Sizing phase converter to Full Load Amp

For the last 5 years I have been running a 10hp American Rotary phase converter. I recently built a new shop and upgraded the service and got all of my machines in one location. I am now looking to upgrade to a 20hp phase converter. My question revolves around how to appropriately select a phase converter.

My question revolves around how machines are labeled as "Full Load Current" vs their actual current draw. The machine in question is a 7.5hp CNC. The book calls for a service of 420v and 32A. The electrical cabinet is marked "Full Load Current" of 55A @ 230v. The previous owner, who is a bit a electrical mad scientist in his own right, was confident my 10hp phase converter would handle running the machine, with issues only occurring at max load at max rpm. He claimed that under 80% spindle load the machine drew 26A.

The 10hp rotary phase converter that I have is labeled as 28A max, 5hp max start, 10hp load.

The crux of the issue comes to purchasing a new phase converter or purchasing a second 10hp to run in parallel.

If I run two 10hp phase converters in parallel, I should have a 52A max load according to what I can find from American Rotary. But according to their website, for a 7.5hp CNC I should only need a 15hp phase converter?

Conveniently, a few of the questions I was going to ask are being answered in the other 20hp phase converter thread! Perfect timing...

The mad scientist knows of what he speaks. Regards, Clark

Keep what you have and see how it works out.

If the 10 hp RPC doesn't work out I would also consider getting a 15hp idler motor and changing the existing RPC internals, which means some capacitors, overloads, and fuses. Maybe the contactor too. With used replacement parts you might even get a better RPC in the long run.

If you have other machines with lower power requirements then what are they? Can you get by with a 5Hp RPC most of the time. Then run a 5 and 10 in parallel with the CNC.

What does the future look like? Any more stuff or do you have everything you need. Is 15Hp going to be your upper limit?

The 7.5 HP is 22A at 230V.

The kicker here is that the unit has at least one VFD in it. Unless the VFD has some form of Power factor Controller in it, which some do, it draws an rms current that is larger than the nominal, because it is a "rectified load". The 22A can turn into something more, because of the lousy power factor characteristic of rectified loads. Probably up to 30% extra.

then, if there are other loads, if the main spindle is 7.5 HP, but there are a couple of 2 or 3 HP axis drives, the total can be higher than expected from the nominal.

The NEC states:
"The full-load current shown on the nameplate shall not
be less than the sum of the full-load currents required for
all motors and other equipment that may be in operation at
the same time under normal conditions of use. Where unusual
type loads, duty cycles, and so forth require oversized
conductors or permit reduced-size conductors, the required
capacity shall be included in the marked "full-load current."

So, basically, they add up all the motors, and that is the nameplate number. And, if you drive a mill in a full HP cut, diagonally, so both axis motors are in action and maybe the Z as well, then you get the number on the plate.

Depending on how you run the machine, you may never even get close to that.,

Congratulations on the new shop

Something else to keep in mind is not just the CNC loading. Are going to want to run other machines while the CNC is chugging along? Last thing you want is to have the CNC trip out because you fired up a lathe or a manual mill. May be worth running separate RPCs? I'm curious what other folks say.

Thank you all for your responses. I am excited to be moving into the new shop. I have been working in a 16x18 "two car" garage for the last 5 years. As time allows I plan on putting a new shop thread up.

It sounds as though I will be OK with the 10hp. This is great news. It will allow me to at least get the machine powered up, cleaned, leveled, and begin running. I do not do much "heavy" milling, so I am not concerned about reaching the upper limit of draw.

I was told by American Rotary that I can only wire similar sized motors in parallel, ie a 10 HP with a 10 HP due to the wave of the motors? something about them being in sync?

I think I will eventually plan on adding a second 10 HP as I will be adding an EDM and in the future upgrading to a newer CNC or possibly a small turning center. The EDM similarly is tagged 60A Full Load Current, but what little I do know, EDM's do draw a lot of power. This may be 3 to 5 years down the road, sooner if business is good, but not planned for a few years. (EDM is in storage right now)