# Rotary Phase Converter Rebuild / Build



## Eddyde (Oct 7, 2018)

Hi all, I just hooked up a used, Snyder brand, Rotary Phase Converter to my newly acquired lathe (more on that soon). It works but makes excessive noise & vibration, especially when running under no load condition. It sounds like its vibrating at 60hz so my thought is its electrical in nature, not mechanical. It is very loud at start up but quiets down noticeably when the lathe motor is running. The converter came pre-wired with a switch and a 3 phase twist-lock outlet, so all I had to do was hook the mains to it, thus I think it is wired correctly. Has anyone had a similar issue? Could it be bad capacitors?

Thanks

Eddy


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## Karl_T (Oct 7, 2018)

It, for sure can be bad caps.

measure these volts L1-L2  L1-L3 L2-L3
goal is to get them within 10%


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## JimDawson (Oct 8, 2018)

When you fire up the lathe the RPC is probably more in balance with the load on it.  It really takes some tuning to get it in balance both unloaded and loaded.  My manufactured leg is about 8 volts high unloaded.  Were it not for the low mechanical rumble from mine you would hardly know it's running, pretty much just sounds like a motor running.

So as @Karl_T said, play with the caps a bit.  Putting some caps across the line (power factor correction) can help also, I think I have about 40mF on mine.


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## tq60 (Oct 8, 2018)

What is it mounted to?

Mounted ours to top of post for car lift and very loud due to shapes of post.

Recounted with isolators and problem solved.

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## Blackjackjacques (Oct 8, 2018)

Sounds like a tuning problem as Jim Dawson says. If it it fine under load, then your caps are likely fine.  Under load, everything shifts including current and power factor.  It is very likely that under the no-load condition, your caps are providing over unity power factor and the machine is bucking against the line frequency - resulting in the vibration.  You may need to disconnect one or more caps when not loaded to see where the vibration stops. Of course, once the load is restored, the caps need to go back


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## Eddyde (Oct 8, 2018)

Thanks for the responses.  I opened up the enclosure and found only 2 capacitors inside, there are no markings on them but they both tested to 50 microfarads, so 100 uF total, I assume they are okay. 




After reading up on info I found around the net, it appears the converter is wired in a simple "self starting" configuration. This is how its wired.


The Info I read says the capacitor value should be 25-30 uF per hp for a self start circuit so that would mean the idler should only be 4hp max (there is no spec plate on the idler), which doesn't make sense as the spec plate says it can, and does, start a 5 hp motor and according to what I've been reading the idler should be twice the hp of the maximum motor to be started?


So I first assumed increasing the capacitor values might help, but then I read, increasing the caps will increase the voltage which is already looking kind of high?
Line voltage, Between L1-L2, 246.1 no load, 243.4 with the idler and lathe motor, load.
Between L2-L3, 273.3v no load, 240.7v w/load.
Between L1-L3, 251.1v no load, 225.6v w/load.


Being this is my first foray into Rotary Phase Converters, I am not sure how I should go about tuning this contraption, do I increase or decrease the caps or do I rewire the whole thing to a more advanced configuration? 


The lathe is very quiet so this RPC is too noisy to use as is. Even under load, it is quite annoying. I am also not too keen on using a VFD for this lathe (I have them on other machines) as I don't need the variable speed option, I would need a 2nd VFD for the coolant pump and I don't want to have to rewire all the controls...

I appricheate your thoughts,

Eddy


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## JimDawson (Oct 8, 2018)

Have a look at this thread https://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/


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## markba633csi (Oct 8, 2018)

Hi Ed, your system there seems kind of over-simplified, like old Mr. Snyder didn't quite get it (or was cutting cost to the max). Most systems I have seen have a separate start circuit set of caps and a relay or switch which drops them out after a short spin-up time, leaving the tuning caps in place.  Check the internet, I have seen many diagrams and schematics
mark


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## Eddyde (Oct 9, 2018)

JimDawson said:


> Have a look at this thread https://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/



Thanks Jim, I remember following that thread. As always, you did an awesome job. I guess I'm ultimately heading down that road too, in pursuit of RPC harmony...


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## Eddyde (Oct 9, 2018)

markba633csi said:


> Hi Ed, your system there seems kind of over-simplified, like old Mr. Snyder didn't quite get it (or was cutting cost to the max). Most systems I have seen have a separate start circuit set of caps and a relay or switch which drops them out after a short spin-up time, leaving the tuning caps in place.  Check the internet, I have seen many diagrams and schematics
> mark



Ha, it sure does looks like Mr Snyder was a little light on the engineering...  
Originally, I was going to build the RPC from scratch, I was hunting around for a 10hp motor when this complete one crossed my path for $200. I figured it was ready to go and would save time and hassle, fool that I am... 
So now I've been reading lots of articles and schematics on RPC design, trying to figure my next move. I would like to reuse the idler but don't know its horsepower rating so Im not sure how to calculate the cap values? 
I'll probably go with a version of Jim's design, though perhaps not as fancy..


