Weak field

[programmes1]

On the Met years ago it seems that there was a weak field button on the master controller, I always thought that they were flags? Were they originally like this for all stocks?

[domh245]

Would it not be similar to the camshaft operated national rail trains, such as the 455s. They have shunt, series, parallel, and weak field notches on their power controller, where field weakening occurs automatically at a determined speed (of about 40mph). I would guess that this early met stock would be similar in principle, especially for operations of the north end of the line, so that it can reach the higher speeds, but with the lower speed control towards the city end, so the driver doesn't need to be going across the cab so frequently

[programmes1]

I'm sorry I have no knowledge of National Rail trains. I have over the years seen the weak field flags used on the underground but have never heard of a button on the master controller, perhaps one of the rolling stock guru's will be able to shed some light.

[domh245]

Its not a button, it is one of the notches. If you look at an old SR power controller, it will be labelled 0,1,2,3,4 which corresponds to off, shunt, series, parallel, and weak field respectively. I suppose it is a bit like the flag switch being integrated into the controller, when the driver selects notch 4, he has put the flag up, but if he's only selected notch 3, he remains without field weakening, which only becomes noticeable at about 40 mph.

[philthetube]

The S8's also have a flag which operates automatically and appears automatically on the tcms screen. it operates all stations north of Finchley Rd. except Uxbridge to Hillingdon where it is kept off because of problems with trains being gapped on the hill leaving Uxbridge.

I dont know if a week field switch was ever on the master controller though.

The 59's were also fitted with a flag though it was not in use on the Northern in 1995. The flags were removed but the hole where they had been was stil there.

[Dstock7080]

Jan 10, 2015 22:49:35 GMT philthetube said:

The S8's also have a flag which operates automatically and appears automatically on the tcms screen.

Under normal circumstances the weak-field indication should appear on the SID screen on S Stock. [/pedant]

[programmes1]

Jan 10, 2015 20:20:53 GMT domh245 said:

Its not a button, it is one of the notches. If you look at an old SR power controller, it will be labelled 0,1,2,3,4 which corresponds to off, shunt, series, parallel, and weak field respectively. I suppose it is a bit like the flag switch being integrated into the controller, when the driver selects notch 4, he has put the flag up, but if he's only selected notch 3, he remains without field weakening, which only becomes noticeable at about 40 mph.

It was a button but details are sketchy almost none at all.

[roythebus]

Having recently been on the T stock at the Spa Valley, there is indeed a button on the deadman's handle. I learnt the T stock back in 1973 but suspect the button was the deadman button rather than the weak field switch.

R stock had the weak field flag which was disconnected on most units but did work on a few!

On the SR,4SUBs and the older stock didn't have weak field and was limited to about 70-75mph by motor gearing. EP stock generally didn't have weak field either, being suburban stock. The CEP/BEP/CIG/BIG/VEP/REP family had weak field on the 4 position controller. They also had a series/parallel switch on the dash. This was only used on the Folkestone Harbour branch and allowed trains leaving the harbour to accelerate rapidly (?) to series to get a run up the 1 in 36 gradient. I sometimes used the switch for a very quick getaway on the SW main line!

Generally speaking AC units automatically go into weak field at a set speed.

Edited to add the CLR motor cars used as ESLs and I believe some battery locos had the button type deadman. just remembered from my training days in 1970!!

[Deleted]

Jan 15, 2015 23:28:57 GMT roythebus said:

Having recently been on the T stock at the Spa Valley, there is indeed a button on the deadman's handle. I learnt the T stock back in 1973 but suspect the button was the deadman button rather than the weak field switch...Edited to add the CLR motor cars used as ESLs and I believe some battery locos had the button type deadman. just remembered from my training days in 1970!!

I think the D stock's shunting controller has a similar deadman's button also.

[John Tuthill]

On the 38's, whenever weak field was employed a flag was displayed in the cab window with vertical black & yellow stripes. I'll see if I can find a photo.

[metman]

I understand the A stock weak field operated a little differently as it altered the rate of acceleration to be slower but could then achieve a higher top speed 70 mph or so.

[norbitonflyer]

Jan 17, 2015 7:58:00 GMT metman said:

I understand the A stock weak field operated a little differently as it altered the rate of acceleration to be slower but could then achieve a higher top speed 70 mph or so.

isn't that what a weak field normally does?

[Dstock7080]

Jan 17, 2015 10:34:52 GMT norbitonflyer said:

Jan 17, 2015 7:58:00 GMT metman said:

I understand the A stock weak field operated a little differently as it altered the rate of acceleration to be slower but could then achieve a higher top speed 70 mph or so.

isn't that what a weak field normally does?

Of more recent Stock, A Stock were unique in automatically selecting the lower acceleration rate when the flag was raised.

[roythebus]

I think you'll find that the "proper" LT stock fitted with weak field flags all operated in a similar way, with weak field cut in acceleration was slower in all positions of the controller. On BR EMU stock, weak field was a fourth position on the controller and was brought into operation by the RPA type equipment when a certain speed was achieved.

On the CEP/CIG etc stock with the series/parallel switch for Folkestone Harbour, with the switch in series only it also cut out the weak field so that maximum power was available up to series only to go up the 1 in 36.

[bigvern]

On the 67 Tube Stock, weak field was operational in Auto only, as the No8 relay was only operational when AUTO was selected, on the 72 Tube Stock Weak Field is automatically selected when the controller is in Parallel position as No3 and No8 relays are both energised, however the Weak Field resistances are of a different value than those on the 67TS and thus achieves a 72ts achieves a lower top speed.