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?
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 consulting http://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
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.
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
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
Dart Controls has something that would work, although overrated for your application, but might be pricy for you at around $150US.
Type "shaded pole motor control" into globalspec.com and see what you get.
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.
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.
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 %.
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.
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.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
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
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