Transformer problems?

[bluesman70]

I was at a customers site and I was troubleshooting on the VFDs ( Variable Frequency Drives) AC of course.It was tripping out on high DC link voltage...anyway that not the issue.I measured the following on the motor.

L1-ground = 440 V AC
L2- ground = 440 V AC
L3- ground = 55 V AC

I went to the power distribution and it was the same there.
The first thought I had but not sure: They must have a open Delta Transformer where one leg is tied to ground..the L3 leg I believe...is that possible?.Or please tell me whats going on.If you can provide me a PDF with any form of drawing that would be great.Thanks so much!

 

[sibeen]

Not sure if this is revelant, but I'll blather on regardless, using a lot of hand waving generalisations.

In older style three phase UPS systems the input rectifier consisted of a controlled thyristor rectifier. This was then inverted using an output transformer. Using this shcheme the DC voltage didn't have to be above the peak voltage of the output AC waveform as the transformer could adjust. The input to the rectifier only required the three phases to operate and no neutral was required.

The output transformer was very expensive and so for economic reasons circuit topologies were developed to do away with this transformer. One of these was to use a standard three phase rectifier followed by two boost circuits which would develop a positive and negative DC voltage both referenced to a neutral point. The DC voltage was at a level to be above the peak voltage of the required output AC voltage.

In this topology (and other closely related topologies) the neutral is required, if the neutral is not present the DC loses control and either the positive or negative legs will go into an over-voltage mode.

Do VFDs now use a boost style input?

 

[rockman7892]

We have several 480V VFD's that are connected to isolation transformers which have a delta corner grounded secondary. To my knowledge none of these drives have ever had a problem and I'm thinking this is due to the fact that becuase one leg is grounded a fault will cause a OCPD trip rather then cause an overvoltage condition and damage components as warros said.

I have other 480V VFD's that are connected on a HRG wye secondary. How can you tell if a VFD is rated to be on an HRG wye or ungrounded delta and weather the components are rated for 480V L-G. It this rating typically on a nameplate somewhere?