speed control of shaded pole induction AC motor

[[email protected]]

i have a small AC motor, its spec sheet 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?

 

[Adrian Tuddenham]

i have a small AC motor, its spec sheet 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.

 

[Paul E. Schoen]

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.

You can probably make a controller for a 35 watt motor in a very small box.
If you have 90% efficiency, you need to dissipate only about 4 watts of
heat.

It is possible to rewind a shaded pole single phase motor to be three
phase. I did this with a 1/40 HP fan motor, and I used #14 AWG wire so that
it ran on about 8 VAC, which could be produced by a 12 VDC battery. There
are some monolithic three phase driver modules that can be controlled by a
PIC, and handle several HP, and without the shading coils, such a motor can
be driven at a wide range of RPM, with potentially better efficiency. But I
did not actually measure the torque, HP, and efficiency of my modified
motor, and I suspect it was not as good as it would have been if it had
been designed originally for 3 phase.

Paul

 

[whit3rd]

The rotor material determines if it burns up, or just gets less
efficient, when it slows down. 35W sounds like a relatively
high power, so I'm guessing it will burn up.

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

Heating thus the disk, AND the rotor as the field slips.

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

That will work, but remember it's 35W; too small for a full
motor control VFD, too big for function generators.

True variable speed on this kind of motor only happens if the
designer put the right rotor material for type (a) speed control.
I've seen stirrer motors that worked that way, but it's not
the usual case.

 

[Adrian Tuddenham]

The rotor material determines if it burns up, or just gets less
efficient, when it slows down. 35W sounds like a relatively
high power, so I'm guessing it will burn up.

Heating thus the disk, AND the rotor as the field slips.

I have seen it used successfully for fine speed control of a very low
power gramophone motor (Garrard 301/401), but the slip frequency was
only a few %.

That will work, but remember it's 35W; too small for a full
motor control VFD, too big for function generators.

I use a 100W P.A. amplifier (or two) to get the power for my particular
appications. ...but a pair of Quad 50Es and a BBC TS/10 signal
generator won't fit in a cigarette packet.

True variable speed on this kind of motor only happens if the
designer put the right rotor material for type (a) speed control.
I've seen stirrer motors that worked that way, but it's not
the usual case.

Like a dog walking on its hind legs - the marvel is not that it works
well, but that it can be made to work at all.

If you want to design a controllable variable-speed motor, don't start
with a shaded pole type unless there is some over-riding reason why you
must. If there appears to be a good reason, try to design-out the
reason.

 

[[email protected]]

Not easily, and perhaps not at all if you want reliable high torque, low
speed operation.

A shaded pole induction motor is going to be more or less a one-input-
frequency, one-happy-speed device. Attempts to make it go slower will
probably make it unhappy (and hot).

--
Tim Wescott
Control systems and communications consultinghttp://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html

Tim

Shaded pole motors are frequency dependent. They are also low torque
devices. Based on your input wattage of 35, this is a small motor.
The big question is, what is your load? Fan, gears, etc?

These motor can be controlled by changing the frequency or (depending
on the load) by adjusting the voltage. Under a no load condition,
changing the voltage will have very little effect. If the motor is
loaded, the increased or decreased voltage will result in an increase
or decrease of the strength of the magnetic field and will result in
the motor speed to increase or decrease. This will usually result in
an increase in coil temperatures.

John