Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Where can i find a US standard AC err transformer or something?

  1. Nov 3, 2009 #1
    i have a vent that is too fast and too loud, and it runs off of US standard ac. i was wondering if there is a (good brand name) device that could limit the rms voltage. i don't want a pair of resistors, that would heat up real fast.

    i need the voltage cut in half, approximately.

    an analog device would be fine, but it would be even better if i could hook it up to the usb! then i bet i could do all sorts of cool things with all of my household appliances, although my budget is super strict.
  2. jcsd
  3. Nov 3, 2009 #2
    A good choice is the dimmer switch used specifically for ceiling fans. Do not use it with an induction motor, only shaded pole motors or universal (ac/dc) motors.
    Bob S
  4. Nov 3, 2009 #3
    uh oh, well i see a HUGE inductor in there. what does a dimmer switch do? if it just alters the voltage then all of the impedances would be the same yes? does a dimmer switch apply a certain voltage to a transistor to allow current to reach the bulb?

    im sorry im a dc guy mostly, i dont know how transistors and things act under ac
  5. Nov 3, 2009 #4
    It has a triac in it that is triggered part way through every voltage sine wave, so the front end of each half cycle is chopped off. Be sure to use the one for motors and not lights. If you want a fixed inductor, you could use a ballast for fluorescent lights. Be sure to check the current rating. Be sure the motor is not an induction motor. I have a 1 KVA Variac that I use for adjusting voltages, but they are very expensive. Yoou could use a step-down transformer, but they are expensive too. What current does the motor draw?
    Bob S
  6. Nov 3, 2009 #5


    User Avatar
    Science Advisor

    A cheap source of high powered resistors is 110 volt lamps.

    If you put a 40 watt lamp in series with your fan it will slow it down with no risk to the fan or the lamp. The lamp wattage would have to be determined by experiment but you would have it in a socket so you can try different wattage lamps.

    Wire it safely with insulated connections and allow room around the lamp for cooling.
  7. Nov 3, 2009 #6
    As long as the motor is NOT an induction motor. Induction motors want to run at about 1200, 1800, or 3600 RPM (in the United States), independent of input voltage. Single phase induction motors draw a high surge current when they start, and the series lamp may limit adequate starting current.

    What kind of fan motor is it?

    Bob S
  8. Nov 4, 2009 #7


    User Avatar
    Science Advisor

    For a domestic vent fan, I'd guess it is a shaded pole motor.

    There is also a type of ceiling fan controller that uses switched capacitors. This would be a neat compact way of doing it. Without knowing more about the fan, it would be risky to suggest this, though, as they are not cheap.

    Triac ones as in lamp dimmers cause sparking in the fan windings and series inductor ones seem to go noisy and start rattling after a while.

    I'd still suggest a series lamp as a cheap solution. Starting torque is not a problem for a fan.
  9. Nov 4, 2009 #8
    I've used the light bulb method with shaded pole motors and it works quite nicely. You can also put a switch in parallel with the light bulb and have a 2-speed fan. I've also use a lamp dimmer but I prefer the light bulb method (if it's feasible).
  10. Nov 4, 2009 #9


    User Avatar
    Science Advisor

    uh oh, well i see a HUGE inductor in there

    A shaded pole motor does look like a big inductor. Usually the rotating part of it is open and not very conspicuous but there is a large field coil at one side of the actual rotating part.

    Even less conspicuous is a single shorted turn of copper wire around one part of the core. This actually starts the motor turning and keeps it turning.

    Yes, I've used lamps for this sort of thing and they work fine. If it doesn't, well you haven't lost much by trying.
  11. Nov 4, 2009 #10

    I have seen shaded pole motors with 4 shaded poles (speed ~ 1600 RPM).
    Bob S
  12. Nov 4, 2009 #11


    User Avatar
    Science Advisor

    I've never seen more than one shorted turn, but interesting that some motors use more.

    There must be more of these motors than any other kind, but they always look like a piece of junk. Yet they just keep on turning for year after year.
  13. Nov 4, 2009 #12
    Stove hoods with variable speed vent fans on are the market. What's the application?
  14. Nov 4, 2009 #13
    Shaded pole motors are the least expensive ac motor to manufacture. The copper rings do very little at near-synchronous RPM, but they provide necessary starting torque. The copper rings retard the B field relative to the main field by a few degrees, but the rings also attenuate the amplitude (~1 neper per radian). The real-part attenuation means heat, so the motor gets hot. But they run a long time, especially if they have ball bearings rather than sleeve bearings.
    Bob S
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook