Hi all,
Just pulled out an old hoover motor, 240V 1000W XH-100, complete with original smoothing capacitor, switch etc.
Plugged it into the mains and started it up to check it still worked. Started arcing across the brushes pretty quickly.

I disassembled it, cleaned up the commutator and abraded it gently with emery cloth, brushes were in good nick (thing looked hardly used) but slightly chipped and a few small grooves in the contact surface, which was worn to match the commutator. Decided it may be a good idea to try and clean them up so wrapped emery cloth around the commutator and gently rotated against the brushes. Then rounded off the corners to help prevent chipping.

Reassembled the thing, fired it up and it jolted into life. Nice, clean high-pitched sound, minor radial orange sparking from commutator (figures it was wearing in) and so let it run under observation. Sparking increased and about a minute in the arcing returned. Pitch dropped indicating a change in angular velocity (?) and when powered down motor shaft, face plate and especially brushes were warm to the touch.

Rotating by hand gives smooth motion.
There seems to be minor discolouration on one of the stationary coils but looks surface and possibly just blackening from the sparks?

Unsure where to go from here, I’d really appreciate any advice. Thank you!

 

hi AM,
Welcome to AAC.
I guess you used a very fine emery paper.? and then wiped off any excess grit after the emery wipe.
Which parts did you round off, Then rounded off the corners to help prevent chipping.

Is the brush spring pressure OK and the remaining 'length' of the brush OK.

E

 

An almost poetic account ...very concise ....

Perhaps the problem could be small particles of carbon or copper getting in between the copper segments on the commutator . This will make the whole comutator conduct /arc ...

A wire brush is called for . Brushing action should be along the gaps , parallel to axis of motor.

 

Hi and thanks, good to be here!

hi AM,
I guess you used a very fine emery paper.? and then wiped off any excess grit after the emery wipe.
Which parts did you round off, Then rounded off the corners to help prevent chipping.

Is the brush spring pressure OK and the remaining 'length' of the brush OK.

Yes, once gently ground down I went over everything with a soft paper towel being sure to remove all grime and powder.
I rounded off the corners of the brushes that meet the commutator first when it rotates to about 1mm radii, as a sharp edge here could catch and fracture.
Yes, the pressure is good and judging by the length of the brush and the lack of debris when I originally opened it, it had hardly been used.

 

An almost poetic account ...very concise ....

Perhaps the problem could be small particles of carbon or copper getting in between the copper segments on the commutator . This will make the whole comutator conduct /arc ...

A wire brush is called for . Brushing action should be along the gaps , parallel to axis of motor.

Hi, thanks for your reply. I did wonder about that and so when I disassembled it I made sure to check between the commutator segments when cleaning. The motor seems to have hardly been used judging by how clean they were. Nevertheless I’ll go over them again in case there’s been any significant buildup while wearing the brushes in again.
Will get back to you shortly

 

I should say, picked the wrong words there! When I said I “rounded off the corners” I mean I rounded off the edges that first make contact with the commutator, to stop them catching and chipping.
Sorry about that, slip up on my behalf!

 

judging by the length of the brush

Do you know what the length of a brand new brush and the spring pressure should actually be?
If the brush spring has got hot it might have weakened.

 

Do you know what the length of a brand new brush and the spring pressure should actually be?
If the brush spring has got hot it might have weakened.

That’s a good point however when pushed fully in the brushes stand out of their enclosures by a good distance and when reinserting the brush units into the assembly the back-force from the springs was such that it took a “notable” effort.

I was noticing that when turning it by hand in one direction it’s completely silent but the other if I turn it by above, say, 1 rev per second, it emits a faint clicking sound. I’ll have to take it apart again to see if I can identify any directional differences. It’s a universal bidirectional motor so doesn’t have any inherent directionality but could be the brushes have more play one direction than the other or some other quirk?

 

If it gets hot the longer it runs with no load, I would suspect shorted turns in the armature.

 

Normally there should be no need to re-bed in brushes on a motor that has run for a lengthy period, and it is never done with emery, only a com stone.
You may have done more harm than good.
Some degree of arcing on a Universal motor is normal, as long as the arc does not extend the full diameter of the com. Which often indicates a shorted armature coil.
As a vacuum motor, it has been running one direction its whole life, so there well be a difference heard in the opposite direction.
If it runs at full rpm and the rpm increases when the air inlet is blocked, then its sounds normal, you would need to take a amperage reading to confirm.
Max.

 

If it gets hot the longer it runs with no load, I would suspect shorted turns in the armature.

Yea, I suspected likewise. I’ve done some low-level tests that show it’s pretty balanced across all the commutator coils and across the stator coils but I don’t have a high-voltage impédance measures like the Megger to get a proper read on it!

 

An AC clamp on ammeter reading would work if available.
Max.

 

Normally there should be no need to re-bed in brushes on a motor that has run for a lengthy period, and it is never done with emery, only a com stone.
You may have done more harm than good.
Some degree of arcing on a Universal motor is normal, as long as the arc does not extend the full diameter of the com. Which often indicates a shorted armature coil.
As a vacuum motor, it has been running one direction its whole life, so there well be a difference heard in the opposite direction.
If it runs at full rpm and the rpm increases when the air inlet is blocked, then its sounds normal, you would need to take a amperage reading to confirm.
Max.

Thanks for your reply, Max!
I didn’t think so but I was advised by a more experienced hand that it would be a good move, just in case. As I say, it didn’t look like the motor had actually been running very long judging by the absence of much carbon buildup and lack of general wear. I don’t believe we did more harm as following that it did run smoothly for at least a minute.
As to the arcing, I’m seeing at most 3 persistent arcs before I turn it off, is that really an acceptable amount?
I’ll try blocking the inlets shortly,

The outside of the smoothing capacitor is damaged enough that I can’t make out the rating but I wouldn’t say enough to damage the internal structure at first inspection however we tried to get a capacitive read across it and we’re getting open circuit and occasionally a small capacitance, but spikes and not a true read. I fear the capacitor could be damaged, which could lead to more severe sparking and arcing, right?

 

I suspect the cap is part of a snubber, if so it would have no real effect on the running of the motor but would not hurt replacing it.
A cap is not used on a Universal motor as it is in a 1ph induction motor for phase shift.
Older commutators had mica between the commutator slots and if the com shows sign of wear, the mica can sit a little proud of the copper causing slight arcing, in these cases the com mica has to be undercut with a small tool made for the job.
Now com's have a wearable material as the copper wears.
Max.