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Why do electric motors burn out

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spec

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Hi all,

Can anyone explain why some motors regularly burn out, I am thinking specifically about electric hand drills and to a lesser extent angle grinders.

I know how they burn out- normally the insulating varnish on one of the coils breaks down and creates a short- I think that is right. If not perhaps one of the ETO motor experts can advise.

But why should it happen? After all the coils of wire do mot move (relative to each other). Obviously over temperature can cause insulation to break down but why should a motor ever get excessively hot. Surely it is simple and low cost to fit a thermal cut-out or am I missing something. Don't hand power tools have thermal cut-outs?

This question was triggered by the failure of a lovely 115mm, 600W light-weight Bosh angle grinder yesterday, right at a critical moment, but I must admit it has had a hard life cutting concrete and steel. I have two other angle grinders, 115mm DeWalt heavy duty and B&D industrial 230mm job, which I do not like using.

What amazed me was that a replacement for the Bosch is only £37UK, so a repair is not sensible, but perhaps I will go for a higher power replacement and put up with the bulk and weight.

spec
 
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debe

Active Member
No they dont have thermal over load. Generaly they burn out due to excessive loading, or just been used to the point of over heating. They generaly only meant for intermittent use not continuous. I can generaly tell when my drill or angle grinder needs a rest as it will start getting quite hot. It will cool quicker if just left running for a while with no load, so the internal fan can cool it.
 

spec

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No they dont have thermal over load. Generaly they burn out due to excessive loading, or just been used to the point of over heating. They generaly only meant for intermittent use not continuous. I can generaly tell when my drill or angle grinder needs a rest as it will start getting quite hot. It will cool quicker if just left running for a while with no load, so the internal fan can cool it.
Thanks Debe,

That makes a lot of sense. At the time the Bosh angle grinder blew I had been giving it some stick.

I suppose the failure mode is that the varnish on the copper wire just beaks down with the heat.

spec
 

DerStrom8

Super Moderator
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It doesn't apply to AC motors but the brushes are a major failing point in most brushed DC motors. Either the brushes are completely worn down or they get stuck. This is the case for many hand drills that run off batteries.
 

spec

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It doesn't apply to AC motors but the brushes are a major failing point in most brushed DC motors. Either the brushes are completely worn down or they get stuck. This is the case for many hand drills that run off batteries.
Hmm, The brushes are a thought (the Bosh is AC but it does have brushes)- I just assumed that one of the coils had shorted.

spec
 

ClydeCrashKop

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Skilsaw 5150 and a variety of Mastercraft circular saw from 20 years ago were exactly the same and had plastic brush holders. If you made a very long rip cut in a 2X6, it would melt the brush holder and let the magic smoke out. The brush holder assembly was cheap enough but I lost faith in the Skil brand.
 

spec

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Can a faulty brush/brush assembly, worn brushes sound like a blown coil- sort off cracking sound like certain fireworks?

Come to think of it, the angle grinder stopped dead when when the cracking started rather than running at low power which is what I have experienced before with blown coils.

spec
 

DerStrom8

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Can a faulty brush/brush assembly, worn brushes sound like a blown coil- sort off cracking sound like certain fireworks?

Come to think of it, the angle grinder stopped dead when when the cracking started rather than running at low power which is what I have experienced before with blown coils.

spec
Definitely. If the brush gets stuck in the holder and only makes intermittent contact to the commutator then it will arc and spark and make all sorts of crackling noises.
 

spec

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Definitely. If the brush gets stuck in the holder and only makes intermittent contact to the commutator then it will arc and spark and make all sorts of crackling noises.
Oh thanks DS- that makes a lot of sense. Maybe the Bosch angle grinder just needs the brushes sorted.:)

spec
 

DerStrom8

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Oh thanks DS- that makes a lot of sense. Maybe the Bosch angle grinder just needs the brushes sorted.:)

spec
Could be. Definitely worth a look.
 

debe

Active Member
More likely you will find open circuit windings on the armature with what you describe.
 

schmitt trigger

Well-Known Member
Spec;
if I were you, I would perform an autopsy on the grinder.

The motors are universal series wound motors in hand tools, because they produce lots of torque in a very compact package. The torque actually increases as the motor slows down.

The downside are the brush portion......it is not only the brushes themselves that wear out, but the commutator. And as others have mentioned, the brushes may become very hot with heavy duty operation, and thus the brush holder also fails.
 

ronsimpson

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Can anyone explain why some motors regularly burn out
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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MaxHeadRoom78

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The mains powered hand tools are predominantly Universal motors and like all series field motor operate in a run away condition, max rpm limited by load, friction, windage etc.
This high speed is hard on brushes and bearings which are usually the first to suffer.
And as already mentioned they are high torque motors and as such pass high current when loaded causing heating of the windings.
Max.
 

shortbus=

Well-Known Member
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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.
 

JimW

Member
I am surprised that no one has mentioned thermal fuses. Modern AC motors have a thermal fuse in the windings, usually easy to find near the surface of the coil. At a Repair Cafe I regularly volunteer at, we find this to almost always be the cause on a motor driven object that suddenly stopped working. Replacing that fixes them every time. I believe that the Thermal Fuses themselves are aging and failing on their own without any overheating event taking place.

JimW
 

DerStrom8

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I am surprised that no one has mentioned thermal fuses. Modern AC motors have a thermal fuse in the windings, usually easy to find near the surface of the coil. At a Repair Cafe I regularly volunteer at, we find this to almost always be the cause on a motor driven object that suddenly stopped working. Replacing that fixes them every time. I believe that the Thermal Fuses themselves are aging and failing on their own without any overheating event taking place.

JimW
Thermal overload protection was already mentioned, and additionally a blown fuse will not cause the device to have "...a blown coil- sort off cracking sound like certain fireworks".
 

spec

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Wow, thanks for all the help everybody.

Luckily the Bosch angle grinder teardown drawings are on the net and the brushes are available for about £5UK. So I will have a look inside to see what has failed.

The angle grinder is around 10 years old and has had a hard life and it is only a Bosch 600W hobby standard (green body) rather than professional (blue body).

A similar specification Bosch angle grinder is only £37UK but I will probably go for a 900W professional model for £75UK, if the original is beyond economical repair.

spec
 
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MaxHeadRoom78

Well-Known Member
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.
 

ClydeCrashKop

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I believe that the Thermal Fuses themselves are aging and failing on their own without any overheating event taking place.
Planned obsolescence. Not many people would try to fix them.
Do they have a micro-controller inside to tell when the warranty has expired?
 
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