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# VFD size - motor size4

## VFD size - motor size

(OP)
Dear all,

as a rule of thumb we have always kept the VFD size (in kW) two times the motor size.
Now we have a project where a 0.37 kW three-phase motor is to be driven by a 1.5 kW VFD - this is because of delivery times,
we can't have a 0.55 kW in time for the job. Any suggestions? Do we have to put an inductor on the load side?

Thank you

### RE: VFD size - motor size

I don't see there will be a problem except higher losses in the VFD.

### RE: VFD size - motor size

2
You will have a problem IF you want to use the VFD in any mode except V/Hz, the simplest.

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

### RE: VFD size - motor size

(OP)
Thank you, actually we are using simple V/f on this application.

### RE: VFD size - motor size

#### Quote:

You will have a problem IF you want to use the VFD in any mode except V/Hz, the simplest.
Can you provide a deeper explanation?

### RE: VFD size - motor size

Sure. For a drive to provide any other type of control like sensorless vector, sensored vector, voltage vector, etc., the drive needs very accurate measurements to carry out the Clarke/Park transforms of Field Oriented Control. They make those measurements on the unpowered phases which are still a noisy, difficult proposition. If you use a larger drive than expected the current sensing will be too coarse since the transducers have to be sized for the maximum load of the maximum allowed FLA of the choosen drive model. For a 5hp motor the minimum accuracy might be 0.1A and that's what the sensing is capable of whereas a 3hp motor might require 0.05A accuracy. Some too large drives you can setup up for a smaller motor then discover that at low speeds or loads it's unstable. Other drives just forbid you from selecting those modes once they see that the FLA you entered is below some minimum threshold.

I never oversize a drive unless I'm trying to play single phase source games. Most drives these days seem to have plenty of headroom when sized correctly. Often the drive's FLA rating is way above the drives rated hp anyway so there's no need to go up in drive sizes.

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

### RE: VFD size - motor size

Perfect explanation Keith, I may steal that because I get this question a lot.

MOST drive mfrs will tell you not to use a drive at more than a 2:1 ration to the motor FLA, ie the drive is 2x the motor nameplate FLA. But as Keith pointed out, this is due to the vector control requirements. If you are using it strictly in V/Hz (scalar) mode, it’s relatively unimportant, especially on low cost small drives. In V/Hz mode the only thing the current sensors have to do is protect the motor from overload.

There are a few “next-gen” drives that are now using a form of Rogowski Coil for current measurement that can maintain accuracy down to a 10:1 size ratio, but they are not the low cost versions.

But I am curious why you ROUTINELY double the size of the drive? I’m sure the drive suppliers love you but it seems like a waste of money.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

### RE: VFD size - motor size

(OP)
Sorry gentlemen I was in a hurry and forgot to type that we double VDF size when the correct size is not available,
normally we use same size as the motor. Or we pickup the next size available. As you pointed out there is current enough.

### RE: VFD size - motor size

Ok then, you should be good to go. For most drives on the market, the 2:1 rule is a good one to try to maintain. Even the ones that I know are 10:1 will state 2:1 in their manual, you have to know someone to get the real number…

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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