Why do electric motors burn out

shortbus= said:

Another thing is the newer models of power tools are all trying to get smaller. mainly for cost saving but some what for the DIYer that is uncomfortable with the weight of a tool. The smaller the housing is, the less the airflow through the tool. The smaller the housing the smaller the heat sink ability. Real industrial tools are some what larger than the DIY, home store variety. And the real tools don't have the burn out problems. You guys may disagree with this, just my opinion from years of factory work.

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Well, I agree with you. I worked in a Garage evenings and weekends for a couple of years and they had a few industrial strength electric hand tools. One was a Wolf hand drill which had a cast aluminium body in the raw. It was made in the early 1950s and was abused to hell, but never missed a beat.

This is not to do with motors, but a trend I do not like is the use of plastics for critical moving parts- some tools, especially angle grinders have plastic gears now.

spec

ronsimpson said:

I have noticed that, if you don't use them, they don't fail.

I think the iron and the copper expand at different rates. So there bust be movement inside the motor. (assuming you use it)

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I had to read your post a couple of times Ron. At first I thought you were joking.

But I understand your point now and can see that differential expansion could cause failures.

The same goes for damp. I inadvertently left a B&D hand drill out in the garden for three months and although it was fully dried out it made the tel-tale crackling noise and only turned very slowly. But when it was stripped down everything seemed to be OK.

spec

MaxHeadRoom78 said:

I don't ever recall seeing a thermal cut out in a Universal, and if it was it would probably just be in the field which leaves the Armature to its own devices, it generally runs hotter than the field.
I have had high speed bearings fail on Universal motors, this and the brushes are common items.
Max.

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I regularly had to renew the bearings on a so-called heavy duty B&D electric hand drill and also the brushes. Mind you, that drill was badly abused. I even used it for grinding because an angle grinder was too expensive in those days.

spec

Some of the worst designed and built hand tools are hedge trimmers. I had a year long affair with my father-in-laws hedge trimmer- an el cheapo model.

It was the worst piece of design ever. Never mind about bearings and motors failing but the mains trigger switch failed regularly and the suppression capacitor regularly broke off- the old dear next door would complain that she could not watch TV. The mains switch was one of those self exploding assemblies held to together with one tiny screw and was practically impossible to assemble, although after a few strip-downs I became quite expert. The final straw came when, due to a poor deign, the whole blade assembly started oscillating back and forth rather than the halves of the blade moving relative to each other.

The brush housing would also vibrate loose and move around so it lost the timing, so at one moment the motor would be screaming and the next it was turning at reduced speed and making a kind of groaning sound.

I hate throwing anything out, but in the case of Pop's %!!** hedge trimmer it was a real pleasure.

spec

spec said:

At first I thought you were joking.

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I often joke......................just kidding.

from reading this it looks like I am wrong, my assumption having always taken dead stuff apart (in case I can use something). Was to do with the commutator, maybe if I explained what I see then you can work out what I am on about.

My assumption is every copper block on the commutator, is connected to a wire on the winding. The brush material and to a certain extent, most the grindings and crud, that get inside and lay between the copper blocks, are conductive. So I assumed over time as the crud built up, it started to short out more and more of the windings, thats why I assumed grinders that are older and heavily used always get hot quicker........


But its all based on observation with nothing to back it up, so maybe I was totally wrong after all.

Little Ghostman said:

from reading this it looks like I am wrong, my assumption having always taken dead stuff apart (in case I can use something). Was to do with the commutator, maybe if I explained what I see then you can work out what I am on about.

My assumption is every copper block on the commutator, is connected to a wire on the winding. The brush material and to a certain extent, most the grindings and crud, that get inside and lay between the copper blocks, are conductive. So I assumed over time as the crud built up, it started to short out more and more of the windings, thats why I assumed grinders that are older and heavily used always get hot quicker........


But its all based on observation with nothing to back it up, so maybe I was totally wrong after all.

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Hi LG,

Shorting between motor commutator segments is possible but not common in my experience. The normal reason why older motors may get hotter is due to high brush/commutator resistance, which can be caused by a number of factors.
(1) worn brushes
(2) weak brush tension
(3) badly scored armature
(4) miss aligned brush housing
(5) brush jammed in brush housing

On a couple of occasions I have transformed a motor by simply putting some appropriate lubrication in the brush housing.

spec

Little Ghostman said:

. The brush material and to a certain extent, most the grindings and crud, that get inside and lay between the copper blocks, are conductive. So I assumed over time as the crud built up, it started to short out more and more of the windings,

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At one time Mica was used as an insulator between com segments, these had to be undercut with a special tool or one made from a saw blade.
Now this usually not necessary as a different material is used that wears evenly along with the copper, so there is virtually no gaps now in a modern commutator.
And undercutting is no longer needed.
Max.

I seem to remember that some motor commutators just had gaps between the segments. These would cause more brush wear and also fill with carbon dust and thus form a connection between the segments.

spec

A few years ago the motor fails on my milling machine. (Permanent magnet motor.) It was working normaly under a light load and there was a bang and a flash. The the way that the fuse had blown (vaporised metal on the inside of the glass fuse.) suggested a very high fault current. If anyone is interested I did my best to find the mode of failure. The information on this failure is here under the heading "Update on motor failure" I think the initial failure was probably a short between windings which fits with ronsimpson's theory.

Les.

spec said:

I seem to remember that some motor commutators just had gaps between the segments. These would cause more brush wear and also fill with carbon dust and thus form a connection between the segments.

spec

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These were the under-cuts as in the previous post!

As to measuring armature resistance it is not recommended to just use an Ohm meter, the better way is to lock the motor rotor, supply a small known DC voltage and measure the current, take the lowest reading.
Max.

When buying an electric drill or other hand tool, I generally purchase the one within my budget that has the highest current rating.
Generally 5-6 Amp drills have never overheated for me.
Another possible problem:
Operating tools with a overly long extension cord with smaller gauge wired can cause a voltage drop that can cause a motor to overheat.
Pat - W9ZO

I had the failed Bosch angle grinder apart this morning.

The brushes and brushes holder are like new and the tension on the brush springs is OK. The field coils look OK too but the armature smells like it has been overheating- time for a new angle grinder I think.

If anyone is interested, after a bit of research, I am going for a Bosch GWS9-115: https://www.screwfix.com/p/bosch-gws-9-115-avh-900w-4-angle-grinder-230v/8624r

spec

spec said:

After all the coils of wire do mot move (relative to each other)

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Well, they shouldn't, but parallel wires carrying the same current produce magnetic fields causing attaction/repulsion forces between the wires. The wires will move, albeit microscopically, if they aren't properly anchored (e.g. by zero separation or by varnish etc).

alec_t said:

Well, they shouldn't, but parallel wires carrying the same current produce magnetic fields causing attaction/repulsion forces between the wires. The wires will move, albeit microscopically, if they aren't properly anchored (e.g. by zero separation or by varnish etc).

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Ah yes, thanks Alec. I missed that aspect.

spec