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ac motor speed control Answered

my uncle recently gave me a compact, functional ac motor with an axial blower attached to it. its a pretty cool thing, and i am planning on putting it in my mac's plexi case (when its built). i have no clue how to vary the speed. from what i found on wikipedia, an inductance motor cannot have speed control. how do i know if this motor is brushed or inductive?? and if its brushed, how do i vary its speed? help!!??


I see that you have found a solution for this speed control (twoax years ago) . Could you tell me what was the solution? I got the same problem with my bandsaw motor.
Thanks Amnon

comeonnnnn you're the tech-king! I'm pretty sure you answered this in another forum use a triac with something like a 555 timer

this was a month ago. i have the perfect triac dimmer circuit for this thread now. thats why i have not responded in a while.

BTW: do you think cruser still needs it for his van de graff generator?


10 years ago

You can use Optocoupler to vary the speed. Optocoupler change the frequency of your AC motor to be an analog voltage. Then use the ADC to be a digital value.


Reply 10 years ago


where do you get one? and for how much?

if you can get one of those (light)switches with brightness control just run the current through that and it should work

You could use an ac axial fan, a petrol driven leaf blower, or the down-draught* from a hovering helicopter to keep your computer case cool.

Alternatively, you could simplify your life by using an off-the-shelf computer powered fan - just a thought** ;-)


Pat. Pending

*draft (US)
** thoft (?)

i want to use this motor. most of the shelf computer fans are axial fans, not blowers. those that are blowers are card slot blowers, which are notoriously slow and underpowered.

Taken from Wikipedia on induction motors speed control

The rotating speed of the rotor is controlled by the number of pole pairs (number of windings in the stator) and the frequency of the supply voltage. Before the development of cheap power electronics it was difficult to vary the frequency to the motor and therefore the uses for the Induction Motor were limited.

There are various techniques to produce a desired frequency available today, the most widely used of which is called PWM (Pulse Width Modulation). This takes a DC signal and cuts it into pulses at varying frequencies, when this is supplied to the inductor windings they smooth the signal which therefore gives an average over that time period. For example, if a 100 V DC signal was cut equally on and off then the average seen by the winding would be 50 V. If it was now cut so that it was on for a third of the time that it is off then the average would be 25 V etc. By changing the mark-space ratio, otherwise known as the duty cycle, a waveform resembling a sine wave (in an appropriate manner) can be produced at whatever frequency (Motor speed) is required.

The general term for a power electronic device that controls the speed as well as other parameters is called an 'Inverter'. A typical unit will take the mains AC supply, rectify and smooth it into a "Link" DC voltage and then using the method described above will convert this into the desired AC waveform.

Because the Induction motor has no brushes and is now easy to control, many older DC motors are being replaced with Induction motors and accompanying circuitry in industrial applications.

what?? that was like reading the description of a dictionary. i don't get it. and what can i use to invert the ac to dc then modulate it? a light dimmer maybe?

Well, in a way I was wondering where you had gotten that an induction motor could not be varied in speed. It does take more than just a potentiometer, I mean, it is more involved, but it can be done as far as I know.

from what i had read, i understood that little could be done to vary inductive motors.

. A light dimmer may work, but they are notorious for having "dirty" circuits/outputs - I wouldn't want those noisy (electrically) things any where near my computer case.
. If the motor doesn't pull too many amps, a computer case fan controller will probably work (although I can't guarantee it). I've bought several from TigerDirect.
. Do a search for "variable speed drive" and/or "variable frequency drive". Append " +circuit" or " +schematic" if you want to DIY.

. If you can get the cover off of the end of the motor where the wires enter (usually the end opposite the output shaft), you should be able to see the brushes, if they exist. Many motors have a centrifugal switch for the starting windings - don't mistake this for the brush assembly. . If it's an induction motor, the power wires will go to a coil that surrounds the rotor. . I'll bet five cents that it's induction.

its inductive is there anything i can do to vary the speed without overheating it?

. See my earlier post. . As far as overheating goes, this should be manageable if you get the voltage right. Run the unit at rated volts/freq and feel how warm it gets. Install your speed control and monitor the temp. . For larger motors, if you can hold your hand on it, it's not too hot. Not sure how this rule-of-thumb holds up for small motors.