Why are trams always electrified with DC, while trains can be electrified either with AC or DC?

Question

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 ?

Answer 1

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.

Answer 2

It's not true that onboard rectification is required to enable speed control. Several early rural electric trams in France ran on fairly high AC voltages. That in Haute Vienne being typical. In rural areas its half-dozen lines were suppled with 10kV 25Hz. An autotransformer in each control cabin stepped that down to between 98 and 360 V AC (in 7 steps) for the nominally 300V series wound traction motors (which can used both AC & DC)

The high voltage enabled a line 60-70 km long to be supplied from one end with just section isolators* and without the expense (for a rural tramway with perhaps four return services a day) of the intermediate transformer/rectifiers every few kms that a DC system would have required. The running rails that provided the return path had be very thoroughly earthed but the idea of 10kV overhead lines running betweeen the buildings in towns and villages is fairly worrying. So, for street running within the city of Limoges (the centre of the system) the supply voltage was 600V 25Hz with trams stopping on the edge of the city to switch the supply from the pantographs to a different feed to the primary side of the onboard transformer coil. Limoges also had urban trams that ran on 750V DC but they ran on completely separate lines.

*The supply for each rural line was three phase carried mainly on wires mounted above the catenary on the same poles.Within Limoges the supply was from several transformers supplied by underground 10kV cables

Answer 3

(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.

Answer 4

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.