What is the function of a soft starter in motor control?

• A. To start the motor at full voltage immediately.
• B. To ramp voltage to the motor gradually, reducing inrush current and mechanical stress.
• C. To increase torque to maximum instantly.
• D. To maintain motor speed constant regardless of load.

Answer: (B)

The main idea is that a soft starter controls the initial surge of current when a motor starts. When a motor is stopped, it can draw a large inrush current that can trip breakers, cause voltage dips, and put stress on windings and mechanical components. A soft starter handles this by gradually increasing the voltage applied to the motor during startup. This voltage ramp means the motor accelerates smoothly, with much less electrical and mechanical stress, protecting both the motor and the power supply. Once the motor is running up to speed, full voltage is applied and normal operation continues.

This approach is different from speed control devices. It’s not meant to keep speed constant under varying load or to instantly boost torque to maximum. For ongoing speed control, a different device like a variable frequency drive would be used. Soft starters are specifically about easing the startup process by limiting inrush and mechanical shock through a controlled voltage ramp.