115 shunt wound motor pwm circuit. Where to start?

Discussion in 'Power Electronics' started by Hamlet, Jul 18, 2016.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    I have a 115v pump motor that I want to drive from about 140v dc.
    I anticipate I might want to replace it in the future with a permanent magnet motor of 90v.

    Both motors are rated for about 250w.

    Should I use Mosfet or BJT to source the current to the motor?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What is the technology of the existing motor?
    If wishing to replace with a DC motor do you need rpm control?
    If not a simple bridge off of 120vac should suffice.
    Max.
     
  3. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Dang, I forgot to post the types of motors!
    Both are d.c.,

    #1 is a 1/3hp shunt wound motor rated at 115v for field &
    armature, 1725 rpm, service factor unknown.
    #2 is also 1/3hp, permanent magnet, 90v, 1725rpm. Service factor is 1.15.

    In the past, I have had them pumping water from 50 to 100 psi,
    using rotatory vane pump heads. In those applications,
    I was using a 251/125v auto transformer with a bridge.
    This was all on 110v mains. I was getting about 55v d.c,
    and about half speed.

    With that said, I would like to run at rated voltage (and speed).
    I could, and have, run these motors up on household mains
    using nothing but a bridge rectifier...

    However, this was all before I learned about "Form Factor" ratings of
    d.c. motors:

    "Form Factor (Power Supply Code): NEMA classification of quality of D.C. drive power quality based on the ratio of ripple D.C. current to average RMS D.C. output. 15 Typical codes are A, C, D, E and K."

    From my understanding, some motors prefer less ripple than others.
    So that got me to thinking that I should get the ripple as low as possible... for longevity of the motors.
    So, sprinkling in a few smoothing capacitors...
    Of course, now the voltage is too high!

    Sigh, maybe I'm overthinking this Wouldn't be the first time.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Have you tried the 90v P.M. motor on 120v with just a bridge? I would expect it to run with not problems.
    Max.
     
  5. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    Yes, I had run it with just a bridge at 120v ac, with pump, driving a series of filters.
    The pressures in the filtration units went too high, and thus I halved the voltage.
    This was before I knew about F.F.

    Only one filter in my new application, so back pressure shouldn't be a problem.

    I probably will not bother with PWM with pumping this time around. However,
    I sure would like to see a PWM circuit that can handle more than 12v, running
    a 12 volt motor. I don't know if you need to isolate the control circuit from gate
    on a Mosfet when controlling high voltages, or not. I can see now that there's a
    large range of experiments waiting for me in Pulse Width Modulation...
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    We seem to be jumping around from 90vdc motors to 12vdc?
    There are many DC controllers out there, both SCR bridge and PWM made by KB who specialize in these motors, for 12v PWM they can be had on Ebay for ~$5.00.
    Max.
     
    Hamlet likes this.
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Why bother is yo have a working system to power the motors?

    What do you expect to gain? o_O
     
  8. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    Most PWM circuit discussion on the web are low voltage.
    Sorry about loosing focus.
    (Most pwm circuit discussion on the web is 12v.)

    I'll look into KB. Thank you.
     
  9. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
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    The ability to chose appropriate voltage, with low ripple.
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Is the lack of ripple all that significant to the motors operation or is it just a personal fixation?

    If it was my project I would be looking at a common variable speed DC motor drive power supply or if you want to go DIY for the sake of learning just take an old power drill speed control trigger unit apart and rework it to use an external pot and if needed use a larger external switching device on its own heat sink if necessary.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The low level logic is 12v but you can use any H.V. Mosfet for the motor driver/control.
    Max.
     
  12. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    Great!
     
  13. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    Some motors don't like ripple. Name Plate specifications/ratings for motors list tolerance for ripple (Form Factor or F.F). Some motors are more
    tolerant than others. I'd like to eliminate ripple as a concern. I think brush & commutator life may be adversely affected by too much ripple.
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    What's the power rating and anticipated service life these will be used in?

    Reference to the motors themselves would be helpful too.
     
  15. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I have two of these shunt wound types. 115v, 3A, 1750rpm, Gould.
    The 90v pm motor is a 1/4hp, 2.5amp, 1750rpm Leeson (sorry, no picture).
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Odd if they are shunt wound motors and the field voltage is not given for the rated 1750rpm?
    And if they are shunt wound it is easy to control the rpm by the field voltage although torque will suffer unfortunately.
    Max.
     
  17. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
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    Yes, annoying that the field voltage is not stamped.
    The field & arm. are wired in parallel from the factory.
     
  18. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I doubt that those motors could care less about ripple. I've seen similar general purpose type DC motors like those used on some pretty crude DC power supplies that ran for years in near continuous use without issues.

    Given their low amp ratings I would just use a simple light dimmer unit ahead of a bridge rectifier and a large capacitor for the power supply. I've got several DC motors I run like that in variable speed applications that work just fine.
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That would seem to imply that they require the same voltage.
    On the question of ripple, both servo's and spindle motors have run off of full wave SCR bridge control for decades, mostly 3ph however.
    Max.
     
  20. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Experience counts. Thanks.

    I used a light 600w triac dimmer ahead of bridge rectifier for testing, but the dimmer eventually died. The uF value of cap I was using at the time
    was small. Nothing got hot, I think it was EMF that killed it.
     
    Last edited: Jul 20, 2016
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