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Post by darwins on Nov 7, 2019 11:13:28 GMT
Hello,
Just trying to get my history around how weak field has been applied on the Underground.
The odd old one - F stock 1920 had a "tapped field" relay that was activated when a certain speed was reached in notch 9.
As far as I can gather from the forums here everything from 1938 stock onwards had weak field available for d.c. motors that was only engaged by raising the weak field flag.
(I can see the case for the Bakerloo going to Watford Junction.)
If this continued on all stock with d.c. motors, was the last stock to have this in the form described the 1992 tube stock?
There is a very good explanation for the D78 stock on trainweb.
It is basically saying that "flag down" acted as a speed limiter and cut off power to the motors above 30 mph.
Was this same thing true for all stock including the A60 stock - which would make some sense for speed limits in central London - or was the A stock able to achieve a higher speed than that with the flag down?
Thanks
Darwin
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Post by philthetube on Nov 8, 2019 2:12:14 GMT
the a stock was considerably faster with the flag up, a common trick during leaf fall was to put the flag down coming out of Chesham and motor all the way up the hill, this would maintain a steady speed of 38 mph, (limit 35) without needing to shut off power at any point and thus greatly reducing the risk of wheel spin and getting stuck on the hill.
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Post by t697 on Nov 8, 2019 7:53:26 GMT
Hello, Just trying to get my history around how weak field has been applied on the Underground. The odd old one - F stock 1920 had a "tapped field" relay that was activated when a certain speed was reached in notch 9. As far as I can gather from the forums here everything from 1938 stock onwards had weak field available for d.c. motors that was only engaged by raising the weak field flag. (I can see the case for the Bakerloo going to Watford Junction.)
If this continued on all stock with d.c. motors, was the last stock to have this in the form described the 1992 tube stock? There is a very good explanation for the D78 stock on trainweb. It is basically saying that "flag down" acted as a speed limiter and cut off power to the motors above 30 mph. Was this same thing true for all stock including the A60 stock - which would make some sense for speed limits in central London - or was the A stock able to achieve a higher speed than that with the flag down? Thanks Darwin
1. I'm sure the F stock tapped field would have been switched at a certain motor current, not a speed control which would have been very difficult with 1920s tech. That said, it would be a certain speed assuming constant line supply voltage (which does vary though!). 2. 1983TS was the last with d.c. series motors. The present 1992TS equipment has d.c. chopper control with separately excited fields. On the old signalling the Central line ran at one setting throughout on the 1962TS and the 1992TS had a software controlled 'interrunning' performance to match that. It may have involved an element of motor field control. With ATC signalling throughout, the full performance characteristic software is used. Maximum speed is then enforced by the signalling as appropriate to the section of line etc. 3. The 30mph (approx) mentioned for D stock was the automatic coasting speed when the weak field flag switch was down. With coasting cut out the train could go faster even with the weak field flag down. The automatic coasting speed setting was higher with the weak field flag up. I think 1983TS also had the automatic coasting feature but no LUL stocks before D78 had it.
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Post by Dstock7080 on Nov 8, 2019 9:34:21 GMT
3. I think 1983TS also had the automatic coasting feature but no LUL stocks before D78 had it. Indeed they did. R Stock had switchable weak-field, although it made no noticeable difference to train performance! Located behind the set-number box, it uncovered a painted plate to the side of the set-number.
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Post by Deleted on Nov 13, 2019 18:04:13 GMT
And full stop
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Post by 100andthirty on Nov 14, 2019 6:37:35 GMT
WEAK FIELD: When the train is running at speed and the motors are connected in full parallel the speed of the motors is limited by the opposing back EMF. To gain further increases in speed the back EMF needs to be reduced. This is done by reducing the current through the field of the motor by connecting a small resistance in parallel across the motor field. This term is field weakening and allows some of the current to bypass or be diverted away from the field of the motor. By reducing the field strength back EMF is reduced and the motor increases in speed until once again the opposing EMF limits the current and therefore the weak field is usually achieved in two stages using field shunt switches FS1 AND FS2 I've taken the liberty of editing FISH7373's post for clarity. Other points. Clearly all this applies in detail to series wound motors.....where the field (the fixed part) is connected in series with the armature (the rotating part). Somewhat different arrangements are made where the field is connected in parallel (not generally used on trains) or where the field is fed independently (termed separately excited and currently used on 1992 tube stock. LU dc series motors were traditional provided with a field that was stronger than is typical in dc traction machines so that they could accelerate and get all the resisters disconnected (off notches) at a comparatively low speed of about 20 miles/hour. On trains with two weak field stages it was typical for FS1 to engage automatically as an "extra notch". Originally the 1973 tube stock has a 3rd stage of field weakening - FS3 - giving a very healthy turn of speed which was demonstrated during testing. It was there to provide a performance uplift when the line was resignalled. When that didn't happen and some spreadsheets needed, the FS 3 contactor was removed. I'm sure t697 will be along to correct anything I've mis-remembered.
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Post by t697 on Nov 14, 2019 20:44:18 GMT
Well there's a challenge! Had to go off and look things up. The motoring only PCMs like A stock, 1959/62TS etc. and also the 1967TS and 1972TS have two stages of field weakening available. Not always used though. For example 1962TS on the Central line didn't. The first stage (FS1) is under notching relay control switched by an auxiliary contact on the last camshaft step, after full parallel been reached. The second stage (FS2) is switched in under control of a separate current sensing relay called the Field Shunt Relay. This drops out at a lower current than the normal Rate 2 notching relay setting. This scheme seems to have two justifications; 1. It made the control circuits easier to implement. 2. The lower setting of FSR allows for quite a big step in field strength without an excessive current peak when FS2 closes.
C stock, 1973TS and D78 stocks have all field weakening steps under notching relay control, sequenced from camshaft auxiliary contacts. 1973TS and D78 stock have up to three stages of field weakening, C stock had two. 1973TS is actually set up to only use the first stage (FS1). D78 stock used only the first two stages.
Only the 1972TS and 1973TS now stopping this thread being more correctly in 'Historical' I suppose. :-)
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