Weak field switch - how does it work?

[yerkes]

I understand that the weak field switch is used for running at higher speeds and know what the flag looks like. But for a long time I've wondered:

(a) How does it work from a technical point-of-view? What is the weak field? (The extent of my electrical knowledge is understanding circuits, resistors and switches, plus a few more random bits and pieces.)

(b) What is the purpose of the flag? As it's on the offside it's presumably not so much for the benefit of the T/O?

(c) Is the weak field switch/flag still incorporated into stock built post C/D stock?

Thanks in advance for any explanations.

[Deleted]

a) doesn't it reduce the resistance in the electric motors resulting in slower acceleration but a higher maximum speed.

b) isn't it to show to inspectors/DMT that the driver isn't exceeding 30mph/slowing down the service by having it up unnecessarily.

c) I don't know if the 83 stock had it. The 9x stocks are governed by power electronics so the amount of power is controlled by computers. The 95/96 stock have limitations on their power because they are not using ATO yet.

[Deleted]

When Phil, stanmore K, BAET, solidbond and company turn up they will post lots of interesting technical info on weak field, but rob actually has the basic concept right.

When weak field is enabled, a series of devices (whose composition escape me at the moment) are electricially inserted into the traction motor circuitry; these devices cause the magnetic field that has saturated the windings of the motor (and prevented it from rotating any faster) to diminish in intensity; as a result, the motor is able to draw more energy (I don't remember if it's electrical power or just electrical amperage), causing the motor's balancing speed to increase beyond normal. As rob says, this weakening of the magnetic field in the motor windings reduces the motor's torque, lowering its acceleration somewhat, but the final speed that the motor can rotate at can be significantly higher. The final result is a faster train that consumes a bit more energy and accelerates more slowly.

rob also has the flag's concept perfectly correct. THIS THREAD goes into more detail surrounding the flag's use on the District Line.

To answer the third question, the 1983TS did have weak field and coasting control, just like the D stock.

fx: stampede of LU T/Ops and infraco staff....

[100andthirty]

TheOneKea has basically got it right except for one thing: the train doesn't accelerate more slowly as a result of weak field. Accelerating more slowly with the flag switch up is/was a feature of A stock but on this, it controls two features.

When the line was electrified to Amersham the power supply was installed on a bit of a shoestring budget and the powers that be of the time wanted to limit the current drawn from the supply but also to have higher speeds.

What they did was to link the flag switch to the rate switch so that with weak field enabled, the train could only accelerate in rate 1. Acceleration on the camshaft (RPA) is carried out in full field and then weak field is introduced and has the effect of an extra notch on the acceleration process.

[yerkes]

That's extremely useful - thanks very much. Thanks too for pointing me to the other thread, also very helpful.

[signalfailure]

I think this post should be answerd in a 1000 word essay by Aspect

[Colin]

Apr 15, 2008 19:56:45 GMT yerkes said:

(b) What is the purpose of the flag? As it's on the offside it's presumably not so much for the benefit of the T/O?

It is the actual switch which turns the weakfield on or off, and it's positioned on the offside and viewable through the windscreen to show when it is being used.

Apr 15, 2008 20:36:39 GMT @rob said:

b) isn't it to show to inspectors/DMT that the driver isn't exceeding 30mph/slowing down the service by having it up unnecessarily.

You're almost right, but not quite!

It is indeed positioned in the front off side windscreen to show when it is being used (as mentioned above) - but remember, the train will go faster! No flags up will show the train as not exceeding 30 mph

So in essence, on the District a driver should have the weakfield (or coasting) flag up east of Bow road or west of Turnham Green/Putney Bridge - if no flags were up in these areas, then it could be considered that a driver was deliberately driving slowly.

[Phil]

I've turned up but can hardly add to TOK's technical description. Making the magnetic field weaker by reducing the current in the field coils (or equivalent on newer stock) reduces the 'back emf', so allows the motor to speed up until the new back emf (the balancing voltage) again equals supplied voltage, but at a higher speed. So higher speed, but the weaker field strength means less force, so less power/torque and lower acceleration.

.......................as TOK basically said ;D

[Colin]

Except as 100andthirty already said - acceleration will improve using weakfield. Only A stocks have poorer acceleration when using the weakfield flag, and that is because it is linked to the lower rate 1 switch.

