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Post by nickf on Apr 20, 2010 8:59:03 GMT
This shows just how long it is since I travelled on LU, but on Friday I was on the Jubilee Line and noticed an entirely different sound from the familiar clicks as the resistances notched out on older stock. You'll know the sound I mean as the motors accelerate, an almost square wave rising an octave then falling back to the original tone and up again and then a third, more subtle change which I find hard to describe, almost as if the nature of the wave form changes as well as its frequency. Can someone explain what is going on, please? What actually produces the sound? Is it chopping the DC supply into increasingly longer fragments or are we talking AC motors with DC - AC converters with variable frequency?
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slugabed
Zu lang am schnuller.
Posts: 1,480
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Post by slugabed on Apr 20, 2010 9:55:02 GMT
Yes....the Jubilee stock does have a very distinctive sound (though I couldn't have described it as precisely as you have) which is shared with contemporaneous stock on the 3rd rail electrics. I have particularly noticed this on the trains running suburban services out of Cannon St/London Bridge. Older sliding-door trains on the Southern and LT,and newer stocks don't seem to have this,almost "fluting" sound.Was it a technological dead-end? What makes this sound?
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slugabed
Zu lang am schnuller.
Posts: 1,480
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Post by slugabed on Apr 20, 2010 10:11:59 GMT
Oh....and whilst we're on the subject of sounds that motors etc make,here's one that's intrigued me since I first heard 4-SUBs back in the distant past..... The train pulls away from stationary as usual,but within seconds (and it always is SECONDS) there is an almighty "POP" and acceleration (presumably power) is lost with a jolt which shakes the whole train.The train will coast a little,and you can often hear clicks of presumably relays,and acceleration restarts from the beginning as normal once more. I've heard this on SR electrics and LT trains.....most recently the other day on a 67 stock. Is it due to elderly equipment or,ahem,overenthusiastic acceleration by the motorman causing some protection to cut in? Could someone describe step-by-step what I am hearing?
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Deleted
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Post by Deleted on Apr 20, 2010 12:03:26 GMT
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mrfs42
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Post by mrfs42 on Apr 20, 2010 12:12:56 GMT
Can someone explain what is going on, please? What actually produces the sound? Is it chopping the DC supply into increasingly longer fragments or are we talking AC motors with DC - AC converters with variable frequency? Very slightly adapted from Wikipedia: The Gate Turn Off thyristor used on 1996 stock achieves this by 'chopping' the supply voltage in order to drive a sinusoidal current in the motor windings (Pulse Width Modulation), creating the characteristic audible whine associated with the stock and with the Class 465 networker trains that share its traction drive system. The noise timbre changes as the pulse length changes. The noise is produced by the switching frequency current ripple and resulting torque pulsation experienced by the rotor of the induction machine.AIUI the DC fragments are being chopped into slightly longer chunks, hence the very distinctive take-off noise. The full wikipedia article is available here
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slugabed
Zu lang am schnuller.
Posts: 1,480
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Post by slugabed on Apr 20, 2010 12:18:55 GMT
Thanks Cabdarren! And Mrfs42!
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Post by nickf on Apr 20, 2010 12:37:27 GMT
Thanks from me too. That Wiki article is tantalising; Quote: "The noise timbre changes as the pulse length changes. The noise is produced by the switching frequency current ripple and resulting torque pulsation experienced by the rotor of the induction machine." So the induction machine has moving parts. Reminds me of two things (hope I don't get ticked off for going off thread). A ship I worked on a couple of years ago was powered by seven diesel/alternator sets. These fed a AC-DC rectification system. The DC was then processed to produce variable frequency AC which fed the three phase motor which turned the propeller. And many years ago when I worked on feature films the camera had a 220 volt three phase motor, for synchronisation purposes. On location we in the sound department got 24 volts from a couple of lorry batteries, which turned a DC motor, which turned an 3 phase generator to supply the camera. Frequency was controlled by hand, watching a Fram wobbling reed frequency meter. Happy Days. Sorry for blethering!
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Post by tubeprune on Apr 26, 2010 6:23:22 GMT
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Post by t697 on Apr 26, 2010 7:35:20 GMT
Yes, a good find there. Jubilee 96TS has GTO inverters similar in principle to the Alstom class 465 inverter on the sound clip. Northern line (Alstom), new Vic and S stock (Bombardier) use IGBT inverters that, as described in that link, don't need the 'gearchange' effect.
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Post by nickf on Apr 26, 2010 9:03:57 GMT
Thanks Tubeprune. That makes it very clear.
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