Is my 3ph motor good?

[Chuck Saunders]

Well I have bought used 10hp 3ph Baldor motor and when I hook it up to 220 and give it a spin the motor rumbles and slows down immediatly to a slow speed and if left on will trip the breaker after 30 seconds. Regardless of direction turned or which 2 leads are used, the same result is obtained. I guess tomorrow I will have to get the meter out and start checking.
Any ideas?

Baldor 10hp 3ph 230/460
cat# EN3313T
spec# 37FG144568

[David DeCristoforo]

Do you have 3ph power? Does not sound like it. You would actually have four leads, three load and one neutral. Sounds like you are trying to run the motor on single phase 220....

[John Kaner]

Chuck,
A 3 Phase motor needs 3-hot wires to run, one for each phase. Industrial motors use 3 ph. Normal residential power supply is single phase 220 v.

Good News: You can run one on single phase with an Inverter. The Inverter takes that singe phase input and sends out 3 phase output.

Sounds like a great deal on a powerful motor.

[Chuck Saunders]

My mistake for not giving the needd information. No, I do not have 3 phase power and I am trying to run it on single phase . This is all in the effort to build a rotary phase converter. I apologize for leading everyone down the wrong path. For clarification, a three phase motor does not utilize a neutral wire, just the three hots and ground to frame. The neutral may be neded for control wiring.
Thanks for your time and assistance.

[David DeCristoforo]

"a three phase motor does not utilize a neutral...just...ground"

That's what I meant to say... ground... sorry.

[Bruce Wrenn]

Chuck, you need to get it up to speed before applying power. A pony motor of some sorts is best suited for this purpose. Pony motor needs to be 1725 RPM, and 1/4-1/2 HP. When I built my neighbor's converter we found some old commercial dryer motors in the junk yard. When they reach speed, voltage appears on one of the leads. This voltage is applied to the coil of a "latching relay" which then applies voltage to leads of motor. Motor becomes a generator. Then power removed from run windings of pony motor. If power is lost for any reason, latching relay drops out and everything coasts to a stop. Biggest cost was fused disconnects, one for single phase and one for three phase. When we built his converter, we were paying $0.15 a pound for used motors.

[Phil Thien]

I'm missing something here. It still appears that he is trying to run a three phase motor on single phase.

I thought the way a rotary phase convertor worked was: A single-phase motor shaft is coupled to a three-phase motor shaft. Single phase incoming current is used to drive the single phase motor, which spins the three-phase motor, which works as a generator and outputs three-phase power.

But I could be wrong, I really don't know anything about this. I just can't imagine a three-phase motor working on single-phase no mater what. In fact, I've heard of factories that have had a phase drop at the pole that have had substantial damage to machines.

[Mike Henderson]

Can someone explain to me how this works, please?

I can understand how you can run a three phase motor on single phase, if you can get the rotor up to near synchronous speed before applying the voltage to the field. So for a two pole motor running on 60Hz, you'd probably get the rotor up to about 3450RPM.

The difference in the speed of the field rotation (3600 RPM) and the rotor rotation (3450 RPM) would be the slip, and because of that slip, current would be induced in the coils of the rotor (as in any induction motor). The three phase motor would continue to run but it would not have the power it would have if running at three phase.

Now, that current in the rotor would create a magnetic field and would cut the stator coils for the other two phases of the motor - and would generate a voltage in those coils. So as someone said earlier, the motor would act like a generator - at least for those other two "phases".

The problem I see (and I may be wrong) is the different hertz of the phases. The phase from the power company will be 60Hz. But the voltage and current in the other two phases is generated by the magnetic field of the rotor, which is 57.67Hz (3450 RPM divided by 60 seconds).

So while you will produce three phase voltage from the motor, the three phases will not have the proper relationship to each other (the two generated phases will have the proper relationship to each other, but not to the supply) and will not have the proper frequency on two of the phases.

So if this works, what am I missing? Or do people run the tool three phase motors on only the two generated phases?

Mike

[Charles P. Wright]

If you google for "rick christopherson phase converter" you will find a page that has some explanations on building them. Some people on another forum have had good luck with his instructions.

Essentially, you need some capacitors to create 3 legs out of your two leg power. This results in a static phase converter. The rotary phase converter adds an idler motor, which runs off of the static phase converter, but cleans up the power.

[Chuck Saunders]

Rick's site has been my guide in large part. He has a lot of other electrical information that is oriented towards woodworking shops.
A 3 phase (3ph) motor will run on single phase (1ph) with a loss of ~20% of the rated power. (this is where a lot of the damage from a dropped leg will come from) The 3ph motor cannot start itself on 1ph unless you add start capacitors.
A rotary phase converter (RPC) will generate the third hot leg to feed 3ph powered equipment. Through the use of run capacitors the voltage differences between the legs can be minimized. Due to losses the size of the motor in the RPC must be at least 125% larger than the motor to be driven.

My problem is since I bought a used motor I do not know for sure that it is actually in good working order. And I am new to the RPC world so I have 1 equation with 2 unknowns.

[Rick Christopherson]

Chuck, Your approach to this is spot-on correct, and is the same thing I would have been doing.

First off, make sure that you are spinning the motor fast enough. Put a dozen or more wraps of string around the motor shaft (not on a pulley), and pull it as fast as you can without breaking the string. For a motor this large, you will need to ramp your pulling speed up to overcome the high inertia.

When you get it spinning fast, then hit it with power. Make sure to not leave power connected if the motor does not instantly begin to speed up on its own, and never let it lope along until the breaker trips.

For a motor this size, there is a good chance that it is 208/360 (or 240/480) dual voltage. Make sure it is configured for the lower voltage.

If you still cannot get it going, then maybe try adding any size capacitor between one hot leg and the missing leg to give it some assistance. Just note that once you put this capacitor in, the motor will spin in only one direction, and reversing the motor direction requires reversing the capacitor to the other hot leg.

[Chuck Saunders]

Thanks Rick glad to know I haven't overlooked a primary step. I have put the meter to all of the leads and they are all <1 ohm and not shorted to ground (via dvm not megger). The motor is marked 230/460 with a additional amp rating for 208. Do you have any suggestions on recommended value for the capacitor?

[William Cummins]

First I would make sure that it's not wired to the high voltage. The DVM will tell you if it's a trashed motor, but will not really tell you as much as a megger will on the windings condition. I would check the voltage and if it's wired on the low voltage settings, I would have some one with 3 phase power spin it up for you. That way you will know that your starting with a good motor. To spin up a 10 hp motor I would try about 3-400mf on the starting caps. You may need more that that when starting it, before you balance it, and less after. Here is a few pictures of the last one I made before I got 3 phase power...

William....

[Chuck Saunders]

You do good work William. There is just something about seeing neat wiring that inspires confidence.

I am sure that I am wired low voltage. I am just wondering who I know that has 3 phase. But I do ned to know that I am working with good motor.

Thanks for your advise, and that goes for everyone here at the creek. Always good to have experienced voices providing light in the dark corners.