# Wiring two motors to a VFD?



## HMF (Jul 21, 2011)

Some machines have two separate motors.
Can two separate motors on a machine be wired to a VFD? If so, how?

*The following question came from another forum:*
(http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/vfd-2-motor-machine-229284/)

"I am taking delivery of an oscillating spindle sander and will need to provide 3-phase power from my 1-phase supply. This machine has separate motors for the spindle and oscillation, with the oscillator motor switchable on/off independently even while the spindle is running. I understand that VFDs are very touchy about having any switches or anything between them and the motor. I'd also rather not rewire the machine if possible - I was thinking I could just short the original starter control into a permanent "on" position and use the VFD as an on/off/reverse control.
Will the dual motors (with one switching on and off during operation) be no good with a VFD? If so, I guess I'm stuck with buying an RPC."

there is an explanation there, but I don't exactly follow it. Can someone explain it more simply?

Thanks,


Nelson


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## Starlight Tools (Jul 21, 2011)

The short answer is "NOT WISE"

I have heard of people running two motors off of VFD's, but they have to be running in parallel so that they both start and stop at the same time. Bear in mind that if there is a larger and a smaller motor, the smaller one will not have any protection.

Safest way to go about this is to either use two separate VFD's each rated for the appropriate motor, plus a line choke on each motor.

Even better alternative would be to invest in a RPC, Rotary Phase Converter. This way you can run multiple 3 phase motors, independant of each other, starting and stopping as needed. You can also hook a VFD up to any motor you desire and run it from the RPC.  this was one of the best investments I ever made!

Yes, RPC's can be made with drone motor and a bunch of capacitors, but be very careful going this route as your fire insurance on your house and shopp just became "Nul and Void"

Walter


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## HMF (Jul 22, 2011)

So basically Walter, this is a BAD idea, and should not be done. And the response on the other forum showing two options is not a great idea.

Interestingly, the OP called Teco Westinghouse, who said pretty clearly that this is an application for an RPC. In other words, Walter was right on the money. The only other option would be 2 separate VFDs, one for each motor, which would mean rewiring the machine pretty much completely, and would require either a 2-stage startup operation, or else some kind of logic-based way to start one VFD when the other one is fired up.

Now, this does not mean that, on a machine where two motors are not running all the time you could not use two separate VFDs. For example, on my VN #12, there is a spindle motor which is large (maybe 2-3 HP I forget) and a tiny table feed motor which is 1/4 HP. There two VFD's are possible, and we have a setup for that on the site. (I intended to follow that one in wiring the #12).

Nelson


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