Definition
A soft starter reduces the voltage applied to a motor during start-up and increases it gradually over a set ramp time, limiting starting current and torque shock before switching to full line voltage for normal running. Unlike a VFD, it operates at fixed line frequency throughout and does not continuously vary running speed.
Key facts
- Reduces starting current and mechanical shock by ramping voltage, not frequency, at motor start-up.
- Runs the motor at fixed line frequency once ramped to full voltage: it does not provide continuous speed control like a VFD.
- Typically bypassed with contactors after start-up to reduce heat losses during normal running.
- Suited to loads needing gentler starts (pumps, fans, conveyors) without a need for variable running speed.
How does Soft starter work or appear?
A soft starter uses thyristors (SCRs) to control the RMS voltage delivered to the motor, ramping it from a reduced starting level up to full voltage over a configured time or current profile. Because frequency is not altered, motor synchronous speed is not changed, only the voltage magnitude during the ramp, which limits inrush current and reduces mechanical stress on couplings, belts, and driven equipment at start-up.
What should technicians inspect for Soft starter?
A VFD continuously varies both frequency and voltage to control running speed across the whole operating range; a soft starter only manages the transition from standstill to full speed at line frequency and then typically bypasses to direct-online operation. A soft starter therefore addresses starting behavior, while a VFD addresses ongoing speed and torque control.
FAQ
Frequently asked questions
How does a soft starter affect a motor's start-up?
It ramps the applied voltage from a reduced level up to full line voltage over a set time, limiting inrush current and mechanical shock, then typically bypasses to direct line operation for normal running at fixed frequency.
Last reviewed: