Shaded-pole motors: failure mechanisms?

[Kiwi BlokeParticipant @kiwibloke62605]

When things go OK, you tend to ignore them. When they don't, you suddenly pay attention – but can easily imagine their behaviour has changed in ways it hasn't…

A fan heater seemed to start running hot, with visibly red elements (simple coiled type), and its cylindrical fan seemed to be going slower than usual. Dismantling revealed the expected eiderdown-full of fluff and muck, but the fan and motor rotated very freely, with no evidence of unexpected friction. After cleaning the inaccessible nooks and crannies, and sparingly lubricating the motor bushes, it all seems to work OK, with barely-red elements (never noticed them red before, but never really looked…), but am I imagining that the fan is slower than it used to be?

Excepting mechanical causes, can the simple type of shaded-pole motor partially fail, so that it runs a bit more slowly than intended? I think I understand how they work, and can't imagine how this could occur…

[Kiwi BlokeParticipant @kiwibloke62605]

[SillyOldDufferModerator @sillyoldduffer]

Shorted turns in the coil maybe? Shaded pole motors are low-torque, making starting problems likely if anything increases the load. Like mucky fluff! A possible failure mode:

• fluff stops the motor starting or causes it to run slowly, causing the
• element and coil to overheat, and cooking
• the insulation of the coil, shorting
• one or more turns of the coil, resulting in
• power loss and more heat.

Cleaning helps, but the coil has partially failed, and the motor blows less cooling air over the element, which gets a bit hotter.

I had a fan heater that glowed a very dull red, but I don't know if that was normal! Wouldn't be surprised – the electric elements in an ordinary bar fire glow orange.

Dave

[SteviegtrParticipant @steviegtr]

The problem is the motor shafts get an oxide on them from the bushes. The only way to rectify is by taking the caps off the motor ends.

Clean the internal faces of the bushes, Clean the shafts which will be a blackish colour.

Re-oil the bushes & refit end caps. Some motors are rivetted together & some have screws & nuts . If rivet type then shear them off to gain access. The motor windings rarely fail on this type of motor & will happily buzz away if the shaft is seized. It is a very common fault with fan heaters.

Steve.

[Robert Atkinson 2Participant @robertatkinson2]

Apart from friction in the moving parts, which you seem to have sorted already, there is another possible failure mode. This is "sort of" electrical. It's poor contact of one or of the rods in the rotor "squirrel-cage". This reduces the current induced in the cage and thus the torque. The cage is typically just bars and two rings that are peened together. Re-peening with a punch may help.

Some years ago I was asked to look at a commercial slushy drink maker. It was freezing up so came to me initally as suspected electrical control fault. After proving the controls were OK I was unable to give up. I finally realised that the large screw was turning the wrong way (I knew nothing about these machines). It was driven by a squirrel-cage motor and multi-stage gearbox. These are not reversable so a bit of a puzzle. Simple gearbox so nothing wrong there. Turned out (no pun intended) that the coil assembly had been flipped over. No idea if it had been wrong from new or somone had been in there before me. The owner had bought it used for a low price. I did get cheap kebabs for a year or so.

Robert.

Edit, shorted turns in the coil will cause increased current and thus more torque, not less. So once any excess load is remove the motor will run at normal speed. It will also cause the coil to overheat which tends to be a runaway condition and obvious fault.

Edited By Robert Atkinson 2 on 13/06/2023 17:44:50

[Kiwi BlokeParticipant @kiwibloke62605]

Thanks Robert, great info. So it seems that these devices are so gutless that they have terrible speed regulation, and will 'slip' increasingly, as load is applied. Is this correct? However, they may not be damaged if they stall – also correct? Just for info, there's no external evidence that this motor has been cooked.

If it seems to be misbehaving in the future, I'll bite the bullet and pull the rotor out, with the intention of frightening it with a punch.

[Martin Johnson 1Participant @martinjohnson1]

Posted by Kiwi Bloke on 14/06/2023 09:39:00:

Thanks Robert, great info. So it seems that these devices are so gutless that they have terrible speed regulation, and will 'slip' increasingly, as load is applied. Is this correct? However, they may not be damaged if they stall – also correct? Just for info, there's no external evidence that this motor has been cooked.

If it seems to be misbehaving in the future, I'll bite the bullet and pull the rotor out, with the intention of frightening it with a punch.

Yes, the amount of slip will be far greater than a lathe motor, for example. Slip also increases with load, so speed decreases.

As to life under extended stall, it rather depends………..

Martin

[duncan webster 1Participant @duncanwebster1]

Posted by Robert Atkinson 2 on 13/06/2023 17:39:39

Edit, shorted turns in the coil will cause increased current and thus more torque, not less. So once any excess load is remove the motor will run at normal speed. It will also cause the coil to overheat which tends to be a runaway condition and obvious fault.

