Anatoliy Sadovskiy
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Why Motors (and Engineers) Appreciate Soft Starters

May 15, 2023
Using a soft-start method to get motors running can reduce the risk of damage and improve efficiency.

This article was updated May 15, 2023. It was originally published July 14, 2020.

It often takes a lot of energy to get a motor up to speed. Across-the-line starters (DOL), the most common starter circuitry, create the highest currents when starting motors, but that puts a lot of wear-and-tear on the motor as well as components and equipment down the line. And wye-delta starting (also called star-delta starting) lowers initial voltage by 33% but adds complexity and unnecessary terminations while taking up more space in cabinets.

But soft starters can reduce inrush currents and limit torque—protecting valuable equipment and extending the life of the motor by reducing motor heating caused by frequent starts and stops and sudden spikes in power and torque.

Soft starters or reduced voltage soft starters (RVSSs) can be added to typical 3-phase AC motors to reduce strain on the motors during typical power-up phases by limiting motor torque and the initial inrush of current. This is done by controlled solid-state switches for each phase that gradually ramp up the initial voltage to the motor and generate so called “soft starts.”

Inside a Soft Starter

Soft starters let motors smoothly accelerate to running speed and avoid any risk of overstressing the power circuitry. Soft starters also give more control to operators, reduce the risk of damages, and increase efficiency. Applications that have high inertial load and a large inrush of current could benefit greatly from a soft starter by eliminating failures and unnecessary delays. And some mechanical equipment requires gentle starts to avoid the torque spikes and tension associated with normal startups.

For applications that require speed and torque control only during motor startup, soft starters are often the economical choice. Additionally, they are often ideal for applications where space is a concern, as they usually take up less space than variable frequency drives.

Comparing Soft and DOL Starters

Engineers who decide to use a soft starter should size it based on the motor’s full-load-amps (FLA) current, not the power rating. The inrush current of a large motor can be as high as 10 times the full load amps, but usually soft starters are sized to accommodate three or four times this amount. If a motor really does pull 10 times the FLA, the soft starter will have to be oversized to accommodate it.

Compared to DOL or wye-delta starting starters, soft starters generate lower inrush current methods, which:

  • Reduce stresses on belts, chains, pumps and conveyors by ramping the voltage up or down during starting and stopping. Large inrush current, on the other hand, generates significant magnetic forces in the motor’s windings, and they can send mechanical shocks through the winding insulation that lead to early equipment failure. And the mechanical shock of the torques generated by large starting currents can damage the motor shaft, belting, gear box, and drive train.
  • Increase equipment uptime and provide motor diagnostics.
  • Eliminate water hammer by imposing an S-curve profile on the start and stop the pump, which also extends the life of the equipment. Water hammer can rattle plumbing and lead to catastrophic component failure.
  • Increase energy efficiency and cost savings. Soft starting can also lower peak demand, and therefore reduce electric bill on larger multi-motor applications.

A variety of applications can make good use of soft starting technology including conveyors, belt-driven devices such as fans, blowers, and pumps. Slow starts minimize torque spikes and tension and decrease motor heating due to frequent stars and stops. ALL of these extend the motor’s life.


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John Bordewick was the lead application engineer for ICD components and assemblies at Eaton Corp. when this article was originally published.

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