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DIY RPC - voltage on case & large imbalance between legs

crusty1007

Plastic
Joined
Apr 25, 2010
Location
Horseshoe Bend
Good morning,
I purchased a mill with a DIY 10HP RPC a few years back that I haven’t used yet. I’d like to hook it to my 5HP lathe. I did see it in use when buying the mill.
I wired the unit to power and before I hooked the outputs up to any machine equipment, I wanted to check the legs against ground and against each other. one of the legs is producing 205 volts to ground, the other 2 are around 120 (I’m headed into shop to document it again, will update) this imbalance caught my eye. Leg to leg there was also an outlier but don’t remember the numbers. regarding my imbalances, from what I’m understanding, this could be an issue with one of the capacitors?
During this, I also checked from single phase ground lead to the case of the machine, while running I’m getting a constant 25 volts and on startup my basic volt meter jumps over 200 momentarily. (I’m going to add a ground strap from the case to the ground lead, although, I’m assuming it will disengage the breaker in the panel?)

starting is managed by a single 3 pole switch; momentary on-off-on. Momentary on engages the capacitors to start the motor and then you quickly switch the unit to the on position.

Over the years I’ve read a decent amount on RPCs and studied the different DIY plans on here and other places but can’t figure out why I’m leaking voltage to the body. Its possible I’m forgetting something glaringly obvious or just outright don’t know and am missing an import diag procedure in locating the issue.

I’ll grab more data and post a few pics shortly.
help is appreciated.

Thanks,
T
 
Leg to ground readings will show the generated leg with a higher than normal voltage, ignore that, test leg to leg only.
Thanks for clarification. I appreciate the feedback.

after chasing the circuits and finding a wire lose, I added a ground to the case and tested the voltages;

input voltage from panel:
hot legs to ground: 122v/122v
across hot legs: 246v

output voltage:
T1-T2: 244v
T1-T3: 240v
T2-T3: 236V

ground to:
T1: 122v
T2: 122v
T3: 204v

so from my limited functional knowledge and tools, it looks like all is good. I’ve read to check the amps on the outputs, but requires a better meter, which I might be able to get next week, is this paramount? Is there anything else I should check?
thanks for the help.
-T
 
output voltage:
T1-T2: 244v
T1-T3: 240v
T2-T3: 236V
Is there anything else I should check?
thanks for the help.
The leg to leg measurements are what counts with the RPC connected to the lathe. If those output voltages were measured under load then
that is about the best you can expect. Under load you will see a change and the capacitor bank may have to be altered. The capacitors in the
box may have been tuned to get good numbers like what you are getting. But maybe the past DIYer did not know about load balancing.

In my experience the generated leg voltage is always higher than line potential with no load and then drops under load.
From your picture the generated leg is T3.

In the future when one or more of those black can capacitors explode I would replace them all with oil filled canisters like on the left side of the box.
It will take more cans for the equivalent capacitance but you have the room for them.

Those pieces of tape (green,white,red) are not conventional unless this box was made in Italy.
I use red and black for T1 & T2 and white for T3. Some people use blue for T3. Or wrap a piece of blue tape on white wire. Using green for T3 is not so good.
Also, notice the cut in the ground wire going out to the purple twist connector. Just seal it.
 
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The ONLY voltage readings that matter are those obtained when the machine motor is LOADED.

A lightly loaded motor is not prone to any failures due to imbalanced leg voltages.
Remember Voltage never did a lick of work, it only drives current.
 
The leg to leg measurements are what counts with the RPC connected to the lathe. If those output voltages were measured under load then
that is about the best you can expect. Under load you will see a change and the capacitor bank may have to be altered. The capacitors in the
box may have been tuned to get good numbers like what you are getting. But maybe the past DIYer did not know about load balancing.

In my experience the generated leg voltage is always higher than line potential with no load and then drops under load.
From your picture the generated leg is T3.

In the future when one or more of those black can capacitors explode I would replace them all with oil filled canisters like on the left side of the box.
It will take more cans for the equivalent capacitance but you have the room for them.

Those pieces of tape (green,white,red) are not conventional unless this box was made in Italy.
I use red and black for T1 & T2 and white for T3. Some people use blue for T3. Or wrap a piece of blue tape on white wire. Using green for T3 is not so good.
Also, notice the cut in the ground wire going out to the purple twist connector. Just seal it.

thanks, appreciate the info. Yes, T3 is generated leg.
noted on the capacitors replacements and balancing under load, did not know that, so appreciate the heads up. Those numbers are all unloaded. I will get it tied into the lathe tomorrow and test it under load.
funny enough, I didn’t take the time to look up the proper color coding so I just grabbed the Italian flag colors for my own traceability purposes 😅
I will update with proper color coding when I’m back in it tomorrow.
 
The ONLY voltage readings that matter are those obtained when the machine motor is LOADED.

A lightly loaded motor is not prone to any failures due to imbalanced leg voltages.
Remember Voltage never did a lick of work, it only drives current.

thanks, appreciate the info. Presumably running the machine with it taking a cut is the best way to put load on system and check voltage. Is this why they are testing the amps under load, is it just a different path to the same result?
 
Yes, if under load, not horrible at all.

I am guessing from the numbers that there are few or no "balance" capacitors installed (there may have been a comment, but I missed it).

If with no load, then you may want to either add some balance capacitors, or use a boost transformer for the generated leg.

The generated leg is naturally lower in voltage at no load (unless specially designed), which means that the voltages will drop farther under load. Most set the balance capacitors to produce about 10% high on the L1-L3 and L2-L3 phases, unloaded. That way, when the load is put on, the expected drop may be within a reasonable range.

A similar amount would be appropriate for a boost transformer.
 
