4. Rotor Resistance Starter

• 1. This starter is used with a wound rotor induction motor. It uses an external resistance/phase in the rotor circuit so that rotor will develop a high value of torque.
  2. High torque is produced at low speeds, when the external resistance is at its higher value.
  3. At start, supply power is connected to stator through a three pole contactor and, at a same time, an external rotor resistance is added.
  4. The high resistance limits staring current and allows the motor to start safely against high load.
  5. Resistors are normally of the wire-wound type, connected through brushes and slip rings to each rotor phase. They are tapped with points brought out to fixed contactors.
  6. As the motor starts, the external rotor resistance is gradually cut out of circuit ; the handle or starter is turned and moves the three contacts simultaneously from one fixed contact to the next.
  7. The three moving contacts are interconnected to form a start point for the resistors.
  8. To ensure that the motor cannot be started until all rotor resistance is in circuit, an interlock is fitted which prevents the contactors from being closed until this condition is fulfilled.