Need lightweight circuit for 18VDC-25amp motor using 120VAC

Discussion in 'The Projects Forum' started by KYrocketman, Apr 26, 2013.

  1. KYrocketman

    Thread Starter New Member

    Apr 25, 2013
    2
    0
    25 years as a mechanical & aerospace engineer has finally caught up with me; I guess I have gotten a little rusty with electrical engineering.
    I need a lightweight solution to power a 18VDC electric motor from 0 to 25 amps via 120VAC. Lightweight means no transformers and small inductors and caps. I don't think a simple chopper circuit would work since I don't think the insulation on the 18VDC motor can handle the ~170 V peak coming from a bridge rectifier.
    As a test: I placed four 10mH chokes in series between a bridge rectifier and the motor. Since the chokes were rated for 5 amps, I used a variable transformer to keep the voltage and current down. This arrangement reduces the voltage on the motor some during high amps, but the voltage is too high at low amps.
    I think I need to build a PWM to increase the frequency to 1000 hz or more and reduce inductor size, but I also need a way of automatically maintaining 18 to 25 volts on the motor when the amp draw is low.
    BTW: I found AVR for generators were either too expensive, too little current, or too heavy.
    Any help with the PWM and voltage regulator circuits would GREATLY by appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    It's called a DC to DC converter. It uses kilohertz through a ferrite core transformer and includes voltage regulation. 450 watts is not considered "small" for these. Personally, I couldn't do this job. Only pointing you at one solution.
     
  3. KYrocketman

    Thread Starter New Member

    Apr 25, 2013
    2
    0
    Thank you #12 for your quick reply.
    But, a DC to DC converter pertains to a whole range of circuits including the buck converter I described in my previous tests. Can you be a little more specific? I need to know how to make the output voltage feedback to a PWM chopper so the output voltage stays nearly the same; maybe? If I knew the specifics or knew this would work, I wouldn't have started a new thread. Thats why I came to the experts.
     
  4. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    OK. It's called a buck converter. First problem, the only resource I know, the Texas Instruments Simple Switcher site doesn't go to 170 volt inputs. I know somebody does because all the retail switchers are rated like, 87V to 264V input. The usual way to make the output feed back so the output voltage stays nearly the same is to use an IC designed to control switching power supplies. Unfortunately, those chips have the PWM circuit built in. Using switcher chips is something I have never done and trying to do it with a separate PWM circuit is certainly beyond me.

    I can only hope someone else wants to design a 450 watt switching power supply for you.
     
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