# Rotary Phase converter problem



## John_Dennis (Jan 8, 2022)

My Rotary Phase converter is putting out high voltage on 1 leg.  I suspect that the capacitors are shot, can anyone provide any wisdom on this?

Ground to 1 123 V
Ground to 2 123 V
Ground to 3 225 V

1+2  246 V
1+3  265 V
2+3  246 V


----------



## benmychree (Jan 8, 2022)

Does it make your 3 phase motors start and run well, if so, no problem.


----------



## John_Dennis (Jan 8, 2022)

I removed the two capacitors, Anderson MFG was unkind and removed the Capacitor labels.  The capacitors are round aluminum run capacitors 2 1/2 inches x 4 7/8 and the horsepower on the nameplate is 7.5

The two capacitors were wired parallel in between 1 leg of the 220 and the 3rd leg output + 3rd leg motor input

Can anyone give me an estimate on the amount of  capacitance needed and the type of Capacitor I should purchase?


benmychree said:


> Does it make your 3 phase motors start and run well, if so, no problem.


what happens when you double the voltage on 1 leg of a 3 phase motor?


----------



## benmychree (Jan 8, 2022)

I doubt that voltage to ground matters if the voltAage between phases is within reason, and I think it is.


----------



## Beckerkumm (Jan 8, 2022)

That's not bad for a non electronic balanced RPC.  Wild legs can be even higher but generally the legs balance better under load.  The ideal voltage on the manufactured leg would be 208 to ground but I've seen numbers higher than yours .  Dave


----------



## John_Dennis (Jan 8, 2022)

Beckerkumm said:


> That's not bad for a non electronic balanced RPC.  Wild legs can be even higher but generally the legs balance better under load.  The ideal voltage on the manufactured leg would be 208 to ground but I've seen numbers higher than yours .  Dave


265 while running a 1 hp bridgeport seems high enough to damage the motor. thats a little over 50 volts too high.


----------



## Beckerkumm (Jan 9, 2022)

Are your measurements while running with a load on the BP?  That does seem out of balance.  Whether it hurts the motor depends on the insulation.  I worry more about undervoltage as that increases the amp draw and causes the motor to run hot.  If the motor is 200 or 208v that is pretty high.  If the motor is 220-240, not so much.

RPC balance differently under different loads so a large rpc doesn't balance well under light load.  My rpc that would start a 10 hp motor, didn't balance well on small motors but was fine at 3 hp above.  I now run a phase perfect and it balances from 30 hp to 0.  Dave


----------



## brino (Jan 9, 2022)

There are some great threads here by @Ulma Doctor where he walks thru making your own phase converter and he gives capacitor sizes based on HP.

If you cannot find them, let me know.

Brian


----------



## markba633csi (Jan 9, 2022)

The measurements to ground are not meaningful. Only be concerned with the phase to phase readings; some variation is ok
-M


----------



## Barncat (Jan 9, 2022)

Are those phase to phase readings while the rpc is under load? If those are unloaded values, they may change to a more acceptable value while running a machine.


----------



## JimDawson (Jan 9, 2022)

Those numbers seem reasonable.  Check the loaded numbers.  Mine runs 240, 240, 256 unloaded, but settles down to 240, 240, 239 loaded.

As said, values to ground mean nothing.


----------



## John_Dennis (Jan 9, 2022)

JimDawson said:


> Those numbers seem reasonable.  Check the loaded numbers.  Mine runs 240, 240, 256 unloaded, but settles down to 240, 240, 239 loaded.
> 
> As said, values to ground mean nothing.


My problem is that the numbers do not go down under a light load such as the BP running a small endmill.  This RPC is a 7.5 HP and my motors range from 1/2 to 3 HP, they may never generate enough load to normalize.


----------



## John_Dennis (Jan 9, 2022)

Is there any good reason to solder all of the spade connectors to the terminals as the manufacturer has done?  I would prefer not to do this.


----------



## JimDawson (Jan 9, 2022)

Your RPC numbers are within reason for 3 phase motor operation.  Your line voltage is a bit high, but there is nothing you can do about that.  For about a year my line voltage used to run over 250, but the power company changed out a transformer and got it down in the 240 range.  I didn't have any problems running at the higher voltage.

I used heavy duty spade connectors on my RPC, and have had no problems with loose connections in the last 4 years.


----------



## John_Dennis (Jan 10, 2022)

JimDawson said:


> Your RPC numbers are within reason for 3 phase motor operation.  Your line voltage is a bit high, but there is nothing you can do about that.  For about a year my line voltage used to run over 250, but the power company changed out a transformer and got it down in the 240 range.  I didn't have any problems running at the higher voltage.
> 
> I used heavy duty spade connectors on my RPC, and have had no problems with loose connections in the last 4 years.


If my service voltage is 235, how does the converter voltage get higher on all 3 legs of the 3 phase output?


----------



## JimDawson (Jan 10, 2022)

John_Dennis said:


> If my service voltage is 235, how does the converter voltage get higher on all 3 legs of the 3 phase output?


Good question, that is a bit odd.  Assuming you incoming power is connected to L1 and L2, I would expect the voltage across L1, L2 to be service voltage.  L3 (manufactured leg) to L1 or L2 could be some higher or lower voltage.


