Can anyone guide me through making an Inverter to control the speed of a 1HP single (or 3) phase induction motor?

I sometimes come across induction motors at a local tip that work but are of fixed speed, and wish I could use them better.  I have one at the moment which I plan to build into a humidity  dependent (with user override for occasional purge venting) ventilation system. 

So I want to be able to control the speed of an induction motor which currently runs at a fixed speed, off single phase 240volts.  I wonder if anyone has experience they might share in putting together an inverter capable of doing this?

Many thanks
Bongo. 

Comments

[steveastrouk]

For a little bit of speed variation, you can use a high power dimmer. For a lot, you have to buy an inverter. If you haven't a lot of experience of inverter design, forget it, its very difficult.

[bongodrummer]

Thanks for the reply. I have heard of using light dimmers, but like you say I don't think they are up to much, with induction motors (unless they are of 'high slip' design).
 
Well I don't have a lot of inverter design experience, but I do like difficult ;)
Is it not possible to build one off the freely available schematics?  This http://freecircuitdiagram.com/2009/06/18/3-phase-ac-motor-speed-control/  Looks to be useful.. And I did find another good source of free inverter schematics, but bookmarked it on a different computer.  Will post later..

[lemonie]

You can control AC motors by feeding them variable frequencies, you can also change the way field-coils are energised. I reckon this is a moderately-advanced project (at the least). I'm not too sure about a DIY guide.

L

[bongodrummer]

Yep, variable frequency is the way to go... And I agree about the difficulty, I just though someone out there might have made something like this and have a recommendation about what chip to build around, or any other general pointers on sourcing the bits etc..   

[lemonie]

Not that I know of, it tends to be an industrial thing. Still, if you do crack it be sure to publish the thing.

L

[bongodrummer]

Industrial secrets ;)   

Will certainly write up, if I can get something going...