Re: speed control of shaded pole induction AC motor

acannell@xxxxxxx <acannell@xxxxxxx> wrote:

i have a small AC motor, its spec *** says its "shaded pole
induction" type. its input power is 35W. i'd like to be able to
continuously control its speed with something small, say the size of a
pack of cigarettes or smaller.. is there some way to do this?

It is possible to control it over a limited range by:

a) Coupling it to a speed-dependent load such as an eddy-current disc.

b) Producing the same effect by passing a small amount of DC through the
windings along with the AC.

c) Feeding it with a variable frequency and variable voltage supply.

d) Using an off-the-shelf thyristor dimmer or a series rheostat.


The range of speed control with a) is very small, just a few percent,
but this was used in the past for accurate speed adjustment of
gramophone turntable motors.

The range should be a bit greater with b) but most shaded pole moters
run their laminations fairly near to saturation; so a large DC component
would lead to saturation, excessive current and overheating.

The best method is usually c), but that will still be limited to a range
of less than +100% or -50% of the rated speed and it won't fit into a
cigarette packet. The voltage must be varied at the same time as the
frequency, so as to keep the magnetic field at about the correct value.
I have occasionally driven small shaded pole and semi-synchronopus
motors from a signal generator and 100v line amplifiers when I needed
variable speed from a fixed-speed tape, belt or wire player. It worked
very well but I had to keep a careful check on the motor to make sure it
wasn't overheating.
http://www.poppyrecords.co.uk/other/Dictabelts/dictabelts.htm

Some motors have, to my great surprise, proved to be controllable to
some extent by method d) when the load was very speed-dependent (fans or
stirrers). It is a bit like the bumble bee which theoretically cannot
fly - but carries on flying because no-one has explained that to it. I
wouldn't recommend it, but if you try it and find that it works for your
motor (without damaging or overheating it), well and good.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
.