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Which type of 3 phase motor starter is suitable for the 3 phase slip ring induction motor?
1. Star and delta starter
2. DOL starter
3. Auto transformer starter
4. Rotor resistance starter

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Correct Answer - Option 4 : Rotor resistance starter

Rotor resistance starter:

  • Rotor resistance starter is used in a high rating slip ring induction motor.
  • It uses an external resistance/phase in the rotor circuit so that the rotor will develop a high value of torque.
  • High torque is produced at low speeds when the external resistance is at its higher value.
  • At the start, supply power is connected to the stator through a three-pole contactor and, at the same time, an external rotor resistance is added
  • The high resistance limits the staring current and allows the motor to start safely against a high load.
  • Hence if resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct line starting the starting current reduces but starting torque increases.

Adding external resistance to rotor:

In the case of slip ring motors, the rotor resistance is increased by the addition of external resistance. This external resistance makes the rotor circuit current low.

During the starting of a 3 ϕ induction motor

\(T \propto \frac{R}{s}\)

Where T = Starting Torque

R= Rotor resistance

s = slip

Hence by increasing the rotor resistance, we can improve the starting torque of the wound rotor induction motor (W.R.I.M.). which can be done by adding external resistance in the rotor circuit.

This method is only applicable to W.R.I.M. due to wound rotor is short-circuited with slip rings, where external resistance is added through the brush whereas, in squirrel cage induction motor, the rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings.

Therefore in the case of the squirrel cage induction motor, it is not possible to add any external resistance in series with the rotor circuit for starting purposes.

Slip ring induction motor

Squirrel cage induction motor

It has a slip ring type rotor

It has a squirrel cage type rotor

Cylindrical laminated core with parallel slots and each slot consist one bar

The slots of the rotor are not parallel but are skewed

Construction is complicated

Construction is simple

We can add external resistance to the rotor

The rotor bar is permanently shorted at the end of the ring; thus, it is not possible to add any external resistance

The rotor resistance starter can be used

Rotor resistance starter cannot be used

Starting torque is high

Starting torque is low

Brushes are present

Brushes are absent

Frequent maintenance is required

Less maintenance required

Power factor is low

Power factor is high

Speed control is possible

Speed control is not possible

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