# Arcing and jolting of trains at section gaps

Discussion in 'General Electronics Chat' started by AidanCroft, Jul 15, 2015.

1. ### AidanCroft Thread Starter New Member

Jan 7, 2015
24
2
Hello all,

I'm writing to ask if anyone knows why trains and trams jolt when passing through a section gap whilst drawing power?

I understand that arcs are drawn out when a load (say a trams) leaves its source of power (say the overhead line). As to why this causes the vehicle to jolt harshly, I don't know.

I've attached a photo of an example. The Voltage is 750V DC and notice how the skids overlap each other - the white insulators are not at the pantograph/overhead line interface but higher than it. When a pantograph with two carbon strips passes over this you can see how the overlap should mean that power is always applied.

So - why does the jolting occur? Bear in mind that both sides of the gap are on the same circuit, joined up by a ground level closed circuit breaker.

Kind regards,

Aidan.

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2. ### strantor AAC Fanatic!

Oct 3, 2010
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I've never been on an electric train so I can't say for sure. But my guess is that at these junctions, the train is switching from one power source to another. If that's the case, then one might be at a slightly higher voltage than the other, hence the jolt.

3. ### #12 Expert

Nov 30, 2010
16,257
6,757
If both sets of feed wires are connected to the same source, but there seems to be a difference at the point of intersection, there must be something different about the power available on the two sides of the insulators, otherwise, why even have the insulators? Either they aren't the same, intentionally, or voltage changes are caused by current flow in the resistance of these very long wires.

4. ### AidanCroft Thread Starter New Member

Jan 7, 2015
24
2
The section gaps closed by a circuit breaker are there so that the circuit breakers can be opened to allow sections of overhead line to be maintained. I have included an attachment to show this.

Aidan.

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5. ### #12 Expert

Nov 30, 2010
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Now it really doesn't make sense. The circuit breakers are at ground level AND they are across the section gaps AND there is no difference of voltage on the lines AND something is different or the train wouldn't jolt.

Beats me.

6. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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Inasmuch as arcing, in this context, is an artifact of "switching" -- it would seem obvious that the described 'jolting action' is corollary to transient interruption of power to the traction motor{s} --- IOW: despite the arrangement/construction of the feed conductor{s}, there is obvious (electrical) 'making and breaking' occurring -- perhaps owing to nothing more outré than pantograph "brush" bounce...

Best regards
HP

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7. ### AidanCroft Thread Starter New Member

Jan 7, 2015
24
2
The contact wire on each side of the SI skids has joined on to it a cable that runs on the inside of the supporting pole to a lineside cabinet containing switches/circuit breakers. The circuit breaker here is kept closed so that power is available both sides of the "gap". I say "gap" because whilst there is a physical gap they were designed with an overlap so that current may always be drawn by the pantograph, even when sitting directly under the SI. And yet, this jolting an arcing still occurs when the driver applies power (or indeed the trams uses regenerative braking) through the SI.

In this example, there is often a clatter from the SI and a bit of a bounce from the SI but the arcing and violent jolting only occurs under applied power or regenerative braking. Just as a thought, even though it would seem that there is constant current drawn by the pantograph because of constant contact, what else is possibly causing a disruption?

Kind regards,

Aidan.

8. ### #12 Expert

Nov 30, 2010
16,257
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You have described very well a condition that only happens at the junctions of the overhead cable and only during times of load. The only other possibility is that being under load and under a junction magically makes an opening or a lump in the tracks under the train. That disruption in the tracks magically doesn't happen under the junctions when the motor is not under load. I don't believe in magic, so I have to believe you have named the cause.

9. ### Aleph(0) Member

Mar 14, 2015
343
322
Mr. AidanCroft you are doing the overthinking The shuddering is because of the arcing and because the connection is being broken. Higher loading current fluctuation makes higher torque differences then more violent the driving of the machine!

10. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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11. ### AidanCroft Thread Starter New Member

Jan 7, 2015
24
2
Thanks folks!

I'm trying my best to understand this, but why is current to the GTO thyristor controlled motors being broken when the skids on the SI actually overlap thus, in theory, providing constant supply to the pantograph?

Kind regards,

Aidan.

12. ### #12 Expert

Nov 30, 2010
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Because they are not perfect.

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13. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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At risk of 'whipping a dead horse' -- The causal agent is the arcing! --- Solve that and you've solved the 'jolting' issue as well

Best regards
HP

14. ### AidanCroft Thread Starter New Member

Jan 7, 2015
24
2
OK, so current is being drawn from the skids on one side causing an arc to be drawn that is then extinguished - before that arc extinguishes current for traction begins being drawn from the skids on the other side so current to the motors should be uninterrupted - apparently it's not as there is a jolt, but why? That's what I can't understand. So yes, there are arcs from skids on both sides so current is being interrupted but across the whole SI current supply appears uninterrupted.

Kind regards,

Aidan.

15. ### #12 Expert

Nov 30, 2010
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Backwards thinking, you are. The current is NOT supposed to be interrupted at the junctions, but it IS, therefore, a change in coupling power to the motor, therefore a change in power applied to the wheels, therefore, the tram jerks.

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16. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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For these purposes current interruption means load current interruption! --- To paraphrase parrot our resident 'Georg Cantor fan' you are, indeed, 'over-thinking' this!!!

Respectfully
HP

17. ### Glenn Holland Member

Dec 26, 2014
353
110
I used to work in mass transit here in San Francisco and we have both electric rail cars and trolley buses that run off a 600 VDC overhead line.

The jolt occurs when a vehicle runs through a section gap (a de-energized and insulated section) with propulsion ON and the magnetic field collapses and creates a high voltage transient (that is opposite to the normal voltage) on the overhead line. The momentary reversed voltage causes the traction motor to reverse direction and that's the jolt you are feeling. These transients have been measured at over 1800 VDC or 3 times the normal voltage and they can also cause torque surges.

On rail lines, there is only one overhead wire (positive supply) and the track is the negative return so there are no section gaps in the overhead wire. However for the trolley buses (rubber tired vehicles), there are two wires and the positive and negative wires must be separated at intersections. There are yellow dots on the street to warn operators to let off the accelerator and coast until they pass through the intersection.

• ###### Trolley Bus.jpg
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