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Electric train operation

  1. Oct 15, 2013 #1
    Hi , I observed an older type of electric train yesterday which is a DC train having a +ve overhead wire with around 3000v in it.
    Now as I was sitting there I saw the capacitor banks below the train , probably used to smooth the incoming voltage for peaks and stuff to feed that further to the motors.
    Now there are two things that come to mind , first one is usually a capacitor is charged using both + and - , in this case we have a +ve wire as the supply wire and then the whole train, chassis wheels and rails at ground potential.Now as far as where the power comes I understand , because of the +ve supply wire there is a potential difference but is that enough do charge a capacitor?
    In other words if I would to take a capacitor and ground one plate while connecting the other plate at some + potential would that charge the capacitor?
    I guess it should as the opposite charges from ground would flow towards the positive plate and the capacitor would charge right?

    Also the other question that I have thought is what would happen in the case of an wire isolation breakdown , in other words if the overhead wire would somehow fall on the train chassis or just some train inner wire loose its insulation , the passengers inside the train wouldn't feel anything right ? Because firstly the whole train is grounded or at ground potential and the passengers are not part of a circuit and also because the train chassis forms a something similar to a faraday cage?
    And even if one passenger would touch the train chassis while standing on ground , he would only feel a shock if the resistance through him would be smaller than that through the train wheels through rails to ground is that right?

    Here is a link just for picture purposes of a similar train , an old dc train.
  2. jcsd
  3. Oct 15, 2013 #2

    Simon Bridge

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    Only in junior science text books ... it is possible to charge a capacitor with both plates carrying an excess of positive charge ... one will still be more positive than the other.
    The electric field depends on the potential difference between the plates.

    The cap is "charged" when there is an electric field between the plates.
    The location of zero potential is arbitrary.

    If the ground connection is good, the passengers would not get shocked - no.

    That's right.
  4. Oct 15, 2013 #3


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    No, that is not right, although depending on the exact resistances involved, the shock might be much less. You're describing two resistors in parallel, and they're both going to carry some current.
  5. Oct 15, 2013 #4


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    You don't even need to ground the negative plate. Charging one plate to +100v would cause a difference in potential to develop between the plates and upon being connected through a circuit charges would flow from one plate to the other.

    Not quite. Imagine the train and the person are two resistors in a circuit in parallel with each other. (Well, don't imagine it. They really are.) Let's say the train, being metal, has a very low resistance of 1 ohm while the person has a very high resistance of 100,000 ohms. (Just throwing out numbers)

    That's an equivalent resistance of 0.99999 ohms. With 3,000 volts that means we have a total of 3000.03 amps of current flowing. Of that, 3000 amps will be through the train and 0.03 amps will be through the person.

    Now, I don't know what the resistance of each would actually be in this case, but you can see that if there is a potential difference between the person and ground current WILL flow, even if there is a path with much less resistance nearby.
  6. Oct 15, 2013 #5
    ok thanks for the responses.
  7. Oct 16, 2013 #6


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    I'm not so sure about these capacitors you saw. I doubt that capacitors are needed for the motors. There needs to be some form of speed controller and I suspect that would be large with multiple steps for speed selection, and with fins for heat dispersal. There may well be caps for spark suppression, but I can't imagine you would see anything of their plates.
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