Question on 30rpm motor

A proper rheostat would work but wastes power. All it does is end up dropping the voltage across the motor or equivalently reducing current to the motor... but will dissipate power in rheostat and it might be too much.

A simple fix: Try a lightbulb or two in series. This will reduce the current and the bulbs usually dissipate the power easily and it can be helpful to light up something too ;) (although it might be too much or too little and it generates a lot of heat) (this is very similar to the rheostat but you know it will be able to handle the voltage if it's 110V)

A better fix: Get a Variac. It lets you step up/down the voltage to whatever you need... it's like the rheostat but doesn't dissipate nearly as much heat... but can be expensive.

Best solution: Use a triac to lower reduce the average voltage. This basically turns on and off the motor very quickly. By varying the ratio(called the duty cycle) of the on to off you can vary the speed(in proportion). It can be controlled by a simple potentiometer and usually waste very little power.

I imagine you can find something that uses a triac at home depot or lowes. They are used in dimming circuits for lights. They'll probably charge you an arm and a leg so alternatively you could buy it online:

Info on operation:

(make sure the dimmer you get is solid state based else you get a rheostatic)

Do you know if it is a DC motor or an induction motor? If it is a DC motor or a universal-wound motor then you can slow it down with a rheostat. If it's an induction motor then you can't slow it down with less work than it would take to rebuild what you have with a different motor.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Most small timing motors are AC induction or synchronous motors with an integral gear box. The motor speed is locked to the line frequency, so the only good way to vary the speed without weakening the torque to the point of stall, is to drive it with a variable frequency source that also varies in voltage in rough proportion to the frequency.

For large motors this is not a beginner's project, but if you have a spare audio power amplifier laying around, you may be able to connect a step up transformer to the speaker connections and drive the motor with that power source, using a variable frequency sine wave oscillator to set the speed (frequency), and the volume control to set the voltage. Slowing the motor to one half to one third might be quite practical this way.

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Regards,

John Popelish

Huh. I hadn't thought of that. You may have to give it a push to start, but it'd work.

More detail from the OP would help.

OP?

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

If it's a 'split-phase' induction motor you might be able to take further what Studer Revox did in the A77 (see

put a bridge rectifier in series with one of the windings and a transistor across the DC legs of the bridge, biasing the transistor to get the desired (lower) speed. A Triac would achieve the same effect, but in a time-discontinuous way. But if the load is constant, surely any sort of effective series resistance will achieve some reduction in the speed, albeit with potentially great inefficiency - perhaps OK if the load consumes only a watt or so?

Chris

If it has brushes in it and seems to run directly from the AC current (120v AC) with no sign of electronics, I would say you have a series (universal) type motor. These motors can be slowed down and they also can be driven via DC volts with no direction change regardless of polarity. If you have a variac auto transformer kicking around you could also use that.

P.S. normally these types of motors are noisy.. If it appears that all you hear is a smooth hum from it or very little electrical noise then it's possible you have an AC motor with no brushes and that would require a AC inverter drive to control the freq/voltage.

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to do that you may need to slow down the frequency of your AC supply this will be most difficult.

sounds like the controller for a sewing machine.

the platter motor from an old microwave oven will be much slower. possibly too slow.

you could look into using reduction gears or belts with your existing motor

or possibly an old record player that's capable of 16-2/3RPM would work for you?

Bye. Jasen

One obvious solution, that works without needing to know what kind of motor you have, is adding a belt drive and experimenting with pulley ratios until you get the speed you want. If you use a cone driving a rubber-tired wheel, you can even make it continuously variable. It may look crude compared to a variable-frequency oscillator driving an audio amp driving a synchronous motor, but you may already have the necessary parts lying around.

And there is a nice sight to view the various types/

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http://webpages.charter.net/jamie_5"