need help troubleshooting a shunt wound dc motor

[hyoid]

I have a shunt dc motor which was working fine one day but not the next. I am a novice when it comes to motors. I took out the brushes which looked ok then opened the motor. I noticed a fine feathery white deposit on the field windings which looked like mold. It easily blew off. I measured the voltage across the brush holders which went from 10-100 increased the speed control. I thus think that the control unit is ok.-but maybe I am wrong.The commutator looked ok and I measured the ohms across the pads which was 1 ohm. I found the wires going to F1 anF2 in the controll box and measured the ohms which was infinity.At one point it suddenly started to run-but only in one direction-I could not reverse it using the controller-I think the controller is ok because I was able to measure reverse polarity with my meter. At one point the motor also sped up without a load.

I would appreciate any help

 

[itsmoked]

Sounds like your field has an intermittent open. I think it's ded boss.

Any water or moisture get to it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Infinity at F1-F2 is not good. That is the magnetizing winding and shall have from a few hundred ohms down to tens of ohms - depends on what size you motor is. But never infinity. Look for loose connections. Does the motor smell "burnt"? (an acrid smell, usually).

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[hyoid]

Thanks Keith

Actually one year ago the motor was submerged in a one hundred year flood. I opened it dried it out and changed the bearings and was good until now.
What is an intermittent open? does it explain the inability to reverse and suddenly speed up?
Joel

 

[itsmoked]

AH one of those 100 year floods that happen every few year... I understand. We get those too.

Thanks for the confirmation Skoggs.

Intermittent is like a connection that comes and goes. This is very possible in motors, of course, as they vibrate and have rotational forces that vary. So the motor may start and run up to some speed and then the field opens and strange things occur.

In shunt wound motor you should definitely NOT have and open in the field. You may have dried the surface off but inside where you can't see you could have some corrosion that cut the wire.

I used to go thru solenoid valves for my truck air horn on a regular basis. Since it was mounted in the engine compartment heat and wet would get them almost annually. They would start to get flaky acting. I would take the end off and they would have white powder here and there. Close inspection would often show corrosion of the copper windings somewhere. Toast.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I suspect heavy corrosion. Running one way and not the other may be a sign that a connection has developed a layer of copper oxide. Early rectifiers were based on copper oxide.
I have seen this on a DC printing press motor.
It explains the motor running in only one direction if the field is reversed to reverse the motor.
You may find a different resistance value than infinity if you reverse the meter leads and check with opposite polarity.
If the connection is in such bad shape as to develop copper oxide, an intermitent connection is also probable.
Check your connections and repair the damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

It sounds as though apart from the connection your field winding is still OK. If your field windings are removable you may be able to remove the insulation covering the coil. Quite often with an intermittent connection you will find the break right at the joint between the lead and the coil itself. If so it's an easy fix.
Roy