Re: magnetism question

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"Autymn D. C." <lysdexia@xxxxxxxxxxxxx> wrote in message
news:1134853316.309531.132130@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Don Kelly wrote:
>> A non-stick pan doesn't have to contend with changes in dimensions of the
>> rotor and stator as they heat up and put the squeeze on. There is also
>> the
>> problem that nothing is "perfectly true". This increases with the size of
>> the machine. The limitations are mechanical.
>
> Pans stretch when they heat, so your objection is ungrounden. The
> stretching to and fro wreaks the teflon to flake off. The limitations
> are economic only. Grandparent clocks use a mercury-steel pendulum to
> fix length-shifts with temperature to keep the same heftmidst
> (barucentre).
>
>> I ran an experimental motor with an aluminum shell rotor- all was good
>> until
>> the rotor heated up- then the squealing started and, if left, the rotor
>> would have jammed on the stator. A teflon coating wouldn't have helped
>
> I didn't say teflon; I said iron glass. And if you had made the same
> corrections to the stator, it wouldn't happen.
>
> -Aut

>
Pans stretch when heated- true- but take two snugly nested teflon coated
pans and heat them - see if you can separate them when they are hot. It's
the old "ball bearing in washer" demonstration. Rotor and stator both
expand--Squeeze play. It really doesn't matter what coating is between
them or what the surface is -glass or teflon. Also consider the fact that
wobble, however small, does occur and that there are large transient forces
which in some cases can and do crack steel parts of the rotor. These are
some of the mechanical factors/ limitations which must be considered.

Iron glass is fine (if you are thinking of the same thing that I am- the
"ferromagnetic glass" which has been used in some distribution
transformers ) but the conditions in a rotating machine are not those in a
transformer and the mechanical factors are more of a limitation in the
former. Even if some form of ferromagnetic glass can be developed (and it
may) which can cope with the mechanical factors as well as being very
slippery even under pressure, will its magnetic properties allow a higher
flux density thant what is now practical ?

The pendulum correction scheme is fine for pendulums but the mechanical
demands and limitations are quite different. Even if feasible, what effect
does this have on magnetic properties of the device? Does it add complexity
with little or no gain?

Now consider material on material contact (glass or teflon) as the contact
pressure increases. The friction will not be negligable (and will be much
higher than with an air gap) and cooling will be compromised. The contact
surface gets hotter and hotter- what happens- does it vaporise or does it
melt? Sure an oil coating can be applied but to be effective, room to
"float" is needed.

In practice, the air gap does require an increase in exciting amp turns and
corresponding I^2R loss compared to a continuous ferromagnetic path.
However the whole design of a rotating machine does require tradeoffs
between mechanical, electrical, magnetic and economical factors. In
addition, what is possible in a milliwatt to watt application is often not
feasible in a Kw to Gw application (i.e. magnetic bearings). My comments are
based on experience with the latter where cost/Kw and efficiency are of
real concern.



Don Kelly @shawcross.ca
remove the X to answer


.

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