Challenge Questions

Street car is on a track system.

yes you got the point i believe!

grounding.

The trolley gets a ground through the tracks on which it runs, whereas the bus has non-conducting tires and has to get both phase and neutral from overhead.

Actually, the term "phase" is inappropriate. These systems run on DC.

Is on circular track or only required to go in one direction.

An electric circuit needs to have a voltage source and a voltage return. The return conductor for streetcars is the steel rails, via the wheels, brushes, and slip rings. For trolley buses, as others have posted, there is no such available return conductor available, so two separate overhead cables are required.

To call the return conductor the ground is quite common, but a poor use of terminology. It is the grounded conductor. For safety, connecting a conductor to the earth (ground) is usually done to limit the total voltage available if a person were to accidentally touch a live conductor. In the USA, the typical streetcar and trolley bus systems have operated on 600 VDC, so limiting voltage to ground is a practicality necessary for streetcars, and of lesser importance for safety.

JMM

your STREET CAR must be TRAM in other words.

POSITIVE AND NEGATIVE ON THE TROLLEY FOR RUBBER TIRES

POSITIVE ON THE TROLLEY AND NEGETIVE ON THE STEEL WHEELS AND TRACKS

STAN HOYLE61@YAHOO.COM

Bonus question !

Trains in Europe run on similar single line systems, with power running from the conductor through the train motor to the steel rails and returning to the power source.

Here's the bonus question:

What happens when those supply lines get struck by lightning? Does it travel through the train motor to the steel rails which are at ground potential? How do they protect the motors?

Happy Trails !

My guess to your bonus question is that there are large capacitors at regular intervals along the DC supply lines connected to earth which do not affect the DC supply but act to shunt any short duration transient power surges, ie, lightning strikes to ground before they can damage the train/motor. Similarly any inductors (coils) in series with the DC supply lines would act as chokes to any current surges along the lines thereby directing any surges along alternative paths (parallel connected capacitors or air gaps).

Thanks, BobD !

Must be Godzilla capacitors to handle that kind of surge. I've never seen one, but I don't work with that kind of power.

Cheers !

Hi Happy Fish,

I agree that any practical capacitor that could be used cost effectively would probably not survive a lightning strike, however, it would provide a ground path as it arced and disintegrated. I think that the principle of an alternative path that has lower resistance to ground than the train/motor for rapid transient surges but not for the DC source is the answer but as this is not my field I am not sure as to exactly the type of device that would be used (an air gap to a ground conductor may be sufficient). Increasing the resistance of the train/motor for the transient is also beneficial, however, given the magnitude of the lightning surge it is generally best to shunt it away rather than trying to block it.

I don't know the field, but given that the equipments run at Voltages that are much higher than electronics, the margins should not be impossibly difficult to manage.
However, I would expect some or all of the following could be used:

The power wires may be partially protected by the supporting wire;
Arc-across paths can be provided between the pantograph and the body of the engine - with a air-gaps sufficiently small to limit the peak Voltage (in principle the spacing could be varied according to climactic conditions- but I doubt this is necessary);
And gas discharge tubes to be used in the feeds-through that bring the supply through each of the outside of the vehicle and into the power controller.
Maybe MOVs would be used as adjuncts to the gas discharge tubes.

The electric lines are supported by steel poles and those are well grounded (or earthed), you may see earth strips on those too.

The spacing of the poles will create shadows enough to cover most of the wire.

In the rare event one is still hit, there will be lighting arrestors (where the gap between a conducting wire and the live wire maintained such that it braks down under high voltage and allows th surge to pass onto the ground) this is the practice in transformers too.

The insulators on the poles will also be working like a capacitor and discharging the current to ground.

The street car has metal wheels which run on metal tracks, so the electrical circuit can be closed through them, requiring only one more connection via the pole. The bus, on the other hand, has rubber tires which are insulators, so the electrical circuit cannot be closed through them, therefore 2 poles are required to complete the electrical circuit.

There were some 3-phase streetcars/trains with double trolleys. The most famous is the Valtellina Railway built in 1906 by Ganz Budapest and Kalman Kando CEO.

If you are interested in this topic you can find several photos and drawings on the link below. It's a 25 MB pdf file. Sorry, the text is Hungarian, but the pictures are self-explanatory

http://www.ganzdata.hu/download/nagyfesz.pdf

Street cars have steel wheels used to complete a circuit through the rails.

The second trolley on the bus is required to complete the circuit. (rubber tires!)

The street car (usually DC) uses the track (ground) as the circuit return/reference and the bus does not. The bus (AC or DC) uses the second conductor as the circuit return/reference.

The street car is on steel wheels and uses the wheel to rail connection as a ground to complete the circuit, the bus is on rubber wheels therefore it needs the 2nd trolley as a return for its power.

Interestingly, in New Orleans (and it appears to work this way in other locations as well) the streetcars have two poles that connect to the overhead conductor. The pole has a rope attached, whereby the operator can lower the pole from the overhead line.

When the car get to the end of a line, the car goes through a small turnout which shifts the car to the parallel tracks for travel in the opposite direction. He then lowers the pole on what is now the 'front' of the car and raises the pole on the new 'rear' of the car. Now the car can travel in the opposite direction.

My assumption is that the set of trucks (and DC motors) on what is currently the 'rear' of streetcar are the ones that actually drive the car. Thus to change direction, the operator changes which set of trucks are powered.

As a kid, I always enjoyed watching the operator lean out of the window and use the rope to raise the connecting arm and then walk to opposite end to man the duplicate set of controls.

If you are interested in reading more, I found a website showcasing the New Orleans streetcars at:

http://www.canalstreetcar.com/canal-line/

Is this a trick question???

a tram and a trolley-bus run on DC power. They require negative and positive lines. Negative line for a tram is the running rail via steel wheels and as long as the trolley-bus is on the rubber wheels it has been provided with two overhed lines: - negative and positive.

The streetcar uses the tracks as the return or ground

The bus has rubber wheels and needs a second trolley to complete the circuit

A street car has only one trolley (the pole connected to the power line above it). An electric bus has two trolleys. Why? Street car grounds through its rails. Bus is insulated from ground by its tires and needs extra trolley for grounding cable.

The return path is the track (Which is at ground potential) in the case of street cars while the busses which ride on tires need two for the return path.

because somebody from the "hood" stole one of the trollies from the train?

Why is this called a "Challenge Question"?

The digression introduced by Happy Fish in #10 has livened it up - but surely the original question is a no-brainer on an engineering forum? Where is the challenge?

Or maybe (as I suggested previously) it's a more complex problem - a trick question.

If the correct answer has already been (multiply) posted, please put us out of our misery; if not, please give us a "sharp intake of breath", & encourage us to think again.

I have no dispute with the answer, but it needs proofreading.

A street car has only trolley only.

When showing your intelligence, try not to appear ignert.

Okay, maybe I'm carping a little bit.