How does a soft starter work?

Written by tyler lacoma
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How does a soft starter work?
A motor can be protected by a soft starter. (motor image by Pablo Armelles from

A soft starter is an electronic device used to help protect motors. Not all motors are initially equipped with soft starters, but they have become common, especially with delicate motors that can be easily damaged by sudden influxes of power. Soft starters are composed of a series of rectifiers that work together to manage voltage.


There are different types of motors, but all of them work by generating some type of electromagnetic force that turns the motor and operates the devices to which they connected. All motors, induction or not, need a starting flow of current to power up (induction motors simply need a larger burst of current to start operating correctly). Electrical connections are designed to provide this current, and motors are designed to take the common voltage levels from household electricity.

Motor Damage

The bursts of power that start a motor can be damaging. The motor immediately turns on, which is efficient and saves time, but the high rush of current can damage the electronic applications in the motor, and the sudden start of the motor's mechanical devices can create wear and tear. Variable-speed drivers are used to start and stop the drive train in the beginning of motor operation to save this mechanical wear, but soft starters are an option for motors that do not have the drivers.


Most soft starters are made from solid-state switches known as silicon-controlled rectifiers, or SCRs. They have a simple job: regulate voltage so the motor first is given a low-voltage surge, which ramps up as the motor starts to turn, saving on wear and tear and often helping electronic components to last longer. The switches turn on in rapid succession to let more and more voltage through.

Phase Designs

Soft starters vary for different phases. A single-phase unit controls starting torque but does not really affect the current much, which makes it difficult to help motors with frequent cycling. Two-phase starters offer more protection but typically are used with a thermal switch to help protect the motor. Three-phase starters control all three phases of the common motor and offer much more complete protection.

Loop Options

Soft starters can be open-loop or closed-loop. Open-loop units have no feedback, so they control voltage only according to a preset voltage setting. If the motor does not match that setting, the soft starter might not operate correctly. Closed-loop versions offer a feedback loop that allows the starter to determine how it needs to protect the motor.

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