Saga of a DC motor

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
User avatar
Lenp
Posts: 1316
Joined: Thu Oct 26, 2006 7:11 pm
Location: Maryland
Contact:

Saga of a DC motor

Post by Lenp » Wed May 02, 2018 6:53 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?
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

User avatar
dacflyer
Posts: 4295
Joined: Fri Feb 08, 2002 1:01 am
Location: USA / North Carolina / Fayetteville
Contact:

Re: Saga of a DC motor

Post by dacflyer » Thu May 03, 2018 5:44 am

i dealt with about the same with a brushless generator,
see post.. http://forum.nutsvolts.com/viewtopic.php?f=45&t=17250
found the armature had a shorted winding, but it was a bitch to find, the issue was intermittent.
finally found it after a week of testing.. i forced power into the generator, this super charged the armature, and i saw the elusive spark fly.
could never see it before, because the unit would never load or generate power. so i forced it.

User avatar
Lenp
Posts: 1316
Joined: Thu Oct 26, 2006 7:11 pm
Location: Maryland
Contact:

Re: Saga of a DC motor

Post by Lenp » Thu May 03, 2018 6:51 am

Indeed that's a time honored troubleshooting shortcut. I repaired fire alarm systems and we had a ton of smoke detectors in one building, unfortunately all on one zone! The detector's alarm relay was supposed to latch when tripped and illuminate a light on the detector so it could be located. Someplace one would go into alarm, trip the FACP then clear itself. No latch, no light, just an alarm relay blip long enough to creating numerous false alarms. These were old school line powered systems with #14 AWG wire so I wired the alarm contacts across 120vac with a 15A fuse. Weld or blow was the detector's only options. If the contacts welded, the short could be found, and if it blew them open, a smoke test would find it. My boss, since he wasn't technically astute, thought I was nuts and would burn down the building. The alarm relay contacts tripped one night the contacts welded and it was 'easily' located and replaced with only a small smoke puff from the detector, but none from the building itself!

This may seem to be a rather crude approach, but it worked in those days. I'm not sure if this would work on more modern low voltage systems in use today since without raising the voltage considerably the wire gauge may present enough resistance a prevent adequate current to do the dirty work . Definitely this technique would not work on addressable systems since their architecture is completely different.

Bring big guns to a big war!
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

User avatar
dacflyer
Posts: 4295
Joined: Fri Feb 08, 2002 1:01 am
Location: USA / North Carolina / Fayetteville
Contact:

Re: Saga of a DC motor

Post by dacflyer » Fri May 18, 2018 7:01 am

LENP >> i would have done the same thing..

Post Reply

Who is online

Users browsing this forum: Google [Bot] and 1 guest