# How do you measure voltages on a rpc ?



## Brain Coral (Nov 19, 2014)

Hello all,

I just finished wiring in a 10hp RPC. From the RPC I have run a 12 AWG 3/wire and ground extension cord with a 30 amp 3/wire and ground plug on the end of it. I can then plug different machines into the plug as I choose.

Here's a pic of the plug...




I would like to know what the voltages are at the plug end, but am not too sure how to do it.

Brian


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## JimDawson (Nov 19, 2014)

Measure;

A to B
A to C
B to C

That will give you the voltage between each leg.  Between 2 of the legs I would expect it to be at line voltage, but I'm not sure what the manufactured leg will measure unloaded.  Under load I would expect it to be close to the other two.

When you do this, please let us know what the voltages are.  I have never taken a reading on a RPC.


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## Karl_T (Nov 19, 2014)

JimDawson said:


> Measure;
> 
> A to B
> A to C
> ...



I put in cheap voltmeters as Jim outlined. They cost about $10 each on eBay. Put them in the RPC panel.

Tuning an RPC is important, that's why the voltmeters. You can then start adding or removing run caps to get all the voltages within 10%. This step costs almost nothing and will improve the performance of your equipment.


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## JimDawson (Nov 19, 2014)

Karl, let's assume no voltage drop for this question. Would the voltage be the same on the manufactured leg in both the loaded and unloaded condition?  I've never set up a RPC, so I've never had a chance to look at the output.


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## Brain Coral (Nov 19, 2014)

Thanks for the replies... 

I guess that I should further explain that while I do own a voltmeter and ran a new circuit from my main panel and wired in the RPC and motor, I don't know how to safely and properly use the voltmeter to test the voltages.

I do know that I must turn the voltmeter to the ACV setting, but then I'm not sure if I have to check the voltage inside the RPC panel or can I check it at the plug end somehow... ?

Brian


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## Karl_T (Nov 19, 2014)

JimDawson said:


> Karl, let's assume no voltage drop for this question. Would the voltage be the same on the manufactured leg in both the loaded and unloaded condition?  I've never set up a RPC, so I've never had a chance to look at the output.


. The "wild
You actually have two manufactured leg voltages. The "wild" leg to ground or neutral seems to be meaningless.

If you have a large RPC motor horse, compared to machine.Voltages will change when loaded, but not terribly. They tend to sag with load.
Anyway, you need to watch both loaded and unloaded.  I add or remove a cap, record numbers:repeat.


I'm not the EE here. I am just repeating the work of a guy named Fitch that became an established expert on this. He's a EE - power specialty. I can give you his email.


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## Karl_T (Nov 19, 2014)

Brain Coral said:


> Thanks for the replies...
> 
> I guess that I should further explain that while I do own a voltmeter and ran a new circuit from my main panel and wired in the RPC and motor, I don't know how to safely and properly use the voltmeter to test the voltages.
> 
> ...



Same number either place, do it where its easy for you


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## Ulma Doctor (Nov 19, 2014)

JimDawson said:


> Measure;
> 
> A to B
> A to C
> ...




Hey Jim,
a RPC will have basically 3 different voltages dependent on measured phases.
i measure them all the time, the generated leg on an UNBALANCED RPC can be as much as 30 volts @ 240v
common readings look like this 242/236/212 unloaded
lightly loaded they look like 238ish/230ish/205ish on a 5hp motor 
of course balanced you can get them within 2% or slightly less with some work.


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## JimDawson (Nov 19, 2014)

Ulma Doctor said:


> Hey Jim,
> a RPC will have basically 3 different voltages dependent on measured phases.
> i measure them all the time, the generated leg on an UNBALANCED RPC can be as much as 30 volts @ 240v
> common readings look like this 242/236/212 unloaded
> ...



Thank you, that's very helpful.)


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## Brain Coral (Nov 20, 2014)

Back again... 

I guess that what I need is the actual techique used in measuring the voltage. I am not entirely comfortable with it at this point. I don't want to short something out, nor get a shock....  :yikes:

Here's a pic of the panel...




And here's a pic of my cheapo voltmeter...




