Rotary Phase Converter Tech

[bgaidan]

Playing with this little RPC I got "free" with a machine a year or two ago. It's a 2/4 hp - 2hp starting and 4 hp max simultaneous total. I'm hooking it up to run my surface grinder and vertical bandsaw for now. But I also have a 20hp motor tucked away in the corner that I'll eventually use for an RPC for the whole shop.


I got it wired up and spinning this morning. With no load on it, it' makes a horrible vibrating/buzzing sound and I'm seeing about 290v between the generated leg on either of the supply legs. My single phase line voltage sits at about 242.


When I run the 3/4hp surface grinder off of it, the voltage drops, but still seems kind of high to me.
I'm seeing 265v and 275v on the generated leg to either of the single phase feed legs.

Should I be worried about this much varriance? How much voltage variation and higher voltage is too much on these?


This particular unit seems simpler than others I've seen on the web. There are only two big caps behind the panel. I swear others I've seen had half a dozen to a dozen or more. Is it just a size thing (fewer big caps vs. more smaller ones) or is this just a cheaply built unit?

 

[cj3a]

I had a cheaper phase converter that did that. I think it was a phase master. I bought a 40hp American rotary and it does not do that bad. But I don’t remember how close the generated leg is compared to the line voltage.

 

[aczlan]

Not an expert, but from what I can find (looking at building one myself) it seems that it's supposed to be within 5% when under load, so it seems you need different capacitors.
It will be different under full load then under a partial load, so if you have a bigger load that you plan to run you should use that rather than the 3/4hp.
A decent writeup: Rotary Phase Converter
The Fitch Williams that is referenced in that article seems to be the gold standard for balancing capacitors in RPCs: http://www.practicalmachinist.com/FitchWConverter.pdf

Aaron Z

 

[cj3a]

You had me thinking about this and I check my American rotary 40hp. No load 246, 248 and 240 volts. 2hp Bridgeport 243, 246 and 240. 20 hp lathe no load 231, 234, 239 volts

 

[bgaidan]

You had me thinking about this and I check my American rotary 40hp. No load 246, 248 and 240 volts. 2hp Bridgeport 243, 246 and 240. 20 hp lathe no load 231, 234, 239 volts

Those numbers sound more in line with what I'd expect.

This little one I have only uses two BIG caps. Nothing else inside the box. Most of the ones I've seen pics of online use several caps for starting and running.

I probably should test the two caps and make sure they're actually still good.



I believe this is the inside of a 40hp american rotary. They do have some electronicals in them, but otherwise just use caps like the others. I'd be curious what the circuit board is doing for it. I'm wondering if it's just some type of protection circuit that trips the contactor if something is too unbalanced.

 

[Dethmachinefab]

Is the start function shutting off once it spools up? That would cause a "don't sound right" buzz and some weird voltage I think.

 

[bgaidan]

Is the start function shutting off once it spools up? That would cause a "don't sound right" buzz and some weird voltage I think.

This one doesn't have any type of start function. There are only two caps in it. Nothing else. Plug it in and the motor spins up and starts generating the third leg.

 

[Dethmachinefab]

This one doesn't have any type of start function. There are only two caps in it. Nothing else. Plug it in and the motor spins up and starts generating the third leg.

It should do it automatically. It can't start rotating by itself without input to 3rd leg from starting cap.

 

[bgaidan]

It should do it automatically. It can't start rotating by itself without input to 3rd leg from starting cap.

Here's the wiring. Pretty simple. Two caps, wired in parallel

I need to do some reading and figure out if this is just a cheaply made unit. Might be able to add another cap or two, ore replace one of these to smooth it out.

 

[sdmuleman]

Cheaply built POS seems likely. Everything I've always seen is that you need 2 different sets of capacitors - one set for starting and one for running, and start caps need a cutout system.

 

[Dethmachinefab]

Cheaply built POS seems likely. Everything I've always seen is that you need 2 different sets of capacitors - one set for starting and one for running, and start caps need a cutout system.

If not sized correctly, they won't cut out and buzz until eventually going pop.

 

[WaterH]

Ive never checked mine. It makes lots of noise when running, but not really much vibration. I don’t recall the capacitors, but I hooked it up a long time ago.

If I get a chance, I will check it. Mine runs my Bridgeport or a 5hp lathe. (Not both)

 

[Pook]

I have a larger American Rotary ADX 75, the circuit board stuff is part of the controls for the capacitors to keep things within 5% of the voltage and running smooth for CNC machines and other stuff requiring cleaner 3phase generated power. Big motor and just makes a quiet hum. Can hear it surge a little and hum louder a bit when the cnc lathe is running, where it is speeding up and slowing down quickly. voltage is 242-245 between the legs.

