Why do S Stock "lose power" so often?

[OneTrackMinded]

Generally the S Stock provides quite a smooth ride experience, however there's something odd I've noticed with them.
When taking power, they seem to have a habit of suddenly loosing traction power and then regaining it, creating a jolt when moving off. There's also a quite obvious spark, which can be seen and heard.

From what I can see, this is caused in places where there are gaps in the electrified rail. However mainline third rail trains that also encounter gaps don't seem to have this problem.

Is this caused by a different method of power management/delivery?

A few years ago I was told this was because of their high power requirement in comparison to the substation limit, which at the time was still at the standard for the older trains, but all these have been upgraded to 750v now.

 

[DanNCL]

If I recall correctly, LU stock doesn’t have a power bus between carriages so each motor car loses power when it’s gapped. Mainline trains have a power bus so as long as one carriage with shoe gear is over the third rail the whole unit will still have power.

Others can probably expand on this, or indeed correct me if I’m wrong!

 

[hexagon789]

If I recall correctly, LU stock doesn’t have a power bus between carriages so each motor car loses power when it’s gapped. Mainline trains have a power bus so as long as one carriage with shoe gear is over the third rail the whole unit will still have power.

Others can probably expand on this, or indeed correct me if I’m wrong!

Correct. To avoid inadvertently energising dead sections through the shoes I believe.

 

[Dstock7080]

If I recall correctly, LU stock doesn’t have a power bus between carriages so each motor car loses power when it’s gapped.

As all cars of S Stock are motored, once the shoe-car goes onto a gap it’s usually two cars that lose traction hence the jolt.

 

[OneTrackMinded]

If I recall correctly, LU stock doesn’t have a power bus between carriages so each motor car loses power when it’s gapped. Mainline trains have a power bus so as long as one carriage with shoe gear is over the third rail the whole unit will still have power.

Correct. To avoid inadvertently energising dead sections through the shoes I believe.

As all cars of S Stock are motored, once the shoe-car goes onto a gap it’s usually two cars that lose traction hence the jolt.

Thank you all for your insightful answers.

 

[Silent]

Generally the S Stock provides quite a smooth ride experience, however there's something odd I've noticed with them.
When taking power, they seem to have a habit of suddenly loosing traction power and then regaining it, creating a jolt when moving off. There's also a quite obvious spark, which can be seen and heard.

From what I can see, this is caused in places where there are gaps in the electrified rail. However mainline third rail trains that also encounter gaps don't seem to have this problem.

Is this caused by a different method of power management/delivery?

A few years ago I was told this was because of their high power requirement in comparison to the substation limit, which at the time was still at the standard for the older trains, but all these have been upgraded to 750v now.

Obvious spark? This isn’t really sparks but why do new trains, after the s stock, seem to sound like they are quietly arcing before moving.

2009ts does this and I notice the same sound on class 710 and 730.

 

[Snow1964]

Obvious spark? This isn’t really sparks but why do new trains, after the s stock, seem to sound like they are quietly arcing before moving.

2009ts does this and I notice the same sound on class 710 and 730.

The newer trains have ac (alternating current) motors rather than dc (direct current). There is electronics that control high power electric switches called thyristors which change the frequency of the ac to control the speed, in simple terms get short (fraction of a second) pulses of power with no volts between the pulses, to some extent this has the effect of making it sound like short bits of motor action.

The duration of the power pulses change as train speeds up, and in some cases they are programmed to avoid pulses at certain frequency so almost gets what sounds like a gear change (the whine of the motor seems to change musical note). These frequency gaps are where it is very similar to those used by signalling circuits to avoid any interference.

In technical terms motor harmonics are rather harder to describe than simple explanation.

 

[contrex]

Correct. To avoid inadvertently energising dead sections through the shoes I believe.

