Phase Current un-balanced problem ..

[place1234]

I have a hermitic motor(compressor motor) which is 635KW , FLA 1091A.
DC resistance measured :8.52/8.51/8.50 microohm, which consider as balanced.
but voltage/current strange as below:
389v/390v/389v
609A/652A/608A

Obviously phase B current is higher. I have other 3 motors had the same problem with phase B current higher. It looks motor has no problem, but I was wondering why this could happen? Anyone helps...

 

[itsmoked]

Do you have a stinger leg?

 

[varri79]

If supply voltage is balanced in all 3 phases, probably stator/rotor winding that could create unbalance.

Since DC resistance maynot reveal really unbalances, check for reactances (Z and phi/Z).
Motor tester (ALL Test) can help in finding the unbalances (in stator winding or rotor bars).

Varri

 

[place1234]

By the way, I've rotated the power line, A-B-C to B-C-A then to C-A-B, still phase B current higher.
For sure it's not related to the motor I think.
Anyone else got opinion/help, thanks.

 

[davidbeach]

Can you check the phase angles of the three voltages and the three currents?

 

[petronila]

Hello Place1234,

For the winding ohmics readings and due to the power line rotation results, I think the motors are OK.

The Voltage media is : 389.3 and differences between Vmedia and the real values are: 0.33,0.7 and 0.33.Then the Desbalance Voltage in percent is: (0.7/389.3)x 100= 0.179% this desbalance don´t cause power problems.The amps readings were taken at 50% Load? I am agree with Varri79´s. Also the Rated Voltage, P.F and Efficiency date could help, could you provide?

Regards

Petronila

 

[quark]

I may be wrong in this context but some hermetic compressor windings are provided with heaters. You should better speak to the compressor manufacturer.

 

[place1234]

Thanks petronila ,
I got following infomation on one of these motors:
460V/509HP/3555RPM/HERMITIC MOTOR/FLA 572/LR CODE:F /HERTZ:60 /SVC FACTOR:1.0

Test result:
394V/395V/392V
242A/259A/245A

Phase B current higher also. I'm not able to measure reactances right now, neither the phase angel of power line.
This thing really confused me

 

[aolalde]

Per NEMA and EASA recommendations, "Current unbalance at full load is six to ten times as great as the voltage unbalance."
394/395/392>>393.66 ave >>0.422% Voltage
242/259/245>>248.666ave >>4.15% Current
4.15/0.422 = 9.83 times (as expected)

I wonder if a circuit analysis calculation could be atempted using the technic of Symetrical Components for an unbalanced three phase circuit. You will need all the impedance parameters of the motor and phase angles of the voltages.

 

[petronila]

Hello place1234,

I think you have to check your power transformer, check the Output voltage and loading, the motor´s Voltage name plate is 460 ,some extrange to me are the voltage readings: if your motors are running at 394,395,392(Below 15%) this is an under voltage operation and this cause effects on the motors efficiency,temperature,current and the motor´s perfomance.I think you have to find if the Voltage problem comes from the Transformer´s Load capability.

Regrads

Petronila

 

[ccjersey]

Does it really matter about the current unbalance when your motors are running so far below their FL amperage. Have you ever measured the unbalance at full load.

Are you running 50hz?

Jim

 

[place1234]

yes, we're running at 50HZ instead of 60HZ.
The way to find out whether power line affects this current unblance is difficult

 

[rick2080]

Did you solve your amp imbalance problem?

 

[rick2080]

I have had numerous situations where the current imbalance is excessive causing motors to fail prematurely. In most cases the cause was either a blown fuse on the power companies power correction capacitors or improper single phase loading on the three phase system. This can be seen if you check the amp phasor balance on a power quality analyzer. In all cases, the power company had to correct the problem. Please let me know what you find.

 

[electricpete]

"389v/390v/389v
609A/652A/608A"

"By the way, I've rotated the power line, A-B-C to B-C-A then to C-A-B, still phase B current higher.
For sure it's not related to the motor I think. "

I assume you rotated at the motor terminals and the power system B phase remained high, even when connected to motor lead T1, T2, T3 right?

Where was the voltage measured? Maybe there is a high resistance connection between the point of voltage measurement and the motor? For example the breaker?



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[pennine]

voltage and current are not in phase these need to be in phase to make full usage of the power being supplied to the motor this is proven by your experiment of swaping phase rotation but still phase b draws the most current