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bridge rectifier project

Discussion in 'General Electronics Discussion' started by fordfasterr, Dec 20, 2011.

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  1. fordfasterr

    fordfasterr

    3
    0
    Dec 20, 2011
    Hi all, I need to convert 120v ac to approx 100v dc.

    I have a high power rectifier (the type with a hole through the middle so you can mount it to a heat-sink)..

    The application will be to spin a 90v dc treadmill motor.


    The power will actually come from an inverter that is being powered by an alternator spun by a bicycle. (yeah, silly project.. but I still need help...)
    (this sounds silly and redundant, but this project calls for such silliness...)

    The actual problem is that the alternator has a 13.5v regulator, and the motor runs at 90v dc. Thus I need to UP the voltage drastically hence the inverter and rectifier are needed...

    I have attached a diagram of a circuit I found that might be suitable..

    So, any thoughts on this idea?
     

    Attached Files:

  2. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi fordfasterr.
    Welcome to the forum.
    I think your bridge rectifier is wrong, problem is you want to drop 20 volts, and on a high current application, the power levels at times are going to be high i would think, so here is the problem, the higher the current and voltage the more demanding the circuit, its not like you can use a regulator or drop a resistor in there, if it was less than 20 volts diodes might be an option, but 20 volts drop is to much for diodes, because its DC as well it makes dropping the voltage more difficult, converter would need to be custom made, transformers are out due to power levels.

    Maybe another member can offer some options, another thing i am not sure about is the dimmer at those varying power levels, the power levels will be up and down all over the place, speed, incline torque, etc.

    I have mad ideas, but this one needs some thinking, ok iam thinking. :)
     
  3. duke37

    duke37

    5,364
    770
    Jan 9, 2011
    If you rectify an AC signal and feed it into a capacitor, you get Vrms * 1.414. If you feed into an inductor, you get Vrms * 0.9.

    Thus remove the capacitor and there may be sufficient inductance in the motor to make the supply look like 120 * 0.9 = 108, only a little bit high.
     
  4. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Yes i forgot the capacitor will push up the voltage, the motor should run ok on unfiltered DC, but i dont know how much of an issue 8 volts is, most motors will tolerate a little leeway. :)
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The dimmer won't work with the capacitor present. Skip the capacitor and the dimmer will reduce the effective voltage delivered to the motor.
    The bicycle alternator & inverter sounds silly indeed. I doubt it would provide enough oomph to run an inverter and a threadmill motor.
    What are the power levels involved? Also, if the inverter does not put out a pure sine it may throw off the dimmer operation.
     
  6. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I thought any rectified AC signal is 1.414 more then the AC Vrms. Was unaware that you needed a capacitor for this to happen.
     
  7. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I don't think motors care as much about voltage as they do current. Dc fans in computers can be driven with a potentiometer at a wide range of voltages.
     
  8. fordfasterr

    fordfasterr

    3
    0
    Dec 20, 2011
    Update.

    I built the circuit. It actually works.

    There were some other issues which are unrelated to the original question I asked..

    When I turn the voltage down using the speed controller, the motor does in fact slow down as desired however it starts to get really hot.

    At a certain point, smoke was coming out of it at the lower speed so I shut everything off.


    All in all, I am still pleased with the help I got on this forum. - Thanks ! :eek:
     
  9. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Induction motors take quite badly to dimmer-type speed reductions, but I figure a DC treadmill motor can not be an induction type.
    Brushed motors tolerate this much better, but if they're internally fan cooled (like hand drills are) they become vulnerable to overheating at low speeds (if fully loaded).
     
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