# Thread: Cost of running a RPC vs a VFD?

1. ## Cost of running a RPC vs a VFD?

Im wondering how much more it will cost me to run my 3hp milling machine on a 10hp RPC compaired to a 7.5hp VFD, I would think it would be cheaper to run the vfd but I thought I would see what you guys think.

Thanks Mike

2. A lot cheaper with VFD.
Al.

3. doing the math.....

10hp 230v motor typically draws 25 amps full load and is 90% eficient:

NEMA Design B Electrical Motors
Electrical motors constructed according NEMA Design B must meet the efficiencies below:

Power
(hp) Minimum Nominal Efficiency1)
1 - 4 78.8
5 - 9 84.0
10 - 19 85.5
20 - 49 88.5

so go with 90%....

vfd is like 97% efficient....

question: why 7.5hp size for 3hp motor> why not pick a 3hp model? Irregardless, it is still 97% efficient.

so you can do lots of math with this.... say 7% higher efficiency on 3hp = .21hp or 15 watts higher efficiency for vfd than rpc. also, rpc has more losses generating 3rd leg so probably only 80% efficient....research it here as example:

Mythical RPC efficiency.... - Practical Machinist - Largest Manufacturing Technology Forum on the Web

so figure 80% (probably way high) vs 97% for vfd.... thats 17% of ur 3hp motor less for rpc or 17%*3hp= 380watts less... so you will burn 380 watts more while pulling 3hp witih rpc than vfd.... BUT.. if spindle not turning, then vfd is not pulling ANY power (ok, if turned on maybe 20 watts for logic supply inside), but if rpc, then it is spinning away pulling power while your spindle is not running - LOTS less eficiency! see post shown above - some suggest rpc is more like 40% efficient when u consider this.....

in summary, Al said it clearly: vfd is much higher efficiency than rpc.

4. Thanks for the replys,I am using the 7.5hp vfd because it will have to be derated in order to make it run on single phase and from what I read it has to be derated to about half the hp of the vfd so I needed 6hp to get a 3hp motor to run and I already have the 7.5hp unit. So I think I will run the mill off of the vfd instead of the rpc.

Thanks Mike