i was attempting to determing the current imbalance of a three phase motor. the mechanical load varied and as you can probably imagine so did the current. this made it difficult to get good numbers because the numbers were always on the move. a clamp on placed around all three motor leads when the current is balanced should read zero. i did this and got a reading of about 750 milliamps. would i be correct in assuming this is the net current imbalance?

Dear Fred F:

Interesting, and logical, but unfortunately not an accurate method of checking for phase unbalance. The reason is that small changes in circuit impedance (including before the motor), power factor and voltage unbalance will impact the unbalance. As a rule, current unbalance can be many times the value of voltage unbalance, depending on the size, design and circuit related to the individual motor.

I have actually been doing a Blog on this topic for Terry O'Hanlon's Motor Blogs. Presently, I am covering all methods of motor tests, with explanations and recommended test limits (as well as the strengths and weaknesses of each).

The Blog can be found: http://maintenancetalk.com/blog.php/motorblog/

It just so happens that the past two weeks covered voltage and current testing.

Sincerely,
Howard

If a current clamp is clamped on all 3 phases, it should measure 0 amps on a perfect system as the sum of the 3 phases equal 0. The 750 mA reading that you got would measure current taking a different return path to its supply, such as leakage to ground. That's my 2 cents worth. Perhaps I'll be shown otherwise.

J-

I agree with Wally that in theory a clamp-on around all 3 phases should read zero, regardless of current balance (even unbalanced currents should sum to zero assuming there is no current path to ground).

It is also a principle used for ground fault protection... a single window type ct enclosing the three phase conductors. The measured current should read close to zero if measurment is perfect and no ground fault.

You didn't say how big your current was so 75% may be only a small fraction of the total and so perhaps not surprising since there is some error in the measurment. Sometimes if you move a handheld clamp-on a little bit you can affect the measurement for whatever reason (something to do with the flux path I think). So it doesn't surprise me a clampon is also inaccurate for the sum of three currents. Another measurement error might be having a double-grounded metallic shield (such as for higher voltage cables) passing through the clamp-on although I suspect this is 480v or below and if so wouldn't have shielded cable.

I think the probability of discovering a ground fault with a clamp-on is pretty small (most likely it would trip something or escalate and not just conduct at 0.75A). I think it is much more likely you are just seeing measurment error. (if measurement were perfect you sould see 0).

Pete, doesn't it depend on the type of grounding used in the three phase system? We use a floating ground delta system, and I've seen motors with one leg shorted directly to ground running with no problems at all.

You are right that the type of system affects how the system responds to a single ground.

It doesn't change any of my answers.

If you put a clamp-on around all 3 leads you expect zero current within the measurement error. If you happen to be reading a motor with a single ground on an otherwise ungrounded system you should still get zero (no ground current is flowing).

If you read non-zero current and you are sure it is above the measurement error, then there is an actual ground current flowing. This is a fault in-progress... very unlikely and the motor would likely trip very soon.