Brian


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## Ulma Doctor (Nov 20, 2014)

you can take the measurements for output at the plug, you'll have little chance of hurting yourself.
you'll notice one leg on the plug looks different,
 it will either be bent inward [leg in,30amp] or bent outward[leg out, 20 amp] that is your ground leg.
you won't use it for testing at this point.
start up the unit, test across 2 hot poles of the plug and record your reading.
move 1 lead to the other hot pole you have not tested and record your reading.
move the other meter leg back to the first pole to get the last reading and record.
that will give you all three phase to phase voltages .

at this point ,you could also put one lead to the ground pole and test ground to phase for each pole as well and record your readings.

i hope i didn't confuse you more, if i did LMK, i might be able to explain differently


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## Karl_T (Nov 20, 2014)

my vote for easiest place to measure is your terminal block labeled CAB
looks like C and A are you L1 L2 line voltage

CB   would be anther voltage of interest
AB   would be the other

Use the pins of your voltmeter to touch the screw heads in this terminal block

The plug will give the same values - do which ever is easier.

Karl


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## Boswell (Nov 20, 2014)

I suggest that if you are not 100% confident in using the meter probes inside an open and energized panel that you measure at the plug This is the safest. I am not saying that you can't measure in the open panel safely, but YOU need to KNOW that you can do it safely, to be safe.

just my .02 USD


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## Phils69 (Nov 20, 2014)

Looking at the pic of the panel you don't have your 3 phase RPC motor connected yet, unless I am just not seeing it. Without that you aren't going to be able to check any generated voltages.


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## Brain Coral (Nov 20, 2014)

I'm back with some readings...

I also have a 5hp RPC and thought that I would measure that one as well, while I was at it.

10hp RPC...

                          C-A   249v
                          C-B   242v
                          A-B   257v

5hp RPC...

                          L1-L2   249v
                          L1-L3   252v
                          L2-L3   260v

I didn't check to ground, but here's a start. Just so you know, before I turned the breaker on and started the RPC, I cut some cardboard to provide a small measure of safety in a shield so that I couldn't come into contact with the capacitors...

Thanks for all of the helpful advice... 

Brian


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## JimDawson (Nov 20, 2014)

I would say that is pretty close.  Normally +/- 10% is close enough.  Your readings are closer than that.


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## Brain Coral (Nov 20, 2014)

Phils69 said:


> Looking at the pic of the panel you don't have your 3 phase RPC motor connected yet, unless I am just not seeing it. Without that you aren't going to be able to check any generated voltages.



Hi Phil,

That pic was taken before I completed the wiring. Both the 3-wires from the motor and the 3-wire extension cord share the CAB block. Also, I ran a 50 amp cicuit with #8awg 3/wire with ground to the RPC panel.

Brian


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## Brain Coral (Nov 20, 2014)

JimDawson said:


> I would say that is pretty close.  Normally +/- 10% is close enough.  Your readings are closer than that.




Hi Jim,

Well, that's nice to know. I've had the 5hp RPC running for a few years now and didn't know how well it was performing.

Brian


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## Phils69 (Nov 20, 2014)

I thought I was missing something. Who made that panel you have in the pic? Looks a lot like my 7.5 hp one.


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## Brain Coral (Nov 21, 2014)

Hello Phil,

The panel was made by WNY Phase Converter and Supply. I am very pleased with the voltage readings and the ease that it starts the motor. You can barely hear the starting power contactor tripping out because it happens so fast. It also has discharge resistors on the start capacitors, which is a nice safety feature.

It retails for $185.00 plus shipping. I forget what the shipping was to my door, here in Canada, but it was very reasonable.

The idler motor is a 10hp Leeson TEFC motor at 1760 rpm. It must have real good bearings, as it takes a long time to come to a stop after shutting the RPC down. I was given this motor, along with 4 others for free from a local monastery.

Brian


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## Phils69 (Nov 21, 2014)

I thought it looked familiar, I have a 7.5 Panel from them. I don't use it anymore but it worked well for the 2 years that I had it hooked up. I've heard the Leeson works well in RPC's, My Baldor was pretty loud I thought. Sounds like you have your 3 phase power well supplied now. Sounds like its time for you to make some chips.


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## Brain Coral (Nov 21, 2014)

Yeah, the Leeson is pretty quiet compared to my 5hp RPC. The name plate on the 5hp motor had been removed at the point of sale, so I don't know what brand of motor it is.

I have to fix an oil sump leak before I can use the horizontal mill on a steady basis, but I am home this afternoon and will fill the sump back up and try the mill at the high speed setting (8hp) on the motor and see how it performs.

Cheers... 

Brian


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