How do you check an armature on a universal motor?

[Andrew TinsleyParticipant @andrewtinsley63637]

This motor suddenly stopped. Took it apart and the commutator is very clean and unworn and the brush gear is in excellent , virtually unworn, condition. None of the windings appear to have overheated.

I would bet that there is a fault on the armature winding. So how does one test an armature for a fault condition? Please only give practical advice, there is enough handwaving theory on the internet. I do have a growler somewhere in my storage system. However it has been many years since I have used it and have forgotten the basics of the test. Seem to remember that it only works if the armature can be made to rotate. However I could well be completely wrong about this!

Any suggestions? Preferably involving measuring resistance between segments.

Andrew.

[noel shelleyParticipant @noelshelley55608]

Hi Andrew, Do you have a megger or insulation tester ?  I would check all the stator windings and all other wiring for continuity first. Then check opposite segments on the com for continuity, go all the way round. Since the com and brushes seem OK I would look else where. Never used a growler but believe it is all to do with induced current in the windings ? Good luck,  Noel.

[peak4Participant @peak4]

I had a quick look for a suitable video without annoying background music; this one seems to cover it OK.
Note that he’s demonstrating on a starter motor, so the coils will be made of large diameter wire, whereas most other motors will have more turns of smaller diameter wire, and thus a higher resistance.
Personally I’d use a higher resistance range for the commutator to lamination test.

Bill

[Roger HartParticipant @rogerhart88496]

Fools rush in,  granny/eggs etc.

Is this a big industrial motor or something like a traditional hoover motor?  Assuming this runs on normal 230v mains.

Essentially for a smallish motor the current comes in,  passes through one field winding then in and out the armature via the brushes and then out via the (usually) second winding.  Seems odd just to have stopped and not tried to stagger round or buzz like mad.  Makes me think it’s not the armature…

Measure the resistance of the field windings – they should be the same,  a few tens of ohms for a small motor.  Then measure between each adjacent segment on the commutator – should be a few ohms or one or two ohms for a smallish motor and the same all the way round.  Also try across the commutator where the brushes go – not so easy to be sure you are ‘opposite’ but a few ohms and the same for all positions.

Re assemble then fire up with a mains filament lamp (100W or so) in series.  The motor should conduct more or less the same as you rotate the armature and buzz and try to ‘go’.  Try not to kill yourself.

I have had a cooked armature where the solder had melted and spun off to the inside housing and the armature wires ceased making contact with the commutator segments.  But you would likely notice and small this…

 

[Andrew TinsleyParticipant @andrewtinsley63637]

Field windings don’t apply here, as it is a permanent magnet motor. I tried the obvious  of measuring the resistance between opposite commutator segments. There was no continuity between any of them. I even tried measuring between nearly opposite commutator segments in case (for some obscure reason) they were not connected diametrically.

I cannot believe that ALL of the armature windings have gone open circuit! The motor stopped with no sign of distress. I can certainly replace the motor, at a price, but I would really like to know what has gone wrong with the armature !

Andrew.

[Ex contributorParticipant @mgnbuk]

I tried the obvious of measuring the resistance between opposite commutator segments.

Measure between adjacent segments – “walk” the test leads around the commutator checking each pair of adjacent segments. Readings between each pair of adjacent segments should be very close to each other – within 10% – and depending on the size of the armature probably low single digit Ohms. Then check between each segment & the shaft – all should be open circuit.

[Roger HartParticipant @rogerhart88496]

A bit of digging with ‘fuses inside dc motor armature windings’ on Google suggests that some Chinese dc motors do have a fuse in the armature windings to prevent fire.  Sometimes on top but more usually buried under the winding.

A new one on me.  Not so bad if they were resetable but if blown – bad news.  Would this clue apply to your motor?

 

[MacolmParticipant @macolm]

If it stopped suddenly and totally, it is unlikely to be the armature (but see previous post!). This is because it comprises a series coil between each pair of adjacent commutator segments right the way round, and thus there are two paths between the brushes, one round each half. Two breaks and you have found the failure! One break and the motor will likely run, but very badly. The commutator might be in the process of falling apart (wires break at the crimps), again unlikely if it looks good, but possible. Checking between each pair of adjacent commutator segments will indeed find where there is a break.

Therefore look at the rest of the circuit for continuity all the way through the fields to the brush holders. Obviously also check for shorts to the frame. See also the various other posts, since there are all sorts if possible faults.

As already suggested, field resistances should match closely. Shorted turns in a field winding lead to a mismatch in the back emf between the two branches of the armature circuit, and thus a large parasitic circulating current which can burn out the armature.

To check an armature for shorted turns (without the professional gear), with the motor assembled and able to rotate, connect the brush holders together (without brushes, no connection to the armature), and feed the fields (they must be in series) with lower voltage AC (perhaps 50V), or from mains via a 100W filament lamp in series. If there are shorted turns, the armature will rotate until the short alights with the field flux. If the armature can be freely rotated and stay in any position, it should be free of shorted turns.

[Andrew TinsleyParticipant @andrewtinsley63637]

Ah ! Mention of shorted turns, reminds me that is what my professional growler is for! Thanks for reminding me. Must be 40 years since I last used it!

The problem is definitely in the armature. Full wave rectified mains is fed to the brushes and the pulsed DC is definitely getting to the brushes. Checked with multimeter and oscilloscope. Checked on the actual carbon of the brushes (after removing the armature!). As stated previously. this is a permanent magnet motor, so NO field windings.

Interesting comment about fuses in the armature. New one on me! There are several blobs of what looks like plastic on the armature windings. Thought they were balancing weights, but could be a fuse hidden underneath!

