Your Repair Partner Must Get the Measure of the Job
Apparently, the electric motor pictured here didn't last very long in service after it was rewound. It wasn't one that we had rewound at Fletcher Moorland, I may add.
Finding out why something has failed has always interested me. I also feel it's important to share the findings to prevent them from happening again. After all, this failure happened unexpectedly whilst the motor was in service, causing unwelcome machine downtime.
It's quite often easy to say 'the windings have blown'. Well, that's pretty obvious as there is the traditional blackend area on the photo, blobs of copper and a hole where the windings should be – finding out why is the interesting bit.
Thankfully, the cause of failure of this motor isn't often seen because most professional rewinders take critical measurements to ensure it doesn't happen.
But it does happen. Let me explain how.
There are clues to what has happened with this motor. The blow is on one phase; you might be able to see that the blown windings are within an area sandwiched between the yellow-coloured phase insulation. The blow has left debris on the inner face of the end shield. This isn't uncommon; however, having copper strands on the end shield is. There are scuff marks on the phase insulation and tape on the right-hand section of the lower winding portion.
What has happened here is that the winder hasn't correctly measured the coil overhang. That's the measurement of how far the coils protrude from the core. As a result, the coils have been touching the inside of the end shield following the repair. This wouldn't cause a direct short-circuit to the frame as the copper is insulated by an enamel covering. However, correct static tests should have shown this. But I can't comment on what testing equipment the rewinder had available at the time. The refurbished motor may have passed a basic insulation resistance test and then been sent back to the customer in the belief that it was ok.
Some of you might not be aware that every time an electric motor starts, or has changes in load, the coil overhangs move slightly. In a very old video from GE Electric, available here bit.ly/CoilMove, you can see the movement.
We try to mitigate this movement with the varnish and lacing of the windings, but they do move. So, from when this motor was installed, the copper windings were moving against the cast iron end-shield every time it was asked to work. This movement wore away the enamel coating to a point where a short-circuit current flowed to earth, and the motor stopped spinning.
The good rewinders out there will take correct measurements, be aware of the internal geometries of the end shields and ensure correct clearances.
I must admit that it is quite uncommon for us to see this error, but it can happen. This is why accurately measuring a wide range of parameters when repairing a motor is absolutely critical.