Answer / vipul jain

In the case starter, the capacitors are often controlled by
the main contactor which is also controlling the motor.
This is because the capacitance and the inductance of the
motor form a resonant circuit. While the motor is connected
to the supply, there is no problem. Once the motor is
disconnected from the supply, it begins to decelerate. As it
decelerates, it generates voltage at the frequency at which
it is rotating. If the capacitive reactance equals the
inductive reactance, i.e. unity power factor, we have
resonance. If the motor is critically corrected (pf = 1) or
over corrected, then as the motor slows, the voltage it is
generating will pass through the resonant frequency set up
between the motor and the capacitors. If this happens, major
problems can occur. There will be very high voltages
developed across the motor terminals and capacitors causing
insulation damage, high resonant currents can flow, and
transient torque's generated can cause mechanical equipment
failure.

Answer / g.v.hema chandu

The unity power factor means the angle b/w voltage and current is unity.As a practical it is not possible for motors because motor is inductive.For an theory if motor is at unit power factor the speed decreases.

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