Hi everyone,
We have a motor that its nameplate: 110kW 380V 202A 2965rpm cos fi:0,87. While it is in operation with load it operates differantial protection. And we took the motor to workshop. We tested the motor and this is the result ;
DC resistance 37mOhm,35mOhm,34mOhm
Megger test 1mn 7900 MOhm 10mn 12300 MOhm
Baker Surge test EAR1-2 1%
EAR1-3 1%
EAR2-3 2%
According to the tests the motor looks normal. But we run it at our test panel we see different currents at three phases.
Phase1 : 37A
Phase2 : 38A
Phase3 : 45A
How can we explain the situation?
Highly appreciate in advance.

Unbalanced current under no-load condition is expected if there is any slight unbalance in the supply system (which there usually is). It is the subject of several recent threads in this forum. I would not be concerned about the current unbalance numbers unless someone told me the motor was powered from a vfd electronic drive... I did observe this weekend a motor shop with special vfd drive used unloaded runs of large motors with flywheels (reduces startup time compared to reduced voltage without increasing rotor heating)... in that case I observed the full-speed, no-load current on a 1200rpm 13.2kv 8000hp motor was very well balanced (137A, 137A, 138A). But for most power supplies there is a voltage unbalance that creates current unbalance similar to what you reported.

As for your other test results, I haven't looked at them closely.

Hi electricpete,
I agree with you but we surveyed the supply system. We run motors like this from direct starts. So there were balance at three phases. They were 390V. I'm concern that this problem is related with motor. But what?

quote:

there were balance at three phases.

Again sticking with the issue of current unbalance at no-load, it was discussed here:
http://maintenanceforums.com/e...=408107006#408107006

And particularly a google book link here:
http://books.google.com/books?...0currents%22&f=false
The google book link suggests that the unbalance of current is 12-15 times as high as the unbalance in voltage. Jan posted some data in the thread where it was around 20 times as high (but still correlated to the voltage unbalance).

So it does not take much voltage unbalance to produce a very large current unbalance at no-load. I personally would not worry about the current unbalance at no-load as a symptom. (If you want to investigate that aspect further, then add a load and see what it does).

You had a differential trip so you need to investigate the motor, cable, relaying and associated wiring and settings, but this particular symptom of unbalanced current at no load should not suggest any problem with the motor... it is normal.

muratti,

Where on all of Gods green earth is izmir?

D

I agree with electricpete about the current imbalance with no load. However, according to your resistance measurements, .034, .035, and .037, you have a 4.7% resistance imbalance. Above 4% is an alarm condition according to PdMA. This could indeed cause negative sequence currents, current imbalance and excessive heating under load, and damage the winding insulation. If you got these readings right at the motor leads and nothing but the motor is in the circuit, I would call a motor repair shop.

quote:

Originally posted by jvoitl:
.... However, according to your resistance measurements, .034, .035, and .037, you have a 4.7% resistance imbalance. Above 4% is an alarm condition according to PdMA. This could indeed cause negative sequence currents, current imbalance and excessive heating under load, and damage the winding insulation. .....

Hoping it may help in distinguishing what is important for the current unbalance, I would like to point out other important facts.
The distribution of the current between the phases depends very little on the resistance of the motor. It can be said that the resistance is totally irrelevant for the current balance, no matter what the PdMA says. Since the initial thread talks about the distribution at no load, we have to look what role the resistance plays at no load:
The impedance per phase of the motor in question at no load is (380 Volts/1.73)/40 Amps= 5.49 Ohms. The 40 is the average current {(37+38+45)/3=40}. This impedance is virtually perfect reactance, because the power factor at no load is very close to zero. Now just compare the 5.49 Ohms with 0.035 Ohms. Calculate sqrt(5.49^2-0.035^2)=5.48988432 Ohms. It is almost 5.49. You have to do the calculations for every resistance that was measured and only then calculate the current unbalance that would result. Similar calculations could be done for full load and for the locked rotor condition of the motor.
(The above calculations are far away from being strictly correct, but they give you virtually correct results.)
Generally, the resistance does not play ANY role in the current imbalance at no load, at full load and at the locked rotor condition. Even the REACTANCE (calculated from the inductance) of the motor plays a miniscule role in the current unbalance. The best way to understand this problem is getting familiar with the induction motor EQUIVALENT DIAGRAM.
The last thing I would like to achieve by this response is discouraging you from posting. Just keep the responses coming. This forum is the place for getting the right information (most of the time).
jank