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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 .
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  • 0
    Is a 3 phase supply available ?

    If so, then rectified three phase will produce much less ripple than rectified single phase.

    Other options include a large switched mode power supply with a smoothed and regulated output. 

    Power supplies for electric railways, even miniature ones, need careful design. A standard SMPSU can be killed by an external voltage applied to its output. Fit a large silicon power diode between the SMPSU, to prevent any back fed voltage if the supply fails and the motor on the train acts as a generator. Also consider a resistance of say 0.1 to 0.25 ohms that is shunted across the  DC supply by a contactor when the mains fails. This will prevent any undesireable voltage being produced. Under normal conditions this resistor will disipate nothing, but it must be able to handle a few Kw for few seconds, a calculated length of cable will serve.

    Presuming a short line and modest speeds, consider on board battery power, a relatively small battery would suffice. I have seen a miniature electric railway powered by a 24 volt nominal battery of about 300 AH. Charge overnight. Fit an interlock to stop the train being driven whilst plugged in to the charger.


    How touchable is the conductor rail ? if protected against easy contact you MIGHT be able to justify the present arrangements.


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  • 0
    Is a 3 phase supply available ?

    If so, then rectified three phase will produce much less ripple than rectified single phase.

    Other options include a large switched mode power supply with a smoothed and regulated output. 

    Power supplies for electric railways, even miniature ones, need careful design. A standard SMPSU can be killed by an external voltage applied to its output. Fit a large silicon power diode between the SMPSU, to prevent any back fed voltage if the supply fails and the motor on the train acts as a generator. Also consider a resistance of say 0.1 to 0.25 ohms that is shunted across the  DC supply by a contactor when the mains fails. This will prevent any undesireable voltage being produced. Under normal conditions this resistor will disipate nothing, but it must be able to handle a few Kw for few seconds, a calculated length of cable will serve.

    Presuming a short line and modest speeds, consider on board battery power, a relatively small battery would suffice. I have seen a miniature electric railway powered by a 24 volt nominal battery of about 300 AH. Charge overnight. Fit an interlock to stop the train being driven whilst plugged in to the charger.


    How touchable is the conductor rail ? if protected against easy contact you MIGHT be able to justify the present arrangements.


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