PWM limits

Discussion in 'General Electronics Chat' started by harry99932, Sep 11, 2011.

  1. harry99932

    Thread Starter Member

    Dec 30, 2010
    Hi all, ive used pwm in its most basic forms a few times and have a basic understanding but im wondering were its useable limits and inefficencies are? for instance how well does it work with bit voltage differences and were does the line come between pwm and smps?
    This has come as a friend would like to run a high current 24v motor and queried wether it could be run using pwm from rectified mains. The is custom built and insulation is has safe working voltage of 700v according to the motor house so it would be ok with the peak voltage.
    can anyone help me with the logistics side of the switching i.e would the mosfet on off times limit the output voltage at all but the lowest of frequencies?

    Thanks for helping on such a vague and varied question
  2. nigelwright7557

    Distinguished Member

    May 10, 2008
    Phase control is usually used for mains motors.

    If you used PWM you would have to sync with the mains or the motor speed would be all over the place.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    Am I getting this right. You want to rectify the mains supply voltage for an effective DC voltage then use PWM to supply the motor for an avg DC voltage of 24V? Where's the transformer for isolation? Is the motor a DC motor or an AC motor?
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    PWM stands for Pulse Width Modulation, which is a technique for representing some variable in terms of the width of a pulse. It can have applications other than power control, although this has become the dominant application in recent years.

    SMPS stands for a Switched Mode Power Supply, a system for delivering electric power where voltage or current is controlled by switching devices on and off, as opposed to linear control. Many, but not all, SMPS circuits use PWM techniques to control their output.

    PWM control may be used to control DC motors: this is not necessarily the same thing as phase control (of the input to a rectifier), as the basic input may be DC in the first place, as in battery-electric vehicles.

    Unfortunately, not all types of motor are suitable to receive a raw PWM input. The relatively high frequency components may lead to excessive losses and overheating. For a motor not designed for this use, it may be safer to filter the PWM so that what you have is essentially DC.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    OOPS, I've just read your post properly - my bad for not doing so in the first place: RECTIFIED MAINS! Are you planning to run a big 24V motor off rectified mains with no transformer?

    No way - this is absolutely not a project for someone at a level of experience where they are still learning basic concepts. Furthermore, this forum does not support discussion of such things - sorry.
  6. harry99932

    Thread Starter Member

    Dec 30, 2010
    Ooops sorry my mistake leaving info out, the plan is to use an isolation transformer which we can source fairly cheaply then rectify and run the motor from pwm 330vdc. Dont worry im a plant engineer by trade im not silly enough not to isolate but but we can get isolation transformers a lot cheaper than a 120amp stepdown transformer especially when you take capacitor loading into account. This way with such a high voltage head over the reqd voltage theres no need to worry about major smoothing of the voltage and a lot lower costs. The only down side is it goes from a highish current lv circuit to a highish current highish voltage circuit.
    My only conern is that the circuit will require such a short on time that the mosfets may struggle to get on and back off quick enough at a high enough frequency. Ive yet to work out the motors time constant but ive got a feeling its going to be low!
  7. plaahemantha

    New Member

    Aug 14, 2011
    how to set PWM to all pins of any PORT in C or MikroC? help