Author Topic: Some musings on the traction current of an electric train  (Read 8043 times)

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Online IanB

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Some musings on the traction current of an electric train
« on: May 26, 2012, 04:07:32 am »
Exhibit 1: This video:



It shows a London Underground Central Line train arriving and then departing from Bethnal Green station. It showed some pretty sharp acceleration as it left. How many amps of traction current was it pulling to do that? Let's find out.

First we need to know about the train. It was a 92 Stock train, and we can learn about that here: http://www.tfl.gov.uk/assets/downloads/foi/Rolling_stock_Data_Sheet_2nd_Edition.pdf

Useful information is that the train is 132 m long and weighs 170 tonnes unladen.

From the video we see that it starts moving at 0m55s and the back of the train disappears into the tunnel at 1m10s. Making the reasonable assumption that the front of the train was at the tunnel mouth when it started, that is 132 m in 15 seconds.

Using our high school physics formula for rectilinear motion, we have:

s = 0.5 * (u + v) * t

where s is distance travelled, u is initial velocity, v is final velocity and t is time. Given an initial velocity of zero and making an assumption of constant acceleration we can find:

v = 2 * s / t = 2 * 132 / 15 = 17.6 m/s (~ 40 mph)

That's how fast the train was going when it vanished into the tunnel. Next, we can calculate the energy the train has at that speed. Again from physics:

E = 0.5 * M * v^2

Assuming the train was unladen (it looked pretty empty), we have:

E = 0.5 * 170 000 * 17.6^2 = 26 300 000 J = 26.3 MJ

From the energy we can get the power. The energy has to be imparted to the train in 15 s, so the power is given by:

W = E / t = 26.3 / 15 = 1.75 MW

That gets us very close to the traction current. We know that the train is supplied at a nominal 630 V DC, so:

I = W / V = 1 750 000 / 630 = 2 800 A

That's quite a lot of current.

We can see from some of the pictures on this page what size cables are used to carry that current:

http://www.strategicrail.co.uk/case-studies/survey-and-design/power-engineering/

Nice thick cables and heavy bolts.

« Last Edit: May 26, 2012, 04:55:03 am by IanB »
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Offline T4P

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Re: Some musings on the traction current of an electric train
« Reply #1 on: May 26, 2012, 04:35:14 am »


My favourite train in my town, just listen to that sound of the motor ! (750VDC folks, that's a bad choice)
 

Offline SeanB

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Re: Some musings on the traction current of an electric train
« Reply #2 on: May 26, 2012, 09:47:29 am »
I know our local Metrorail units can get up to 130 plus kph, although they are supposed to do 100 max. Limited by the rail there, and many sections are limited to 60, 30 or lower, as the majority of the turnouts are low speed units, only the mainline being high speed rated to handle 100kph as a straight through.

Interesting is the traction units are 130 tons mass, and the cars are around 30 tons empty. The reason the cars are around that mass is because the drivers are so heavy, and a lighter car will buckle in shunting.

I think SA has the longest trains in the world though, carrying iron ore to port.

I know a few drivers who had to stop driving main line and change to long distance, too much stress driving passenger rail. They have dedicated counsellor teams for the drivers, as they all have killed multiple pedestrians on the track. Pretty much all the time the driver is not guilty, but still takes 2km to stop and call for assistance to help with the basin and tongs.
 

Online NiHaoMike

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Re: Some musings on the traction current of an electric train
« Reply #3 on: May 26, 2012, 02:07:15 pm »
I'm surprised they're not using higher voltages to reduce the current. Japan is using 25kV.
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Offline T4P

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Re: Some musings on the traction current of an electric train
« Reply #4 on: May 26, 2012, 02:37:33 pm »
I'm surprised they're not using higher voltages to reduce the current. Japan is using 25kV.

Hmm... because when my train system started out it was open platform therefore worried about the people jumping in and getting killed by the high energy available from 25kV ( ohms law anyways )

So 750V ... they are 3rd rail because it's hard to get zapped by the third rail since it's under the plastic cover ... but you still can be zapped anyway

And a 2005 line was added using an solid bar overhead centenary, alstom rolling stock 1.5kVAC ... why didn't they use more ? Maybe because of ohms law again but it's solid and not wires as with japan but i think they are worried about insulation
 

Online IanB

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Re: Some musings on the traction current of an electric train
« Reply #5 on: May 26, 2012, 04:12:04 pm »
Third rail systems always use lower voltages. It is mainly due to the difficulty of controlling the voltage, preventing flash overs, arc faults, ground leakage and things like that. The London Underground is somewhat unusual in using a fourth conductor rail for current return so the traction supply is completely isolated from ground and from the running rails. Historically this was done due to fears that stray earth return currents might cause galvanic corrosion in nearby infrastructure and since then it has stuck.

I did the calculation because I have always been curious about how much current a train like that must draw from the rails. "Big" electricity is more fun than small electricity. Those particular trains have DC traction motors and control the power using a variant of PWM. How's that for fun, using chopped DC power regulation on nearly 3000 A? And just like Dremels there is a minor issue with the carbon brushes wearing out on the motors and needing regular replacement. Those were probably the last trains to be designed with DC traction motors.