Lightbulb moment about checking resistance between adjacent commutator segments! This is what was bothering me about a basic resistance check. I could not remember what it was and that comment has now reminded me!

Thanks for all the feedback. It has certainly jogged the memory. I will do further checks and report back

Andrew.

[john fletcher 1Participant @johnfletcher1]

Dig out your growler. Get a hack saw blade or some thing thin but similar, place the armature in the VEE, connect the Growler to the mains supply. Next, slowly rotate the armature with the blade just slightly above the it, and if there is a fault you will hear the sound and the blade will be attracted to the rotating armature. Hole in one. Also, there is normally a pair of probes with the Growler, to carry out an insulation test one probe to the shaft other to the comm. As the motor suddenly stopped have you check the supply to it ?

Ex contributor above, his idea is a good one and works.   John

[Andrew TinsleyParticipant @andrewtinsley63637]

Checking the resistance of each of the adjacent pairs, I did find two that had gone open circuit. So Malcolm has hit the nail on the head and problem solved! No chance of a repair so a new motor is the next move.

Thanks everyone for the help and suggestions.

Andrew.

[Roger Hart]

On Roger Hart Said:

…Google suggests that some Chinese dc motors do have a fuse in the armature windings to prevent fire.  Sometimes on top but more usually buried under the winding.

…

Not fuses exactly, they’re thermal cut-outs intended to break the circuit when they get too hot and close again when things cool down.  Either a bimetallic strip or a thermistor.  They prevent fires, and reduce the risk of heat damaged insulation.

Often glued on the outside, making repair possible.  A thermal cut-out inside the windings used to be a sign of extra-cost quality.  More ordinary these days, possibly necessary to meet modern safety standards.

My limited experience of cut-outs is in transformers.   Usually fail open circuit rendering the transformer useless, easy to detect with a multimeter.  But I had one that passed resistance tests and produced the expected volts only to break contact on load.  Very confusing, I guess a loose bimetallic strip.  Worth noting the possibility that a faulty cut-out tests OK on a multimeter and then fails in action.

If testing doesn’t find another explanation, a duff cut-out is likely.  And then the pragmatic answer is to replace the motor or transformer. Unless worth rewinding; small ones rarely are.

🙁

Dave

 

[duncan webster 1Participant @duncanwebster1]

Pedant alert, if it has permanent magnet field it isn’t a universal motor.

[duncan webster 1]

On duncan webster 1 Said:

Pedant alert, if it has permanent magnet field it isn’t a universal motor.

It hasn’t been a universal motor since Andrew told us it was permanent magnet type earlier in the thread.

I’m mystified how this motor ‘suddenly stopped’. If the armature is only partly open circuit then surely it would have complained and given some warning?

What voltage is it and please can you post a picture.

Ian P

[noel shelleyParticipant @noelshelley55608]

That it should suddenly stop seems odd, permag dc motor there is little to go wrong, unless it has some sort of overload/overheat device. Brushes and comm stated as being OK. Noel.

[Andrew TinsleyParticipant @andrewtinsley63637]

I never said it was a universal motor. Like the last couple of posts, I too am surprised that the motor didn’t show signs of distress before it stopped working. Usually a motor will run rough before expiring. I have had a couple of angle grinders do this when one of the armature windings goes open circuit, followed by total failure when other windings fail.

What really surprised me on this motor was the brushes were in excellent condition and the commutator showed only the merest trace of graphite. Usually the brushes show severe roughness and the commutator is usually pitted and covered in carbon!

Sorry Ian, I am a bit of a Luddite when it comes to photos. I don’t have (or want!) a smart phone and my camera gear is old fashioned 35mm kit. So no photos I am afraid.

Andrew.

[duncan webster 1Participant @duncanwebster1]

Thread title?

[duncan webster 1]

On duncan webster 1 Said:

Thread title?

+1

[Roger HartParticipant @rogerhart88496]

There remains the unsettling question – why did the windings/fuse/thermistors blow?

If you do fork out on a new motor I would advise caution before firing it up.  Perhaps check out the controller with a filament type light bulb.  Personally I might try tormenting the controller a bit,  a poke or rattle with a stick to see if anything odd happens.  I might even wire in a lowish fuse until the – whatever – has been running for a bit.

Something caused that motor to fail and to me it smells of a sudden voltage overload – but I have been wrong many many times before.  The why is a bit unsettling.

BTW,  I suppose we should hide away a few filament type light bulbs for the future,  still common enough but that is changing.

This puzzle made me look up wave and lap windings on dc motors,  something I had forgotten about for over 50 years.

 

[noel shelleyParticipant @noelshelley55608]

Have the electronics in the DC supply failed ? Rectifier gone ? Noel.

[Andrew TinsleyParticipant @andrewtinsley63637]

I have already said that the full wave rectified DC is present at the carbon of the brushes. Checked with multimeter and oscilloscope so nothing wrong with the supply.

Sorry Duncan! You are quite correct I did say “Universal Motor” in the title, mea culpa!

After examining the blobs on the armature, they are not thermal cutouts or fuses, just balancing weights. So that isn’t the cause of the failure. It appears to be a couple of the armature windings gone O/C, simple as that.

Me too Roger. I had forgotten all about wave and lap windings on DC motors. I would not lose any sleep over the puzzle. Looking up the motor specs, the claimed life is only 600 hours! So cheap and nasty springs to mind! There is a 24 V DC version of the motor and it is used in some of the expensive Dyson pet hair, vacuum cleaners. Surprise,  surprise there is plenty of moaning on the net about the short life of the motors!

I was interested in the failure mode as it didn’t line up with other expiring DC armature motors, that I have encountered. Lots of very interesting suggestions and information which is much appreciated.

Andrew.

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