Detect shorted turns in motor windings...

[Jim Adney]

Nah. A commutator motor happens to be shorted but that is a byproduct
of how it's wound, not a requirement. A squirrel cage rotor is a bunch
of copper bars embedded just under the surface of a solid steel rotor
running length-wise. They are shorted at the end plates. Since these
are induction motors, they depend on the current flowing in the bars
as the secondary of a transformer to provide the torque.

I'm in total agreement with the last part of this paragraph.

For brush-type motors, current is applied to the rotor windings via the
brushes. It's possible to build such a motor without "shorted" windings.

Can you put your money where your mouth is? I've certainly never seen
a commercial motor which was wound this way, and it strikes me that
such a motor would quickly destroy its commutator as you tried to
suddenly turn off the current in each winding segment.

I'm prepared to eat my words, however, if someone can come up with an
example. ;-)

Note that I put in the word "commercial" so that you guys don't point
out the home-build from 2 nails and a bunch of wire type motors which
are used to demonstrate basic motor principles. Yes, I agree that it's
not "necessary" unless you want to make a practical motor.

I'm not trying to be pedantic, just sticking to the original question
about a shorted turn in a motor, which I was willing to assume was a
commercial motor of some kind.

So, to get back to my question: Has anyone ever seen an
electromagnetic failure in a squirrel cage rotor? Does it EVER happen?

-

 

[Sam Goldwasser]

I'm in total agreement with the last part of this paragraph.


Can you put your money where your mouth is? I've certainly never seen
a commercial motor which was wound this way, and it strikes me that
such a motor would quickly destroy its commutator as you tried to
suddenly turn off the current in each winding segment.

I didn't say that any commercial motors were built this way. I don't
know. What I'm saying is that the motor will turn, which would not
be the case if the squirrel cage rotor were open.

I'm prepared to eat my words, however, if someone can come up with an
example. ;-)

Note that I put in the word "commercial" so that you guys don't point
out the home-build from 2 nails and a bunch of wire type motors which
are used to demonstrate basic motor principles. Yes, I agree that it's
not "necessary" unless you want to make a practical motor.

I'm not trying to be pedantic, just sticking to the original question
about a shorted turn in a motor, which I was willing to assume was a
commercial motor of some kind.

So, to get back to my question: Has anyone ever seen an
electromagnetic failure in a squirrel cage rotor? Does it EVER happen?

I have a bench grinder that seems to have such a fault. I have eliminated
mechanical causes as best I can and it still intermittently stops and
hums loudly when the rotor gets hot.

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[NSM]

| I have a bench grinder that seems to have such a fault. I have eliminated
| mechanical causes as best I can and it still intermittently stops and
| hums loudly when the rotor gets hot.

Bearings?

NM

 

[Sam Goldwasser]

| I have a bench grinder that seems to have such a fault. I have eliminated
| mechanical causes as best I can and it still intermittently stops and
| hums loudly when the rotor gets hot.

Bearings?

That's what I thought originally but I couldn't find any problems.

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[NSM]

| > | I have a bench grinder that seems to have such a fault. I have
eliminated
| > | mechanical causes as best I can and it still intermittently stops and
| > | hums loudly when the rotor gets hot.
| >
| > Bearings?
|
| That's what I thought originally but I couldn't find any problems.

If it's a cap run motor maybe the cap is shorting, otherwise a shorted turn
in the motor I think.

NM

 

[Sam Goldwasser]

| > | I have a bench grinder that seems to have such a fault. I have
eliminated
| > | mechanical causes as best I can and it still intermittently stops and
| > | hums loudly when the rotor gets hot.
| >
| > Bearings?
|
| That's what I thought originally but I couldn't find any problems.

If it's a cap run motor maybe the cap is shorting, otherwise a shorted turn
in the motor I think.

Could be though the effect seems to depend on the position (angle) of the
rotor. It's a split phase motor.

Also should note that if the rotor gets up to speed before it heats up,
the motor runs fine. Once it gets into funky mode, though, it can't
be spun up by hand unless it's done with power off.

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[NSM]

| Could be though the effect seems to depend on the position (angle) of the
| rotor. It's a split phase motor.
|
| Also should note that if the rotor gets up to speed before it heats up,
| the motor runs fine. Once it gets into funky mode, though, it can't
| be spun up by hand unless it's done with power off.

