Unit 3- Three phase slip ring induction motor
Starting Of Slip Ring Induction Motor
These motors are usually started with full line voltage applied across its
terminals. As these motors have external resistance connected to its rotor
circuit, the value of starting current is adjusted or kept minimum, by increasing
the resistance of the rotor circuit. This external resistance can be assumed to be
a form of rheostat, connected in star, thus kept at maximum when the motor
starts and gradually cut-out as the motor gathers speed. It has been already
discussed in my last article on “Slip Ring Induction Motors..!!," that increasing
the rotor resistance, not only reduces the rotor current, but the stator current too.
This implies that the starting current of the motor is reduced when an external
resistance is added in the rotor circuit. Thus because of this, the starting torque
is increased due to the improvement in power factor.
The slip ring induction motor has two distinctly separate parts, one is the stator
and other is the rotor. The stator circuit is rated as same in the squirrel cage
motor, but the rotor is rated in frame voltage or short circuit current. The frame
voltage is the open circuit voltage when the rotor is not rotating and gives the
measure of turns ratio between the stator and rotor. The short circuit current is
the current flowing when the motor is operating at full speed, with the slip rings
shorted and the full load applied to the motor shaft.
Unit 3- Three phase slip ring induction motor
Qualities Of Slip-Ring Motors:
1. suited for high inertia loads as it caters with excellent starting torque,
2. Low starting current when compared to squirrel cage induction motors,
3. Speed is varied with the help of external resistance, from 50% to 100% of the
full speed,
4. Higher brush and slip ring maintenance required,
Unit 3- Three phase slip ring induction motor
5. As the brush wears out, it may lead to intermittent contact, and thus heavy
sparking.
Thus we have seen in detail the starting arrangement of slip ring induction
motors. In my next article, we will discuss on the speed control of slip ring
induction motors.