Rotor Resistance Starter in Three-Phase Induction Motor

Circuit Diagram and Working Principle

In a rotor resistance starter, a star connected variable resistance is connected in the rotor circuit through slip-rings. The full voltage is applied to the stator windings. The connection arrangement of the rotor resistance starter is shown in the figure.

At the instant of starting, the handle of variable resistance (rheostat) is set to ‘OFF’ position. This inserts maximum resistance in series with each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased due to external rotor resistance.

As the motor accelerates, the external resistance is gradually removed from the rotor circuit. When the motor attains rated speed, the handle is switched in the ‘ON’ position, this removes the whole external resistance from the rotor circuit.

Slip Ring induction motors are mainly used for driving high inertia loads or the loads which require a starting torque across a full speed range. Hence, by correctly selecting the starting resistors inserted into the rotor circuit, the maximum torque can be obtained from the motor at a relatively low starting current. Thus, the magnitude of starting torque is controlled by the value starting resistance.

The starting resistors need to be designed to allow for the starting duty cycle without overheating or too large change in the value of the resistance when hot.

Advantages of Rotor Resistance Starter

Following are the chief advantages of the rotor resistance starter −

• High starting torque may be obtained with the rotor resistance starter.
• It improves the power factor of the motor.
• Absence of line current harmonics.
• Reduced starting current.
• Smooth and wide range of speed control is possible.

Disadvantages of Rotor Resistance Starter

The disadvantages of the rotor resistance starter are given as follows −

• Lower efficiency due to the I²R losses in the external resistance connected in the rotor circuit.
• This starter can only be used for starting a slip ring induction motor.
• It needs frequent maintenance due to presence of slip rings and carbon brushes.