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Design choices for running an Electric train (and network)?

  1. Sep 29, 2016 #1

    I'm going to pose my questions with a number of assumptions, so please pick me up on any that are wrong.
    I want to know the design choices behind electric trains historically and current, this will mostly require some context of induction motors and inverters. I don't know exactly how EMUs work but I have some theories.

    I noticed that 25 kVAC is becoming popular, I assume they rectify and invert this to control an induction motor to drive the train(?) and I was wondering why they don't rectify it for HV DC overhead, to save the train from having to invert it? Now that we can control the frequency easily with solid-state technology (inverters) I assume they use squirrel cage induction motors controlled by frequency because they are really robust?

    I assume the switch to AC motors over DC is based around efficiency, or is this also because changing commutators all the time would be expensive?
    I know that some older networks use 1500 VDC, did/do these invert then use AC motors, or do/did they always use DC motors? If they pretty much always inverted to use AC motors did they ever have to use wound rotor induction motors (in the old days before you could vary inverter frequency so easily), to vary the resistance of the rotor to get maximum torque for start up? Or was there some problem like maybe it was too inefficient or they couldn't get resistors big enough.

  2. jcsd
  3. Sep 29, 2016 #2


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    Distributing AC is cheaper than DC because an arc will not extinguish with DC and the insulation is more reliable with AC than with DC.

    Use high voltage because the distribution wires can be lighter and will cost less in copper. Transform the 25kV AC down to a lower voltage so it can be controlled.

    Some old systems used DC distribution and motors because regenerative braking was possible using simple technology. Multiple DC traction motors could be grouped in series or parallel and regulated by adjusting the field current.
  4. Sep 29, 2016 #3
    Thanks or the reply
    That is something I didn't think of, I presume it is because AC has a zero crossing. Although I didn't know that AC insulation was more reliable than DC though.
    I remember a bit about series and compound DC motors. So they did use DC motors in the past, I'm wondering how long ago did they stop, or if older DC trains still use them.
    You don't have any idea why in older systems they didn't just use AC in the first place and use wound rotor induction motors to vary the field current (torque characteristic) with a resistor, do you?

    Thank you
  5. Sep 29, 2016 #4


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    There was one contact above the train and one to the rails below. That limited power to two connections, so two or three phase systems were not possible. Wound rotor motors for single phase AC would be more difficult than DC. Regenerative braking was easier with DC systems.
    Before the advent of the national grid, synchronisation would be required if several steam AC alternator plants were distributed in the region. The early DC supply to the regional rail network did not need to be synchronised.
  6. Sep 29, 2016 #5
    Ah, of course; how silly of me not to have thought of that. So as far as motors go, I suppose in years gone by the purpose of using a wound rotor induction over a squirrel-cage would have been advantageous, but the fact that you can't really get three phases onto the EMU in the first place was the problem. So now days rectifying and inverting to three phases to drive a squirrel-cage motor would be more or less a piece of cake.

    So some of those 1500 VDC trains from the 1980s still in use today, you reckon they'd still be using DC motors?

  7. Sep 30, 2016 #6


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    Plant is continuously maintained and retrofitted with new equipment when it is economic to do so.
    The answer to your question will depend on what country you refer to. We have no electric trains here.
  8. Sep 30, 2016 #7
    That's a fair point. Thanks
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