Unbalanced Currents

V

Thread Starter

vwillet

I have a customer whose compressors are experiencing current levels at 117A/144A/79A over the three legs of the 3 phase motor. The voltages are running pretty close at 506V/503V/488V. The motor is a 480/3/60. Supply voltage at the meter is good. Any idea what is going on?
 
B
Collect a motor checker (this is an instrument by which you can measure winding resistances and inductance.)

measure the winding resistance between three phase terminal say Rab, Rbc, Rca.

Similarly measure the winding inductance.

These values must be balanced, if there is any unbalancing more than 10% then your motor winding need to be corrected.

Otherwise check the terminal connections and cable joints. if any in the path of the motor supply, because of looseness or other reasons resistance of path increases and motor current becomes unbalanced.
 
You say "compressors". Does that mean there are more than one and you are reading the current from the panel to the compressors? Or, is it just one?

I would start with connections. Do any look burnt or hot?

Also what are the hp, kw and current ratings on the motor(s)?

>I have a customer whose compressors are experiencing
>current levels at 117A/144A/79A over the three legs of the 3
>phase motor. The voltages are running pretty close at
>506V/503V/488V. The motor is a 480/3/60. Supply voltage at
>the meter is good. Any idea what is going on?
 
VWillet

The AUF (Voltage-Unbalance-Factor) is about 2% using both NEMA and IEC standards.

If the problem were only related to supply voltage imbalance, then the corresponding AUF (Ampere-Unbalance-Factor) should be about 5%. Instead it is over 30%! The readings are indicative of a motor in immediate danger of failure!

Thus, if you want help, additional info is required:

1) Were the readings taken with a 'True-RMS' meter?

2) Is a VFD involved?

3) Can you measure the temperature of the motor's carcass (Frame)?

4) What is the ambient temperature of the motor's location?

Phil
 
VWillet

Many times the power issues are difficult to resolve unless all variables are considered. For example, we all believe the line to phase voltage is at a factor of 1.732, this is ONLY true when the phase voltage vectors/phasors are equally separated by 120 degrees. 360 degrees divided by 3 equals 120 degrees. When this is not the case part of the motor is doing more work and part of the motor is doing less work. When a power quality analyzer is connected it is easier to determine the phase angle differential distortion. If this is powered from a VFD and the phase angle separation is more or less than 120 degrees by over 5 degrees some/most VFDs will become confused and fault.

Without data everyone is just guessing.
 
Actually, the original data was sufficient to complete two "Negative-Sequence" computations... one covering line-voltage unbalance, the other line-current unbalance.
The original 2015 study can be posted if anyone is interested !
Regards, Phil Corso
 
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