Constant current motor drive circuit

Discussion in 'The Projects Forum' started by LenS, Aug 3, 2008.

  1. LenS

    Thread Starter New Member

    Aug 3, 2008
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    0
    I'd like to run a 2.5 HP permanent magnet DC motor in (approximately) "constant horsepower" mode, where the speed varies inversely with the load. Commercial motor controllers like to regulate speed instead!
    I think I basically just need a constant-current supply. Any designs for a regulated-current DC supply that can push about 18A into a variable inductive load that takes 50V to 130V? Input is a house circuit: single phase 120 or 240 VAC. I don't think the output waveform cleanliness is very important, since motor controllers seem to get away with all kinds of waveform mutilation at higher frequencies using SCRs, PWM, etc.
    Thanks,
    Len
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, just running it from a stable DC supply puts it in "constant horsepower" mode.

    Under no-load conditions, the motor increases in speed until the back-EMF created by the motor itself balances the source EMF. As the load increases, the back-EMF decreases, and the motor draws more current as a result of the reduced back-EMF.

    If the motor is loaded to the point of stalling, there is no back-EMF generated and the motor draws maximum current, which will likely burn it out quickly.

    You would not want to run a DC motor with a true constant current supply, as the voltage across it would continue to increase until the windings burned up or the motor flew apart.

    Now a current LIMITER along with a controlled voltage - that would be a good thing. If the motor was nearing stall, the limiter should trip and turn the power to the motor off.
     
  3. LenS

    Thread Starter New Member

    Aug 3, 2008
    2
    0
    Aha, so I really need an ordinary constant-voltage but current-limited supply. Any pointers to a good design for 120/240 VAC in, and 130 VDC 15A out? It seems like a good approach to avoid giant capacitors would be pulse width modulation controllling IGBTs, since at a frequency much higher than the rotation it doesnt need to be smooth DC. Aren't there ICs these days that do all the control logic for that?
    Thanks,
    Len
     
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