# How Muh Does It Cost To Run A Rpc ?



## rock_breaker (Apr 21, 2016)

I have a 15 HP three phase motor that I am considering for a RPC but am curious about electrical costs. It would seem to me that it be close to 15 HP x (0.746 KW/1 HP) x ($0.115/1 KW Hour) = $ 1.33 per hour where my power cost is $0.115 per KW hour. Granted the 746 watts per HP does not take in efficiency so the cost would be somewhat higher.
Have a good day!
Ray


----------



## British Steel (Apr 21, 2016)

Morning Ray,
I don't think I've ever seen proper figures, but my thinking is that it would only draw that under full load (a 10+HP motor running from it, itself at full load) - unloaded I think it would pull much less, only the current to spin the unloaded idler motor so dependent on idler motor's losses in bearings, magnetisation current for the rotor, iron losses, windage? a 15 local-currency-unit AC clamp meter would tell you, but obviously only after you've built and tuned the RPC...

Dave H. (the other one)


----------



## Karl_T (Apr 21, 2016)

The only way to easily measure is to watch the speed of the wheel on the power meter at your service entrance. I have a 35 hp unit that uses about the same as a few 100 watt light bulbs with nothing running in the shop.

My neighbor had a shop with a 50 hp. unit. With nothing running in the shop, it actually made the meter run BACKWARD. It was a somewhat rare case of poor power factor on the electrical lines at the time. The unit was improving the power factor enough to overcome was power was actually used.

https://en.wikipedia.org/wiki/Power_factor


----------



## Bob Korves (Apr 21, 2016)

Put your hand on the RPC after it has been running for a while.  All the power used is converted to heat.  I suspect it will barely be warm.


----------



## timvercoe (Apr 21, 2016)

I have an interesting story, facts a little sketchy..........   A  few yrs ago I lived with my family in an all electric house.  We typically burned about $115 in power a month.  The rate was 10 cents a KW hr.  Wife and kids went on 3 week vacation.  I spent every waking moment in the shop running 15hp phase converter, 3 phase compressor, bridge port and two manual lathes.  Sometimes all at once. Power bill dropped to $50 for the month.  Maybe only one person taking shower each day and one persons laundry and cooking for one?  Any way it seems to me that rotary phase conversion is actually pretty efficient way to do things.   

Have new wife and new garage and same old rotary phase converter.  Some much larger machines and some CNC all which I intend to run off phase converter.  It will be interesting to see what happens to the power bill here.   Tim


----------



## Reeltor (Apr 21, 2016)

My experience with my 15hp RPC is similar to Karl T's; while I don't know if the meter is running backwards with the RPC running, I don't see any increase in usage on the bill.  I don't have anything that I can point at, but in the summer when the A/C is running 24/7 and the pool pump is doing it's thing, I think the RPC actually reduces the amount of metered electricity.


----------



## rock_breaker (Apr 21, 2016)

Good morning guys!
Your responses are certainly encouraging! My equipment is either 120 or 220 single phase but my son in law brought home a large drill press that has three phase motors on it so it may be the first load.
I had overlooked the running at no load vs loaded amperage draw. At my former job the primary crusher had a 600 HP 4160 volt 3 phase motor with an ammeter in the main switch panel that would peg (I believe) at 200 amps on start-up then settle to 82 amps when running empty but it was spinning a 5 ton impactor rock breaker.  Again memory seems to recall 140 amps as "rated" load.
Thanks for the help and have a good day!
Ray


----------



## Keith Foor (May 4, 2016)

While your mileage may vary, I actually see the meter slow down when I am running the RPC and any of the large resistive loads (dryer or electric stove) in the house.  Meters are calibrated to see inductive loads.  Most loads in a house are inductive (motors) when you add capacitance to the load you offset some of the inductance.  For some reason it causes my meter to lag.


----------

