VFD showing overvoltage

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ElectricMatt

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Waco, tx
I am look at vfd for a 75 hp 480v 3phase Exhaust fan direct drive.

It trips on overvoltage almost instantly. I have checked all my readings
A-b 493
A-C 492
B-C 492

A-G 308
B-G 318
C-G 316

The line to ground readings look high to me, but my question is wouldn’t a floating delta give you these high readings to ground?


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ElectricMatt said:
I am look at vfd for a 75 hp 480v 3phase Exhaust fan direct drive.

It trips on overvoltage almost instantly. I have checked all my readings
A-b 493
A-C 492
B-C 492

A-G 308
B-G 318
C-G 316

The line to ground readings look high to me, but my question is wouldn’t a floating delta give you these high readings to ground?


Sent from my iPhone using Tapatalk
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If truly floating the voltages would be indeterminate, meaningless.
 
most drives you would typically use for that application will require being connected to a grounded wye system.

Major reason is MOV's installed for surge protection are connected line to ground and would be selected for 277 volts nominal to ground as the input voltage.
 
Have you checked to see if the motor is spinning before starting. Air flow in the duct can cause the motor to spin backwards and this will cause overvoltage when starting.
 
493 / sqrt(3) = 285, so what explains the L-G readings of 308 to 318? An ungrounded delta where the act of taking the L-G measurement causes the system to float that phase a little farther away from ground?

Cheers, Wayne
 
wwhitney said:
493 / sqrt(3) = 285, so what explains the L-G readings of 308 to 318? An ungrounded delta where the act of taking the L-G measurement causes the system to float that phase a little farther away from ground?

Cheers, Wayne
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In a truly ungrounded delta system, L-G measurements will depend on capacitance at the moment and location of the measurement, but are generally meaningless.

It does appear you have an ungrounded delta system, in which case the other comments are also very valid, this is BAD NEWS for feeding a VFD... Over voltages are just the tip of the iceberg of potential problems. I would be rethinking this all around, as in possibly installing a Drive isolation Transformer, which would be a 1:1 ratio, but Delta primary to Wye secondary. That's the recommended practice when feeding VFDs from ungrounded delta sources.

Side note; How are they getting an ungrounded Delta system? Utilities won't do it around here, it has to be corner grounded. I'd bet that what happened here is that the end user has a 208 or 240V system and bought their own transformer to step up to 480 (for some reason) and then set it up themselves as an ungrounded Delta.
 
Thanks for all the replies. Just to answer some of your comments, I did check the fan initially for rotation prior to starting and that was not the case.

I know that the line to ground would be meaningless in a floating delta. I checked that to verify what kind of system voltage I had.

There is no transformer form the service to the vfd. I think the POCO has lost their ground, because the other service feeding the plant is a corner grounded delta.

I was able to get it running for now by increasing the ramp time on the vfd and that solved the issue for now. POCO is coming out to check their transformers.

Thanks again for all the good advice.


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ElectricMatt said:
Thanks for all the replies. Just to answer some of your comments, I did check the fan initially for rotation prior to starting and that was not the case.

I know that the line to ground would be meaningless in a floating delta. I checked that to verify what kind of system voltage I had.

There is no transformer form the service to the vfd. I think the POCO has lost their ground, because the other service feeding the plant is a corner grounded delta.

I was able to get it running for now by increasing the ramp time on the vfd and that solved the issue for now. POCO is coming out to check their transformers.

Thanks again for all the good advice.


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To me, increasing the ramp time on deceleration indicates a load problem. Only if the ramp time you now have is too long for the process...in other words you may need a braking resistor to get the speed control you want.
 
ElectricMatt said:
Thanks for all the replies. Just to answer some of your comments, I did check the fan initially for rotation prior to starting and that was not the case.

I know that the line to ground would be meaningless in a floating delta. I checked that to verify what kind of system voltage I had.

There is no transformer form the service to the vfd. I think the POCO has lost their ground, because the other service feeding the plant is a corner grounded delta.

I was able to get it running for now by increasing the ramp time on the vfd and that solved the issue for now. POCO is coming out to check their transformers.

Thanks again for all the good advice.


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Decelerating too fast is a common way to get overvoltage error. You said in op you get the fault almost immediately, assuming this means either upon energizing the drive or after issuing a start command. If the load is already spinning when start command is given - I could see that being a cause, if that is the case you need to enable flying start features, otherwise the drive is going to attempt to brake the motor immediately because it isn't in sync with the output. Will be even worse if the already spinning load is spinning the wrong direction.
 
OP send over volts on Startup. Not decel. Lengthening ramps got him running. Sounds like Drive is in sensorless Vector mode and not tuned properly.

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