3-Phase convertor help...

Old 12-10-08, 03:04 PM
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3-Phase convertor help...

I have a few old 220V - 3 phase electric motors attached to some old home-built machines that I am looking to restore, the only problem is that I do not have 3-phase electric running into my garage and the cost of doing so would be in upwards of $6000 (or more according to JCP&L).

I have a 200-Amp, 220V, single phase service in the garage and have been looking at rotary convertors from Americanrotary.com. The largest motor I have in my garage is 15Hp, 37 Amp, 220V, 3-phase and I will only be running one machine at a time. The motor is quite old, I called GE to see if they could date it though they could not.

1) Is a rotary convertor my best bet in this situation?
2) If so, what size motor should I purchase? American rotary suggests a 40hp - does that seem like overkill?

Would appreciate any help or leads as to a better solution.

Old 12-10-08, 04:12 PM
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There is a way you can run a 3 phase motor from a single phase supply.
When I took my electrical training in Germany many, many moons ago I remember us doing this.
I can't recall all the details but you can hook up your single phase supply wires to any 2 of the 3 phase terminals of the motor and connect a capacitor of the appropriate voltage between the remaining 3rd phase and the hot terminal of the supply. It introduced enough of a phaseshift to run the motor. I think the motor we had was delta configuration, I don't know if it would work with the Y config.
In any case, it worked when we tried it although I'm fairly sure that the motor didn't develop full power. Also the motors we tried it with were low HP motors whereas yours is one hefty pup.
I also don't remember the values of the capacitors we used.
It would be fun to try it though
Old 12-10-08, 05:11 PM
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The only way to get true three-phase power from a single-phase supply is by using a single-phase motor to drive a three-phase generator, either two machines with a shaft coupling or a specially made machine that has multiple coils and a wound rotor.

That stated you can often simulate three-phase and that will be good enough for the job. Unless you have a dual-voltage star (wye) wound motor and you re-connect the coils you will have a definite loss of power. Static converters using both running and starting capacitors can achieve full power in smaller (7-1/2 hp. or smaller) motors IF the proper connections are made. This is preferable to using a separate motor as a rotary transformer if the motor allows the reconnection.

Most DIY three-phase converters use a separate three-phase motor idling to act as a rotary transformer and simulate the three phases. This often works well although it does have the disadvantage of having reduced power from the motor on the tool. It also means having the idler motor running with the additional noise and power consumption and any mechanical problems that might arise.

I have built both kinds of converters and been happy with them both. I prefer the static converter when it can be used.

One thing that I can assure you of is that unless you live in an industrial area (and probably even then) if you run a 15 horsepower motor on a single phase line your utility is not going to be happy, perhaps very UNhappy.

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