Phase converter

[Dave Bureau]

I'll looking at a 30 hp rotory phase converter. what size 3 phase motor can i run off that?

[Rob Russell]

A 30 HP RPC? That's a big 'un! A 30 HP motor has a Full Load Ampere rating from the NEC tables of 80 amps at 230v. You'll need a good sized service to start and run that baby plus your load motors.

You could start up to a 30 HP motor, depending on the load and motor you're trying to start. There are dual winding motors that reduce the amount of inrush current needed to get the motor spinning. It also depends on whether the motor is under load or not - for example trying to start a honker of an air compressor that didn't have a pressure relief valve would be an example of starting under load.

Beyond that, remembering some of the posts from a machinists forum where they do a lot with RPCs, you should have no problem running up to 20, even 25 HP motors.

Better get some help moving that 30HP RPC!

Rob
Addy protocol: unlicensed, homeowner electrician

[Bernhard Lampert]

First off, it is more reliable to compare and base comparisons of AC electric motors on NEMA T frame sizes. HP rating may be exagerated by the particular manufacturer and there is no 'standard' way to determine HP across manufacturers. NEMA frame sizes are a standard and a AC motor of a particluar frame size will have the same power regardless of source.
In general, the rotary phase converter should be 50% oversized to account for start-up/in rush currents/voatlge fluctuations, etc. So the 30hp should start a 15hp motor.
..my 2 cents
Bernhard

[Dave Bureau]

I havent seen it yet but by your discription I'ts probably overkill for what i might use it for. I only have a 100 amp service in my shop. I'ts was $800. the guy said he would take $500. but if its to big its to big. I dont even know if thats a good price or not.
Dave

[Rob Russell]

In general, the rotary phase converter should be 50% oversized to account for start-up/in rush currents/voatlge fluctuations, etc. So the 30hp should start a 15hp motor.

At 50% oversizing, a 30HP RPC would start a 20HP motor ... at least 20 * 1.5 = 30 when I do the math :D.

[Rob Russell]

Dave,

What 3-phase motors are you trying to run and for what kind of machinery?

$500 for a 30HP RPC is definitely a good price, but that could be like having a Caterpillar bulldozer to scrape snow off of your driveway.

Rob

[Dave Bureau]

I dont have 3 phase equipment yet but i always see good deals on 3 phase stuff. all i really need is a good planer about 18-24" would be good. that would be about 7-10 Hp. what kind of service would i need to run this 30 hp converter and 1 peice of equipment at a time?
Dave

[Rob Russell]

If you're looking for an industrial planer in the 18-24" size, think of a good ole' Powermatic. The 180 or 225 would do what you want. Those will have motors ranging from 7.5 HP to 15HP on some that were configured for really heavy duty.

I'd frankly give up on that 30HP RPC. Without digging into the details, you're probably looking at a 150-200 amp service to get that going. On top of that, unless you used a pony motor (a separate, smaller motor you use to get the RPC's idler motor up to speed before you supply power to it), the startup current required to get it going would literally make your neighbor's lights blink.

To give you an idea of amperage draw:
- a single-phase, 230v, 7.5HP motor has a FLA rating of 40 amps.
- a 3-phase, 230v, 7.5HP motor has a FLA rating of 22 amps.

If that's the sort of motor you want to run, a 10 HP RPC should be big enough to start and run a 7.5HP machine. You'd need a 50-60 amp circuit to drive the RPC and machine. A 'rule of thumb' I've seen posted (on a machinist forum that has a specific subforum dedicated to 3-phase power conversion, RPCs and VFDs) is 5 amp @ 240v single-phase for each HP of 3-phase.

[Neal Flatley]

Be careful how these things are rated. To some " 30Hp" means you can run up to 30 HP combined, but starting, its only rated for a 10HP motor.

Typically the "15HP" Converters are rated to start a 5 HP motor and run 15 HP Combined, ie starting one machine at a time up to 15 HP, with no one machine exceeding 5 HP. In this configuration the idler motor is a 7.5 HP, not a 15HP, and thus the start up currents are less.

I would be very suprised if that converter actually had a 30 HP idler motor.

You might want to check the actual motor nameplate for the electrical specs.

Neal

[Dave Bureau]

what are those vfd's all about. would i be better off going that route?

[Rob Russell]

what are those vfd's all about. would i be better off going that route?

Probably not. A VFD that could run a 7.5HP 3-phase motor from 240v single-phase is going to be expensive. You'd have to get a 15HP VFD and make sure that it would allow you to program it to generate 3-phase, but derated by a factor of 50%. You're looking at $900+ for something like that.

[Rob Russell]

Be careful how these things are rated. To some " 30Hp" means you can run up to 30 HP combined, but starting, its only rated for a 10HP motor.

Typically the "15HP" Converters are rated to start a 5 HP motor and run 15 HP Combined, ie starting one machine at a time up to 15 HP, with no one machine exceeding 5 HP. In this configuration the idler motor is a 7.5 HP, not a 15HP, and thus the start up currents are less.

I would be very suprised if that converter actually had a 30 HP idler motor.

You might want to check the actual motor nameplate for the electrical specs.

Neal

Valid points. I think of a "15HP RPC" as using a 15HP idler motor because that's what I started off building.

If the RPC that the guy is selling is really a 10HP idler motor, that would be a good size for Dave's application and $500 is a good price IF the RPC is well-designed, well-balanced and in good condition.

[Dave Bureau]

the guy said its a 30hp motor.

[Rob Russell]

the guy said its a 30hp motor.

Bigger than you need or really want.

[Scott Long]

I noticed under the philly CL there is a 15hp SINGLE PHASE 230v motor. If you are in the market for a BIG motor this might be what your are looking for.

http://philadelphia.craigslist.org/mat/597613908.html

[Dave Bureau]

Ya, i'm gonna pass on that beast.
Thanks all, for all the help
Dave