View Full Version : RPC question

05-17-2009, 06:13 PM
Presently building a 7.5 HP RPC. Right now I have one three phase drill press but will be accessing more 3 phase equipment. To wire several machines to the RPC do I need a sub panel that the individual machines are wired into. Or just how is that done.

Also the 3 phase drill press has a 1 HP motor, can I just wire the drill press directly to the RPC until I really need the sub panel or junction box. The 220 single phase input is on a 30 amp breaker. The drill press is currently on a small VFD, but with all the new equipment I plan to buy, I invested in the RPC.


05-17-2009, 06:18 PM
Sure you can wire the RPC directly to the DP so long as there is a switch or disconnect between the two.

Easiest method for adding machines later is to find a small 3~ breaker panel and wire the RPC to it.Then it's just a matter of adding a breaker when you need another circuit.

05-17-2009, 06:21 PM
Why the switch between the two (DP and RPC) if the converter is off the drill press is off right. Or I'm missing sometime here. Can't I just use the off/on on the drill press if the converter is running.

05-17-2009, 06:26 PM
The switch between the Drill Press and the RPC is required by Code. My understanding is that as long as you have some way of disconnecting the DP from the RPC, (like a plug and outlet), you are Okay. The RPC qualifies as a power source.

05-17-2009, 06:34 PM
Ok got it...thanks for the clean up. Did some surfing for a 3 Phase breaker panel. Pretty expensive stuff. Anyone have a source for one of these in the cheaper range. I don't need a big panel just one big enough for three machines to be attached....gear

05-17-2009, 11:32 PM
Ebay,prolly somebody on there has breakers too.

http://cgi.ebay.com/Square-D-QO312L125G-THREE-PHASE-LOAD-CENTER-PANEL-QO_W0QQitemZ220415559465QQcmdZViewItemQQptZBI_Elec trical_Equipment_Tools?hash=item3351ca8729&_trksid=p3286.m20.l1116#ebayphotohosting

J Tiers
05-18-2009, 12:39 AM
There is no real reason you HAVE to have a panel....... in the US at least.

So long as the output wires are properly protected, each machine has protection, and some acceptable disconnect means is provided for each, you could treat the RPC output as a single branch circuit.

IF the machine is rated above a certain current, which IIRC is 50A, then you need a dedicated branch circuit for each such machine. But below that number, you can have a multi-outlet circuit right off the RPC.

Why would a panel be required?

05-18-2009, 07:23 AM
Get frendly with your local junk yard! (I'm surprised that someone else has not already mentioned this!)

Since scrounging seems to be a necessary trait of good machinists in this area, often times the local junk yard is the best source for some things very useful!

I'm fortunate to be in a rural area and have a friendly junk/salvage yard operator. He allows me to check out his stuff, and since he sells stuff to steel mills for the going price per pound, I ususlly get the things I can use for that same going price.

One practice I use, and consider a cardinal rule! [B][U]Don't haggle the price!
Knowing what the item is worth, if you were to buy it from another source is important in knowing just how good a deal you are getting.

Another practice that I think is helpful, after I finish a project, I take pictures, and show the salvage dealer how I used the stuff he sold me!

A natural scrounger, Doc-Zeus

05-18-2009, 07:56 AM
Very good information here. Right now I just have the one 3 phase machine. Can I just put on a female 3 phase plug on RPC 3 phase out end and then plug in the different machines as I get them. I know this is not very progressive thinking but until I come up with something else is this idea still good. Or is there other issues on this....gear

05-18-2009, 07:59 AM
I have a "remote switch" for my RPC that is right over the outlet (RPC is in a space on the other side of the wall). If/when I need a second 3PH circuit I plan to add a second outlet. Along with the switches on the machine(s) themselves I don't see the need for anything fancier.

J Tiers
05-18-2009, 08:53 AM
Very good information here. Right now I just have the one 3 phase machine. Can I just put on a female 3 phase plug on RPC 3 phase out end and then plug in the different machines as I get them.

