wiring run caps in RPC

Investigator

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