Using a dimmer switch to control electric motors

[Diogenes James]

I am building a large fluid-bed roaster and at this point am trying to use a leaf blower for my air supply. I have tried dampering the flow, but I still have too much air. Can I use a dimmer switch to slow the motor down? I seem to recall hearing that you can't use a dimmer switch on just any electric motor. Is this true? If any of you have ever used a dimmer switch to control an electric motor (or more specifically a leaf blower), I'd love to hear about it.

[martin]

Yeah. Dimmers reduce the effective voltage not by uniformly reducing the whole sine wave, but by cutting off a piece of it. Also, the triac switching element, once turned on at some point in the wave, stays on until the current drops to zero. Motors are more or less reactive loads, so this is more or less far from the zero voltage crossing, and the trigger circuit is designed with the assumption that turn off is near the zero voltage points. The inductive load may draw heavy surges when the triac turns on with up the peak line votage suddenly hitting its colis. And maybe other stuff I'm forgetting...

Long story short, everything's likely to go more smoothly if you can just use a variac for the motor - it doesn't need to be rated for the motor's brief turn-on surge, just the steady running current. OTOH, the simple dimmer will work just fine with a resistive load like a heater, and is loads less costly at a size to handle a high power heater than a variac.

[oldgearhead]

Just do a Google search for 'Router Speed Control', and you should find a nice assortment of 120 VAC
'switching' controllers. A Variac, on the other hand is capable of raising the line voltage, as well as controlling it. A router controller will not increase the line voltage.