Hello Peter & PIC.ers,

I beg to differ with some of the bandiment around AC motors..

>
>>    "shaded pole 'synchronous' motors"

..
A shaded-pole motor has a rotating field on the (generally..) stator
which is synchronous to the incoming mains frequency.
No torque exists until the rotor slips somewhat.
The rotor turns due to the torque, but *not* at the synchronous speed.
It is called an `asynchronous motor', because at rated mechanical load the
rotor runs at a speed below synchronous.

This also applies to sq.cage motors.
..

>>
>>I believe we are not being correct here ...
>>
>>Do you believe that all AC motors are 'synchonous'
>>motors?

..
There *are* synchronous motors, and generators.  Power station generators
are all synchronous.., but it is possible to generate using an asynchronous
machine provided you already possess a solid-frequency grid to feed into.
Y'just overhaul the `motor' & suddenly it's a genny.

Synchronous motors aren't common, but were popular in spinning mills to keep
bobbins winding in sync., and did a real good job in mains-powered
mantelpiece clocks about a generation ago. Plug-in timers still use them
& this sort have permanently magnetised rotors, this lets them be
synchronous.

The biggies have the property that, by over-exciting the wound rotor (field)
you can create leading power-factor conditions, i.e. the current leads the
line voltage in phase.  This can be used to compensate for a customer's
other lagging p.f. loads & save him a pile of max. demand tariff costs.
..mmm.. I don't think anyone uses clock motors for this.. it's not viable
till you're in the megawatt range.
( now, a megawatt clock would really be something :)  wonder if Bob Blick's
listening )
..

>
>No, only those that do not employ a commutator of any kind and have a
>defined number of poles in the rotor and stator ;-). F.ex. the motor in
>the electricity wattmeter is not synchronous because the rotor does not
>have a defined number of poles (it's a solid disc or cylinder).
>
>>It is *only* with a magnetized rotor that a motor
>>becomes a synchronized (synchonous) motor - otherwise
>>they all 'slip' (the rotor lags and *loses* entire
>>cycles in relation to the exciting AC) some ...
>
>The squirrel cage is a shorted transformer secondary winding and is
>'magnetized' by the induced primary current from the stator. The principle
>is the same as in the physics experiment where a metal ring jumps off an
>electromagnet when it is energized. The ring is not magnetized at all (it
>can be made of copper) and it will jump quite far. Some people hope to
>make weapons like this (see 'rail guns' etc). Squirrel cage shaded pole
>motors are as synchronous as their 2 and 3 phase cousins. Except they are
>usually designed to operate with considerable slip (and this is why they
>usually run hot and have low efficiency).

..
...but t t  ... the instant you have slip, you are no longer `synchronous'
...


>
>Peter
>

..
:)
Did I miss anything?


        best regards,   John


e-mail from the desk of John Sanderson, JS Controls.
Snailmail:          PO Box 1887, Boksburg 1460, Rep. of South Africa.
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email:                jsand@pixie.co.za
Manufacturer & purveyor of laboratory force testing apparatus, and related
products and services.

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