1350 A Variable speed drive failure

[maypot]

We got a breakdown on one of our 400 V variable speed drive yesterday morning. The drive tripped on starting and the following faults were recorded namely overcurrent,angu , ground fault, overvoltage and undervoltage . The motor was disconnected to the load and the motor started .It again tripped before starting and the same faults noted.After some attempts, it finally ran and at 40 Hz ,the current on the first and second phases were 480 A but the current on the third phase was only around 240 A. The output module of the third phase has been dismounted but the IGBT appears to be fine, with no apparent short or open circuit. We are suspecting the firing card.There are four IGBTS on each phase and if two IGBTs were not firing, I understand that the drive would trip on overcurrent but ground fault and over and under voltage trips seem strange.
Grateful to have some inputs on your side.

Bob

 

[pirates]

I have a book which says that because of its structure the IGBT cannot be reliably tested with an ohmmeter and basically refer it to a qualified field service technician however there are none of those here where I live in the first world so I looked further and found this at

http://www.galco.com/circuit/igbt_testing.htm

however I have not yet tried it so I take no resposibility for the method.

 

[Marke]

Hello maypot

On each phase, you wil have parallel IGBTs from the positive rail to the output and from the output to the negative rail.

Your description does sound like either the positive or the negative leg is not switching. This will result in a DC polarisation of the iron and excess DC current flow can result. This will not show up o an AC meter on the output.

This could cause an over current trip, and it could also cause ground fault and under voltage trips as well depending on the circuitry used.

I would suspect that one of the driver circuits has failed, but it could be as simple as a connection problem between the drver circuit and the main board. I have seen issues with ribbon cables and connectors.

This sort of thing can be difficult to find if you do not have the full service information.

I would personally be using a small motor for testing provided that the drive can be set up as a V/Hz drive. This way there is less chance of causing serious damage to the drive. If at all possible, involve someone with service information and/or experience in servicing that breed of drive. They should have the appropriate data and spares.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[maypot]

Thanks Marke for your reply. Can you be more precise about th dc polarisation phenomenon.

Thanks.

Bob

 

[Marke]

Hi Bob

If one leg of one phase is not switching, you will be putting pulsed DC through one phase of the motor. This is going to polarise the iron and can cause all sorts of effects as some of the iron goes into saturation.

Best regards,

Mark Empson

http://www.lmphotonics.com