Fan Controller wirering 240V need help

[lord nicon21]

i have a 240v fan and want to make it have 2 speeds how would/could i go about this?

thanks guys

 

[breezer]

get a new fan that has 2 speeds it will be easier, not all motors can have their speed varied, and chances are if you did it you would run it at the fastest speed any way

 

[ban-all-sheds]

It already has 2 speeds.

 

[breezer]

It already has 2 speeds.

what, stop and fast?

 

[ban-all-sheds]

I kill myself sometimes, I really do....

 

[breezer]

I kill myself sometimes, I really do....

? i dont follow

 

[lord nicon21]

hahhhaha sorry lol

oh my

ok so lets say i want off slow and fast

it will be on slow most of the time to keep noise down.

i need a cross flow fan with 2(or for some of you 3) speeds any ideas where to get one from ?

 

[breezer]

not wishing to "suck up" but hows about electrical wholesaler.

 

[lord nicon21]

i look on RS but they cost too much whay too much and they did't say anythink about 2 or 3 speeds

 

[breezer]

who said anything about RS? i didn't.

 

[theshogun]

Whats the make of fan and what/where is it's application?

 

[mapj1]

Those with brushed motors can sometimes be controlled successfully with a triac style lamp dimmer. Synchronous mains motors (no brushes) can only be speed controlled by altering the mains frequency. If you are an electronics enthusiast, you could make up a vari-speed inverter to achieve this (basically a variable tone generator (oscillator) driving a power amplifier), but I suspect this is not a route you wish to explore. A low voltage DC fan might be easier to control with a variable DC voltage - probably a route worth investigating..
regards M.

 

[felix]

Synchronous mains motors (no brushes) can only be speed controlled by altering the mains frequency

This is technically correct. In fact by definition a synchronous motor is locked to the supply frequency. On the other hand, an induction motor (which is more likely to be found in a fan than a synchronous one) always runs a bit slower than the synchronous speed.

It's a terrible way to do things but I've seen fan speed controllers which simply add a capacitor in series with the motor to drop the voltage. I've actually got one of these things in my conservatory. It's a three (or four for BAS) speed fan and the capacitors in the switch are, if memory serves me right, 1uF and 2.2uF.

The capacitor size must be chosen to suit the fan so you'll have to experiment and there are no guarantees that it will work at all. A lot depends upon how the motor is wound.

 

[mapj1]

Yuk, what a dreadful method.
OK I missed that trick you are right, if you allow a lot of magnetic slip, then the armature is not seeing DC, but the supply freq, minus the rotation. I wonder if the caps are semi-resonant with the windings, they sound rather small.

It is news to me that you can get more than a small speed reduction in this way before all the torque is lost. Will it start up from stalled on the slow speeds OK though?

As an aside for single phase machine control (upto perhaps 0.5HP) we would normally assume by 75% of synchronous speed, i.e. 25% slip, armature seeing ~ 12Hz, for coils driven at 50 all is lost...
Oh well, I don't design conservatory fans, and I stand corrected .
Lord Nichon, try some caps. - look in the CPC catalog at the polypropylenes.
regards M.

 

[ban-all-sheds]

Virtually every desk fan in the world has multiple speeds - at the prices of them I can't believe they are very sophisticated...