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Ripple on low voltage

Peterd
I look after a small fairground type railway. It is powered via a transformer and full wave rectifier and runs at around 35 volts. At recent inspections it has been noted that it fails as there is around 24 volt ripple on the 35 volt dc, which is not surprising as there is no smoothing. Apparently according to the person undertaking the inspection, who states it is to bs7671, the ripple should be no more than 10% ripple, however I cannot find any such requirement.

Has anyone got any experience of this type of thing, I am loathe to add a capacitor to smooth the dc - it would have to be probably around 0.1F, and ripple current in it could well be a problem.

Any thoughts on this would be appreciated, is there an actual requirement on ripple, or is it OK to just have full wave rectified dc as long as the peak voltage does not exceed the elv limits of 120v dc or 50 v ac rms .
  • 0
    30A is not as much as I was expecting - if you have LT spice, you can experiment with capacitors, series inductance and  and resistors in series with various diode to see what the effect of a smoothing cap would be. (see effect of 10mH below, some way from resistive ;-)  )


    Mike.

    d1f64397bc01a0e5f263f8ee24c5b5a9-original-motor_sim.png

    You need surprisingly little motor inductance to keep the motor current (I(R1)) near constant even though  the voltage goes the full p-p

    If you want to play with the sim yourself the files are attached you'll need to
    download LTSpice too.

  • 0
    And if anyone can tell me how to remove the extra copies of the attached files that would be great - they do not appear in edit either in the text editor or in the source view, but there they are.

    Just take the top zip for the LTspice model (the asc file)  and the plotting options.


    Edit fixed thta part , thanks Elizabeth.


    PS in the world of spice you need to define sine waves by their single sided peak, so 230V RMS is about 330pk, transformers you specify by the unloaded inductance and a magnetic linking factor 'k' nearly one but not quite in a real system.

    Such sims are very good as demonstrating the peak voltages and currents on switches and so on during switching, if you put in all the components that are not on the simple schematic, like the capacitance of cable cores to each other and the small series resistances that all real parts have.

    M.
  • 0
    mapj1:

    And if anyone can tell me how to remove the extra copies of the attached files that would be great - they do not appear in edit either in the text editor or in the source view, but there they are.

    Just take the top zip for the LTspice model (the asc file)  and the plotting options.


    mapj1‍ To delete the files, you should be able to see a dustbin next to the file name under the 'attached files' heading in your published post.


  • 0
    Elizabeth Morgan:
     


    mapj1‍ To delete the files, you should be able to see a dustbin next to the file name under the 'attached files' heading in your published post.




    Thanks for that. I can see no dustbins ? Any tricks I need to do  to make it appear ?

    What I see is this  (Firefox, Linux mint 17in monitor... it should not matter. perhaps it does.)

    Later EDIT

    AHA changing the attachements to a post is not "editing" it at all. You do not start the editor.

    I must say that is not obvious, and I would not have guessed it. Mind you 3 dots for 'edit' is not the most obvious either.

    cheers

    Mike.

     


  • 0
    It hasn’t had an outing lately 

     

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