Jim Adney writes:

On 12 Nov 2004 08:27:24 -0500 Sam Goldwasser
wrote:

Jim Adney writes:

On 11 Nov 2004 08:08:07 -0500 Sam Goldwasser
wrote:

By definition, a squirrel cage rotor is all shorts.

Right, but so is a regular commutator rotor. The only difference, I
think, is that the squirrel cage rotor is wound with much more rugged
"wire."

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 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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