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# Using a VFD to balance output of RPC??

#### specfab

##### Titanium
I have been having some difficulty with a Tree CNC machine, and in the process of sorting that out, a question crossed my mind:

Can a VFD be used as a power smoothing/balancing device for the output of a whole-shop RPC?

I have a 10HP phase converter operating in my one-man shop. It is supplied with 240V single phase, and its output results in 241/241+/267V measured between the legs. In wrangling the issues of the Tree machine, which I think has a bad input circuit breaker, I wondered if using a VFD downstream of the RPC would allow for "tuning" of each leg of the 3-phase output from the RPC, to get closer to factory spec 230V +/- 5% on each leg. This is sort of a thought question from a theoretical standpoint. Obviously, if I were to be considering a VFD that could handle the shop needs, I wouldn't need the RPC in any case. I mostly want to know if the VFDs these days have this sort of voltage adjustment between legs, for a 3-phase to 3-phase unit.

Since it has to be big enough to handle the machine your are hooking it too, I fail to see the advantage of running it through the RPC first

The VFD takes all of the inputs and converts them to a common DC. Then it generates the outputs from the DC, so it would produce more equal outputs, but not by adjusting individual phases. The advantage of 3phase input for the VFD instead of 1 phase input is that there are six peaks per 60Hz cycle instead of two, so it's simpler to make a smooth DC voltage from them.

Nothing to gain...Phil

The VFD takes all of the inputs and converts them to a common DC. Then it generates the outputs from the DC, so it would produce more equal outputs, but not by adjusting individual phases. The advantage of 3phase input for the VFD instead of 1 phase input is that there are six peaks per 60Hz cycle instead of two, so it's simpler to make a smooth DC voltage from them.

That would be true except that if there is a weak generated leg, which will be lower in voltage than the others at any given current, those six peaks drop back to two, with weaker peaks in between. So the 3 phase from the RPC is not giving a consistent DC when rectified.

That means the DC will have a "ripple" to it that will "telegraph through" to the load. Not the same as an unbalance, but may have just as bad an effect in a slightly different way.

Bottom line is that there is no advantage with most VFDs. If you want smooth output, get a Phase Perfect. Produces 3 phase that is no worse than the powerco may allow, and probably better, directly from single phase.

You would do better to go over to a transformer balanced system on the RPC, and not try to fiddle-fart around with capacitors.

There are static var generators/compensators that can do funky things with active and reactive power to shore up weak phases and adjust voltage so that your loads see a near-perfect voltage and the grid sees a near-perfect current.

But the kVA required to correct an RPC is large enough that you're essentially just using a phase converter.

I have been having some difficulty with a Tree CNC machine, and in the process of sorting that out, a question crossed my mind:

Can a VFD be used as a power smoothing/balancing device for the output of a whole-shop RPC?

The easiest, fastest, and simplest way to accomplish this, is to just add an additional idler or two. Get a couple big 3-phase motors and connect them to the RPC after it's started. Let them spin.

If you have some other 3ph machines on your system, turn them on... let them run without a load... it accomplishes the same thing. ;-)

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