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I am interested in multiple aspects of trolleybuses as a way of transitioning from fossil-fuel powered buses to "green" public transport.

I am not an electrical engineer myself but I do have an, albeit limited, understanding of electrical engineering.

Reading through the specifications of available trolleybuses, it seems like the standard way of transmitting power to them is through DC (600V) wires. From what I know, transmitting current over long(er) distances is more efficient using AC.

What is the reasoning of using DC for power transmission in this case?

Is DC power transmittion in this case more efficient (no heavy power converters inside the vehicle; cheaper to produce DC motors; more efficient DC motors etc.), safer for the pedestrians, or just legacy?

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    \$\begingroup\$ Found an interesting discussion here about Tesla motors switching to AC current driven engines: electronics.stackexchange.com/questions/58236/… \$\endgroup\$
    – mangafaua
    Jan 3, 2021 at 13:04
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    \$\begingroup\$ AC isn't intrinsically any better than DC over distances, but higher voltage is more efficient. With AC it's relatively easy to convert voltages using a transformer. Interestingly, electric trains often use low frequency AC (<50Hz). \$\endgroup\$
    – Frog
    Jan 4, 2021 at 0:38
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    \$\begingroup\$ @Frog in fact, while convenient, AC is actually less efficient. Skin effect increases resistive losses, transmission line effects hinder transmission of power, there's inductive and capacitive losses (especially for underwater cables), etc. There's some HVDC transmission lines that handle power as DC in part because of this (en.wikipedia.org/wiki/High-voltage_direct_current). \$\endgroup\$ Jan 4, 2021 at 1:08
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    \$\begingroup\$ Insulator requirements for AC are also tied to the peak voltage, which for AC is 41% higher than the average. 600V AC has a peak voltage of ~850V. This matters especially for public transport systems such as trolleybuses, where the voltage by necessity must be out in the open. \$\endgroup\$
    – MSalters
    Jan 4, 2021 at 12:02
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    \$\begingroup\$ Part of this question is answered when we realise that the overhead wires are power distribution rather than power transmission - that's why we don't step up to higher voltages that we'd use in a transmission line. \$\endgroup\$ Jan 5, 2021 at 14:28

3 Answers 3

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Trolleys with overhead wires often disconnect momentarily and the high inductance creates an arc of the same current that allows some continuity but with some drop of voltage or power. Whereas AC might cutout in < 10ms.

But the real reason is DC traction motors have greater efficient torque needed to start.

You might see some DC motor Torque capability curve. These match the load vs speed requirements better than AC motors of accelerating a trolley car with constant current is applied by a control method until speed is reached.

Also another advantage in the Netherlands are the DC trolleys that have batteries to extend routes. An 18-meter bus with IMC500 technology only has to travel 20 percent of its route beneath overhead lines.

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    \$\begingroup\$ I understand. But why not putting a DC rectifier inside the vehicles instead? \$\endgroup\$
    – mangafaua
    Jan 3, 2021 at 13:04
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    \$\begingroup\$ One less thing to fail on each bus. \$\endgroup\$ Jan 3, 2021 at 13:13
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    \$\begingroup\$ It makes sense, but nowadays there are DC-DC converters inside the vehicles to drive the low power electronics (like screens etc.). I guess it is still preferable not to have a more powerful (and heavier) rectifier inside to carry around. But given that Tesla motors is switching to AC motors, the argument with the batteries doesn't seem so strong to me. \$\endgroup\$
    – mangafaua
    Jan 3, 2021 at 13:25
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    \$\begingroup\$ How about also, regenerative braking? Much simpler to feed power back into a DC supply than an AC supply. Assuming the tram has "reversing" switchgear, just throw it into reverse when you want to slow down its forward motion. Only, don't ask me what happens at the central station when all of the trams in the city want to slow down at the same time. \$\endgroup\$ Jan 4, 2021 at 21:47
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    \$\begingroup\$ @SolomonSlow, "don't ask me what happens at the central station when all of the trams in the city want to slow down at the same time." It's easy enough. When regenerating the local line voltage rises. On the Dublin Area Rapid Tranisit (DART) you could feel a blast of heat from under the seats during regenerative braking due to the increased 1500 V DC nominal voltage rise. When the line voltage reaches some pre-determined max voltage - 2 kV, perhaps - the air brakes are blended in and re-gen is decreased. \$\endgroup\$
    – Transistor
    Jan 4, 2021 at 22:16
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From what I know, transmitting current over long(er) distances is more efficient using AC.