Re the lack of a train bus line on trains used in 'underground' type tunnels - I was always under the impression that the reason was historical - some nasty fires on trains both here and abroad (e.g. Paris 1903, 94 dead) led to the Board of Trade banning a DC bus.

 

[Dstock7080]

Re the lack of a train bus line on trains used in 'underground' type tunnels - I was always under the impression that the reason was historical - some nasty fires on trains both here and abroad (e.g. Paris 1903, 94 dead) led to the Board of Trade banning a DC bus.

Surface O/P (later CO/CP) Stock had a bus line within a unit (2-car or 3-car).
R Stock didn’t although all cars were motored and each had shoes.

 

[Railguy1]

I've used these trains for a decade and I have never noticed a "jolt" when leaving a station. Does it happen from any particular station? Is there a video that describes/ shows the problem?

 

[boiledbeans2]

Obvious spark? This isn’t really sparks but why do new trains, after the s stock, seem to sound like they are quietly arcing before moving.

2009ts does this and I notice the same sound on class 710 and 730.

I believe what you're hearing is the air hissing from the brakes releasing. They make multiple short hisses, "sss......sss...sss", unlike older trains which might make a single long hiss.

 

[Silent]

I've used these trains for a decade and I have never noticed a "jolt" when leaving a station. Does it happen from any particular station? Is there a video that describes/ shows the problem?

I’ve noticed it like the train accelerates with motor noise then it cuts off and goes on again.

== Doublepost prevention - post automatically merged: 18 Nov 2024 ==

I believe what you're hearing is the air hissing from the brakes releasing. They make multiple short hisses, "sss......sss...sss", unlike older trains which might make a single long hiss.

Yeah you are right.

Why is it different?

Also the ac whine, I think from the thyristors, are much less distinct and are quieter than older trains that have ac motors.

 

[boiledbeans2]

Yeah you are right.

Why is it different?

My suspicion is that it's because the newer trains use the EP2002/CubeControl braking system. This is the only commonality I see among the trains with the "sss......sss...sss" sound. Maybe someone who's familiar with the technical details can confirm?

As mentioned in the article below, CubeControl is installed on S stock, 2009 stock and Aventras.

Newsforum

rail.knorr-bremse.com

Also the ac whine, I think from the thyristors, are much less distinct and are quieter than older trains that have ac motors.

To be precise, newer AC motor trains (post-2000s in general) use IGBTs, where the last 'T' stands for transistor. 90s-era AC motor trains use GTO thyristors. IGBTs use much higher switching frequencies, mostly beyond the range of human hearing. That's why they are quieter.

 

[Silent]

My suspicion is that it's because the newer trains use the EP2002/CubeControl braking system. This is the only commonality I see among the trains with the "sss......sss...sss" sound. Maybe someone who's familiar with the technical details can confirm?

As mentioned in the article below, CubeControl is installed on S stock, 2009 stock and Aventras.

Newsforum

rail.knorr-bremse.com

To be precise, newer AC motor trains (post-2000s in general) use IGBTs, where the last 'T' stands for transistor. 90s-era AC motor trains use GTO thyristors. IGBTs use much higher switching frequencies, mostly beyond the range of human hearing. That's why they are quieter.

But even a class 378 from the late 00s has louder igbts than a class 710 or s stock. I guess the newer trains might have even higher pitched igbt’s?

 

[GFE]

Perhaps also issues affecting the whole train when "controlling cab" gear/electronics get disturbed by the Sags/surges as that part of the train passes over the gap ??
I assume in theory the "controls" should use an backup supply from another part of train or battery

 

[Dstock7080]

I assume in theory the "controls" should use an backup supply from another part of train or battery

Yep battery backup for cab supplies

 

[sjoh]

In technical terms motor harmonics are rather harder to describe than simple explanation.

There's a rather good explanation on YouTube I found recently here:

 

[Silent]

Interesting ^

 

[Silent]

There's a rather good explanation on YouTube I found recently here:

A different way of explaining certain motor sounds that I just found.