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Rheostat vs PWM

Discussion in 'General Electronics Discussion' started by Bremme, Jan 10, 2017.

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  1. Bremme

    Bremme

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    0
    Oct 6, 2016
    I have a Seperately Excited DC motor. Instaed of using a large rheostat, I would like to use PWM to control the field. Is it possible? Any suggestions.
     
  2. Minder

    Minder

    3,144
    679
    Apr 24, 2015
    Presumably a shunt field motor? what is the reason for controlling the field? higher rpm at the cost of torque?
    Normally the field is varied very little unless higher rpm is required and the rotor is controlled, PWM
    etc.
    M.
     
  3. Bremme

    Bremme

    10
    0
    Oct 6, 2016
    Thank you for answering. Yes it is a shunt field excited motor (same source but field and armature in //). The reason for controling the field is to have a slight speed variation and a smooth start. I haven't done deep calculations but with a start at 9.2 ohms (8 for the rheostat + 1.2 for the field itself) I have a smooth start. When the motor reaches 1000 rpm I increase the rheostat resistance to 40 ohms for more torque. The load is then applied.
     
  4. Minder

    Minder

    3,144
    679
    Apr 24, 2015
    What voltage is the motor operating at?
    I have never used PWM on a wound field, you could possibly pick up one of the $5.00 ones off ebay and try it.
    What is the reason for not applying full torque and load at start?
    M.
     
  5. Bremme

    Bremme

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    0
    Oct 6, 2016
    My motor operates at 96V. I chose not to apply full torque and full load at start to prevent heating. My motor is linked to a large flywheel and runs at almost constant speed. Field weakening to maintain the speed seems to work well. But I would like to get rid of my old lab rheostat and replace it with somting more modern. I already bought a PWM good for 40 amps which is roughtly the max for the field. Since my PWM is quite expensive, starting with a small one is a good idea.

    Best regards
     
  6. Minder

    Minder

    3,144
    679
    Apr 24, 2015
    That current seems extrordinarily high for a shunt field, what HP motor is it?
    M.
     
  7. Bremme

    Bremme

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    0
    Oct 6, 2016
    In order to limit the inrush current at start, the field amperage should be at max. My motor, a D&D model ES-84-12, has a full field max amperage of 50A (and a min of 5A). The HP rating is 14 continuous and 40 momentary.
     
  8. phin

    phin

    7
    3
    Mar 29, 2016
    A long time ago I worked on big shunt field DC motors. I would be concerned about heating the windings from the start-stop of the PWM and inductive kickback from the windings to the PWM switch. I am thinking that adding an inductor in series to limit both effects. If you can monitor current and voltage with a good scope and a ¨sense¨resistor, you can determine the right inductor for the job.

    Good Luck!
     
  9. Bremme

    Bremme

    10
    0
    Oct 6, 2016
    Thank you for your answer
     
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