# Thread: Capacitor sizing and power clarification....

1. ## Capacitor sizing and power clarification....

Ok after lots of searching and reading on here... i think i have a pretty good idea of things, but i want to verify / clarify....

For a start cap on a RPC... you need a single cap, of AT LEAST the line voltage (but more doesnt hurt) and at 60 to 70 uf per HP.... some say 100uf/per hp...

so questions remaining are...

1. Is there a limiting factor on the voltage thing? like if you had a 200v cap, and you needed a 110v cap... do you lose or sacrifice anything going with the larger one?

2. the other side of it... I have a cap rated for 200v, 250v max... would this be dangerous to use on a 220/240v system?

3. to figure out #2 you need to know the voltage your dealing with... in a household "220v" system using 2 hot "110v" lines from the breaker box... is it really 220v? or is it 240v?

4. Why isnt it called 110v 2 phase? as that seems to be what it really is now learning about 3phase... and why is 3phase considered 220v 3 ph... seems to me that it should be 110v 3ph... or 330 3ph... if 2 legs of 110 =220v then 3 legs of 110v should = 330v? no?

5. My original question was can you have too many uf's? like is there an advantage of 70 over 60 per hp? is there anything wrong with 100uf/hp?
I gather that higher uf ratings can start a motor faster... but draw more power... but i heave also heard it said that too large a cap can actually hurt starting, and make things take longer to start up and still use tons of power.
I have a whole bank of 5400uf caps that i pulled out of an old machine that i updated. I need a start cap for my RPC, and was hoping i could use some of these... But i assume these are WAY WAY overkill for a 7.5hp idler. I just hate to go out and buy components when i have a box of them already sitting in front of me.

6. assuming i cant use them for my RPC, any idea what would actually need 5400uf caps?? and like 6 of them!!! im just amazed at what could possibly use that. they are rated for 200v, 250v max.
If i cant use them for my RPC, i would really love to figure out another use for them... just seems too cool to have a box full of them and not do something cool with them.

2. Using a higher voltage rated capacitor is perfectly OK, but need to be rated for the voltage you are working on with allowance for surge margins etc.
You need to tune a RPC, see the post by Fitch Williams here.
www.metalworking.com - /dropbox/_1998_retired_files/
Look for the FRW files.
N.A. residential supply is now generally 120/240, the old standard was 110/220.
The origin is a 240v centre tapped single phase transformer, the phase difference is still 180 whether 120 or 240, hence single phase.
Even if you were to use 2 phases of a 3 phase supply it would be termed single phase if there is no other reference involved, i.e. neutral etc. As the phase difference would still be 180.
The phase angle difference of 3 phase is 120 deg, but this is referenced from a 0 or neutral point.
The Run capacitors Must be AC continuous rated, only the start cap. can be intermediate rated, due to the large value, a continuous rated start cap. would be quite expensive.
Al.

3. Originally Posted by Al_The_Man
The origin is a 240v centre tapped single phase transformer, the phase difference is still 180 whether 120 or 240, hence single phase.
Even if you were to use 2 phases of a 3 phase supply it would be termed single phase if there is no other reference involved, i.e. neutral etc. As the phase difference would still be 180.
The phase angle difference of 3 phase is 120 deg, but this is referenced from a 0 or neutral point.
Al.
very interesting.... but now i am more confused.... if that is the case... then how does a RPC even work? or a static for that matter...

if the 2 phases of 220v are phased at 180... then how does a 3phase idler even run on single phase? and creating the third phase with a idler... wouldnt that just give you a 180 180 and 120?

guess i am still confused on why 120v + 120v = 240v @ 180 phase but
120v + 120v + 120v does not = 360v @ 120 phase.

but the single phase and 3 phase difference makes sence now...

4. Originally Posted by Ksoggs
very interesting.... but now i am more confused.... if that is the case... then how does a RPC even work? or a static for that matter...

if the 2 phases of 220v are phased at 180... then how does a 3phase idler even run on single phase? and creating the third phase with a idler... wouldnt that just give you a 180 180 and 120?

guess i am still confused on why 120v + 120v = 240v @ 180 phase but
120v + 120v + 120v does not = 360v @ 120 phase.

.
You have to study it graphically and observe where you are measuring or observing from using a vector diagram.
3 phases do not occur with a simultaneous peak, hence 3x120 does not = 360, they have a angular peak difference of 120 deg. if you observe from a common or neutral vector point.
Look at the graph output in the FRW files.
When you input 1 ph 240 into a RPC you would be trying to rotate it with 1 phase, which will not work, BUT once you start the rotor turning by some means the stator field induces a field into the rotor bars and produces a generated output on the 3rd phase leg.
If you look at the incoming 1 phase relative to the artificial leg with a central 0 imaginary vector point, you will now see three phases displaced 120 deg from each other.
Just as if you input 1 phase to a 1 phase induction motor, it will just sit and hum and not rotate, unless a means of introducing a simulated 2 phase, (split phase), typically via a phase shift capacitor to a second winding to start it rotating.
Once it is running, you can actually remove the cap.
Al.