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## Eddyde (Oct 9, 2018)

tq60 said:


> What is it mounted to?
> 
> Mounted ours to top of post for car lift and very loud due to shapes of post.
> 
> ...



Thanks,

It has soft rubber isolator feet and is sitting on a concrete floor, looks like Mr. Snyder knew vibration would be an issue...


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## JimDawson (Oct 9, 2018)

Eddyde said:


> Thanks Jim, I remember following that thread. As always, you did an awesome job. I guess I'm ultimately heading down that road too, in pursuit of RPC harmony...



Thank you for the kind words 



> Ha, it sure does looks like Mr Snyder was a little light on the engineering...
> Originally, I was going to build the RPC from scratch, I was hunting around for a 10hp motor when this complete one crossed my path for $200. I figured it was ready to go and would save time and hassle, fool that I am...
> So now I've been reading lots of articles and schematics on RPC design, trying to figure my next move. I would like to reuse the idler but don't know its horsepower rating so Im not sure how to calculate the cap values?
> I'll probably go with a version of Jim's design, though perhaps not as fancy..



From the looks of the motor it doesn't look like it could be over 5 HP, just judging from the size.  I would start there for caps, then add or subtract as needed.  The fact that it seems to have no start relay makes me wonder if is some kind of purpose built idler just for an RPC.  I have heard that with a special winding configuration you can spin one up without a start circuit.


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## Karl_T (Oct 9, 2018)

Yep, not having start caps that drop out after a few seconds is surely the problem here. The system I built for my son vibrates terribly until the start caps drop.


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## markba633csi (Oct 10, 2018)

ED: first thing I would try would be to put a switch in series with the cap bank and throw it open when the idler is up to speed. See if the vibration/hum is reduced or even eliminated.  Obviously there will be a voltage imbalance with the caps out of the circuit, but this test would give you an idea of how smooth the system could potentially be, with some circuit changes...take care not to get shocked
mark


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## Eddyde (Oct 11, 2018)

JimDawson said:


> From the looks of the motor it doesn't look like it could be over 5 HP, just judging from the size.  I would start there for caps, then add or subtract as needed.  The fact that it seems to have no start relay makes me wonder if is some kind of purpose built idler just for an RPC.  I have heard that with a special winding configuration you can spin one up without a start circuit.


Yes it appears to be a dedicated idler as no shaft is protruding at either end. Thanks, I'll start at 5 hp and see how it goes...



markba633csi said:


> ED: first thing I would try would be to put a switch in series with the cap bank and throw it open when the idler is up to speed. See if the vibration/hum is reduced or even eliminated.  Obviously there will be a voltage imbalance with the caps out of the circuit, but this test would give you an idea of how smooth the system could potentially be, with some circuit changes...take care not to get shocked
> mark



Thanks, I'll give that a try.  I'm also going to change the bearings on the idler, years of vibration must have beat em up pretty bad...


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## Eddyde (Oct 11, 2018)

Karl_T said:


> Yep, not having start caps that drop out after a few seconds is surely the problem here. The system I built for my son vibrates terribly until the start caps drop.


Thanks, I'm definitely going to add a start circuit relay.


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## markba633csi (Oct 12, 2018)

You could just have a momentary pushbutton to start, save a few $


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## Eddyde (Oct 12, 2018)

markba633csi said:


> You could just have a momentary pushbutton to start, save a few $


I may, depends on what I find in my "miscellaneous electrical" bins.


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## Eddyde (Oct 14, 2018)

So here's where I'm at, I ordered a bunch of run caps, enough to give me 5 to 200 µF X 2, a 270-324 µF start cap & 15k Ω 2W resistor. I have a momentary start-stop switch and I think I have a 3 phase magnetic motor starter (just have to find it) I also have a time delay relay I could use for the start circuit. 
I have most of the design more or less figured out. 
One grey area I'm still trying to understand is how to figure the power factor correction cap values?


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## JimDawson (Oct 14, 2018)

Eddyde said:


> One grey area I'm still trying to understand is how to figure the power factor correction cap values?



The way I did it is to put the amprobe on the incoming line and start switching in caps until I ran out of caps to switch in.  Every cap I switched in lowered the incoming amps.  I have 25 MF connected.


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## Eddyde (Oct 15, 2018)

Thanks Jim, I thought it might have to go that way, I'll just start with 5µf and go up...


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## Eddyde (Oct 15, 2018)

Here s a first draft schematic:


I may add a time delay relay for the start cap, as I have a couple on hand and may want to mount the start/stop switches remotely.  
I over spec'd the motor start relay incase I need to ditch the idler I have and go up to a 10hp motor.
There will be disconnects and fuses between the idler and lathe motors.


