When would you use a soft starter over a VFD?

• A. For gentle starts when speed control is not required; use a VFD when speed control and torque regulation are required.
• B. For precise speed control across a wide range, use soft starter; for simple on/off operation use soft starter.
• C. To save energy during running; use soft starter for torque control.
• D. Only with DC motors; VFD only with AC motors.

Answer: (C)

Soft starters and VFDs serve different purposes. A soft starter lowers the voltage (and current) during the motor’s startup to reduce inrush, mechanical stress, and voltage sag, but once the motor reaches speed it essentially delivers full voltage and provides no ongoing speed control. A VFD, on the other hand, changes both voltage and frequency to control motor speed and torque across a wide range and can ramp performance as needed.

This makes soft starters ideal for simple on/off applications or situations where you want a gentler start but don’t need to vary speed or regulate torque. If you actually need to adjust speed or torque during operation, a VFD is the better choice because it can precisely control speed and provides true torque regulation.

The idea that a soft starter saves energy during running isn’t accurate—energy savings come mainly from controlling speed with a VFD, not from running a motor softened at startup. Likewise, soft starters don’t offer continuous torque control; they only affect the startup ramp, whereas a VFD provides ongoing speed and torque management. And soft starters and VFDs are used with AC motors; DC motors require a DC drive, so the notion that soft starters are for DC only and VFDs for AC only isn’t correct.