Using VFD with RPC

It depends on what the manufacturer did when designing it. And how conservative the ratings are. The part which probably makes more difference than anything else is how much capacitance is there to hold the DC voltage generated from the power line before it is used to generate the three phase.

With true three phase input, you can get away with aobut 1/3 the capacitor size needed for single phase to control the ripple.

Some makers also tend to way overdesign, so it will run at full specified load when the input power is a bit on the low side and when the shop is too hot for any office person to stay there long. :-)

Personally, I like to buy about 50% more horsepowewr rating than I really expect to use when I plan to us a VFD for generating three phase from single phase.

Enjoy, DoN.

That would be a huge waste when you could just buy a larger VFD or one that is rated for 100% of single phase input for a few dollars more.

I don't understand the problem. Buy a single phase in, 3 phase out VFD. Same price. Steve

The problem is price. I already have the 7.5HP RPC. The next problem is: if running a VFD from 230v single phase you have to derate the VFD usually by 50% is my understanding. I can pick up a 3ph in/ 3ph out for $179. A single phase in to 3 phase out is twice that much. If I can use the RPC to generate the 3 phase and use the $179 VFD then I can save $$$.

My question is will I get 2HP out of the VFD if I run it from the RPC outputting 3 phase???

how many HP is the largest VFD single phase input unit?? the largest i can find is a 3HP unit, he is running a 7.5 HP unit.

So if he wants a VFD and he has the RPC already inducing the third leg, wouldn't it be easier to buy a 3 phase in/3 phase out 7.5 HP unit?

-S

There is a new class of VFD inverters. They are expensive in the states and in Europe, but I have sourced some made in China that are very well priced. This new class converts the input power directly to DC. Internally, they use a microprocessor to construct a variable frequency sinosoidal AC signal which is then amplified to drive a 3 phase inductive load. These generate very little heat and virtually no RF noise. They are very efficient and small in size. I am going to the Canton Industrial fair in April and try to put a deal together. I will gladly let you know how successful I was when I return. Standby. Steve

I've seen VFDs at least as large as 30HP that would run on single phase. Most manufacturers don't specifically mention running on single phase in their literature, but will make a recommendation if you ask. Yaskawa is one mfr that does give derating numbers in their manuals.

While oversizing is the conservative way to go, for intermittent use in a home shop I expect most drives would be just fine at full rated output with single phase input.

Ned Simmons

VFD's have always done what you describe below. They may be new to your Chinese sources, but this kind of VFD has been around for quite a few years. The "new class" aspect may refer to price.

Not complaining, just making sure you aren't getting the wool pulled over your eyes.

Steve

Steve Lusardi wrote:

Why bother with the RPC? Buy a VFD single phase in, 3 phase out. I wouldn't go through all that monkey motion .... also you wouldn't be paying for electric to run the RPC.

Carl

Yeah, but . . . . he'd already have a RPC ready to run more than 1 machine; whereas if he chucks his RPC and goes with a VFD, he'll have to buy a seperate one for every new machine he might add to his shop. IMO, the best route, since he already has the RPC, is to go ahead and balance it out, optimized for the lathe. RPC's, when idling, aren't very bad consumers of "electric". For example, the single-phase load of my 7-1/2 HP RPC, when idling, is 3.5 amps at 240 volts. That is 840 watts. At 10 cents per KWH, the standby (idling) cost of the machine would be 8.4 cents per hour. In actuality, it would be even less, because unloaded induction motors have a relatively poor power factor.