wiring run caps in RPC

I think for the time being, I may just leave the capacitors out. I had always planned on starting the idler with a pony motor, so no start circuit's are needed. I think I will just wire the 3 phase motor in and see how it sounds, as well as what voltages I have on the legs.

Maybe Ulma Doctor will chime in.
Hi Investigator,
your machinery as described will operate just fine with only a slight bit of electrical loss when running unbalanced. :grin:
the only thing you need to know is the the generated leg is not suitable for powering control circuits and/or transformers
a secondary bit of advise is to use oversized input wiring for the single phase circuit.
the single phase input wiring will carry the entire load of the circuit that's run from the RPC
 
I need to make sure understand, since I am about to wire things together.
I have a 7.5hp idler motor for my soon-to-be-built RPC. My questions concern the run cappacitors

Question 1: When sizing the run caps to help balance the 3rd leg, my calcualtions show I need between 90 and 120 mF. Does that mean I need that value per leg or total. For instance, I have 2, 50mF run caps and plan to put one between L1-L3 and the other between L2-L3. Is this correct that it will give me the 100mF I need, Or do I need 100mF on each leg?

Question 2: My run caps are duel run 50/5mF. Where to plug wires? From L1 to Com on cap, out HERM on Cap. to L3?
Hi Investigator,
(all dependent on load and service voltage!!!)
Answer:1- you will have 2 different capacitor values when balancing a RPC.
phases A and B will be supplied voltage (for this purpose) their running voltage is 240vac between poles
phases A and C will have one supplied leg and one generated leg, their running voltage may be as low as 200vac between poles
phases B and C will have a generated leg and a supplied leg, their running voltage may be around 226vac between poles
if you were to treat phases A-C, exactly like Phases B-C, you would throw the motor back out of balance by application of too much capacitance on the B-C phase that may only require 30 or 40 mF of balancing capacitance.
phases A-C in this scenario would require the most capacitance maybe as much as 80 or 100 mF (all dependent on load and service voltage and rotation!!!)

Answer 2: when using air conditioning dual capacitors,
Com will get the supplied leg, the Herm terminal will connect to the generated leg.
the other terminal on the dual cap is 5mF, that's normally for the Air conditioner's fan- watch out it can still bite you even though it may not be connected.

please be careful when handling capacitors, discharge them before handling them- it could save your life.
 
Hi Investigator,
your machinery as described will operate just fine with only a slight bit of electrical loss when running unbalanced. :grin:
the only thing you need to know is the the generated leg is not suitable for powering control circuits and/or transformers
a secondary bit of advise is to use oversized input wiring for the single phase circuit.
the single phase input wiring will carry the entire load of the circuit that's run from the RPC

Thanks for the info. I currently have 8-2 from the main panel to the RPC panel, protected at the main with a 30A breaker, through a 30A switch to the bus bars. From the bus bars to the 7.5 idler motor is 10AWG (this motor shows 21 amps on name plate). From the Logan to the bus bars will be 12AWG (this motor shows a 6amp draw).

I know the wiring is over sized, to me thats better than under.

Ulma Doctor, will the 2 50mF caps be useful at all?
 
your wire is very nicely oversized, well done!
i'd try one 50mf cap first and test the voltage and see if you need the 2nd cap
 
sorry for the delay, feeding face...
yes, phases A-C
i'd try that first, so that you can understand the voltage relationship between the phases as you add the capacitance
 
I should note that my RPC build is going to run a lot of really expensive electronics. That is why the system complexity, I'm going for the best balance I can get. For running a standard 3 phase motor, I would have designed a much simpler system.
 
I should note that my RPC build is going to run a lot of really expensive electronics. That is why the system complexity, I'm going for the best balance I can get. For running a standard 3 phase motor, I would have designed a much simpler system.

I also go for great balance on CNC machines and EDM machines, but would NEVER put electronic control using the wild third leg. I really test for this. I've not seen any electronics needing the third leg, whatcha got?

A couple of these style meters make it a piece of cake to test your system as it runs:
https://www.ebay.com/itm/100A-AC-LC...037538?hash=item2cc44e76a2:g:mssAAOSwr~dZiyUY

BTW, the FitchWConverter document above was written by Fitch Williams. He currently lives in PA. he helped me with my second RPC built in about 1995.
 
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I also go for great balance on CNC machines and EDM machines, but would NEVER put electronic control using the wild third leg. I really test for this. I've not seen any electronics needing the third leg, whatcha got?

A couple of these style meters make it a piece of cake to test your system as it runs:
https://www.ebay.com/itm/100A-AC-LC...037538?hash=item2cc44e76a2:g:mssAAOSwr~dZiyUY

BTW, the FitchWConverter document above was written by Fitch Williams. He currently lives in PA. he helped me with my second RPC built in about 1995.


Take a look at my build thread, I am using meters similar to those http://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/#post-520986

It will be running my CNC lathe http://www.hobby-machinist.com/threads/new-toy-er-tool.62792/page-2#post-520205

The electrical cabinet
1510092024301.png
 
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Hi
I don't mean to jump in, but here is a converter that I built, I wouldn't worry to much about the run capacitors until the unit is up and running. Your better to start off with minimal run capacitance and then add more as you see how the voltages are effected. The start caps are just the opposite. You can take away capacitance in the start circuit if need be. If you don't have enough capacitance in the start circuit the motor will start slow and you may blow the start caps in the circuit. I also wouldn't try to balance the system over 10% of the line voltage. The circuit just has ballpark capacitance values.
grotaryconverterfinal.jpg
 
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