Shaft Voltage measurements

[Marke]

I am looking for a simple means of measuring shaft voltages for EDM investigation.
Any recommendations?

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[rasevskii]

What is an EDM investigation? Shaft voltage can only be measured if the NDE bearing pedestal/frame is insulated from the machine frame. If this a large generator you are dealing with, the NDE bearing is normally insulated to prevent bearing currents which will damage the bearing metals/rollers/races with time. ON some machines in fact a small slipring or machined surface is included on the NDE shaft nearest the bearing with a brush gear to measure the shaft voltage. This voltage will normally be present when the machine is at speed and excited. If it is not present then this could indicate a bearing insulation fault (shorted out). There are protective relays available to supervise this arrangement.

Another version uses a large custom-made split CT around the shaft at the bearing to measure shaft current via a special relay. Current here indicates a bearing insulation fault. This I believe was a patented system by ASEA which was standard on their hydro generators.

It is also often that the brush at the NDE end is used for directly grounding the shaft, if no supervision of the shaft voltage is applied.

rasevskii

 

[jraef]

Have you spoken to Gunnar (skogsgura for the group)? He has or had been making a box he called the "Little Beppe" for this exact task, along with some interpretive software. Don't know if he is still making them, but it's still on his website as of a few weeks ago.

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