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wiring run caps in RPC

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Investigator

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#1
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?
 

markba633csi

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#2
Investigator: Which calculation did you use? 100 mF sounds a bit low, even per leg
Mark S.
 

Investigator

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#4
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.
 

markba633csi

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#5
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

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#6
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 :cautious:
 

markba633csi

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#7
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
 

Investigator

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#8
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
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 :cautious:
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.
 

markba633csi

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#9
Jim: is there a good article he can read on this topic? Something that also covers the PF correction too?
Mark S.
 

Ulma Doctor

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#11
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
 

Ulma Doctor

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#12
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.
 

Investigator

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#13
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?
 

Ulma Doctor

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#14
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
 

Ulma Doctor

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#16
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
 

JimDawson

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#17
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.
 

Karl_T

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#18
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

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#19
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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Tink

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#20
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
 

Investigator

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#21
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!!!)
.
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.
 

Ulma Doctor

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#22
you did everything correctly!!! :aok:
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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Tink

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#24
You could add some run capacitance across L2-L3 to try to balance the voltage a little better.
 
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