I had an older converter with less caps running a big powerhammer before. it was a louder buzz, but it also had a start/run cct, and the legs where probably 10% out between the legs most of the time, but it worked.

Really matters what your trying to run how good it needs to be.

 

[SomeGuyFromOlympia]

I have a North American 60hp that runs the CNC stuff here

customer support was legit during the install

 

[bgaidan]

I've been ordering parts for my 20hp RPC build....accelerated due to an unexpected need.

I'm piecing this together mostly with EBAY surplus where I can find them.

Gonna track costs here just for fun

1 ea. - 16x16x6 Hoffman Enclosure - Used, Surplus - Ebay - $62.94
2 ea. - 270-324 uf 220-250vac Start capacitors - Ebay - $19.21
1 ea. - ABB 120v coil 80a 3 pole contactor - Used - Ebay - $28.30
1 ea. - Allen Bradley 120v coil 100a 3 pole contactor - Used - Ebay - $32.61
1 ea. - Supco 9063 Potential Relay - Ebay - $13.38
10ea - TEMCo 100uf 370/44 Run Capacitors - Amazon - $100.78
3ea - LED Volt/Amp displays - Amazon - $16.04


I need to order a few more mixed variety of run caps. It's kind of a crapshoot on getting it right without some trial and error and having spares. Plus it'll change when I actually get a full load on it. I bought the 10 pack of 100uf on Amazon because it was cheaper than buying just the 5-6 that I know I'll need. It's a start. I'll probably order a pile of 20uf ones and just play with series/parallel to get close. Caps in parallel just sum up to the total uf all all connected. Series, they're the "inverse sum of all inverse capacitances" (had to look that up) - to keep from making it too complicated, if you use the same uf value for two caps in series, you effectively get half the uf. So two 20uf caps in serries will give 10uf. Two 20uf in parallel gives you 40uf. So, with a bunch of 20's, I can make 10uf steps to fine tune. And the 100's can give me steps of 50. From what I've read, there's a little bit of science to building the cap bank, but it's mostly trial and error get get it as close as possible.

I also need to order some drain resistors to drain the caps when power is off. There's a big of math on those that relates to the total capacitance you end up with, but I think it's wide enough that I can order a small variety pack off amazon and end up with something that will work.

And then I'll need add a switch. Since the supply contactor will be 120v, it gives me a few options. I'm planning to remote mount the switch since the cabinet and idler will be tucked away in a back corner of the shop. I have a bunch of SJO cable that I can run to a wall mounted switch by the door of the shop. Either with a regular toggle switch or possibly a timer so I don't forget to shut it off and leave it running all night. It'd also give me the ability to tie it in to an automation system later.

Should have everything in next week to start assembling and figuring out cap values.

 

[aczlan]

Got my little 1HP RPC working yesterday (at least enough to say it works), need to order plugs for the power cord, some bleed resistors, find the 240V to 120V transformer that I had at one point for control voltage and clean up the install a little:

It took a 80uf cap for starting it (A-C, 250V cap), a 40UF running cap (A-C, 370V cap), and a 25UF running cap (B-C, 370V cap).
Voltages above are all with nothing connected, just the motor idling.
These are the voltmeters/amp meters, the CTs are on the output side, might be interesting to add another pair of amp meters for the incoming L1 and L2, but not right now:

Amazon.com

For controls it has a 30A 2 pole switch as the main disconnect, a N/C "Stop" button and a N/O "Start" button.

Aaron Z

 

[bgaidan]

Nice. I ordered that same V/A meters. Like $20 for all 3.

Everything showed up this week. I know I'll need to order more caps, but I should be able to play around with the 10 100uf ones I have an get an idea of what else I'll need.

Pretty sure I need to source some hook up wire and the main feed wire. Looks like it'll take 2 or 4 awg wire for the single phase feed and 4 for the idler hook up. The mill is only 10hp so I should be able to get away using the 10/4 SOOW that I already have.

Pretty happy with this $50 surplus box from Ebay. These things run like $350 if you buy them new. Most expensive part of the build.

 

[aczlan]

Yep, my box was about $10 at a yard sale last summer, made me very happy to find that.
I think the only new things are one of the caps, the main power switch, the start/stop bottons and the meters, everything else was used, or leftover from another project.
After I get this one working, there is another one in the works.
I picked up a 10HP motor that I plan to make into a bigger RPC.
That will have more new parts in it.
The motor:

The guy that sold it would argue that it already is a RPC, but it was done in an "interesting" fashion, so it will be redone before it gets used.

Aaron Z

 

[Dethmachinefab]

I've been stuffing little vfd's in ammo cans because if I can't salvage a box from a job I ain't buying one.

 

[bgaidan]

I've been stuffing little vfd's in ammo cans because if I can't salvage a box from a job I ain't buying one.