----------



## Barncat (Jan 11, 2022)

I would assume that a higher than anticipated incoming voltage would make the rpc behave differently than designed-across all three legs. You originally stated that you thought maybe the capacitors are shot, but if they didn’t work at all then all voltages would be lower. I bet the capacitors are working just as they were designed to, but are just all slightly oversized based on your higher than average incoming voltage. Buy a couple different sizes from surplus center and play around until you are happy with it.


----------



## John_Dennis (Jan 11, 2022)

brino said:


> There are some great threads here by @Ulma Doctor where he walks thru making your own phase converter and he gives capacitor sizes based on HP.
> 
> If you cannot find them, let me know.
> 
> Brian


Thanks for the suggestion about Ulma Doctors RPC.  Unfortunately the site logs me out any time I click on the link, some error about cookies which I have enabled for Hobby-Machinist.  Do I have to be a paying member to access documents or archives?


----------



## Janderso (Jan 11, 2022)

John,
I had some equally goofy numbers coming out of my American Rotary RPC.
All my three phase equipment runs perfectly on the RPC.
I took the advice you have been given.
Make chips man, don’t stress it.
My 2 cents


----------



## JimDawson (Jan 11, 2022)

John_Dennis said:


> Thanks for the suggestion about Ulma Doctors RPC.  Unfortunately the site logs me out any time I click on the link, some error about cookies which I have enabled for Hobby-Machinist.  Do I have to be a paying member to access documents or archives?



I have seen this problem.  Try logging out and log in again, select accept cookies.  The log in screen seems to be the only place you can accept cookies.


----------



## MikeInOr (Jan 11, 2022)

It looks/sounds like your RPC doesn't have any balancing capacitors.  A lot of RPC's are made this way and run for many years without any issues to the RPC or the 3ph motors being powered.

Are you measuring the voltages when the RPC is powering a 3ph motor?  Do they change when the powered motor is under load?  The current readings on each leg are much more meaningful if you have a clamp on amp meter?

Bottom line is you really don't have anything to worry about.


----------



## John_Dennis (Jan 12, 2022)

I found an interesting link describing what I am measuring.  https://www.electram.com/1/different-voltage-on-line-3-phase-converter.php


----------



## John_Dennis (Jan 15, 2022)

I now have a 100 uf capacitor between 3+2 and a 55 uf between 3+1  voltages are now as follows while running a 1 HP bridgeport with a large flycutter with the pulley in the highest speed no cutting.

1+2 = 248
2+3 = 250
3+1 = 251

The largest motor I own is only 3hp and I never run more than 1 machine at a time. 

The original design of this RPC had an estimated 150 or 200 uf between 2+3 and nothing between 1+3.   Without the 55 uf between 1+3 the voltage with the bridgeport running

1+2 = 246
2+3 = 231
3+1 = 248

Adding an additional 100 uf. (200uf) resulted in

1+2 = 246
2+3 = 258
3+1 = 276


Thanks for the help, it helped me understand this very abstract concept.  I still can't figure out how I get an additional 15 volts on each leg, If I wanted to run a 208 motor, is there a practical way to drop the voltage a little?


----------



## Barncat (Jan 15, 2022)

A three phase transformer would do the trick, although finding an affordable one to go from 240 to 208 would be tough. You could always wind your own if you are up to the challenge.


----------



## MikeInOr (Jan 15, 2022)

John_Dennis said:


> I now have a 100 uf capacitor between 3+2 and a 55 uf between 3+1  voltages are now as follows while running a 1 HP bridgeport with a large flycutter with the pulley in the highest speed no cutting.
> 
> 1+2 = 248
> 2+3 = 250
> ...



I have been running a 208v 7.5hp radial arm saw motor on my 10hp RPC for near 30 years with voltages higher than yours.  You are overthinking things!  You can run a 208v motor with the voltages you have currently.  No need for a transformer!   If you are really worried use a clamp on amp meter to see how much current each leg is pulling.  Excessive current will heat up the windings and melt the insulation... not excessive voltage.

My 7.5hp RAS is a 1947 Redstar... back then wire insulation wasn't nearly as advanced and heat resistant as it is in modern motors.  A modern 3ph motor will have no issue at all with the voltages you are showing.


----------



## John_Dennis (Jan 15, 2022)

I just finished replacing the bearings and tidying up everything.  It is now very quiet and the voltages seem ok.  What started me looking at this was the fact that the two spare 3 phase motors that i have show very different amperages on each leg.  I will have to hook them up and test them to see if there is any difference.

Thanks again for all the advice.

JD


----------



## MikeInOr (Jan 15, 2022)

John_Dennis said:


> I just finished replacing the bearings and tidying up everything.  It is now very quiet and the voltages seem ok.  What started me looking at this was the fact that the two spare 3 phase motors that i have show very different amperages on each leg.  I will have to hook them up and test them to see if there is any difference.
> 
> Thanks again for all the advice.
> 
> JD



I built my 10hp RPC 30 or so years ago to power my pretty rare 1947 RAS with a 3ph motor custom designed to hold and swing a 16" saw blade (there is really no way to replace it)... I had the EXACT same concerns then that you are having now!  I hope I was able to mitigate some of your concerns a bit.


----------

