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Trains can be electrified either with DC voltage or with AC voltage. Various networks across various countries uses various standards.

My question is however for trams, which are basically a lighter version of a train who is able to criculate on streets. (I am fully aware some networks are in between trams and trains). All of them seems to be electrified with low-voltage DC, with the exeption of T4 in Paris and the T11-express in Paris which are more like a light-metro than a tram.

Why are trams never electrified with AC ?

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(Your T4 article states they use 750 V DC.)

DC is easier to use on-board the tram. Old designs would use DC motors which could be fed directly from a DC supply. Modern designs can use AC motors with AC generated by on-board electronic inverters. It's easier to feed the inverters with DC rather than rectify and smooth a single phase AC supply. It's also easier to use DC with batteries and super-capacitors.

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  • $\begingroup$ I never rode the T4 so I can't know but apparently the "new branch" east of Gargan uses 750V DC while the "old branch" between Aulnay and Bondy uses 25000V AC. To be verified. $\endgroup$
    – Bregalad
    Feb 10, 2021 at 10:56
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AC is for power transmission across vast distances. DC is for shorter distance it can transmit a few miles with less parasitic power losses, hence why it's easier to change the speed of a DC driven electric motor than an AC one. This is useful in many applications, such as electric and hybrid cars, which is why virtually ALL battery operated devices run on DC current. Trams and trolleys run on dc current, namely transit stations often had their own power plants on site to run lines.

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  • $\begingroup$ A tram could be powered with AC, convert the curent to DC internally and fed a DC motor. Actually it's probably what the T4 in Paris does. $\endgroup$
    – Bregalad
    Mar 10 at 21:01
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Trams often have to inter-work with other road traffic, so good-quality speed control is very important. In the early 1900s when the tram networks were created, the only sensible way to do this is using series-wound DC motors, with hand-controlled switching of series resistors. If the supply has been AC, there would have to be on-board rectification from AC to DC, and this would involve mercury-arc rectifiers, which were large and expensive.

So it was best to do the rectification in substations, rather than in the tram, and once a DC power network was set up, there was little incentive to change it.

To maintain the supply voltage, there had to be sub-stations at frequent intervals, which wasn't practical over longer distances, so higher-voltage AC had to be used, with transformers and mercury-arc rectification in a locomotive. This wasn't a problem as only one locomotive per train was needed, and it had plenty of space for equipment, unlike a tram-car where space is at a premium.

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  • $\begingroup$ Aren't series-wound motors universal motors? $\endgroup$
    – user253751
    Jul 15 at 12:07
  • $\begingroup$ No, there are also shunt-wound DC motors, with a completely different torque-speed characteristic. Series-wound are preferred for traction applications, since they provide a lot of low-speed torque, which means good vehicle acceleration. Shunt-wound are primarily used in fixed-speed applications, such as fans, as they self-regulate their speed. $\endgroup$
    – jayben
    Jul 15 at 15:35
  • $\begingroup$ "universal motor" is a specific type of motor. It doesn't mean "the only type of motor" $\endgroup$
    – user253751
    Jul 15 at 15:48
  • $\begingroup$ The thing that makes them universal is they can run on either AC or DC. $\endgroup$
    – user253751
    Jul 15 at 15:48

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