Thread: Mill problems, again:

I'm not so sure thats the cause. Wouldn't extra resistance reduce the motor current? Maybe a shorted winding? (or maybe I am full of it)

Stuart

So, what actually tripped? Was it a power circuit breaker or the RCD?

The normal circuit breaker trips with excessive current being drawn while the RCD will trip when it "senses" 10mA (I think it's this amount. Can't remember) or more being leaked to earth.

Simon

Hi Guys,

Here are some more pictures of the inside of the motor top case where the brush holders are situated.



You can see that there has been considerable heat generated at the brush holders and the sleeves on the push on tags are badly charred. If you look very closely at the third and fourth pictures you can see where the insulation has charred and then carbonised, causing a leakage current to earth via the motor case. This is what probably tripped the circuit breaker.

Simon:
The ELCB trips at 3ma for the workshop and the breaker for that circuit is a 16 amp one. The ELCB is one of those whole building things, it only feeds the workshop. Its only the second time since I built the workshop that its tripped. The first time was my fault when I plugged in a drill battery charger with a faulty transformer that had shorted windings.

Stuart:
The motor current has to flow through the motor to get it to turn, as the pigtail into the carbon brush starts to loose contact the current then starts to flow through the spring. In order for the motor to keep turning the current has to increase to compensate for the voltage drop across the now higher resistance of the brushes. This in turn generates more heating through the brush connections. In this case the spring, which gets red hot and looses its tension, reducing the pressure on the brush. So the controller raises the voltage to keep up the motor speed, increasing the current and raising the temperature. The whole thing becomes a runaway situation until something fails. In this case hot enough to cause charring and insulation failure.

In hindsight its easy to see how this could become a fire hazard. The rest of the motor is fine and I'm sure that it can be cleaned up. With new brushes it should continue to be serviceable.

BaronJ, if the problem was just the "copper pigtail loosing continuity", then only one brush holder would have melted. Not both.

I think you should measure with an ohmmeter across the armature commutator. Always measure across two 180 degree opposed commutator strips. You may find that one winding is shorted.

I assume this is a Sieg minimill? At least the motor looks suspiciously like one made by Sieg (they make their own motors inhouse).These are just very cheaply made DC motors. Same as treadmill motor quality. A good quality motor would probably cost half as much as the whole mill. If out of warranty, you may consider an upgrade to the newer brushless motor and controller. Or do as I did with my Sieg X3 mill, toss that whole DC motor/controller/gearbox and make a completely new belt driven VFD drive for it.

Agree with CBA that you could have winding failures occurring and high currents are causing the brush failures. Also worth checking that the brushes slide freely in their guides, as they can arc a lot and still somehow keep running. Damage to both brushes suggests excess current flow, (short circuits). Hope you can sort it out.
Combustor.

Hi Baron,

What cooling is there for that motor? Looks to me like the whole thing is cooked. I'd guess that insulation is just pvc, not high temperature..

Ray

Originally Posted by cba_melbourne

BaronJ, if the problem was just the "copper pigtail loosing continuity", then only one brush holder would have melted. Not both.

Mmm, I disagree ! It is obvious that both brushes suffer from the same problem, and that the springs have had to carry the whole of the current through the motor. The result is that the springs have got hot enough to loose their temper and reduce the pressure on the carbon to commutator contact. The heat produced has been enough to damage the plastic wiring inside the case.

Neither brush shows any continuity between the carbon and the pigtail. I would have expected to see an ohm or two if the pigtail was properly secured.

I think you should measure with an ohmmeter across the armature commutator. Always measure across two 180 degree opposed commutator strips. You may find that one winding is shorted.

I admit that I haven't checked the armature yet for any fault,but will do so as soon as I can get the end frame cleaned up and the wiring replaced with some heat resistant stuff.

I assume this is a Sieg minimill? At least the motor looks suspiciously like one made by Sieg (they make their own motors inhouse).These are just very cheaply made DC motors. Same as treadmill motor quality. A good quality motor would probably cost half as much as the whole mill. If out of warranty, you may consider an upgrade to the newer brushless motor and controller. Or do as I did with my Sieg X3 mill, toss that whole DC motor/controller/gearbox and make a completely new belt driven VFD drive for it.

No this machine is not a Sieg, though the motor is very similar. When the plastic gears failed, I did consider doing a belt conversion, but on this mill there is no access to the spindle that would allow that. You may remember that I made a metal gear replacement for the failed plastic ones. I can't find the original thread for some reason, but it was around April last year.

Now a brushless motor conversion is interesting and something that I should look into. So thanks for that suggestion.

Originally Posted by RayG

Hi Baron,

What cooling is there for that motor? Looks to me like the whole thing is cooked. I'd guess that insulation is just pvc, not high temperature..

Ray

Hi Ray,
The motor armature is alright, but yes the wiring is just pvc. I should have realised from the smell at various times that the insulation was burning somewhere in the workshop. But of course nothing visible so no alarm. Certainly not enough to set the smoke alarm off.

Originally Posted by Combustor

Agree with CBA that you could have winding failures occurring and high currents are causing the brush failures. Also worth checking that the brushes slide freely in their guides, as they can arc a lot and still somehow keep running. Damage to both brushes suggests excess current flow, (short circuits). Hope you can sort it out.
Combustor.

Hi Combuster,
The motor does have high peak currents through it, but the brush springs are not intended to support any current flow. The pigtails are there to ensure good continuity to the brush. In this case I think that its a case of poor materials and manufacturing. I've never seen carbon brushes as weak and crumbly as these. They are more like the carbon rods that are used in arc lamps.

But yes I hope that I can sort it one way or another.

Originally Posted by BaronJ

When the plastic gears failed, I did consider doing a belt conversion, but on this mill there is no access to the spindle that would allow that. You may remember that I made a metal gear replacement for the failed plastic ones.

https://www.woodworkforums.com/f65/gear-183460

Michael

Originally Posted by Michael G

https://www.woodworkforums.com/f65/gear-183460

Michael

Thankyou Michael,

Its embarrassing not being able to find my own thread.

Hi Guys,

Just to finish this thread off, I made new brush holders and replaced the wire and brushes with new ones. Its back in action now.

Thanks all for the support.