Page 149 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Hunting or surging or phase swinging
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. 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 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 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
However, they serve the additional purpose of making the synchronous motor self-starting. As said above, almost
all synchronous motors are equipped with dampers or squirrel cage.
Comparison between synchronous and induction motors
1 For a given frequency, the synchronous motor runs at a constant average speed whatever the load, while the
speed of an induction motor falls somewhat with increase in load.

2 The synchronous motor can be operated over a wide range of power factors, both lagging and leading, but
induction motor always runs with a lagging power factor which may become very low at light loads.

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CITS : Power - Electrician & Wireman - Lesson 76-85