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## markba633csi (Oct 19, 2018)

Looks fine Ed- you could also use a 2 pole relay if you don't have a 3 pole
M


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## Eddyde (Oct 21, 2018)

Okay, here is the final draft before I build:


Components ordered...


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## JimDawson (Oct 21, 2018)

Looks good to me


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## markba633csi (Oct 21, 2018)

Ah I see now- RY3 is a time delay relay
If I'm not mistaken, you could eliminate RY2 if the TD relay had hefty enough contacts
I didn't know they made TD relays with both instant and delayed contacts- a handy device


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## Eddyde (Oct 21, 2018)

Sadly, RY3's contacts are only rated for a measly 2A "Non inductive"... No worries though, a proper contactor is on its way along with a whole bunch of other stuff, the gloves are off on this one!


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## Blackjackjacques (Oct 29, 2018)

Eddyde said:


> One grey area I'm still trying to understand is how to figure the power factor correction cap values?



As JD posted, connect the clamp on current meter to any incoming phase, and install caps one by one while watching the apparent power current drop. When you reach the point where another cap is no longer dropping the current, or the current increases, or the amount of current drop is not substantial, then stop adding caps.  You do not want to go into the other direction with leading power factor as that can also cause vibration problems.


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## Eddyde (Nov 1, 2018)

Progress has been made, I hooked it up temporally to test it out, it worked very well, now the idler runs much smoother and quieter than before.


First I got the control relays and terminal blocks in place.


For extra safety, I made "capacitor boots" out of a bicycle inner tube.


I did a preliminary capacitor balancing for the test, I'll do a more thorough job when I do the final assembly.

Control test





Run test





Next chance I get I'll hook it all up for keeps.


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## JimDawson (Nov 1, 2018)

That sounds pretty good.  Nice job !


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## markba633csi (Nov 1, 2018)

Looks nice Ed,  old Mr. Snyder would be blown away I'm sure


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## Eddyde (Nov 2, 2018)

JimDawson said:


> That sounds pretty good.  Nice job !


Thanks Jim,  
The idler has an open frame with an internal fan so I think it's as quiet as it'll get. 
Just saw a 10 hp tefc for $100 on CL, getting in car now...


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## Eddyde (Nov 12, 2018)

New Problem.
The 10 hp motor I went to look at appeared to have a bad bearing(s) and the guy wouldn't knock anything off the price so I passed on it. I know  bearings aren't hard to change but I don't need more projects now and 10 hp was really more than I need anyway. Besides, He did have a smooth turning 7.5 hp available. I tested it out as best I could: Each winding to motor housing, no continuity. Between each winding, .5 ohms Apparently, no opens and no shorts so I bought it. 

However, when I hooked it up to the phase converter, in place of the original idler, it won't get up to speed. It just turns slow, about a hundred rpm then trips the breaker (30 Amp) after about 20 seconds.  I did try adding some more caps on the start circuit but still was a little shy of the estimated 562.5 µf. (@75 µf per hp) Could it be a bad motor or do I simply need more start-cap farads?


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## JimDawson (Nov 12, 2018)

That's odd.  Maybe a trip to the motor shop would be in order.


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## Eddyde (Nov 13, 2018)

Update: I got the new idler motor running, its fine. First I tested it by hooking up the old idler back up, I put a manual starter switch on the new idler motor and connected it to the bus. I started the system with the old idler then fired up the lathe motor and then kicked in the new idler, she started right up and purred like a kitten. 
I surmised it was indeed a case microfarad deficiency of the start cap... So then I disconnected the old idler, hooked the new idler back in and connected all my remaining run caps in parallel with the start cap, that gave me over the recommended value. Hit the start and It worked, the new idler now starts on its own. 
It was totally worth the effort, as the new idler runs much smoother an quieter then old one even after it was balanced. The lathe motor now runs smoother an quieter as well!

New start capacitor on its way from McMaster...


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## Eddyde (Nov 14, 2018)

The additional start cap arrived and I pretty much wrapped up the project. Only a few minor details remain.


I mounted the start-stop switch next to the cabinet until I figure the best location. I could even have one by each machine.


I mounted the idler motor on a pair of hardwood skids and set it on a piece of rubber sound attenuation matting. I'll also make a cover for exposed shaft. I milled out the back of a 5" junction box and mounted it to the terminal box that was missing it's cover.


Next time I get in there, I'll shorten & neaten up the cap wires and make a retaining strap to keep them in place when opening the cabinet.


Eventually, I'll place the idler under the lathe, where there's plenty of dead space.


The final schematic, pretty much as built. I didn't mount S3 (E-stop) for now as the Start-Stop switch is right next to the cabinet.


This is how it balanced out, I'm very happy with the results. I wasn't able to measure the current on phase B (the generated phase) as I couldn't pull out enough wire to get the ammeter on it, perhaps next time... But it probably isn't necessary as the system runs smooth and quiet, which is what I was after.
Now it's back to Craigslist with the George Snyder unit...


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