Low pass filter

Discussion in 'General Electronics Chat' started by Rich2, Nov 20, 2014.

  1. Rich2

    Thread Starter Member

    Mar 3, 2014
    50
    1
    Hi all,
    I have put an extractor fan in my Motorhome ceiling and connected it to a/c motor speed controller to slow it down and quieten it for night time. I've looked at the output on a cheap LCD oscilloscope and its cutting into each sine wave which is making the motor have a noisy whine or buzz on low speed. It's a 40w 230vac 3 speed motor, maybe shaded pole, there's no run capacitor anyway.
    Can anyone suggest a low pass filter circuit to block the noise? I've next to no experience with making inductors but fine soldering and using other components.
    I'm an air conditioning engineer by trade but studied electronics when I was 16 and have remained a keen hobbiest ever since.

    Thanks in advance
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Maybe a choke type speed control would be less noisy. Some have multiple taps with switch selection.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    I agree with Bernard but that solution would be bulky.
    I get the same buzz in a ceiling fan in my RV when I use my modified sine inverter. So, I am thinking a small amount of inductance in series with the fan would knock down the buzz a good bit. I would see what I could find in the 1 mH range and give it a try. It could be as simple as winding 10 turns of house wire around a 16 penny nail.
     
  4. jeremynguyen

    New Member

    Nov 21, 2014
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    0
    Maybe you should try the choke type speed control. It will reduce noise a lot
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    I had the same buzz problem when using a light dimmer with a floor fan. Instead I used a series capacitor to slow the fan down. You may have to experiment to find the correct value, which should be in the neighborhood of a few uF, using one or more in parallel. I used 250V (for 120VAC) film crossover capacitors such as one of these. I placed them in a nice-looking painted aluminum junction box (sealed type) connected in series with the fan motor. You can use a parallel switch across the capacitor(s) to switch them in or out of the circuit to control the speed.
     
  6. Rich2

    Thread Starter Member

    Mar 3, 2014
    50
    1
    Thanks for your replies folks. I've made a 16 turn inductor with enamelled copper wire round an M6 bolt with the threads filed off, soldered it onto veroboard and put it in series with the fan. It made no difference, so I made an 80 turn one, made no difference.
    Maybe a RC filter would be better?
    I just want to explore the possibilities of the filter before moving onto maybe a different speed control method.
    Thanks
     
  7. Rich2

    Thread Starter Member

    Mar 3, 2014
    50
    1
    An update if anybodys interested;

    I tried various LC filter circuits and they made sod all difference to the noise.
    Experimenting with just inline resistors, and no speed controller circuit, I found that a 1000 ohm 10w resistor worked best, the fan motor is smooth and quiet especially as I can now use the 3 speed switch on the motor again (I didn't use it with the speed controller as it made the buzzing even worse)
    Sometimes the simple solutions are the best.
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    The reason I suggested capacitors is that they drop the voltage without dissipating any power. But if you don't mind the power loss in the resistor then the resistor is fine.
     
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