Burned motor on DS 400 Q.

[AlexThePalex]

I was given a DS 400 Q with a burned motor. I'm trying to understand what exactly happens when an electric motor burns. I learned a lot about motors and electricity in college, but I don't know about this. Could anybody who understands how it works explain it to me? They always call it "burned" but it's all metal, that can't burn. Maybe the copper wires melt?

When I turn the sander on the rotor doesn't rotate anymore but vibrates in place, causes enormous heat very quickly and smoke arises. I'm trying to figure out which part I need to replace, the stator or the rotor, or both, anybody got an idea? When I took it apart I couldn't see any damage at all, so I got no clue what is broken. Is there a way to measure it with a multimeter?  

Really sad picture:

[attachimg=1]

[doh] Stupid thing is, it's broken because somebody lubed it up on the inside.  [doh] The entire inside of it, EVERY part, is covered with some lubricant. NEVER DO THAT!

 

[Alan m]

im no expert so take this with a lot of salt.

im guessing that if it is vibrating then the control wiring is ok so it must be in the rotor or stator
check to make sure that both sides of the stator are complet loop

then each segment and its oposite  around the ring of the split ring comutator represents a loop in the rotor check to make sure they are complete loops also

 

[USToolandFastener.com]

Hi Alex,

Sounds like you have a bad "field" (the part that's in between the top cover and dust port in pic).  I used to repair quite a few small power tools (mostly self taught) and it is the magnetic field that helps spin the armature.  If the magnetic field is toast then there is no rotation.  Make sure all the bearings are good and the brushes move freely in the brush holders.  If all that seems good then it's probably just the field which is usually a fairly cheap fix.  Hope that helps, now get that baby running!!!!  Sorry. can't help you with the part from this side of the pond!

 

[rrmccabe]

Burned out is a term that actually has zero value.  Yes sometimes parts inside heat up and melt but even if you get an open somewhere including the wire going to a brush breaks, people say the motor is burned up.  Bearing seizes and the motor is burned up.

 

[oldie]

Like you, my 'theory' days are long past.
Having said that...my first thought was 'shorted' turns.
Wonder if the 'lubricant' had compromised the insulation
- totally agree, wouldn't want any sort of 'chemicals' near coils;
and would want to get rid of all of it, before further testing.
(maybe the smoke is comely from 'local' overheating of the lubricant,
because of 'shorted' turns - obviously speculative on my part.)

In the past have tried to verify shorted turns using multimeter;
the problem we found, was because of very low resistances involved:
1/   needed a very good ohm-meter
2/   needed to check, very carefully, against known good coils - to provide
reference readings.
It worked sometimes, but a pain.  Another fly in the ointment was that,
of course, using a ohm-meter in this way, doesn't mimic the true 'working'
conditions that the coil is experiencing when the tool is fired up.
In this context a meggar test would be better perhaps??

Just a thought - don't you have a similar sander?
Maybe can swap parts to isolate faulty component.
Sometimes, can't beat substitution for 'proving' a fault.

Richard

 

[AlexThePalex]

Lots of useful suggestions, thanks guys. I'll do some rigorous testing to see if all the coils are still connected properly. I do have another sander around to swap parts, but I hesitate to put the good parts in the lubed up sander for fear of contamination. I'll have to do some rigorous cleaning first and maybe then I'll try swapping parts.

Burned out is indeed an overused word. But here there might be some truth in it. When I turn it on after half a minute it becomes extremly hot, and smoke comes out. But perhaps it's just the lubricant burning up, who knows.

Would be great if I only had to get a new field assembly, that's pretty cheap at 35 euros.

 

[ajones]

My money is on the armature I have never seen a field coil fail..

 

[jonny round boy]

I would agree with festoolfootstool, and guess that the armature windings are shorted. That will significantly reduce the resistance, causing more current to flow through the copper windings than it's designed to take. This generates heat, and melts the fine insulating coating that covers the wires. This causes further shorting, compounding the situation. The smoke will be the coating burning up (as well as the lubricant!).

Replacing the armature would be my first suggestion, but bear in mind that the additional heat could have melted/distorted the plastic housing, particularly around the bearings (depending on how long the motor was overheating). If the bearings heated up sufficiently, the lubricant inside them could have burned off/evaporated, so you may need to replace those.