Operation of Rotors Resistance Starter

 A rotor resistance starter uses a ladder diagram to control the gradual removal of external resistance from a slip-ring motor's rotor circuit, starting with high resistance for low starting current and high starting torque, and ending with the rotor short-circuited for normal running. The ladder diagram consists of rungs that energize contactors sequentially via timers to gradually cut out resistors. 

Ladder diagram ladder rungs

A basic ladder diagram for a rotor resistance starter would include the following components: 

Power supply: Three-phase AC power is connected to the starter.

Motor Contactor (MC): The main contactor that connects the stator to the power supply. This is the first contactor to close.

Timers: These are used to control the time delay for each stage of resistance removal.

Resistor bank: Multiple steps of resistors are connected in series with the rotor circuit at the start.

Shunt Contactors (S1, S2, etc.): Each shunt contactor is connected in parallel with a step of the resistor bank. When a shunt contactor closes, it shorts out the associated resistors. 

Operation

Start up: When the start button is pressed, the Motor Contactor (MC) energizes, connecting the stator to the main power supply. The timer starts. In this initial state, the entire resistor bank is in the rotor circuit, which limits starting current and increases starting torque.

Resistance removal (stage 1): After the first timer (e.g., Timer 1) times out, it energizes the first Shunt Contactor (S1). The S1 contactor closes, shorting out the first set of resistors in the rotor circuit. This reduces the overall rotor resistance.

Resistance removal (subsequent stages): A second timer (e.g., Timer 2) is energized through an auxiliary contact on S1. When Timer 2 times out, it energizes the second Shunt Contactor (S2), which shorts out the next set of resistors. This process repeats for all subsequent stages.

Normal running: As the motor accelerates and reaches its rated speed, the final shunt contactor closes, completely short-circuiting the rotor circuit. The brushes are often lifted from the slip rings to reduce friction, and the motor then runs like a squirrel cage motor. 

Why use a rotor resistance starter?

It is used for slip-ring induction motors, which have a wound rotor and can have external resistance added to their circuit.

It provides a high starting torque with a moderate starting current, which is beneficial for high-inertia loads.

The starting torque and current can be controlled by adjusting the value of the external resistance at the start.