using "dimmer" in AC motor control

back in the old days, ( and we still have one machine running like this at work) you would have a wound rotor motor where you control the torque of the motor via the 3 slip rings and variable rheostats. with the load on the motor, you simply adjusted this rotor to adjust how much load torque the motor had. This in turn adjust your speed.

Now, I'm sure you don't have a wound rotor motor there! :)

Okay... Well then what method is used in the commercially made home variety "ceiling fan" wall switch? Triacs are used for incandescents (right?). What's in those other controllers? I know PWM would be ideal, but we are talking pedestrian ceiling fans. And the windings in my case are on the motor casing.

Les

REFER TO OHM'S LAW TO DO CHECKES ON YOUR VOLTAGE AND RESISTANCE AND AMPS, MOTORS WILL SAFELY OPERATE AT 10% +/- OF THERE SPECS. ( ELECTRONICS +/- 5% )

This will usually work for shaded pole motors. If the dimmer is meant for resistive loads you may have to modify it for it to work with an inductive load. I saw a link somewhere for modifying a dimmer to control a neon transformer, that would probably work.

Given that one of the controllers in question is for low voltage lighting that uses transformer reduction, rather than the switch mode "transformers" that a lot of this lighting uses when the loading goes up much above about 3 lamps, then you would expect that that particular controller was designed to handle an inductive rather than resistive load - yes ? If the controller in question is not too expensive, I think that I would be inclined to 'just give it a go'.

Arfa

This is what I was thinking, but I'm wondering if the reduced fan speed, hence reduced air circulation thru the windings could create overheating in the windings. The cost of the motor is multifold vs, the switch cost. I want to avoid taking actions that seem totally absurd. I wanted to see if the brilliant people here have any arguments to poke holes in my logic....... Besides, I'm still curioius about the method used in fan controllers.

Les

"transformers"

to

in

You may get away with it for shaded pole induction motors, which are what many smaller fans use. Just be aware you may blow the dimmer but probably won't damage the motor.

I use a small Variac for my large recycled computer fan. That works very well.

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I use a variable autotransformer (Variac) on my shop fan.

I agree. Except for the possibility of overheating due to reduced air flow - which is easy to check - this is the safest.

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Lower motor power with a load that lessens with RPM such as a fan means less heat in the windings, so it should all even out. If in doubt, check the motor temp with your calibrated finger and compare it to full speed.

That's a cracking bit of 'thinking outside the box', James !! I bet it wasn't early morning when you came up with that one ! Well, it wouldn't have been for me, anyway ... d;~}

Arfa

It's been my experience that it will work with shaded pole motors and very small split phase induction motors and work with fans since it greatly cuts down starting torque and fans don't require a lot of starting torque.

Try it. Downside is it may buzz and run warmer since you are introducing more slip - and if the objective was to save money it probably won't do that.

If you put an ammeter on it . . . as you tweak down the voltage the current will start to drop as well until it reaches ~100 volts then the current shoots UP disproportionately with respect to the reduction in voltage.

I employed a schematic from one of the "encyclopedia's of electronic circuits" that was very similar to a lamp dimmer but had feedback to maintain a constant voltage - the stated purpose was to keep a movie projector's lamp at a constant voltage. The fan I used it on was a belt driven 26" with a 4 amp motor (I believe it was split capacitor). I just adjusted the voltage down until the current started to rise - just to save a little money - a sort of poor man's power factor controller. That worked very well and the motor ran cooler.