Fixing a broken rotor coil of 775 motor

See how I replaced a piece of sheared wire from the rotor winding of a 775 motor

I powered a 775 motor and strange things happened. The motor was new, never used before. In less than 10 seconds while it was supplied with voltage, it looked as if something was attempting to block the axis from spinning, I heard crackling noise from the motor then smelled some smoke. Not good... yet it was still spinning. It's worth mentioning the context in which this happened. I was testing a power supply I'm building, and because something went bad the 12 V motor received 30 V with plenty of current (about 7 A). I would expect it to fail under these circumstances, but not in this way and not as fast. Overheated rotor coils and short-circuited windings is what should have happened.

Yet the motor was completely cold and still spinning when I powered it again (this time from 12 V). A short piece of copper wire came out of it - something bad has happened inside. So, I ended up with a partially broken motor. The next thing I did was to open up the case.

775 motor case opened

Easy to say, but very hard to open the case. I had to bend the metal case notches with some pliers, however I still wasn't able to pull the back cover easily. Hitting the motor axis with a hammer helped, although this resulted in bending the back cover (I eventually straightened it back). A first look revealed a brand new motor (see the brushes) with a destroyed rotor (wires coming out). As ugly as it is, the coil does not look "fried". It still has the normal colour of insulated copper wire.

I'm still wondering whether the higher than nominal voltage produced this or it is poor manufacturing. It seems like a loose winding wire was pulled outwards by centrifugal force and hit the stator magnets, which caused the shear. The windings are not glued together as it was once done. I doesn't matter, the good news is that I can fix this.

The broken rotor wires

The same wire that got sheared in one place was pulled away again and sheared in another place. This resulted in an unconnected piece which I will remove. The windings are made with 0.7 mm diameter copper wire. All I have to do is join the two endings together with a small loop of identical wire.

I cut the flattened ends of the wire, scraped the insulation on a distance of about 7-8 mm then used some pliers to bend the ends and connect with my new wire. This will ensure mechanical strength. But I'm not done yet. I will solder this joint and wrap it in heatshrink tube. No wire must bulge out from the rotor nor any other added material (my heatshrink tube got partially cut because of this).

The process of fixing the rotor winding

I didn't stop here. I poured some Cyanoacrylate glue over the windings and mostly over my connections, hoping that a similar event would never happen again. I put everything together and from what I can see the motor axis can be easily rotated by hand (I added some lubricant) and no strange noises can be heard. I will wait about a day before powering it to let the glue harden enough.

This is just an example that sometimes you can repair what hasn't not been made to be repaired. The hardest part was opening the case and putting the cover back. Hope you enjoyed the tip! By the way, here you can get such motors.