That's true if you can step up the voltage. In the case of the trolleybus the wires are energised at the "end-user" voltage, 600 V. Note that the overhead will be broken up into isolated sections and these will be fed from the nearest trolleybus substation. The feed to the substation will probably be at somewhere in the region of 25 - 100 kV (AC) depending on the nation grid standard voltages. In this manner losses are kept low as the 600 V DC sections are reasonably short.

What is the reasoning of using DC for power transmission in this case?

  • Constant power. DC is always on. AC is in pulses.
  • A more balanced three-phase load on the substation. A polyphase rectifier is used and one phase is always supplying power.
  • Possibility of easily regenerating into the line while braking.
  • For a given insulator breakdown voltage DC allows transmission of more power.

enter image description here

Figure 1. Three-phase to DC conversion loads the 3-phase supply in a more balanced way. A single phase AC supply would provide a pulsing load to the supply. Image source: Electronics Tutorials.

Is DC power transmission in this case more efficient (no heavy power converters inside the vehicle; cheaper to produce DC motors; more efficient DC motors etc.), safer for the pedestrians, or just legacy?

Probably a combination of all of the above except for pedestrian safety. DC has the problem that switching is more difficult as there is no zero-cross where the current drops to zero to help extinguish switching arcs.


From the comments:

"Note that the overhead will be broken up into isolated sections and these will be fed from the nearest trolleybus substation." That looks like an argument for AC transmission again. Less substations to be maintained along the way.

The substations are required for sectional circuit-breakers in any case so there wouldn't be much of a saving. They also allow for redundancy in the case of a substation failure where the one each side can supply power to the dead section (by bridging out the sections) although there will be some voltage drop as a result.

I guess this made sense in the early days of trolleybuses, but nowadays with the availability of cheap electronic components I am wondering if there isn't a stronger argument for AC power transmission.

You're still missing the benefit of the more balanced loading of the three-phase system, regeneration and cost of high-voltage insulation of the network.

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    \$\begingroup\$ You are confusing distinct topics. DC actually has lower loss than AC for transmission. But what is really key to low loss is high voltage, and AC permits easy use of transformers for high voltage lines. A trolley system doesn't have the clearance to use the very high voltages employed by inter-city heavy rail with catenary wire. \$\endgroup\$ Jan 3, 2021 at 14:41
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    \$\begingroup\$ @Christian, I specifically said "into the line", not the grid. Otherwise I agree. \$\endgroup\$
    – Transistor
    Jan 3, 2021 at 21:41
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    \$\begingroup\$ @Transistor: Actually it's a bit simpler. I managed to confuse trolleybus with trolley. \$\endgroup\$
    – Joshua
    Jan 4, 2021 at 19:42
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    \$\begingroup\$ @Joshua, I was going to confess that I don't know what problems would be solved either by connecting one leg of an AC supply to Earth or, by isolating the supply from Earth; but looking back at Transistor's answer, I see that we're talking about 3-phase power, and that renders the whole question of a single overhead wire moot. \$\endgroup\$ Jan 4, 2021 at 22:01
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    \$\begingroup\$ @SolomonSlow "There's no reason why an AC system couldn't also use one overhead wire and use the rails as return. " - that is, in fact, what they do. \$\endgroup\$ Jan 5, 2021 at 19:02
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At least in The Netherlands (also referred to as "here" where I live), it is for historical reasons.

Trolley busses date back to 1882, the one in The Netherlands in Arnhem (still operational) was started in 1949. The speed controls were easier for DC then, in the absence of electronics we know today, and the available series-connected DC motors were great for traction.

In the early days of trolley busses there were no easy ways as we have today for rectifying AC, so it was decided to use DC on the overhead wires.

Using AC on the overhead wires for this type of traction didn't really take off here until the 1950s, but replacing the existing fleet and infrastructure was considered too expensive, so the DC system stayed on.

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    \$\begingroup\$ In ex-USSR countries it's also 600 V DC. I'm not sure why exactly, haven't found an explanation or rationale. \$\endgroup\$ Jan 5, 2021 at 16:31

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