I see no reason why you could not do that.

The machine needs a local motor starter with overload protection, but that would be the case even with a panel.

To simplify everything, you can just leave the VFD on the DP... it functions as motor overload protection, and it offers some distinct operational advantages.

Rich Carlstedt
05-18-2009, 02:53 PM
All my rotary phase convertors (RPC) have plugs on them

I use a standard 240 V -30 amp plug for the inlet and a 240-20 Amp recepticle on the outlet of the RPC. this prevents plugging in a 3 phase motor into the single phase wall supply, as all outlets are 30 amp (twistlock).

It also allows flipping RPCs for tests
RPCs also are on small skids for easy moving with a hand truck


05-18-2009, 04:03 PM
A change in the program, I came across a guy who has a 30 circuit 3-phase panelboard at a just about give it way price. The circuit breakers will be $5.00 for each one I need. This project is really starting to come together.
The panelboard is really much bigger than I would ever use, but its avail.

J Tiers mention I would still need somekind of overload protection even with a panel. Don't the breakers work as the overload protection? I realize some of the questions are painfully stupid, but I'm new to the RPC world and I'm expanding my shop so I'll take all the advice I can get. gear

05-18-2009, 06:14 PM
Just about ready to pass on the panelboard it's 34 inches long, 21 wide and 5.3/4 deep. WAY to big for my needs...Back to the plug ins I guess...

05-18-2009, 07:57 PM
I took the output of my RPC to two banks of fuse holders. By output I mean after the balancing caps but inside the box that houses most everything.

Each bank of fuse holders houses three 10 amp fuses, one for each leg of the output.

I used 10-3 (with ground) solid copper from the output of each fuse bank to an outlet. One outlet is behind the mill the second behind the lathe. The plug/outlet acts as the disconnect.

Also, in my small shop when I am standing at the lathe I can reach the breaker panel; so I could shut off the entire building if necessary.

My RPC is "permanently" installed. The idler is on a shelf near the ceiling, out of the way. The panel box housing the caps is mounted near the shelf as is the motor controller.

Don't know how "right" that is but it seems to work.

Jim Stabe
05-19-2009, 10:46 AM
I have a 7.5 hp homebuilt RPC running a 3 hp mill and 3 hp lathe. The RPC output is a branch with three plug in outlets, no panel. I guess I need to buy another 3 ph machine...


05-19-2009, 06:06 PM
In answer to one of your recent questions....circuit breakers are not available (typically) in very small increments. They are used to protect the *wiring* from exceeding its rated values. Since there are very few wire sizes, with some pretty standard current carrying ratings, the breakers for standard panels are to match those circuit values.

Motor protection, on the other hand, is usually accomplished either with fuses (in a fuse block, often in a fused disconnect) for each unit. A given circuit can theoretically serve more than one motor...which is another reason the circuit breakers in a panel are not used to protect the motor. You would then need to size the circuit (and circuit breaker and wire) to serve two motors. By the time one drew enough current to trip the breaker feeding both motors, the motor would be toast....but I digress

The more common means of protecting motors today is with heater-based overload protection. Often this is an item that attaches directly to a magnetic motor starter (self-latching relay). The heaters heat some low-temp alloy that melts and allows a spring loaded mechanism to disconnect the load. The heaters are available in tiny increments so that you can properly protect any motor. The heaters are specific to a given vendor of the motor protector...and often to a given series of that protector. Charts show you which heaters to use with a given current load...for single, three phase, etc.

When these heated protectors do their job, they are resettable by pushing a button that sort of rewinds the spring mechanism...which is then held back in its operating position by the low-temp alloy that has re-hardened.

More than you wanted to know...but lots of folks never see this stuff so thought I ought to describe it so you will know what you are looking for.


05-19-2009, 06:59 PM
I'm talking all the advice and information available. Never to much information. I have had some really great individuals in this forum reach out to me regarding the fusing systems they use. I'm very new to the RPC way of doing things, but I'm learning with each new post. Thanks again for the info........Gearhead