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Electronics Q about 240v AC 120mm fan

Discussion in 'Modding' started by ufk, 10 Jan 2004.

  1. ufk

    ufk Licenced Fool

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    A 240vAC 50/60hz 120mm fan just sorta fell into my possesion from my friendly neighbourhood AirCon engineer, Ive done some measuring and it'll fit lovely in my case for use as an intake fan, what i was wondering is... can i use a variable resistor to control the fan speed?
    It runs nice and quiet motorwise but the induction noise is a bit loud
    I was thinking of something along the lines of a dimmer switch used for lights to regulate the voltage/pulses from the mains supply
     
    Last edited: 10 Jan 2004
  2. vonkaar

    vonkaar New Member

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    Not really... you could probably use a dimmer switch to do something similar, but it would work a bit different than your typicaly ceiling fan.

    Variable resisters don't work on AC motors. Dimmer switches, while they look like potentiometers, are different. They 'shorten' the wave in pulses (terminology is probably off) so the motor constantly pulses slower than if it were constantly on. The problem with this is you might not have enough initial 'pulses' to turn the blades at the start. Meaning... if you start the fan at 15% 'on', it probably won't spin. The way around this would be to get momemtum working for you. Start it up at 100% and gradually bring it back down to your 'quiet' levels. If it manages to keep the blades moving, cong... it worked. I doubt it will work below 30% or so.

    Good luck ^_^
     
  3. ufk

    ufk Licenced Fool

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    Ta muchly for the info,
    Its just an idea at the moment so i'll be doing some research into the cfm rating of the fan and looking at the possibility of a startup voltage override to get the fan to actually start then spinning letting whatever method of speed control i use kick in once its spun up going down to the diy store to get me a dimmer switch to play with l8r
     
  4. TheAnimus

    TheAnimus Banned

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    a Light dimmer might not work, as most use a triac to cut out part of the sign wave. As such the motor might stall just stall.
     
  5. ufk

    ufk Licenced Fool

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    Ah well the dimmer switch idea didn't work the fan starts allright on 100% setting but when u crank the dimmer down it stops working, so have to put up with the induction noise till another solution raises its head
     
  6. BjD

    BjD New Member

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    You know i was going to post a thread asking if those dimmers would work with a fan :D Bad news about it not working. I did see some AC motor speed controllers on eBay a few months back, unfortunately i didnt win the bid. Not sure how they work either :S Will investigate further, need to quiet down the one ive just installed :D

    EDIT: lots of good stuff on good 'ol ePanorama, this looks interesting at first glance. If i didnt have lots of quite important exams next week id have a good look at it :D
     
    Last edited: 11 Jan 2004
  7. Kookalamans

    Kookalamans New Member

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    I disagree with the above it is possible. I've actually done this using a phase control circuit, which is what all light dimmers are. They use a triac, which is fired by a diac. Basically, you have a capacitor charged by a resistor in series with it from the mains. The capacitor is then across the diac, which feeds the gate of the triac. The triac is symmetrical, and has two terminals: MT1 and MT2 (MT is main terminal). You connect the load (you're fan) to the live mains in series with one terminal, and the other terminal to the neutral wire (mains). The capacitor resistor combination is chosen so that you can set the point at which the triac is fired. The resistor is variable so that you can throttle up or down. If the triac fires at the start of the AC wave, you get full power. If it fires a 1/4 of the way through the wave you get 3/4. If it fires half way you get 1/2 power. If you fire at the end you get 0 power. It's difficult in practice to get a seamless transition from 0 to full power, there's usually a slight gap between 0 and slightly on, but this is insignificant. The technique is similar to pulse width modulation, which is used for lower dc voltages. I used this for my sunon 240VAC, and find that I can throttle the fan down until I can't hear it above ambient noise levels. AC fans like this have a very smooth running action as opposed to dc ones which seem to run a little roughly (they turn in small regular jumps as you move from winding to winding). For this reason, I think that AC fans seem to have less turning resistance, which is perhaps why in my experience they suffer less from low speed starts at stall. I can turn mine down until you can see individual blades rotating. You just have to be careful about the resistor/capacitor combination. If you can find a light dimmer though, this shouldn't be a problem, as this will already be done.

    Phase control is great because you can control masses of current with it, plus it gives great control down to low speeds when done properly. One problem to look out for: you mentioned rf interference from the fan (induction noise)? It will get worse with this circuit because triac generate their own rf interference. I can actually hear it on the mini sound system that I use for sound from my PC. To get around noise from the fan and the phase control circuit you need a snubber network, which is a capacitor and inductor in parallel. These need to be connected across the live and neutral as close to the circuit as possible. This is something I need to do, but have been too lazy to bother with :nono: . If you can get hold of a light dimmer, it may already have the snubber network built in. Get a book from the library or look for circuits on google. I use a book by Neil Storey called "Electronics A Systems Approach".

    P.S., DON'T KILL YOURSELF WITH THE MAINS :worried:
     
  8. ufk

    ufk Licenced Fool

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    Ta I'll give it another go the dimmer switch i used was a cheapassed unit that I've been sly about and returned to the store as faulty they didn't even test it and gave me a refund so I'll try a more expensive one from another store and see what happen

    /me heads for the library for some reading material
     
  9. mrplow

    mrplow obey the fist!!

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    mains is fun :)
     
  10. BjD

    BjD New Member

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    Just tried it with a dimmer from Wilko, seems to work fine. however the package does say not to use it with fans or itll burn out :worried: On the link i posted above it says a 100 ohm 1/2W resistor in series with a 0.22uF 200V
    will protect against it. Not sure if thatll help and i dont think ive got any capacitors that size to try it with. Also would mounting the dimmer in a metal box cut down the rf noise?
     
  11. ufk

    ufk Licenced Fool

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    Result!! :clap: It worked, fan starts up fine and i can now reduce the induction roar to an acceptable level though it gets cranked up for heavy use
    TY for the help guys. If it burns out they're cheap enuf to replace anyway :D

    Now to figure out how to get rid of that annoying buzzing sound from the dimmer switch
     

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