Edited By Robert Atkinson 2 on 13/06/2023 17:44:50

If you have a shorted turn then current goes up but turns goes down, so the magnetism (ampere turns) stays the same?

Edited By duncan webster on 14/06/2023 15:48:17

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 14/06/2023 15:44:37:

Posted by Robert Atkinson 2 on 13/06/2023 17:39:39

Edit, shorted turns in the coil will cause increased current and thus more torque, not less. So once any excess load is remove the motor will run at normal speed. It will also cause the coil to overheat which tends to be a runaway condition and obvious fault.

Edited By Robert Atkinson 2 on 13/06/2023 17:44:50

If you have a shorted turn then current goes up but turns goes down, so the magnetism (ampere turns) stays the same?

…

I've been worrying about this too. The idea that a shorted turn would increase torque feels wrong, but I just don't know!

In an AC motor winding I'd expect a shorted turn to behave like a step-down transformer with a near-zero resistance load, causing high current to flow in the loop at a very low voltage. The loss in watts would be limited. Certainly lots of heat, maybe enough to melt the loop. Though there would be a lot of magnetic flux in a shorted turn too, I can't visualise if it adds to the normal ampere-turns flux, or is out of phase with it.

Every time I think I understand electric motors, I'm proved deficient!

I like Steve's Oxide explanation. It doesn't say much about the quality of the bearings, because oxide can only come from tiny fragments of metal being scraped off. Entirely possible – most of the shaded pole motors I've looked at were, putting it politely, inexpensively made.

Dave

[SteviegtrParticipant @steviegtr]

Posted by SillyOldDuffer on 14/06/2023 16:47:19:

Posted by duncan webster on 14/06/2023 15:44:37:

Posted by Robert Atkinson 2 on 13/06/2023 17:39:39

Edit, shorted turns in the coil will cause increased current and thus more torque, not less. So once any excess load is remove the motor will run at normal speed. It will also cause the coil to overheat which tends to be a runaway condition and obvious fault.

Edited By Robert Atkinson 2 on 13/06/2023 17:44:50

If you have a shorted turn then current goes up but turns goes down, so the magnetism (ampere turns) stays the same?

…

I've been worrying about this too. The idea that a shorted turn would increase torque feels wrong, but I just don't know!

In an AC motor winding I'd expect a shorted turn to behave like a step-down transformer with a near-zero resistance load, causing high current to flow in the loop at a very low voltage. The loss in watts would be limited. Certainly lots of heat, maybe enough to melt the loop. Though there would be a lot of magnetic flux in a shorted turn too, I can't visualise if it adds to the normal ampere-turns flux, or is out of phase with it.

Every time I think I understand electric motors, I'm proved deficient!

I like Steve's Oxide explanation. It doesn't say much about the quality of the bearings, because oxide can only come from tiny fragments of metal being scraped off. Entirely possible – most of the shaded pole motors I've looked at were, putting it politely, inexpensively made.

Dave

My father had an Electrical shop in Leeds many years ago. I sometimes worked there on Saturdays. One of the jobs he left for me was doing the fan heater clean. It was very rare that it was anything else than the shafts needed cleaning & bushes. Most of those type of heaters had self aligning bearings in the end caps. Also they were usually bronze. Remember back then most of those heaters were made in the UK. Not many far Eastern goods came here.

[John HaineParticipant @johnhaine32865]

The squirrel cage rotor is a shorted turn already, another shorted turn in the winding would surely reduce the rotor current and torque. Also it would work to reduce the flux in the airgap – induction motors are essentially transformers and transformers try to arrange the primary and secondary currents so that the core flux is zero.

However the current in the shorted turn itself is likely to rapidly make it into an open circuit turn!

Depending on the design of the rotor the torque may not be very high and it may tolerate being stalled. I used to have a tape deck where the feed and takeup spools were driven by shaded pole motors designed as "torque motors" – the feed spool would actually be driving the motor backwards! Also had series resistors in the windings to "control" the speed, which you can do with this sort of motor as long as you don't care about efficiency. Does your fan have some species of speed control?

[Kiwi BlokeParticipant @kiwibloke62605]

Update. It was mechanical, after all. Following a quick clean and lube, things looked good. Then, a couple of days later, the motor wouldn't start. Poking the fan (power off) revealed that it wouldn't initially turn easily. Then it would, apparently freely. Today, complete strip down, including motor. No corrosion, but what lubricant there had been was gummy or varnish-like. So, acetone wash, brushing out bushes, etc., and gun oil to the bearing felts, reassemble. Now the fan spins really freely! Switch on and it goes like the clappers! Happy wife, who no longer resembles a couple of chapel hat pegs when she gets out of the shower (unheated houses are miserably cold in NZ winter!).

Moral. Simple things shouldn't be discounted, diagnostic examination should be careful, even on a rush job, and assumptions are dangerous…

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