EMA Inc.

When should you use a VFD as a Softstarter?

A VFD can be used as a softstarter. Medium and low voltage softstarters are less expensive than medium or low voltage variable frequency drives (VFD) . If a softstarter will do the job, then that’s often all you need. However, at EMA, we’ve seen situations where a softstarter was purchased and installed, only to discover that it would not start the load. Here’s why.ema softstarterThe image on the right is a medium voltage softstarter manufactured by Motortronics. Motortronics makes medium voltage softstarters for a number of companies, including Toshiba, ABB, WEG, and others. They’re relatively low cost, very reliable, and easy to install, start, and maintain.

A medium voltage softstarter limits the initial current during start, by using Thyristors to reduce the inital voltage. Most medium voltage softstarters then close a vacuum contactor once the motor is up to speed, placing the motor across the 4160 volt input. Because of this, they are normally limited to a set number of starts per hour, because the Thyristors and their heat sinks are not designed for continuous duty. This works wonderfully, IF your load has a low starting torque. It doesn’t work at all if you don’t.

gas compressorThe figure on the left is a reciprocating gas compressor, like that found in refineries, chemical plants, polymer plants, and in the oil and gas fields. These are often started into pressurized systems, and therefore have a very high starting torque requirement. Medium voltage softstarters will not work in this case, because the starting torque of the motor is reduced along with the current and voltage during start. IF you need this soft-started, then you need to use a medium voltage VFD as a softstarter.

This is NOT a sales pitch for a VFD over a softstarter. (heck, we sell softstarters, if you need one… please call us!) If all you need is a softstart for the motor, and your load has low starting torque, then by all means, a medium voltage softstarter is a less expensive and very reliable solution.

We have seen instances however, where people bought a softstarter for a compressor or positive displacement pump, only to discover to their dismay that it wouldn’t start. The first response was to blame the medium voltage softstarter, but that’s incorrect. The medium voltage softstarter, by its nature, reduces the available starting torque for the motor. If you don’t need the torque, such as on a fan, then it works fine. However, in the case of a reciprocating compressor starting into pressure, you do.. and it will not work at all, much less fine.

Particularly on compressors in remote locations within the oil and gas fields, the power line will not take an across the line start of a high horsepower medium voltage motor. Therefore, you need softstarting capabilities. In this instance, a medium voltage VFD will function wonderfully as a medium voltage softstarter.

Why? Medium Voltage VFDs operate by changing both the frequency and the voltage applied to the motor. By using a modern, properly tuned VFD, the motor can develop its full starting torque, at only rated current. In other words, you never see the inrush or locked rotor currents associated with an across the line start. But, you DO get the full starting torque.. something you will never get with a softstarter.

This means that a hard torque start, such as that of a reciprocating compressor, can be “soft started” with a VFD when it cannot with a softstarter. (by the way, the same thing is true if using reactor, or autotransformer starters.. the available torque is reduced during start.)

If you have several medium voltage motors on high torque loads needing a softstart, then one medium voltage VFD can be used to start the individual motors, and then use your existing contactors / switchgear to transfer them to the 4160 volt line. This is a very cost efficient way to reduce the line starting problems.

Here at EMA we both sell and service medium voltage VFD s and Softstarters. We’re great at application issues, and we carry good stocks for quick delivery. Contact us by calling 770-448-4644, or better yet, hit one of the contact methods on this site. (try the online chat.. we’ll be glad to get a technician to talk with you about your VFD or softstarter problems)

No-One, Anywhere, is Better at Drives than we are… (not bragging.. just a fact!)

One response to “When should you use a VFD as a Softstarter?”

  1. Mark Halvorsen says:

    EMA great job of educating the end user and the maintenance engineer. I have applied both inverters and solid state starters. Medium Voltage AC Drives were not in application when I left the Drives business at Siemens in 1999. Benshaw was the company I sold and applied Medium Voltage starters to the Steel Mills in Northwest Indiana. Eddie I was absolutely blown away when I heard your commercial on WSB. Who would have thought a niche business like Medium Voltage AC Drives would have been advertised on radio in Atlanta. You guys are impressive. Drives guy from 1984 until 1999. Mark Halvorsen

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