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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.

Motortronics Soft Starter

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.

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4 Responses

  1. We are working on a Chiller Plant for a government building in Frankfort KY.
    The plant was built in about 1994.
    There are two pumps with overheating starters.
    The pumps are 40HP, 480-3phase motors.
    They both have the same starter.
    Solid State Starter
    Catalog No. 3RW2-J045
    65 amps
    460 V
    3 ph.

    Could you venture an opinion on repairing this starter?
    Should we? What might it cost?
    Is it cheaper to just buy a new starter?

    Thank you
    Ed Clautice
    Connolly Engineers

    1. Ed
      Frankly, our opinion would be it is not worth the money to repair. Sofstarters and across the line starters (or even drives for that matter) of that size are very inexpensive. I’d be happy to quote you on a suitable alternative if you’d like. Email us at info@emainc.net
      Trey Mayfield

  2. Do you repair WEG soft starters? We have a 250 hp 460v SSW07 that shows “phase loss”. We have a new one is its place. Just wondering about having this one fixed for a backup. Thank you.

    Rodney Pederson

    1. Yes we do repair softstarters. At that size, it may be more economical to just replace it. It depends on what is wrong.

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