For many years, medium voltage VFDs were very expensive. The general rule of thumb was below 2500 HP, it was less expensive to use a low voltage VFD with an input and output transformer to match the medium voltage supply and motor. But no more. 
For a number of reasons, it’s become commonplace to find medium voltage VFDs on motors rated 300 horsepower and above. Per Toshiba, their most heavily used medium voltage VFD is their 1000 HP frame size, when can easily be adapted for lower horsepowers. (Most application engineers recommend MV above at 500 HP and above)
That’s not to imply that we don’t find medium voltage VFDs on large
horsepower applications. We were looking at a 25,000 HP medium voltage VFD last week used on a natural gas compressor station. Obviously, at that horsepower, there is no such thing as a low voltage VFD.
I think there are at least three factors that drive the application of medium voltage VFDs in lieu of using low voltage. (I’m sure there are many more)
1. Cable and motor size. Power = Volts times Amps.. consequently, the higher the voltage, the lower the amps are proportionally. If you’re installing a 1000 HP, 480 volt drive, your cabling is going to huge. Not to mention the size of your motor. At that horsepower, a medium voltage VFD is a no brainer.
2. Medium Voltage VFDs pricing is coming down. This is driven by both manufacturing efficiency, and competition. Considering the installation costs, medium voltage VFDs are normally less expensive in higher horsepower applications than attempting to use a low voltage VFD. Along with the declining price, has been a huge jump in reliability. Here at EMA, we normally sell the Toshiba T300MVI medium voltage VFD. This isn’t to imply that other brands are not reliable, but we have yet to have the first reliability issue with any that we’ve sold. Low voltage VFD reliability has been an established fact for many years now; medium voltage VFDs have caught up.
3. Harmonic mitigation. At one time, medium voltage drives were considered by some to be so electrically noisy, that it rendered them unusable in some applications. With technological advances, for many medium voltage VFDs, that is no longer an issue. (read HERE how Toshiba attacks harmonics in their T300MVI medium voltage VFD)
At EMA, we used to be among those that discouraged medium
voltage below 2500 HP. That was based both on costs and on our experiences with reliability issues. Needless to say, we no longer feel that way. As I said earlier, in our experience, the medium voltage VFDs are as reliable as their low voltage cousins.
Control Engineering magazine had a nice article on this subject… read it HERE.
EMA repairs almost all brands of medium voltage VFDs, and we are becoming a major supplier world wide. We carry great stocks on both new and refurbished medium voltage VFDs. See our product offering HERE. For equipment or repair on medium voltage VFDs hit any of the contact methods on the right of this page OR call 770-448-4644770-448-4644.
Eddie Mayfield, EMA
No One, Anywhere, is Better At Drives than We are