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Unbalance Input Phase Current


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#1 ABBdrives

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Posted 06 September 2010 - 01:16 AM

I just replaced the input bridge(diode and scr) for a drive but found that the input current is unbalanced on one phase. The line voltage is good but current values are 30, 30.4 and 25.4 amps. The output current is very balance. I am just thinking whether one of the diode or scr is not too good or possible the firing angle of one of the scr.

#2 frankjay

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Posted 06 September 2010 - 01:29 AM

QUOTE (ABBdrives @ Sep 5 2010, 09:16 PM) <{POST_SNAPBACK}>
I just replaced the input bridge(diode and scr) for a drive but found that the input current is unbalanced on one phase. The line voltage is good but current values are 30, 30.4 and 25.4 amps. The output current is very balance. I am just thinking whether one of the diode or scr is not too good or possible the firing angle of one of the scr.


Do you have a single phase transformer or some other peripheral device that is also operating from one phase of the input? Often, I will find a fan or a single phase transformer( usually used for a control voltage) across one of the phases and this accounts for the imbalance in line current.

#3 ABBdrives

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Posted 06 September 2010 - 02:25 AM

QUOTE (frankjay @ Sep 5 2010, 09:29 PM) <{POST_SNAPBACK}>
Do you have a single phase transformer or some other peripheral device that is also operating from one phase of the input? Often, I will find a fan or a single phase transformer( usually used for a control voltage) across one of the phases and this accounts for the imbalance in line current.



But i imagine then that phase would have a greater current reading. There is a 10 ohms resistor connected from one phase to diode then to the dc bus, i assume that is a precharge circuit. However that is not the phase with the low current.

#4 jOmega

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Posted 06 September 2010 - 06:25 AM

QUOTE (ABBdrives @ Sep 5 2010, 07:16 PM) <{POST_SNAPBACK}>
I just replaced the input bridge(diode and scr) for a drive but found that the input current is unbalanced on one phase. The line voltage is good but current values are 30, 30.4 and 25.4 amps. The output current is very balance. I am just thinking whether one of the diode or scr is not too good or possible the firing angle of one of the scr.


First thing I'd want to do is get my hands on a good oscilloscope and make a thorough examination of the incoming mains voltage .... meter reading is not that useful for this problem. It could be that the voltage waveform is unequal and/or that there is harmonic distortion of the incoming voltage waveform.

Next, I'd use the o'scope to look at the ripple voltage on the d-c bus ....The ripple pulses should be of equal height, assuming that the d-c bridge is conducting in all legs equally and that the a-c mains (supply) voltage is equal and balanced.

Also check the voltage phase-to-neutral (ground) ...

And don't forget to check the mains fuses.

Could be something in the bridge is mis-wired or there's a problem in the bridge or dv/dt network... if you are fortunate enough to have an ISOLATED o'scope (perhaps one that runs on batteries) or have an isolation transformer into which you can plug-in the o'scope (capacitive coupling s/b , 50 pf.) ......... the isolation would allow you to look across each diode in the bridge without blowing up the o'scope or the front-end of the vfd.

You do not say WHY you replaced the input bridge ... that might reveal some clues ... Also, which model ABB vfd ...??

#5 yuri

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Posted 07 September 2010 - 06:06 PM

Maybe the problem is that current drawn by some induction motors may be very sensitive to the supply voltage, thus I have seen, and it still is puzzling, me how a Bitzer compressor and a circulating pump nearby had a current imbalance of about 40 percent while the difference in voltage - measured not with oscilloscope although, unfortunately just with a multimeter - was only some 2 percent.

#6 ABBdrives

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Posted 08 September 2010 - 12:46 AM

Thanks alot JOmega, the drive is an ACS600 and the problem was a fault displaying supply phase. I find one of the SCR defective and so i replaced the three units(scr/diode). I recognized the oscilloscope may be the key in these situations by monitoring the waveform of the voltage and current.

#7 jOmega

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Posted 09 September 2010 - 04:46 AM

QUOTE (ABBdrives @ Sep 7 2010, 07:46 PM) <{POST_SNAPBACK}>
Thanks alot JOmega, the drive is an ACS600 and the problem was a fault displaying supply phase. I find one of the SCR defective and so i replaced the three units(scr/diode). I recognized the oscilloscope may be the key in these situations by monitoring the waveform of the voltage and current.



Please be sure that your oscilloscope is completely ISOLATED. It cannot be grounded..... even if you use differential input ... still a safe practice to unground the scope ... Also, when using the scope, keep one hand in your pocket ... so you don't accidentally touch ground with it; i.e., the cabinet, or anything nearby that is grounded (earthed) ....

I used to have a library of literature, (manuals and drawings) for the ACS 600 ... but they may have been discarded some years ago.

One other suggestion ...

Electrically Separate (isolate) the three sections of the drive; the converter - the dc bus - the inverter.

The converter is the front end with the rectification used to convert the mains ac to dc ...

Make yourself a test load .... by connecting several light bulbs 100 watts or greater (200 watts would be even better) .... in a series configuration.

I don't know what your mains voltage is .... but let's assume it is 415 vac ... if the light bulbs are of the 240v variety, you will need three in series
i.e., 3 x 240v = 720v ..... (wouldn't hurt to have a fuse in the series with the light bulbs) ......

you can use this as a load across the output of the converter bridge ... and observe the bridge output waveform. You'll want to be set-up to observe the waveform as you apply mains power. Once you are satisfied that the bridge output is proper, shut down the power ..... and connect the d-c bus to the output of the bridge..... connect the light bulb load across the d-c bus ..... use the scope to check the ripple current on the d-c bus..... if that is correct ... and the d-c bus voltage is equal to 1.414 x the mains voltage, you can shut down .... and connect the inverter section to the d-c bus. You can leave the light bulb load connected across the d-c bus .... or you can connect the light bulb load to the output of the converter bridge.

Reapply power ... and continue your investigation..... if the converter output was correct .... and the d-c bus was normal .... and you still have the problem .... then I'd be looking for a problem in the inverter section ....

IMPORTANT: for this testing, please disconnect the motor connections at the drive terminals .....

If all looks well (on the o'scope) during these tests .... then I'd be very suspicious that the motor or wiring to the motor has developed a fault; perhaps even a ground (earth) fault. Before reconnecting the motor leads to the ACS600 ... it might be advisable to megger the motor.

Good luck ...




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