# wiring run caps in RPC



## Investigator (Nov 5, 2017)

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?


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## markba633csi (Nov 5, 2017)

Investigator: Which calculation did you use?  100 mF sounds a bit low, even per leg
Mark S.


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## Rick Berk (Nov 6, 2017)

markba633csi said:


> Investigator: Which calculation did you use?  100 mF sounds a bit low, even per leg
> Mark S.


I have a 15 HP and it takes 770 mf to start and 440 mf to run.


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## Investigator (Nov 6, 2017)

markba633csi said:


> Investigator: Which calculation did you use?  100 mF sounds a bit low, even per leg
> Mark S.



The information on Run caps I got from this site:  How to build RPC  about half-way down, under "making your rotary phase converter better" and under the heading of 'add run capacitors'.  This info says to add 12-16mf per rated horse power of the idler motor.

If this is in error, please let me know.


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## markba633csi (Nov 6, 2017)

I'll check it out, in the mean time maybe someone who knows this stuff better than me can chime in- Jim D. is building one now and he has 
a huge bank of caps 
M


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## JimDawson (Nov 6, 2017)

You might take a look at this thread
http://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/#post-520986

The capacitors are roughly calculated by this formula:
CL1-L3 = 4 to 5 times the motor horsepower = 75 (20+20+10+10+10+5+5+5+5)
CL2-L3 = 10 to 15 times the motor horsepower = 180 (60+60+20+20+10+10+5+5+5+5)
PF COR = (10+5+5+5+5)

This is not to say that I am correct in the way I'm doing this.  We'll see later this week when I fire it up


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## markba633csi (Nov 6, 2017)

I read that article and my feeling is there are better instructions out there-  For example, they mention making a RPC without any caps at all.
Yes in theory it's possible but I don't think it would run well enough for any machine shop application.
It does look like they mean per leg and not total when figuring cap size.  So for 7.5 HP x 15 mF =  112 mF per leg.  Usually one leg requires more capacitance than the other depending on the rotational direction.
Mark S.
ah I see Jim D. just posted- his figures are quite different from your article- It seems the cap values can vary quite a bit


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## Investigator (Nov 6, 2017)

markba633csi said:


> I'll check it out, in the mean time maybe someone who knows this stuff better than me can chime in- Jim D. is building one now and he has
> a huge bank of caps
> M





JimDawson said:


> You might take a look at this thread
> http://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/#post-520986
> 
> The capacitors are roughly calculated by this formula:
> ...



Very interesting.  On the one hand I have read how very simple it is, and pretty effective, to simply wire the idler motor to 220v and hook up the tool to that.  This is supposed to result in very usable 3ph for the tool with the understanding that this way might not be best for machines with electronic controls.

On the other hand, I see people with various complexities of schematics for auto starts and balancing caps.  

I wish I knew which was correct.   For the sake of this argument, This will only be  powering a 12" Logan circa 1964.  No DRO's, no electronics, just a simple factory 3 phase 2 speed motor.

This particular RPC build has been meant to be a "starter just to get me going" from the start.  I hope and plan to either build a fancy RPC, or even buy one of the panels from Ebay or such.

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.


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## markba633csi (Nov 6, 2017)

Jim: is there a good article he can read on this topic? Something that also covers the PF correction too? 
Mark S.


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## JimDawson (Nov 6, 2017)

Here are the documents that I used for research:
http://www.waterfront-woods.com/Articles/phaseconverter.htm

FitchWConverter courtesy of @Ulma Doctor 

Then I finished the engineering to meet my specific needs.


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## Ulma Doctor (Nov 6, 2017)

Investigator said:


> 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. 
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


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## Ulma Doctor (Nov 6, 2017)

Investigator said:


> 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?
> ...


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.


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## Investigator (Nov 6, 2017)

Ulma Doctor said:


> Hi Investigator,
> your machinery as described will operate just fine with only a slight bit of electrical loss when running unbalanced.
> 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?


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## Ulma Doctor (Nov 6, 2017)

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


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## Investigator (Nov 6, 2017)

Ulma Doctor said:


> 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



one 50mF cap between A and C?  just on one leg?


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## Ulma Doctor (Nov 6, 2017)

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


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## JimDawson (Nov 6, 2017)

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.


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## Karl_T (Nov 7, 2017)

JimDawson said:


> 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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## JimDawson (Nov 7, 2017)

Karl_T said:


> 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
> ...




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


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## Tink (Nov 8, 2017)

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.


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## Investigator (Nov 10, 2017)

Ulma Doctor said:


> 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
> ...



Well, got everything wired up and running.  I didnt put any run caps in, only using the idler motor.  I turned it on and took some readings with my meter, hopefully folks can tell me if and what I need to do to make improvements.

With just the idler on, and no other motors i have:

ground/com -L1    123Vac
Grnd/com-L2         124Vac
Grnd/com- L3        170Vac
L1-L2                      249V
L1-L3                      220V
L2- L3                     208V


When I turned on the lathe (Logan 12") and took readings at slowest and fastest speed, the readings between l1-l3 and l2-l3 were the same as without the lathe on.


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## Ulma Doctor (Nov 10, 2017)

you did everything correctly!!! 
those are normal readings for a small unbalanced RPC idler in motion @240vac
your lathe could run like this and it wouldn't cause catastrophic problems.

use L1 and L2 as the power legs to transformers or the control circuit, the generated leg is not suitable for these circuits


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## Investigator (Nov 10, 2017)

Ulma Doctor said:


> you did everything correctly



It seems a little surprising how simple it is.


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## Tink (Nov 10, 2017)

You could add some run capacitance across L2-L3 to try to balance the voltage a little better.


