Saga of a DC motor
Posted: Wed May 02, 2018 7:53 pm
I do some industrial repairs and recently serviced a DC drive system with a German 180VDC variable speed 1Hp motor running a paper feeder in a bindery folding machine.
The DC drive control had a blown fuse and resistance tests show that the motor armature was shorted showing around 0.2 ohms across the brushes and to ground.
Time was of the essence so an OEM unit was ordered and installed. The owner wanted to get the old motor repaired to keep as a spare and I had a tough time finding anyone that would repair this DC motor. I did find a shop that would rewind the armature, but not repair the motor itself. In preparation to shipping it to the shop I checked the resistance again, same readings so I disassembled the motor and pulled the armature. It looked great. No signs of heating, or burns. Yes there was a lot of carbon dust from the brushes but nothing else was remarkable. But..now the armature showed infinity from the commutator to the shaft and core? Further inspection revealed the brush holders are mounted on a fiberglass like insulating ring, and one of the holders developed a carbon deposit, burning a path through the edge of the ring and it connected with the grounded end bell. This shorted the brush holder which reflected the short to the armature so it appeared, in the initial tests to be shorted
No parts are available so I used a rotary tool (Dremel) and routed out all the carbon until I got back to virgin fiberglass. It was filled with the infamous J-B Weld, which IS NOT conductive and then cleaned up the excess with rotary tool again after curing. The motor was washed in a motor cleaner, baked for several hours reassembled and tested with at 500V Megger with excellent leakage readings. It was powered ran well and is being returned.
If anyone has a similar tale, I'd like to hear about it, and I have photos if anyone is interested.
The bindery has another one on the shelf, also with a shorted armature that I pulled last year ..Want to bet what I'll find?
The DC drive control had a blown fuse and resistance tests show that the motor armature was shorted showing around 0.2 ohms across the brushes and to ground.
Time was of the essence so an OEM unit was ordered and installed. The owner wanted to get the old motor repaired to keep as a spare and I had a tough time finding anyone that would repair this DC motor. I did find a shop that would rewind the armature, but not repair the motor itself. In preparation to shipping it to the shop I checked the resistance again, same readings so I disassembled the motor and pulled the armature. It looked great. No signs of heating, or burns. Yes there was a lot of carbon dust from the brushes but nothing else was remarkable. But..now the armature showed infinity from the commutator to the shaft and core? Further inspection revealed the brush holders are mounted on a fiberglass like insulating ring, and one of the holders developed a carbon deposit, burning a path through the edge of the ring and it connected with the grounded end bell. This shorted the brush holder which reflected the short to the armature so it appeared, in the initial tests to be shorted
No parts are available so I used a rotary tool (Dremel) and routed out all the carbon until I got back to virgin fiberglass. It was filled with the infamous J-B Weld, which IS NOT conductive and then cleaned up the excess with rotary tool again after curing. The motor was washed in a motor cleaner, baked for several hours reassembled and tested with at 500V Megger with excellent leakage readings. It was powered ran well and is being returned.
If anyone has a similar tale, I'd like to hear about it, and I have photos if anyone is interested.
The bindery has another one on the shelf, also with a shorted armature that I pulled last year ..Want to bet what I'll find?