View Full Version : Electric motor Question " what would you do??"

11-13-2004, 11:03 PM
Ok guys, here's the rub:
3hp 208 volt 3ph motor running on a 240 volt RPC.
It gets noisier and vibrates as it gets warmer.

I've tried to balance the converter as best I can, and get about 215-225 volts on the generated leg with the lathe running.

I'm not happy with the performance of this setup and need to change something here (probably the motor)

To make matters worse, I only have a 30 amp 240 volt circuit to my garage.

As I see it I can do one of the following:

1) replace the motor with a 240volt single phase, but I'm not sure if I have the power for that. ( motor currently on sale for $319 cdn)

2) replace the 208volt 3ph motor with a 240volt 3ph , and run it on the existing RPC ( motor sells for about $220cdn)

3) Buy a VFD and try the existing 208 volt motor on the VFD ( about $400)

4) buy a new 3hp 3ph motor and a VFD and enjoy all the benefits that I've heard about.

As always cheaper is better, but "fixed" is the best bargain.

any advise would be appreciated


11-13-2004, 11:38 PM
When you step up into 3hp, the inverters get pricy.. My 2hp I think was about $239.. I THINK a 3hp is over twice that. Make sure you get a single phase input model.

You can't make real three phase at home. With a phase convertor and a Idle motor you can generate pulses but they never actually time out to real 120 degree pulses.

My bridgeport sounded like a bucket full of marbles on a convertor. That is why I put a inverter on it. Motor got hot too. I never installed a idle motor thou.

I have since installed a 10 hp idle motor and am running the 3ph pump on the hyd press just fine with it, no RPC capacitor "magic" box. As a matter of fact I don't know where the one off the bridgeport went? I have looked for a couple hours for it. THE pump motor does not get too hot to touch. It is quieter than it was with the inverter driving it off the chain-link conveyor thou. Not sure what kind of silly pump that is, it runs and builds same pressure both directions... Of course you have to realize the pump sees actual hp duty cycle of ten seconds per minute.. WHEN it is on bottom thumping it builds pressure. The valving dumps it otherwise..
I can't wait to put it on ebay and run the shop off the Ingersol Rand power unit outside... Noisy rascal that it is.. you need ear muffs when it comes under pressure..

David Cofer, Of:
Tunnel Hill, North Georgia

11-13-2004, 11:49 PM
I run a RPC (5hp) on my Boss 3. I built the convertor and balanced it as close as possible. The mill runs smooth and quiet, the RPC is louder.

So, back to your problem. If you are certain the three legs are "puttin out" then the 3hp motor is suspect. You could have it checked and rewound if needed. You could go with a VFD but I still suspect your motor as being in need of repair.JRouche

11-14-2004, 12:03 AM
My original home made 3ph converter was built using a 5hp idler motor. When I got my horizontal mill it came with a 3hp 3ph motor. My converter would only get it running if the mill was in low speed and no load (belts slack). It would occasionally trip the 30 amp breaker and the converter motor would rattle. When I bought the BP clone it also had a 3hp 3ph motor. It would not start it at all (kept blowing the breaker). I then had to hook up a roto-phase converter (5hp capacity) and it worked like a charm. The Roto-Phase had to be wired to a 60 amp breaker with heavier wire. I have had no problems since. The size of the idler motor is definitly a factor. I often wondered about the size of idler motor as compared to the size of machine motor. Also if the older design motors (sleeve bearing/heavy cast iron case) would make a better idler. I do realize the amperage draw has a lot to do with the size of the idler but I'm not that electrically inclined.


J Tiers
11-14-2004, 12:20 AM
You said "vibrating", but vibrating HOW?

Like unbalanced mechanically?

Or buzzing electrically?

it makes quite a difference, as mechanical obviously would point you at bearings and so forth.

Electrical buzzing or vibrating could be loose windings, unbalanced phases when warmed up (loose/bad/wrong parts) etc, etc.