Heh...my belt grinder....

 

[bgaidan]

So, small update. For anyone that saw my thread in GCC, it turns out that the 20hp motor I bought that was labeled as dual voltage was rewound at some point and was permanently wired for 460v, or at least permanent without tearing in to the case and hoping the leads were actually still there and identifiable. I debated trying to find a motor shop and seeing if they could rewire it, but that was probably going to be several hundred bucks.

Instead, this popped up on FB about 2 hours from me. He wanted $1200. I threw $1k at him and he took it.

It's a beast. A tad more than I need, but it'll give me some head room for future machines. 20hp biggest motor and 60hp total combined.

Full of mouse turds and some of the caps have been bypassed. I may have to do a little balancing on it....or that may be a sign that someone already did balance it.

Only thing I don't love is that it doesn't have a starting circuit with a low voltage switch and load sensing relay or timer like the fancier ones have. It just has what I'm assuming is a centrifugal switch to disconnect the starting caps via a contactor. PO said he just threw the breaker at his main panel to turn it on.

Fortunately I have all the shit from my failed build, which includes the proper relay and an extra contactor.


So now I just need to pick up the feed wire and a breaker for it. To run it at full capacity, I'd need a 125a breaker and associated, properly sized wire.

A quick read says I need #2 copper or 1/0 aluminum. Since it'll be a very short run (~8') from the panel, I can probably get away with stepping down one size. I may under size it for now (with a properly sized breaker for the wire, of course) if I can get away using some of the stuff I already have. If I need more capacity or find myself popping the breaker I can always upsize it. At some point I plan to build a little doghouse behind the shop, right behind the main panel and stick it there to cut noise, so I'll wait until then to give it all the beans.

I'd also like to find a deal on a small 3 phase panel.....something with maybe 6 to 8 slots so I can do some proper distribution on it.

Both the big CNC machines have their own breakers built-on on the supply so I can just feed them straight off the RPC for now though.

 

[aczlan]

I'd also like to find a deal on a small 3 phase panel.....something with maybe 6 to 8 slots so I can do some proper distribution on it.

I cant remember where you are located, but if you are near CT, this might be the ticket:

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Aaron Z

 

[bgaidan]

I cant remember where you are located, but if you are near CT, this might be the ticket:

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Aaron Z

Fuck. I'm in NC...way too far away. Bet there's some good stuff in there.

There's in industrial salvage place I have been too for a while. They'll probably have something I can pick up cheap.

 

[bgaidan]

Buuut.....in other news, I looks like I might have gotten fucked on the "new" RPC.

The seller told me it worked fine, he just upgraded to an American Rotary unit that would be friendlier with some of his electronic controlled equipment.

I tried spinning it up off a 30a breaker. Way under-sized, I know, but I figured it'd be enough to start it with no load. Tripped the breaker immediately. 😠 Pulled out the multimeter and I'm getting continuity between all 3 legs.

So I pulled all 12 of the leads off and found that individually, I'd get continuity on a pair of wires leg to leg. Every single wire paired up to another one on a different leg. So where I have 4 wires each on T1 - T2 - T3, two of the wires on T1 would individually have continuity to 2 separate wires in T2 and the other two to T3. And T2 would have two to T1 and two to T3, etc.


This doesn't seem random, but it also doesn't seem right either. I'm not fully up to speed on motors/winding an whatnot to know how to read this either.

 

[aczlan]

Post a picture of the wiring plate?
Are the wires numbered say with numbers 1 through 9?

Aaron Z

 

[bgaidan]

Post a picture of the wiring plate?
Are the wires numbered say with numbers 1 through 9?

Aaron Z

No wiring plate....I'm guessing because the idler was made specifically for this commercially produced RPC.

The wires are labeled. I'll need to pull them back off the studs to break them apart and write down the wire number going to each leg.

In my GCC thread, [486] it would act like a dead short trying to start it on a 30a breaker, so I may just not be feeding it enough beans.


I would have thought ramping it up with the VFD would have been enough of a soft start to get it running, but apparently not.

I should also add that there's not short to ground on any of the leads.


Gonna have to pick up a 125a breaker and run some temp wire just to make sure it actually works before spending any real money.

 

[aczlan]

Here are some common wiring patterns for 3 phase motors (to compare what you find when you open it back up to): Three Phase Electric Motor Wiring Diagrams

Aaron Z

 

[cj3a]

Running No load 40hp. Let me figure how to measure in rush

 

[cj3a]

In rush for the 40hp.

 

[bgaidan]

Running No load 40hp. Let me figure how to measure in rush

What's that on? The NL on this one supposed to be 18a.


What size breaker is running that? And how do you convert 3 phase amps to single phase?