What makes that sound?

[nate7713]

Hi there,

So this is what lead me to find this website in the first place, unfortunately I believe the thread I found was talking about a different sound.

A fewyears ago I was riding the tube on the way to work experience and, quitehonestly, was scared half to death when the train, as it pulled out, made what sounded like the start of a siren of some kind!

I asked my partner about it, and she mentioned that she has heard that too. she thinks it is only present on older models, while I think it's the newer ones that feature it.

I've put a link below to a video where the sound can be heard just after the doors close.


I swear they used to sound more like this video:


I'd love to know what that sound is, and what causes it if anyone knows.

As a side note, is it me or do the new doors sound a lot angrier? It's like, doors opening: grrrrr! Doors closing: grrrr! *slam*. 

Thanks in advance for any help.
Nathan    

[t697]

Hi there. Not sure why the question was originally posted in the SSR group, but here's an answer:
The train in the upper clip is a Jubilee line 1996TS train. The traction inverters use GTO thyristors and these switch at around 300Hz. However, to minimise electrical interference, the frequency is varied during the acceleration and braking so that other than the very first and last phase, the frequency is matched to an exact number of 'chops' per cycle of the 3 phase traction motor. This gives the 'gear changing' effect as the frequency changes to more chops per cycle and so on. This is not an actual mechanical gear change of course. The acoustic noise is vibration of electrical components such as the motor windings, inductors etc. This is the only fleet with GTO Inverters on LUL.

The second clip is a Northern line 1995TS train. Despite the year number this was actually the next generation of a similar train. The traction inverter uses IGBT devices which can switch at higher frequencies without excessive switching losses. This means the characteristic noises and 'gear change effect' of the Jubilee trains aren't there and instead you get a more constant hum.

Both these fleets have pneumatic operated doors. Most of the roar as they open and shut is from imperfections in the linear bearings the doors run on.

To bring back a link to SSR, the S stock trains have electric motor driven doors so do sound slightly different.

[nate7713]

Hi There,

Thanks for the answer! I'd read a little about acoustic variation in relation to aparrently the sound can actually cause problems for the sensors and signalling devices on the tube?

The reason I put it in SSR was I went by the summaries and figured this didn't really fit into any one category.

thanks for the response.    

[t697]

It's generally electromagnetic noise (EMI) that has the potential to affect signalling and other systems, not acoustic sound.
A lot of work and cost goes into the propulsion system design to keep EMI within safe limits and monitor that it stays that way or self isolates if something goes wrong.

Moderators: You might want to move this little discussion to Rolling Stock?

[rincew1nd]

Dec 5, 2021 12:01:13 GMT t697 said:

Moderators: You might want to move this little discussion to Rolling Stock?

It was moved to the Rolling Stock board about five hours ago.

[ijmad]

Class 456 'Networker' trains operated by Southeastern have a similar traction package and make similar noises:



(Skip to about 50 seconds in!)

[t697]

Yes, the Jubilee line equipment was a sort of second generation GEC-Alsthom Networker equipment. I see less than half the class 465 fleet had that. The others having initially a Brush Electrical Machines equipment replaced later by a Hitachi IGBT package. I think scrapping of Networkers has started.

[nate7713]

Just wanted to drop in a post to thank everyone for their really informative responses.

ijmad your video at 50 seconds sounds spooky as all eek

[cobolot5]

Sorry on a slight tangent. I read somewhere because of the AC traction packages ability to provide a lot of torque (probably the wrong word). Coupled with seltracs ATO characteristics, couplers between units were beginning to crack? Is there any truth in that?

[t697]

Dec 9, 2021 4:30:40 GMT cobolot5 said:

Sorry on a slight tangent. I read somewhere because of the AC traction packages ability to provide a lot of torque (probably the wrong word). Coupled with seltracs ATO characteristics, couplers between units were beginning to crack? Is there any truth in that?

Not much.  It's true that the Jubilee trains have experienced underframe cracks near the coupler attachments and there's a big programme addressing that.

The AC traction package has proper control of tractive effort and jerk rate so is no different to a DC drive in that respect.  ATO applies and removes tractive effort on all cars together so has minimal effect in principle. 

Implementation of ATC did allow use of the full specified design performance though which is a small factor.

Most significant is the cyclic coupler load changes when individual cars' traction is cut suddenly as the car enters a current rail gap.  The 7 car formation with 2 trailers in the 4 car unit has increased the effect compared to the 6 car case.