Single Phase input to VFD

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cpickett

Senior Member
Location
Western Maryland
Occupation
Electrical Engineer
Customer has 2 gas compressors at a remote station that only has a 120/240V single phase service. One motor is 10HP and the other is 15HP. The customer prefers Powerflex 525 drives, but in order to run the drive on a single phase input, we have to derate the drive by 65%, so the 10HP motor would require a 15-20HP drive, and the 15HP motor would require a 25HP drive. Is that correct?

If so, I would most likely recommend that the customer step up to a Powerflex 755 drive, which is available in both 20HP and 25HP ratings, for 240VAC input.

I suppose the alternative would be using a rotary phase converter in order to stick with the cheaper drive (PF525), but I really don't think there'd be any cost savings, plus now there's another failure point.
 
kwired

kwired

Electron manager
Location
NE Nebraska
I'd only consider the phase converter if you don't need to vary speed of the motor, but then you don't need the drive at all.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You are opposite on the 65% de-rate issue. The PF525 VFD size is not 65% OF the motor, it is de-rated BY 65%, i.e. 35% of the motor size.

This issue has to do with the fact that in most "component class" (meaning throw-away made in Asia) drives, like the PF525, there is no DC bus choke. So the capacitors must work harder to smooth the DC bus ripple from a single phase input, and the caps will get hotter in the process. The added de-rate is just to get even more oversized caps so that they last longer. As an alternative, you can ALSO de-rate the operating temperature. The PF525 is rated for 50C (122F) operation as standard. If you use it with an A/C on a box or some other cooling means to keep it at 25C (77F), you can go ahead and use a 50% de-rate.

The reason why the PF750 series drives only need a 50% de-rate is because they DO have a DC bus choke. That is ALSO true of the "Frame E" PF525 drives, the 48 and 62A 240V versions, so those can actually be used at a 50% de-rate. They just don't say it because it's confusing to most people. So I would not hesitate to use a 48A VFD on the 10HP drive, but there isn't a PF525 big enough for the 15HP 230V motor no matter what de-rate you use; you need at least an 84A rating. So since you will have to use the PF753 for that one, I would just use them for both of them.
 
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cpickett

Senior Member
Location
Western Maryland
Occupation
Electrical Engineer
Jraef,
Thank you for the correction, after I found the spec for derate on the 750 series drives, I was doubting what I said initially. As always, thank you for the detailed explanation which helps me make sense of the reason behind the ratings. It's nice to be confident in my recommendations when I propose them to the customer!
 
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