, denoted by δ) with respect to the poles of the forward rotating magnetic field. If the load from the shaft is suddenly thrown-off, the rotor poles are pulled into their original position, but due to the inertia of the rotor, the rotor travels too far ahead of the poles of RMF. Thus, they pulled back again, and so on. Consequently, an oscillating action is set up about the equilibrium position of the rotor. This oscillating action of the rotor about its equilibrium position is known as hunting. Sometimes, hunting is also referred to as phase swinging.
In synchronous motors, hunting is an objectionable characteristic of motors. This is because it increases the probability of losing synchronism. Hunting in synchronous motors also produces large mechanical stresses. On the electrical side, hunting produces large fluctuations in current and power drawn by the motor.
After getting insights about hunting in synchronous motors, let us know the causes, effects, and remedies of hunting.
Causes of Hunting in Synchronous Motors
The following are the major factors that cause hunting in synchronous motors:
- The sudden changes in the load on the motor can cause hunting.
- The abrupt change in the field current of the machine also causes hunting.
- Loads that contain harmonic torques cause hunting.
- Faults occurring in supply systems also cause hunting.
Effects of Hunting in Synchronous Motors
Hunting in synchronous motors has adverse effects. Some adverse effects are as follows:
- Hunting may cause a loss of synchronism.
- Hunting produces severe mechanical stresses in the motor shaft.
- Hunting increases the chance of resonance occurrence.
- Hunting increases losses in motors that result in an increase in temperature.
Finally, let us know some methods to avoid hunting in synchronous motors.
Methods of Reducing Hunting in Synchronous Motor
We can reduce hunting in synchronous motors by employing these practices:
Using Damper Winding
Hunting can be reduced by providing damper winding. Damper winding is also known as squirrel cage winding or damper grid. Damper winding consists of short-circuited conductor bards made up of copper or aluminum. These damper bars are embedded in the field pole faces of the synchronous motor rotor. These damper winding reduces the hunting by producing a torque opposite to the slip of the rotor. However, the damper winding cannot completely suppress the hunting because its operation depends upon the presence of oscillating motion in the motor.
Using of Flywheel
The use of a flywheel on the shaft of the rotor increases the inertia of the rotor. Consequently, it maintains the rotor speed constant.
Machine Design with Synchronizing Power Coefficient
The third method of reducing hunting in synchronous machines is to design the machine with a suitable synchronizing power coefficient.
Hence, this is all about hunting in synchronous motors. In the above sections, we discuss everything about hunting such as its definition, effect, causes, and method to reduce hunting.