In spite of the current draw, the acceleration of those trains is limited by passenger comfort, not technical constraints. If they designed the trains to accelerate any faster the passengers wouldn't like it.

All the trains in the UK that use overhead catenary power supplies do use 25 kV AC. For bigger trains like the Eurostar this can supply much more power. Eurostar sets at full power draw an insane number of megawatts from the supply. So they are still drawing 100's of amps from the overhead cable.
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Online NiHaoMike

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Re: Some musings on the traction current of an electric train
« Reply #6 on: May 26, 2012, 04:42:34 pm »
Quote
In spite of the current draw, the acceleration of those trains is limited by passenger comfort, not technical constraints. If they designed the trains to accelerate any faster the passengers wouldn't like it.
That's surprising given how most Americans accelerate really quickly when driving.
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Offline T4P

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Re: Some musings on the traction current of an electric train
« Reply #7 on: May 26, 2012, 05:01:19 pm »
Third rail systems always use lower voltages. It is mainly due to the difficulty of controlling the voltage, preventing flash overs, arc faults, ground leakage and things like that. The London Underground is somewhat unusual in using a fourth conductor rail for current return so the traction supply is completely isolated from ground and from the running rails. Historically this was done due to fears that stray earth return currents might cause galvanic corrosion in nearby infrastructure and since then it has stuck.

I did the calculation because I have always been curious about how much current a train like that must draw from the rails. "Big" electricity is more fun than small electricity. Those particular trains have DC traction motors and control the power using a variant of PWM. How's that for fun, using chopped DC power regulation on nearly 3000 A? And just like Dremels there is a minor issue with the carbon brushes wearing out on the motors and needing regular replacement. Those were probably the last trains to be designed with DC traction motors.

In spite of the current draw, the acceleration of those trains is limited by passenger comfort, not technical constraints. If they designed the trains to accelerate any faster the passengers wouldn't like it.

All the trains in the UK that use overhead catenary power supplies do use 25 kV AC. For bigger trains like the Eurostar this can supply much more power. Eurostar sets at full power draw an insane number of megawatts from the supply. So they are still drawing 100's of amps from the overhead cable.

2.5MW at least  :o That's shocking
 

Offline SeanB

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Re: Some musings on the traction current of an electric train
« Reply #8 on: May 26, 2012, 05:07:50 pm »
But are most standing in the car? The trains are capable of pretty good acceleration, and the braking is even better until the wheels lock up, which is why a lot of trains have an ABS module on board, or deliberately poor brakes. An uncoupled loco is a surprisingly nimble item, put full power too fast and it just wheelspins in place. You apply power slowly and it is faster than many cars on take off though. Spinning wheels or locking them up is frowned on, as this damages the rail track, and this in turn damages all wheels that go over that spot in future until a detector train is run over. Most of the yards have hotspotting detectors, though there is still a need for wheel tappers, who spend the day hitting wheels with a hammer to find cracks. Funny thing is people do not realise train wheels do have tyres, even if they are solid steel, and on a rigid axle.
 

Online IanB

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Re: Some musings on the traction current of an electric train
« Reply #9 on: May 27, 2012, 12:28:36 am »
Some interesting electrical stuff in this article, including (who'd o'thunk it?) a mention of 2 700 amps!

http://www.lurs.org.uk/documents/pdf%2007/june/THE%20UNDERGROUND%20ELECTRIC%20TRAINjune.pdf

Even more interesting is a future prediction of 4 500 amps! It must be fun designing power electronics.

Look out for the bit where the resistor banks caught fire because the cooling fans sucked up tunnel dust and dumped it on the resistors  :)

These days they have a tunnel cleaning train (a giant vacuum cleaner) that runs through the tunnels and sucks up dust and other rubbish to stop things like this from happening.
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Offline tesla500

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Re: Some musings on the traction current of an electric train
« Reply #10 on: May 27, 2012, 03:50:21 am »
I love the sound of the old Mk I Skytrain cars in Vancouver. They use linear induction motors with old SCR inverters that have step changes in switching frequency during acceleration, for a very unique sound! The system runs on 600V (two dedicated power rails at + and - 300V), an I read somewhere (can't remember where anymore) that there's 4 000A available!

I think what they're doing is having a certain number of PWM cycles per half cycle of the AC output waveform, with the PWM frequency locked to a multiple of the AC output frequency. Since the SCRs are limited to only a few hundred Hertz switching frequency, the number of PWM cycles per AC half cycle has to be reduced in steps as the output frequency increases.

Mk I Skytrain accelerating


The new Mk II cars use much more boring IGBT drives, but they do use a spread spectrum PWM at lower speeds, leading to another unique sound:

Mk II Skytrain accelerating
 

Online IanB

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Re: Some musings on the traction current of an electric train
« Reply #11 on: May 27, 2012, 04:38:13 am »
It seems like there was a very short interval of time where they were using that specific drive system for traction control. There is one set of trains that have it on the London Underground, on the Jubilee line:


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Offline SeanB

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Re: Some musings on the traction current of an electric train
« Reply #12 on: May 27, 2012, 09:32:25 am »
I used to have dead SCR's from electric forklifts, and they drive the motor in the same way, but with a lower frequency whine. I hear them every time i go to Makro and they are picking stock, audible above the annoying beeper.
 


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