Electric Motor Softstarters: Repair and Operation

We grew up primarily as an electronic motor drive repair company but began to repair soft starters almost from our inception. Modern softstarters are reliable, effective, and provide great benefits.

First: If you need the motor to vary in speed, you don’t need a softstarter, you need a VFD. But, if you’d like to reduce the inrush current, and do not need to vary the motor speed, then a softstarter is a much less expensive solution.

There are however, some gotchas. First, softstarters work by reducing the voltage to the motor, thereby limiting the inrush current; but along with this comes reduced torque. This isn’t an issue as long as you’re running a load that requires little or no starting torque, but otherwise, you might have a problem.

We have seen them used on constant torque loads, such as positive displacement pumps, but in every case, they started the pump unloaded. If you need to softstart a load that requires high starting torque, you need a VFD not a softstarter. VFDs are inherently soft starting. But, since they reduce both the voltage and the frequency they allow the motor to develop its rated torque at low speeds.

EMA is a service center for WEG and Motortronics softstarters. We routinely repair softstarters of other manufacturers, although for now, Motortronics seems to be making them for just about everyone.

The diagram on the right shows a typical Motortronics softstarter. Note that the power devices are Thyristors (SCR) and that a bypass contactor is in parallel with the Thyristors. Once the motor is up to speed, the contactor closes.

This allows for a small physical size, because the power devices do not have to be cooled for constant duty. In medium voltage softstarters, the contactor will usually be a vacuum contactor. The Thyristors have a long service life because they are only used during starts, being bypassed once the contactor closes.

EMA repairs softstarters of most brands, and sells both the WEG and Motortronics low and medium voltage softstarter. (see products page) Softstarters are generally easier to troubleshoot than VFDs, and repair costs are consequently lower.

As you can see in the diagram, the six Thyristors are easily checked for shorts from the input and output terminals. Modern softstarters do require programming, so some familiarization with motor control protocols is helpful.

You can contact your nearest EMA facility (phone numbers at the bottom of this page) or by “Click Here to reach us” on the top right. You can also click the “LIVE SUPPORT” button to talk with a technician.

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