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## MikeInOr (Jan 4, 2018)

JimDawson said:


> You might take a look at this thread
> http://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/#post-520986
> 
> The capacitors are roughly calculated by this formula:
> ...



This is the way my 10hp rotary phase converter is wired and it has been running great for the past 20+ years.  All 3 legs are within a few volts of each other.  If L1 - L3 = L2 -L3 I don't think the legs would be balanced very well.  I recall trying about 3 capacitance values each for L1 - L3 and L2 - L3 before the voltages were in a range I felt comfortable with.  I spent longer finding a value for the start capacitor that started the idler in a reasonable period of time without a massive start current.


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## Blackjackjacques (Jan 4, 2018)

Investigator said:


> 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?
> ...



I'm going to come in from left field here and totally mess this up.  For one thing, I never was comfortable with the notion of two-legging a 3-phase motor for purposes of deriving three phases.  It is inefficient, taxes the devil out of three-phase motor, incurs incredible complication wrt power factor capacitors, and in general - does not make sense.  I say it does not make sense because the typical RPC uses two motors anyway - a drive motor which everyone refers to as an Idler, and the larger 3-phase motor itself.  If you have two motors to begin with - why not run a motor-generator arrangement where the single phase motor drives the 3 phase motor through a mechanical coupling and the rating of the  system is limited by the drive motor rating.  Using this arrangement, you do not need power-factor correcting capacitors, but if you want to be dandy, you can go ahead and use them anyway to keep your currents low.   I just did a design for a client with a 5 hp 3-phase Rix compressor using this MG arrangement and two 5 hp motors (a 1-phase and 3-phase) and it works without need for pf compensation.  Granted that it is compressor duty over continuous, but you folks are not running NEMA design C motors in your lathes.


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## MikeInOr (Jan 4, 2018)

3ph, or any induction motor, does not have any permanent magnets.  Don't you need to excite the armature coils some how so they produce a magnetic field that will induce a current in the primary windings?  Then wouldn't you also need some mechanism to regulate the output voltage?  Seems like there would be more involved than just spinning a 3ph motor with an engine or another motor?


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## markba633csi (Jan 5, 2018)

I think we are going in several different directions here.  Jacque:  The "typical" RPC is comprised of only one motor called an "idler". The synthetic 3 phase output produced from this idler is used to power the lathe or milling machine motor in question.  The idler usually has a horsepower rating equal to or larger than the load it powers.  Some RPC installations have an idler so large and with so much mass that a second smaller "pony" motor is employed to help get the idler up to speed and reduce the huge startup current surge.  This pony motor is then disconnected after a few seconds and freewheels with the idler.  
As far as I understand it, a motor/generator system like you mentioned is less efficient than the RPC scheme, hence they are not used much anymore. The RPC is rugged, relatively inexpensive and decently efficient.  
Mike:  I believe a 3-phase motor does not make a decent generator due to the lack of excitation.  The alternator in your car requires a small amount of excitation from the car battery or there would be no output.  3-phase ac alternators have either permanent magnets (as you mentioned) or a separately excited field winding.  
Some generators operate from "residual magnetism" in order to self-excite, I believe these are DC units although there may be AC ones too.    
Mark


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## Blackjackjacques (Jan 5, 2018)

MikeInOr said:


> 3ph, or any induction motor, does not have any permanent magnets.  Don't you need to excite the armature coils some how so they produce a magnetic field that will induce a current in the primary windings?  Then wouldn't you also need some mechanism to regulate the output voltage?  Seems like there would be more involved than just spinning a 3ph motor with an engine or another motor?



You are on target and excitation is necessary.  In your standard RPC, excitation is derived from the external single phase applied that also serves to rotate the motor.   Typically, capacitors are used in parallel with the armature to offset the lagging vars just as in pf correction.  High-efficiency motors will usually self-excite without need to momentarily overspeed the motor. However, regulation can be  problem with the biggest problem being overvoltage if the system is not tuned for a specific load or if the load is changed out.   In retrospect, the MG arrangement would indeed be more involved and more complicated than your conventional RPC.


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## Blackjackjacques (Jan 5, 2018)

markba633csi said:


> I think we are going in several different directions here.  Jacque:  The "typical" RPC is comprised of only one motor called an "idler". The synthetic 3 phase output produced from this idler is used to power the lathe or milling machine motor in question.  The idler usually has a horsepower rating equal to or larger than the load it powers.  Some RPC installations have an idler so large and with so much mass that a second smaller "pony" motor is employed to help get the idler up to speed and reduce the huge startup current surge.  This pony motor is then disconnected after a few seconds and freewheels with the idler.
> As far as I understand it, a motor/generator system like you mentioned is less efficient than the RPC scheme, hence they are not used much anymore. The RPC is rugged, relatively inexpensive and decently efficient.
> Mike:  I believe a 3-phase motor does not make a decent generator due to the lack of excitation.  The alternator in your car requires a small amount of excitation from the car battery or there would be no output.  3-phase ac alternators have either permanent magnets (as you mentioned) or a separately excited field winding.
> Some generators operate from "residual magnetism" in order to self-excite, I believe these are DC units although there may be AC ones too.
> Mark



You are correct, the standard RPC is far more reliable and less complicated than what I had proposed.  Aside from the extra efforts needed to induce excitation, the voltage regulation in response to varying loads would introduce far more complexity than needed.


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## markba633csi (Jan 5, 2018)

I believe some Monarch EE lathes used a controlled motor-generator system that was complicated and inefficient, but did work well 
M


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## KBeitz (May 7, 2018)

I build rotary converters.The way I balance the output is with 3 voltage meters and 3 amp meters.
One one on each leg. When things are up and running I add or remove capacitors as needed to balance the system.


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