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powering a dc motor from ac?

Discussion in 'Power Electronics' started by moose842, Jan 13, 2022.

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  1. moose842


    Jan 13, 2022
    I need help powering a DC motor from AC. Trying to power an old vacuum motor running 120V DC at 8A. I know I need a bridge rectifier and then some sort of smoothing capacitor, HOWEVER doing the calculations I found, I'd apparently need somewhere in the neighborhood of 66,000 microfarad capacitance to smooth it out. Formula: C=I/(2*f*vpp), where I=load current, f=AC frequency, and vpp=desired voltage ripple. I used 1V for voltage ripple, but honestly i have no idea how much ripple is acceptable to still run a DC motor like this without too much issue or overheating.

    Unfortunately, I did not keep or even look at the circuit powering said motor, so I can't reverse engineer this thing, but I SERIOUSLY doubt a simple household vacuum had 66,000 uF worth of capacitors in it... (I'm no engineer, but it just don't seem right..)

    Can anyone answer 1: how to smooth out my rectified DC for my needs and/or 2: how much voltage ripple is acceptable for running my motor.
  2. Martaine2005


    May 12, 2015
    Typical back of the envelope calculation is 1000μF for each Amp.
    I have lots of blender motors and Hoover motors, none have smoothing caps just X2 caps.

  3. Bluejets


    Oct 5, 2014
    Can be done but not a good approach however you look at it.
    Get a motor designed for the supply and loading on ac.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You don't have to.
    On the contrary, this would be detrimental. Rectifying 120 V AC gives you ~ 170 V peak. Using a large smoothing capacitor means you'd operate the motor from 170 V (DC +- ripple) which is way to high for the motor. Without capacitor the effective voltage (RMS) after rectifying will still be 120 V. Perfect for the motor.

    100 %. DC motors are controlled by pwm signals in many applications.
  5. Ylli


    Jun 19, 2018
    As others have said, no smoothing capacitor is necessary. In fact, if this is a "universal" motor (brushes, wound rotor and stator), then it most likely run on the raw AC just as well as DC.
  6. Minder


    Apr 24, 2015
    Plus a typical DC motor only needs a rectified supply, no cap necessary.
    From the other site, it IS a Universal motor.
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