VSD output is erratic in no load test only.

[ahendawy85]

Hi all, I have a problem with a VSD at work that's been confusing.
I am a field service engineer for an oil service company, my line of work is in electric submersible pumps (ESP). The ESP motor is a 3 phase induction motor. Power here is 380V 50 HZ.We were using a VSD for motor control. The control scheme used by this VSD is constant V/HZ.
When performing a no load test the 2nd phase outputted a lower voltage than the other two.
For example when operating the VSD in no load at 50 HZ output frequency, measuring the output terminal phase to phase yields these readings:
U-V 270VAC
V-W 270VAC
U-W 380VAC
Obviously the V phase has a problem so I disconnect power and start doing some static checks on the VSD. I check the converter's SCR's and the inverter's IGBT's they all check out according to the VSD manual. Measured the DC bus and it does have the correct DC voltage value.
The confusing part is when I connected a small motor to the VSD and started testing it it checked out fine, readings are all balanced and the motor runs smoothly.
U-V 370VAC
V-W 380VAC
U-W 379VAC
Tried a no load test after and same thing happened, the V phase is giving erratic readings. And one last observation; the V phase has DC volts in it, when measuring with my AVO meter for DC volts at the output I get those readings:
U-V 80 VDC
V-W 80 VDC
U-W OL

I'm sorry if it took too long to explain, I just can't seem to find the problem. According to my static test results all components are intact. Any thoughts?

 

[Jobrag]

I'm guessing that your "no load test" actually draws more current then your "small motor test". If so (and I hope I'm not teaching my grandmother to suck eggs here) it looks as if one of the diodes that carries actual current is failing under load. Sorry if that's too basic but without schematics I'm only guessing.

 

[jim hardy]

if the parts are all good your symptom suggests timing.
but the DC suggests a winding-to-winding fault in transformer

is this drive the same make as your others? I O W is this unusual for that type VSD?

Perhaps it needs a small load to pull the voltages into phase -
imagine in your mind a star phasor diagram
three arrows pointing out from center 120 degrees apart
... if they re-arranged themselves into a tee instead of a star, eg 90 deg apart instead of 120, V would be closer to U and W than it should

but i can't get past the DC in output, that sounds suspicious

you said static checks
one failure mode for SCR's is "transistoring" where they act as a transistor instead of a switch so don't stay on after the gate pulse ends. Therefore in your static check you have to apply enough current to latch the scr, usually 50 ma or less but still several ma, and a dmm won't do that you need an old timey analog on RX1 . A Simpson 260 never let me down... but even it won't latch big SCR's.

good luck - I'm curious about this one keep us posted...
is there a neutral on this thing?
If phase to neutral voltages are okay but phase to phase unbalanced then you KNOW it's timing.

i'd lift leads and check that transformer for winding to winding short,,,,
then swap SCR's from another unit...
then swap SCR firing board

scratching my head with you... especialy about that DC component
i guess the motor shorts out the DC, so it must be coming through some substantial impedance else something would get real hot.

also- some true RMS voltmeters will correctly calculate the DC component of an offset AC waveform like you describe. That could complicate your observations, so check your DMM manual.


old jim