The only thing that "matters" is amps across legs under load.

The rest is just gibberish and wishful thinking.
Thanks, noted. I’ll have to locate a meter with the ability to check amps.

Yes, if under load, not horrible at all.

I am guessing from the numbers that there are few or no "balance" capacitors installed (there may have been a comment, but I missed it).

If with no load, then you may want to either add some balance capacitors, or use a boost transformer for the generated leg.

The generated leg is naturally lower in voltage at no load (unless specially designed), which means that the voltages will drop farther under load. Most set the balance capacitors to produce about 10% high on the L1-L3 and L2-L3 phases, unloaded. That way, when the load is put on, the expected drop may be within a reasonable range.

A similar amount would be appropriate for a boost transformer.

appreciate the info. did not know that about using the boost transformer. I’ve only ever read about using balance capacitors.
The generated line is 4v to 8V lower than the other two legs with no load. I’ll check to see how far off it becomes under load.
 
noted on the capacitors replacements and balancing under load, did not know that, so appreciate the heads up. Those numbers are all unloaded. I will get it tied into the lathe tomorrow and test it under load.
Just turn on the RPC and lathe and dial RPM to what you use most often with no metal cutting. Then take measurements.

There have been posts here about why a problem comes up with a motor that has been faithful for years and then becomes sluggish or starts to smell or smoke.
The member ignored all of this and just went about their way of not knowing or caring, "everything will just be fine".

You can read about how unbalanced currents cause heating and eventual failures. Starts on page 77.
 
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Just turn on the RPC and lathe and dial RPM to what you use most often with no metal cutting. Then take measurements.

There have been posts here about why a problem comes up with a motor that has been faithful for years and then becomes sluggish or starts to smell or smoke.
The member ignored all of this and just went about their way of ignorance or not caring, "everything will just be fine".

You can read about how unbalanced currents cause heating and eventual failures. Starts on page 77.
Thanks! I have read about the outcomes of the imbalances in the past, but not that specific document, I will give it a read through.
 
Thanks! I have read about the outcomes of the imbalances in the past, but not that specific document, I will give it a read through.
Your box doesn't have any overload protection. After you read that document you may consider adding that to your box.
You might outgrow that box for the next size up. Don't worry about it. Happened to me and I would do it again.
 
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You can amp clamp the three wires going to th load motor, and the results will be a real head scratcher for you. Short answer is even if the converter is operating fine (and based on the voltage readings given, I believe it is), the amp clamp numbers won't make obvious sense. I would not bother doing that but if you do, and ask about them here, it will lead to a 'lively' discussion.

(cannot see the cut ground wire....)
 
From your post history, this looks like a regular, manual engine lathe and probably in a garage for hobby use. Buy a static phase converter, hook it to the lathe and move on with your life. The RPC is unnecessary complication for nothing.
 
You can amp clamp the three wires going to th load motor, and the results will be a real head scratcher for you. Short answer is even if the converter is operating fine (and based on the voltage readings given, I believe it is), the amp clamp numbers won't make obvious sense. I would not bother doing that but if you do, and ask about them here, it will lead to a 'lively' discussion.

(cannot see the cut ground wire....)
Jim, the cut in the insulation is on the green/yellow wire. Where the main bundle power wires break away from the black enclosing jacket.

crusty1007, Don't order an expensive Fluke true RMS meter from Amazon. Mine was stolen from a USPS employee. And they sent out another one.
Amazon gets a kick-back on stolen stuff from USPS. It would be easier if they didn't mark their shipping boxes with their logo.
 
Your box doesn't have any overload protection. After you read that document you may consider adding that to your box.
You might outgrow that box for the next size up. Don't worry about it. Happened to me and I would do it again.

I was looking at adding a 3 phase panel for this RPC to power and break out of. Right now I’m just in need of the lathe in new building as I’ve been running smaller equipment off a smaller RPC, which works fine. My backup compressor is 3 phase 7.5HP. So it’s inevitable, from what I understand, that I’ll need a bigger unit. Plus, larger 3 phase equipment keeps following me home.

You can amp clamp the three wires going to th load motor, and the results will be a real head scratcher for you. Short answer is even if the converter is operating fine (and based on the voltage readings given, I believe it is), the amp clamp numbers won't make obvious sense. I would not bother doing that but if you do, and ask about them here, it will lead to a 'lively' discussion.

(cannot see the cut ground wire....)

thanks, is there an equation required to make the amps make sense? Some of the posts/write ups I’ve read really make a point about measuring amps, but give no indication how they utilize the info, other than “balancing legs”

From your post history, this looks like a regular, manual engine lathe and probably in a garage for hobby use. Buy a static phase converter, hook it to the lathe and move on with your life. The RPC is unnecessary complication for nothing.

Yes, manual lathe from the 60s, You’ll see it in some of my old posts, if photobucket didn’t dump them. The garage it came out of used a small 2hp RPC (240v in) fed direct from the panel and also a static converter feeding from another (120v) outlet to serve this unit. I didn’t have time/knowledge to figure out how they were utilizing them in unison. So here I am with this RPC. I’d like to use it more than a once in a while hobby. Plus, I have multiple gear shapers I’d like to get up and running.

Thanks,
T
 
Jim, the cut in the insulation is where the main bundle power wires break away from the enclosing black jacket.

Op, Don't order an expensive Fluke true RMS meter from Amazon. Mine was stolen from a USPS employee. And they sent out another one.
Amazon gets a kick-back on stolen stuff from USPS. It would be easier if they didn't mark their shipping boxes with their logo.

correct, there’s a slice in the ground wire insulation. I’ll get that sorted When I properly color code the wiring.

Noted on not ordering a new Fluke for this.

Thanks,
T
 








 
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