What is Soft Starter? & Working Principle

What is Soft Starter?

The slow starter, also known as a soft starter, is an electrical device used to control the acceleration of electric motors. It works by gradually increasing the voltage supplied to the motor during startup, which reduces the mechanical stress and current surge that occurs when a motor starts at full voltage.

Related: Current Transformers and Voltage Transformers

The working principle of a slow starter typically involves the following components and steps:

1. Voltage Control: The slow starter is connected in series between the power supply and the motor. It consists of power semiconductor devices (such as thyristors or silicon-controlled rectifiers) that control the voltage supplied to the motor.
2. Ramp-Up Voltage: During startup, the slow starter gradually increases the voltage applied to the motor. This is achieved by controlling the conduction angle of the power semiconductor devices. Initially, the conduction angle is minimal, allowing only a small portion of the AC voltage to pass through to the motor.
3. Current Limitation: By limiting the voltage at startup, the slow starter restricts the current drawn by the motor. This prevents the motor from experiencing excessive inrush current, which can cause mechanical stress, voltage drops, and tripping of protective devices.
4. Smooth Acceleration: As time progresses, the conduction angle of the power semiconductor devices is increased, allowing a higher portion of the AC voltage to pass through to the motor. This gradual increase in voltage leads to a controlled and smooth acceleration of the motor, minimizing sudden torque changes and mechanical shock.
5. Full Voltage Operation: Once the motor reaches its desired operating speed, the slow starter typically bypasses itself or transitions to a fully conducting state. This allows the motor to operate at the full voltage supplied by the power source without any intervention from the slow starter.

The slow starter provides benefits such as reduced mechanical stress on the motor, decreased power demand during startup, and improved control over the acceleration process. It finds applications in various industries where controlled motor acceleration is required, such as pumps, compressors, conveyors, and fans.

Related: SWITCH GEAR AND PROTECTIVE DEVICES