3 HP RPC running a 5 HP motor

I have a 3 HP Phase-a-matic that i am using to run my tools. I have used it to run my 5 HP Cincy Drill Press with no issues.

I am now going to use it to power a 5 HP air compressor. My question is, is the 3 HP RPC only supplying enough to make the 5 HP motor acutally run @ 3 HP?

I have not noticed any problems with the 5 HP drill press when it's running. Seems like plenty of power although i have never had it hooked to a 5 HP source to compare it to.

Note: i only use one machine at a time, so the RPC doesn't get stressed.

Thanks
Caspian

My guess is that you are only running the 5hp drill press at around 3hp or less, if the 3hp converter is holding together. I don't know how robust the Phase-a-matics are built, but I am surprised the starting current does not trip out a breaker when you started the drill.
My guess is the compressor will draw a lot more power, but that depends on how the compressor motor was sized, and what pressure you are running versus the design pressures of the compressor. Running might be possible, but starting is often the issue, assuming proper breakers installed for protection.

RPC is made out of a three phase motor ( with caps..ect ). On the nameplate of the (idler) motor, you would have information on voltage input, rpm, amps..ect. Since you are using the motor as a RPC, the full load current output would be the same as if it was a standard three phase motor.
On your drill press, I bet the load was never over 3 hp... thats why the 3hp RPC can run that application.
Now on a air compressor, its a different story. As the air compressor manufacture knows how much hp is required to turn the compressor pump a certain rpm, in order to get max psi and cfm. RPC used on a air compressor should be sized 2 to 3 times the hp of the compressor motor, because of heavy startup demand.

the compressor is a partial "parts compressor". the owner said when he orgianlly bought it, it had a 1.5 HP motor on it. he replaced it with the 5 HP, replaced the check valve and electronics.

Here it is: http://www.practicalmachinist.com/v...-worthington-compressor-60-gallon-tan-208575/

the pump is the same on that was orginally with the 1.5 morot, so even powered @ 3 HP, it should be o.k., right?

thanks
caspian

Starting current for the motor might be the downfall. Once running, it should be fine. But if the motor has a nominal 3x running current for the starting current, that could cause problems.
If it does, switch to a smaller motor.

Hmm...I know alot of guys dont like the S word...but since your motor is 3 times larger than the first motor. I would go with a static converter, with the hp loss thru the static your 5hp motor would be derated to about 2.5 or 3hp, plus with the static you would not have to turn it on before the compressor motor started ( as with a RPC you would ).

Is the motor rpm over 3000 RPM? many statics will not start a 2 pole motor with any load attached without starting a pony motor first. This means building a control panel to start the pony motor then after a delay start the air compressor when the compressor turns off everything is shut off.

Last question Is the compressor running faster than it did with a 1.5 HP motor? If not change the motor to something more suitable.

i think the load comes from the compressor not the motor
fire it up, put a clamp on amp probe on it, watch what it does
the tricky spots will be start up and when its get up to pressure just before it shuts off, start up is only an instant but the top end could be half a minute or so.
i'd suppose a 3 hp rpc should be able to handle 15 amps or so on 230volts, should be some rating on a store bought device.
and before someone flames me saying "thats kinda lousy goosy".
the best thing to do is call mfg. and abide by there opinion, which will be 3 hp max is 3 hp max

"My guess is that you are only running the 5hp drill press at around 3hp or less, if the 3hp converter is holding together."


Exactly! Using an amp meter I have found your average motor on a piece of machinery runs on about 40% of name plate hp unless it is heavily loaded. A 3hp mill taking light cuts is usually barely making 1hp.

I have had problems in the past with small Phase-o-matics staring up a US made mill motor (unloaded) when the RPC had the same rating as the mill motor. Sounds like your's is better. You might consider scrounging up a spare 2 or 3 HP 3 phase motor that you start after the RPC has started as the unloaded "slave" 3 phase motor willl act as a third leg generator and allow you to run a three phase motor larger than the rating of the RPC alone. If you search this forum you will find more info on that technique.

i'm with crr
a little idler in the loop makes sense

3 hp max is 3 hp max

Click to expand...

+1

The Phase-A-Matic website rates the R-3 to run a maximum 3 hp motor, but goes on to say that:

Almost all machines require sizing the converter 50% larger or more than the largest HP motor of your machine.

problems in the past with small Phase-o-matics

Click to expand...

This is where you need to step back & take a close look at the label on the RPC

Phase-O-Matic is made by Karam Mfg Co in Berlin, Wisconsin. The company's been in business for 35 years, meaning that they started ten years after Phase-A-Matic (located in Palmdale, CA). Not a bad idea to let one company get started, make a solid reputation in the market, and then copy their name changing only one letter.

Why do I know this? Only because my first commercial RPC was made by Phase-O-Matic ... which the seller told me was Phase-A-Matic. The difference is night and day. POM rates their RT-3 to run 3 hp (same as PAM) but rates it for only 6 hp total load. Phase-A-Matic rates the R-3 to run a total of 9 hp, a 50% higher rating for total load.

The Phase-O-Matic was the nosiest commercial RPC I've ever used. Thank goodness a friend wanted it more than I did & I sold it in a heartbeat. What a dog

Phase-A-Matic, Inc. Rotary Phase Converter Installation Instructions

Karam Manufacturing - THE PHASE-O-MATIC -  Product Specifications - The easiest way to get three phase power from a single phase source! - Karam Manufacturing, Karam Manufacturing Inc., karam mfg, www.karammfg.com, rotary phase converters, phase conv

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Barry Milton