Dead squirrel? Seriously, if the squirrel cage bars are soldered, perhaps
they have failed.

N

 

[Sam Goldwasser]

| Could be though the effect seems to depend on the position (angle) of the
| rotor. It's a split phase motor.
|
| Also should note that if the rotor gets up to speed before it heats up,
| the motor runs fine. Once it gets into funky mode, though, it can't
| be spun up by hand unless it's done with power off.

Dead squirrel? Seriously, if the squirrel cage bars are soldered, perhaps
they have failed.

That's what I thought might have happened but there certainly isn't
any visual evidence of it. I even tried to heat the rotor up with a
propane torch but that was a joke - it barely got warm.

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[Jim Adney]

I have a bench grinder that seems to have such a fault. I have eliminated
mechanical causes as best I can and it still intermittently stops and
hums loudly when the rotor gets hot.

Do you have access to a growler? If so, I wonder if you could remove
the rotor and check it cold and then again after warming in an oven.
Might a simple physical inspection reveal something, perhaps even
something that could be fixed.

-

 

[Sam Goldwasser]

Do you have access to a growler? If so, I wonder if you could remove
the rotor and check it cold and then again after warming in an oven.
Might a simple physical inspection reveal something, perhaps even
something that could be fixed.

Physically it looks fine. It's not that important to spend time on
at this point. I picked up another grinder for $6.

Probably no need to put it in an oven. Just run it up a couple times until
it fails and let it cook in that state. Then pull the rotor and test.

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[NSM]

| Physically it looks fine. It's not that important to spend time on
| at this point. I picked up another grinder for $6.
|
| Probably no need to put it in an oven. Just run it up a couple times
until
| it fails and let it cook in that state. Then pull the rotor and test.

It sure sounds like it's pulling out of sync, so there's no out-of-phase
current to keep it running. Does it have a capacitor?

N

 

[Sam Goldwasser]

| Physically it looks fine. It's not that important to spend time on
| at this point. I picked up another grinder for $6.
|
| Probably no need to put it in an oven. Just run it up a couple times
until
| it fails and let it cook in that state. Then pull the rotor and test.

It sure sounds like it's pulling out of sync, so there's no out-of-phase
current to keep it running. Does it have a capacitor?

No capacitor. Split phase meaning two windings, one with much higher
resistance to produce the phase shift.

The thing is, it's not only stopping. It acts as a very good brake
when the condition occurs - hard to turn. This unlike an induction
motor with only one winding energized which will turn freely and
then spin up to normal speed once its going fast enough. This one
will slow down if you try to spin it by hand. Then make a loud hum
or buzz at a certain angle of the rotor.

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[NSM]

| No capacitor. Split phase meaning two windings, one with much higher
| resistance to produce the phase shift.
|
| The thing is, it's not only stopping. It acts as a very good brake
| when the condition occurs - hard to turn. This unlike an induction
| motor with only one winding energized which will turn freely and
| then spin up to normal speed once its going fast enough. This one
| will slow down if you try to spin it by hand. Then make a loud hum
| or buzz at a certain angle of the rotor.

I wonder if the centrifugal switch is failing, but I don't see why that
would slow it down. Most odd.

N

 

[Sam Goldwasser]

| No capacitor. Split phase meaning two windings, one with much higher
| resistance to produce the phase shift.
|
| The thing is, it's not only stopping. It acts as a very good brake
| when the condition occurs - hard to turn. This unlike an induction
| motor with only one winding energized which will turn freely and
| then spin up to normal speed once it's going fast enough. This one
| will slow down if you try to spin it by hand. Then make a loud hum
| or buzz at a certain angle of the rotor.

I wonder if the centrifugal switch is failing, but I don't see why that
would slow it down. Most odd.

If it failed to cut out, then the motor would just smoke.

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[NSM]

| > I wonder if the centrifugal switch is failing, but I don't see why that
| > would slow it down. Most odd.
|
| If it failed to cut out, then the motor would just smoke.

I've heard of them closing randomly while the motor is running.

N

 

[Sam Goldwasser]

| > I wonder if the centrifugal switch is failing, but I don't see why that
| > would slow it down. Most odd.
|
| If it failed to cut out, then the motor would just smoke.

I've heard of them closing randomly while the motor is running.

But all that would do is create smoke when the starting winding overheated!
The motor would still run.

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