How to understand VFD De-rating? OR...Can I use single ph input on a 3ph input safely

I have a nice Baldor 5hp 3ph 230V motor That I really don't want to use a static or Rotary convertor on. Trouble is I cannot seem to find a single phase input to 3ph output VFD that is available for a 5hp motor...(Without it costing a kidney).
I have found lots of good used units for pretty decent prices on Fleabay..but they are ALL 3ph input! Question is can I still input single ph into these? Some have responded to me when asked that you can , BUT it derates the motor? Really? Truth or BS?
Here is one of the listings I was entertaining to buy for my application:
5 HP 3PH 23V VFD Teco Westinghouse Variable Frequency Drive N3 25 C | eBay

Running a VFD on single phase pulls all the current through the 2 rectifier diodes you are using and adds extra voltage ripple on the DC bus capacitors. The extra current through the diodes is easily dealt with by derating the VFD. The extra ripple on the capacitors reduce their life somewhat. Some people say to increase drive size by 50%. I just double the VFD size. Turn off phase loss protection. You should have full use of the motor.

caddguy said:

I have a nice Baldor 5hp 3ph 230V motor That I really don't want to use a static or Rotary convertor on. Trouble is I cannot seem to find a single phase input to 3ph output VFD that is available for a 5hp motor...(Without it costing a kidney).
I have found lots of good used units for pretty decent prices on Fleabay..but they are ALL 3ph input! Question is can I still input single ph into these? Some have responded to me when asked that you can , BUT it derates the motor? Really? Truth or BS?
Here is one of the listings I was entertaining to buy for my application:
5 HP 3PH 23V VFD Teco Westinghouse Variable Frequency Drive N3 25 C | eBay

Click to expand...

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*NOT AN ELECTRICIAN - SO TAKE THIS ADVICE WITH CAUTION AND CONSULT AN ELECTRICIAN - SOME EXPERIENCE WITH VFD AND BRIDGEPORT*
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Lots of advice on this forum about this. For the most part that is true, but you would need a 10HP VFD to run a 5HP motor... See this post http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/single-phase-power-into-vfd-rated-3-phase-153478.. consult an electrician.. Also, unless listed/advised in the Manufacturers manual, I would not use it in that fashion... And it's probably going to cost a arm and a leg as you pointed out.

VFD's when converting from 1phase to 3 are very inexpensive for say 2HP and below, but almost exponentially increase in cost after that.

My recommendation would be a 10HP rotary phase and wire up a panel as it will be easy to add additional machines in the future and I believe a 10HP rotary is right around $800 shipped on ebay.

I am going the rotary route...wiring and stuff doesn't bother me...but big $$$ does!

Other option, is to buy a used 7.5 or 10hp 220 v VFD, that will accept single phase input (some models won't, some can be programmed to, and some can be fooled into single phase input)

So you get ~ 5hp after VFD is derated...

A N3 205 CS would have been single phase capable 5 hp VFD..

Since new VFD's are available in single phase input models up to 5 hp.....

Just buy a single phase input VFD... ODE-2-325-1H42

A 5hp single phase drive is going to be around 500... A DRO is twice that.. VFD should pay for itself quickly.. RPC is heavy, $$$ to ship..

Unless you have an electrical junkyard nearby, 10 hp+ idler motor, Switches, enclosures, heaters/contactors, cost money too, quickly getting that cheap RPC, into VFD price levels.. And not giving you the over/underspeed, full motor protection, and some braking you get with a VFD.. Spend less, get less, take up more room...

On the other hand, RPCs are usually quite bulletproof as far as tolerating abuse. And if the idler motor craps out, or a start capacitor does, the parts are readily available, although they may cost more the faster you want them. Plus, an RPC can power a whole shop with one cost. But you have to either pay a bunch, or make the controls yourself.

That has to be considered when thinking about a VFD vs an RPC.

VFDs are reliable, but not as tough as motors. So if you get a great deal on a VFD, and later you need to replace it, you may have to pay a lot.

Repairs to VFDs are not even to be thought about unless there is NO other option due to some strange requirement. You replace them if they have problems.

VFDs , to get the most advantage, need to be one per motor. That adds up fast. If you have machines with multiple 3 phase motors, you obviously have some decisions to make. Spindle motor on the VFD, feeds and coolant may need their own converters, either a small RPC, or a static converter for each, etc.

With a VFD, your construction cost and effort may be limited to mounting it in a box. If you get an IP55 unit, you might not have to do even that.


Motors are usually available for some premium over scrap, used, and a used motor will work fine. Used VFDs are a crap shoot. often they have been removed for cause, and then that is forgotten or unknown to the eventual seller, who has a pallet of them.

Bad motors often smell bad, and your nose can pre-select them. Most motors that do not smell burnt, and that spin without noises, are good. Bad VFDs CAN smell, but may have crazy deal-killing faults that you cannot find unless you hook them up.

So motors for an RPC can be had in scrapyards, VFDs are best bought new. There is a cost difference there, the significance of which is basically a business decision.

caddguy said:

I have a nice Baldor 5hp 3ph 230V motor That I really don't want to use a static or Rotary convertor on. Trouble is I cannot seem to find a single phase input to 3ph output VFD that is available for a 5hp motor...(Without it costing a kidney).
I have found lots of good used units for pretty decent prices on Fleabay..but they are ALL 3ph input! Question is can I still input single ph into these? Some have responded to me when asked that you can , BUT it derates the motor? Really? Truth or BS?
Here is one of the listings I was entertaining to buy for my application:
5 HP 3PH 23V VFD Teco Westinghouse Variable Frequency Drive N3 25 C | eBay

Click to expand...

It is truth, mostly. You would have get a 7.5 or 10 Hp VFD. Look at the specific manual pages to see what the de-rating is.
Electronic phase converters are another choice. Get ready for a quadruple bypass operation.

The actual derating will depend on the unit, and how it is designed.

3 phase, when rectified, has very little ripple voltage inherently, even if hardly filtered at all. One of the VFDs I did has a 50 uF high current film capacitor as a filter, because it was supposed to get 3 phase input, and had to work over a wide temperature range, with a long life expectancy. Later some customers wanted single phase, and it could not do that at any credible power, so we actually made a single phase adapter that was really just a bank of electrolytic capacitors.