# 30hp Rpc Vs 20hp Rpc?



## Uglydog (Sep 5, 2016)

I've been offered a used commercially built 30hp RPC from I guy I know just down the street at what seems like a very good price.
I'm wondering if there is any advantage over the used 20hp RPC I've been running for several years.
Currently my incoming power is limited to 60amp. 
Based on the charts a 20hp RPC should power the 3rd leg of the largest motor I'm currently running, a 10hp Ram Turret Lathe.
Looks like if I someday move to a CNC I might want more.
Any opinions on a 30hp? Is it a safer bet for a CNC?
Will a 30hp RPC without a load draw much more electricity than the 20hp RPC?
Will 60amp power the 10hp Ram Turret Lathe and a 30hp RPC? (perhaps the same question as above) 
Is there a downside to over-sizing an RPC?
I'm not sure that I need her. But, the price I've been offered seems hot!
Opinions?
If I make the jump, then I post the 20hp RPC in HM classifieds, and pass along my savings, if there is anyone interested?

Daryl
MN


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## Ulma Doctor (Sep 6, 2016)

my opinion is don't fix something that is not broken-
if you are working just fine with a 20 hp unit, you will gain absolutely nothing by powering a 30 hp unit until you have surpassed the ability of the 20 hp unit.
the 30 hp idling will pull somewhere around an extra 10 amps more than a 20 hp unit.
save your money until you need more juice unless the deal is a unicorn


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## Uglydog (Sep 6, 2016)

Thanks for the straight forward response!

Anybody in the MN area who needs a 30hp RPC?

Daryl
MN


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## Keith Foor (Sep 7, 2016)

Couple thoughts.  And it seems that you know what you are doing so I am just gonna share for anyone else.

RPC's are expandable.  Meaning if I have a 20HP RPC and I connect another 20HP idler motor and the run caps for it, I now have a 40 HP RPC.  This even applies when you are turning on machines but to a lesser degree because there are not run caps on the machines, but it still adds to the RPC you are running.   CNC equipment however does not do this nor does any equipment that is running off electronic control like a VFD.  And here's an interesting fact.  You can start an idler motor that is TWICE the size of the RPC you have running with it.  So if your RPC is 10 HP, starting a 20 HP idler motor with it is NOT going to be a problem.  And once the second idler motor is running you can just switch in the run caps for it and the RPC is now a 30HP setup with a 10 and a 20.  When doing this motors should be the same type and RPM though.  Just some food for thought when you decide you need a bigger RPC.


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## Uglydog (Sep 7, 2016)

Know what I'm doing? 
Nope, I'm just a guy with a good haircut.



Keith Foor said:


> And once the second idler motor is running you can just switch in the run caps for it.



Please explain and elaborate!
As I'm eager to learn.

Thank you,
Daryl
MN


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## Ulma Doctor (Sep 7, 2016)

simply put, you can add 3 phase motors to a circuit to increase total capacity- of course you are limited to total circuit amps by the incoming wire size to the first RPC
you can't run 100hp total on #10 wire (it draws way too much power)
but you can add motors up to input wire capacity.
each motor, large or small, that you add - increases the capacity (strength) of the total RPC circuit to auto generate the third leg.
the run capacitors would need a interrupt of some sort so that the run caps would not interfere with the starting of the subsequent motors.
their direction of rotation may be induced to turn in the opposing rotation, in essence running backward.
when the run caps are out of circuit there is only bias by the circuits normal flow on startup.
after rotation has begun the run caps can be added back into circuit to perform the task of giving an extra push for the generated leg, thus better electrically balancing the motor load


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## Keith Foor (Sep 7, 2016)

