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#1 yady239

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Posted 01 March 2006 - 01:35 PM

we have a 250 kw 6.3 kv asynchron inductionmotor when the motor runs the power factor on power supply variates we reppaired the motor but the problem remained would you please help me and tell what causes the matter?

#2 jraef

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Posted 02 March 2006 - 04:21 AM

Power factor varies with load on an asynchronous motor, but is usually not any better than a .8pf. If that is a problem for you, you need to correct it, usually with power factor correction capacitors. You can also consider using a VFD, which would present a .95pf to the line, or replace that asynchronous with synchronous motor, but at that small size it doubt it would be cost effective to do either one compared to capacitors, unless there were some other complelling reason.
"He's not dead, he's just pinin' for the fjords!"

#3 marke

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Posted 02 March 2006 - 05:36 AM

Hey Jraef,

What is this high power factor on drives????

VFDs will have a displacement power factor of better than 0.95, but a distortion power factor that can be less than 0.75 due to the discontinuous current flowing in the three phase inputs.
Ther is a variation between VFDs depending on whether they use inductive input filters or capacitive input filters etc.

The only VFDs that can truely claim a power factor of better than 0.95 are active front end drives.

Best regards from the back blocks,

#4 yady239

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Posted 04 March 2006 - 04:59 AM

Hey Jreaf

Our problem is not power factor correction ,the matter is playing of power factor .The pf variates by the time independent of load.
We think it is caused by broken bars of the rotor.

Best regards

#5 marke

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Posted 07 March 2006 - 10:05 PM

Hello yady239

I wold not expect that rotor bar problems would cause power factor variation, but I could be wrong. Rotor bar problems usually show up as a swinging current where the frequency of the current swing is dependant on the load on the motor shaft. A high load will make the ammeter swing faster than a light load.

The usual cause of power factor variations with an induction motor, is variations in the supply voltage.
As the supply voltage increases, the magnetising current (reactive current) increases and the load current (resistive current) reduces. Monitor the supply voltage with the power factor and you may find a corelation.

You do not mention how much the power factor is changing. If you are finding major variations, this could be due to a severely distorted supply waveform. I recently witnessed such an installation where there was a large four quadrant DC motor with fast acceleration and deceleration and the harmonics from this were affecting the voltage waveform and measured power factor.

Best regards,

#6 yady239

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Posted 11 March 2006 - 04:25 AM

Hello Marke

Thank you very mauch for your answer to my question.But the variation of the power factor is not by the variation of the suply voltage because the voltage was fixed .The power factor variation at offload and discoupled motor was from .25 to .87 but at the last test that we run the motor fulload we did not have the pf. variation.

#7 marke

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Posted 11 March 2006 - 05:10 AM

Hello yady239

The two major causes of this type of performance, is a variation in voltage, or a variation in frequency. Where is your supply coming from?
How frequently is the power factor changing and for how long is the power factor high?

Best regards,

#8 yady239

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Posted 11 March 2006 - 02:07 PM

Dear Marke

The supply is coming from a local power network and we do not have variation in voltage or frequency. Besids we other motors and they do not cause such a problem.May be the problem caused by a unbalanced magnetic feild?When we were measuring the vibration of the motor,after making the motor off,immediately the vibration went to low, do's this show that we have a vibration caused by magnetic feild?Do's this cause the variation on the pf?

#9 marke

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Posted 11 March 2006 - 07:40 PM

What is the rate of variation in pf?

Best regards,

#10 yady239

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Posted 12 March 2006 - 03:48 AM

The variation was about 0.2 to 0.87.

#11 marke

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Posted 12 March 2006 - 04:13 AM

How quickly was this happening and does the speed of the variation change with load?
If you have a variation of say half to one second on an unloaded motor but much faster when the load is applied, then perhaps there is a rotor problem. If it is significantly slower than one second, then I would not expect a rotor problem.

Best regards,

#12 yady239

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Posted 12 March 2006 - 09:00 AM

Dear Marke

The variatio was very high and fast enough at offload and dicouple running of motor but now at fulload ther isn't such a problem.

Best regards




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