Which type of 3 phase motor starter is suitable for the 3 phase slip ring induction motor?

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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.

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