When working on projects that require the use of VFDs, I was always under the impression that the input voltage needed to match the output voltage. Is this necessarily true? I recently worked on a project in which we had specified a VFD capable of accepting a 240-volt, single-phase input and producing a three-phase output. Could such a VFD control a 208-volt, three-phase motor? In other words, can VFDs control motors rated for voltages lower than the input voltage of the VFD?

The input does not need to match the output on a VFD, but the output rating needs to match the motor.

Good on ya,

Goober Dave

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Yes Lower.

Keith Cress
kcress - http://www.flaminsystems.com

Usually no problem to feed a lower voltage motor. But, when going to extremes, like feeding a 230 V motor from an inverter connected to a 690 V grid (European) it should be remembered that the PWM pulses now have close to 1 kV amplitude and even though the RMS voltage can be set to the motor's rated voltage, it is not at all guaranteed that the insulation of the 230 V motor can withstand the high PWM pulse voltage.

Usually, there is a limit as to how much one can "detune" the voltage. But that limit can usually be overridden and sometimes doesn't even exist.

Be careful. Check what the insulation is designed for before proceeding.

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

What about a VFD serving a motor rated for a higher voltage? I am assuming that this is not possible as the VFD does not contain any means (A transformer) of increasing the incoming supply voltage. Is this correct?

Yes, that is correct.

There is a slight exception, only in that some SMALL drives use what is called a "voltage doubler" on the front end to boost the voltage AS IT ENTERS the VFD converter stage. It is a rectifier that then uses capacitors to double the DC side (do a search on "voltage doubler circuit" to see a schematic). The now doubled DC is then fed straight through the rectifier in the VFD direct to the DC bus at a voltage high enough to allow the drive to deliver a PWM AC output at twice the AC input voltage. This is why you see SMALL drives offered here in the US with 115V input and 230V output, where 115V single phase is ubiquitous. Technically, this can be done at any size, and there are at least two mfrs that I have seen which offer this method on larger drives, ie that accept 230V in to run 460V motors. But the caps become ridiculous and a transformer is cheaper, so I don't know why anyone would bother.

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