September 21, 2010

Why do trolley buses require two wires?

Okay, not the most pressing question on your agenda.  But, nonethess, how come Vancouver’s trolleys need two parallel wires when light-rail lines only need one?

Jarrett Walker explains all, in his review of Transport Revolutions by Anthony Perl and Richard Gilbert First of all, though, Jarrett lays out the basic dilemma:

Petroleum has a very high energy density…   Batteries, by comparison, are just too heavy compared to the amount of energy that they can deliver, so when you put them in a mobile vehicle, they lose a lot of their efficiency to the work of transporting their own weight, and don’t have much left over to transport us or our cargo.

All this is carefully explained, and leads Gilbert and Perl to a striking conclusion:  We will need to shift most of our mechanical transport to “grid-connected vehicles” (GCVs), vehicles — like trolleybuses and electric rail lines – that can draw power from the grid continuously (and increasingly, return surplus energy back to the grid as well). …

But their vision goes further, to a network of “grid connected” roads, and … into something that looks to the authors like Personal Rapid Transit (PRT).

Jarrett is skeptical of anything like a network of personal vehicles that require connection to some sort of wired grid.  And at this point, he explains “one nasty technical detail.”

… grid-connected vehicles on tires need to route both directions of the electric circuit through the overhead catenary.  That’s why trolley buses have two wires, while overhead-powered rail has only one. 

With two wires, you have to connect to them with poles, because the two sides of the circuit have to be kept apart.   Trains, by contrast, are grounded through the rails and therefore need only one wire above.  That means trains can meet this wire with a large horizontal structure —  called a pantograph -– which easily accommodates lateral motion.  

It’s a fine joke, really, by the technology gods:  The power source that offers the most lateral flexibility works only on rails, which have the least need for it. 

And hence the problem.  Anyone who’s been on a Vancouver trolley knows how delicate the double-wire technology of trolleybuses can be when the vehicles have to move laterally.  Even turning a corner can dewire the trolley poles – and this is when all the drivers are trained by the transit agency.  Imagine all of us in our separate vehicles, driving the way we want, with minimal training – and unable to pass.

Once again, the romance of technology confounded by the reality of human behaviour.

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  1. It is my observation that the new trolley buses rarely derail and it has yet to happen to me while riding transit. The old ones, by contrast, would derail all the time and it frequently happened to me while riding transit. Perhaps the new models provided by a European company have a superior design compared to the ones that New Flyer used on the old trolleys. Then again, perhaps the things just wear out over time and Translink did a cost benefit analysis and decided not to fix the problem on the old fleet when their replacement was just around the corner. By the way, thanks again, Gord, for the part you played as a Vancouver Councilor on the Translink board when the decision to save or scrap the trolley system was discussed.

    I am not sure whether I am correct or not, but it was my understanding that the second trolley wire was used to return current to the grid when the bus brakes. This is called regenerative braking and it is used on SkyTrain, Prius hybrids, etc..

    1. While we are able to remote control vehicles even on the planet Mars, why can we not design bus poles that find their own wire. Thus we can eliminate all that switch-jungle at intersections; poles down and up again after the unnecessary switches. Back on straight lines afterwards. The overhead installation would be way cheaper, busses could pass each other, and all the frustration of drivers to step out in the rain and fiddle around with the poles is eliminated. Good idea…????

  2. While LRTs and streetcars have the rail as a ground – the rail is also a source of stray current – which (if imbedded in the roadway)without proper insulation can cause premature corrosion in undergound water pipes and metal conduits. I’ve also heard of people and pets getting small shocks from rails too.

    I think the trolley bus wire “derailments” are caused primarily bythe geometry of the corner (i.e. right hand turns on tight corners).

    1. While LRTs and streetcars have the rail as a ground – the rail is also a source of stray current – which (if imbedded in the roadway) without proper insulation can cause premature corrosion in undergound water pipes and metal conduits.

      The rails imbedded in roadways are not insulated from roadway materials, they must be properly grounded, grounding the rails insures that electrolytic damage to underground metallic piping will be theoretically eliminated or minimised at best. When rails to be imbedded in roadways are constructed, electrically conductive cables are securly fastened to the lower parts of the rail, the purpose being to provide an electrically positive return path from the rails back to the substation.

  3. Coming off the wires, as trolleybuses sometimes do, is called dewirement, of course, not derailment. It generally happens at switches in the wire, which is why trolleybuses often slow down to go through these.

    Yes, the newest generation of trolleybuses dewire vastly less often; the same can be observed in San Francisco and Seattle. They also have small motors that let them go a short distance after coming off the wire. So in most cases, you no longer see buses dewired in the middle of a turn at a busy intersection, blocking all directions of traffic, romantic as that was as an image of municipal futility.

    1. Frequent dewirements can be caused improperly maintained or excessively worn overhead “fittings” at turn-outs (switches).

  4. Not all trolleybuses dewire that easily. Watch some videos of trolleybuses in Belfast, they were very flexible because of the design of the network. They were double deckers which meant that the wires didn’t have to be far above the buses which meant the trolley poles had a good turning circles

  5. It’s just ugly.. very very ugly having all those cables on top of your head and blocking all the view. Most European cities have moves away or are moving away from this stupid system and are switching to Ethanol busses instead. I don’t understand why we in vancouver have to stick to 70s technologies. Is it because Seattle is doing it too?

    1. Why is Ethanol better ? It is energetically less efficient than gasoline. The only benefit is that it is often made from food waste or other waste products.

      Electric buses, especially in a hydro based electric system like BC is far cleaner, but yes, you need the wires. As pointed out by the blog poster, you need two connections for electricity to flow, as you can see in any electric plug on a smaller scale.

      Whistler cancelled their hydrogen fuel bus a while back due to cost. Other options are natural gas based buses which would also work well in BC as it is fairly clean and abundant in this province

      Hers Translink’s view

    2. Using the word “cables” makes it sound worse than it really is, they are not cables as in a larger diameter feeder cable. They are wires, trolley wires, they are a grooved wire with a fig. 8 cross section, the technology dates back to the 1920s. Many people don’t even notice the wires and many of us feel the wires have a useful albeit industrial look to them. To some of us they are sign of permanence, knowing we can enjoy good clean modern electric transport with vehicles that offer rapid acceleration and braking with “whisper” quiet motors compared to noisy, smelly lumbering diesel buses.

    1. Rick, not that I’ve ever heard of, I’ve heard that back in the day men did not like to take mechanical pocket watches on streetcars because they assumed stray magnetic-fields on board the streetcars would ruin the watch.