OK  Multi-Idler phase converters.  
RPC's or rotary phase converters are a reasonably simple unit for creating 3 phase from single phase within reason.  There are a lot of pluses to them and a couple minuses.  One of the BIG negatives to them is the required starting current is several times that of what starting the same motor on 3 phase would be.  This rears its head around 30 HP and by 50 HP its real dominate.  So long story short, while not impossible, starting a single motor 100 HP RPC would draw somewhere in the neighborhood of 400 amps, would require a HUGE bank of start caps and even the wiring would be  difficult as you will never get 400 amps across a bunch of 12 or 10 gauge wire running to the caps, and finding 1/4 inch slip on connectors for 0/2 copper wire,,, well I never seen them.  So how do you do a 100HP RPC?  With 5 20HP idler motors that are in parallel.  The wiring is a bit of a challenge but the idler branch circuits are only going to require #8  wire even though the output and input are 0/2 copper.  The best way to understand who it works is to understand how it starts, at least how I do it.  We are going to assume that all motors are the same model here and the run capacitor values are the same for each of them.  So when you push the start button, the input contactor closes and the start capacitor contactor control timer closes connecting the start caps to the first idler.  The timer runs for the time needed to start the first 20 HP motor.  Once that timer runs out, the start cap contactor opens, which sends power to the first output branch contactor which closes and applies power to the output bus.  The rest of the idlers are all attached to the output bus via there own contactors and the run caps for those motors are all connected to the idler motors via another contactor.  The reason for the run capacitors to have their own contactors is the output voltage on the first motor if it had 100 HP worth of run capacitors connected to it would be somewhere around 600 volts.   This voltage could back feed and cause serious issues.  SO back to the start sequence.  Once the first branch output contactor closes and applies power to the output bus.  There is 3 phase power to start the next motor.  So a second set of contacts on that branch contactor sends power to the second idler capacitor control timer that holds open the second idler motor capacitor contactor.  At the same time it sends power to the idler motor branch output contactor that connects the idler to the output bus starting the motor.  Once the branch run capacitor countdown timer expires the run cap contactor closes and you now effectively have a 40 HP RPC running.  That timer also sends power to the next branch run cap timer and branch circuit output contactor starting the next idler motor and then switching the run capacitors into the branch circuit.  This continues until the 5 motors are running and then the master RPC output contactor is closed connecting the 3 phase output to the 3 phase circuit breaker panel feeding the shop.  Power is NOT applied to the RPC output until all motors are running.  Now, there are some serious benefits to this design.  First is that it's very expandible.  If you have enough input power available, you could build this design to ANY size.  Second is that if you wire control bypass switches in, you can control the size of the RPC based on the current need for power.  Of course you can run one 5 horse lathe on a 100 HP RPC but it's gonna cost you a mint in electric bill to do it.  

I have never really sat down to consider how to do automatic control.. and I never thought much about bringing more capacity online if only one motor is running and the rest are bypassed, but I don't think it would be hard in either case.  I ahve only done one 100HP setup.  The rest were 40's and a couple 60's to run big CNC machines.  But again, if you had 400 AMP service in the shop. There is no reason you couldn't build one that was 200 HP like this.  And because the branch circuits are only 20 HP each the internal wiring on them wouldn't need to be  huge welding cable.  Only the input output bus would require the large wire.


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## John Hasler (Sep 7, 2016)

Keith Foor said:


> So how do you do a 100HP RPC?


Spin it up with a much smaller single phase motor and close the contactors after it is up to speed.


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## Keith Foor (Sep 8, 2016)

John Hasler said:


> Spin it up with a much smaller single phase motor and close the contactors after it is up to speed.



Yes, this works too.  And if you are always needing 100HP then it's great.  I am not gonna say that how I do it isn't complex, because it is.  But with bypass switches, I can start any part of my design and have lower power usage when it's not needed.


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## Keith Foor (Sep 8, 2016)

Another side note to RPC's and starting 3 phase motors with them.  I have started without issue up to double the size of the idler motor.  I had a 10 HP converter for a while and it would start a 20 HP motor that I was testing that had no load and a bare shaft with no problem at all.


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## Ulma Doctor (Sep 8, 2016)

just don't try to utilize the generated leg for control voltage from any RPC.
in other words you'd normally not hook up the generated leg of a rpc to power a:
transformer
magnetic contactor
relay (with the exception of potential relay)
control switch
(unregulated) computer controlled device or other sensitive unregulated electronic or other expensive devise you wouldn't want to loose by accident


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## dieselshadow (Sep 8, 2016)

^^^ Great advice. ^^^

That generated leg isn't as smooth and has a "dirty" sine wave of you will. You don't want to mess up critical components if you can prevent it.


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