[Deleted]

Apr 17, 2008 17:32:20 GMT @signalfailure said:

I think this post should be answerd in a 1000 word essay by Aspect

I am obliged to Signalfailure for this confidence and the thought I can manage close to 1000 words !!!

However I prefer to defer in detail to the likes of Solidbond !

The info given by other I think sums it up !

The acceleration improvements of having the Weak Field flag raised (ie. cut out) are sometime necessary to overcome having a motor or two out, so may ocassionally be seen other than in the stated areas. The coasting control flag may be raised (ie. cut out) anywhere on the line. Drivers tend to fall into two distinct groups, Coasting Control permenantly cut out (like me) or Weak Field cut out where permitted.

C stock flags do not all work now, some having been disconnected.

The off side nature of the flags generally means their selected positions can only be changed when stationary.

[Ben]

I'm fairly certain I already know the answer to this, but theres nothing to stop the flag being pulled during motion, even on A stock is there?

[100andthirty]

I'm sure this won't be the last post but I'll add that 92 tube stock doesn't have a flag switch, but does have field weakening. The current in the armatures (the rotating part of the motor for the non tekkies!) is controlled by thyristor current controllers (chopper) - there are teo of these per car - and the current in the fields (the static part) of all 8 motors on a 2-car is controlled by another chopper. At low speed the field current will be held at the maximun allowed and the armature current will be steadily increased to accelerate the train without over accelerating. Once full voltage is applied to the armatures, the field current will be steadily reduced until the required speed has been achieved.

(this, believe it or not is a very simplified description)

thus you might describe 92ts as having infionitely variable field weakening.

There is no such concept for AC motors!!!!!

[metman]

Is the weak field still used on the A stock all the time? I saw it wasn't used on the ELL, but I see it on City services-I thought the flag wasn't supposed to be used in the City!

[Colin]

Apr 18, 2008 6:06:08 GMT Ben said:

I'm fairly certain I already know the answer to this, but theres nothing to stop the flag being pulled during motion, even on A stock is there?

On D stocks, the flags can be operated at anytime (ie, when motoring, coasting or at a stand) and the effect is instantaneous - so I would assume other stocks are the same. Of course to operate it without letting go of the deadman does require long arms or good physical dexterity, but it's certainly more than possible!

[Deleted]

Apr 18, 2008 9:13:27 GMT metman said:

Is the weak field still used on the A stock all the time? I saw it wasn't used on the ELL, but I see it on City services-I thought the flag wasn't supposed to be used in the City!

The weak field flag is supposed to be put down south of Finchley Road, but is generally left up by most drivers.

[Deleted]

I gather R stock drivers used to leave the flag up at all times as they considered the trains too slow otherwise

[Dstock7080]

Apr 18, 2008 23:00:22 GMT @aspect said:

I gather R stock drivers used to leave the flag up at all times as they considered the trains too slow otherwise

I honestly could NEVER feel the difference on an R stock!, unlike the D which have a kick forward when the flag is raised.

[stanmorek]

www.twoof.freeserve.co.uk/TRACTION3.htm

For the record civil engineers generally are not happy when things are on the move.

[Deleted]

Apr 17, 2008 18:02:07 GMT Colin said:

Apr 15, 2008 19:56:45 GMT yerkes said:

(b) What is the purpose of the flag? As it's on the offside it's presumably not so much for the benefit of the T/O?

It is the actual switch which turns the weakfield on or off, and it's positioned on the offside and viewable through the windscreen to show when it is being used.

If the actual flag is the switch i'm curious to know how it works. Was it placed in the slot and made a pair of contacts underneath or is there another switch that is mechanically or electricaly linked to the flag to raise it out of the slot or did the driver just pull it up out of the slot and that made some sort of contacts underneath.
Darren.

[neilw]

Just been in the garden for a look! On 62 TS at least, the flag is lifted up by hand, turning from a horizontal position where it is hidden, to a vertical one where it is on display. On the spindle about which the flag rotates are the electrical contacts which do the switching. The 62 TS on the Central all had their flags removed to prevent weak field being selected, does anyone remember why? I also recall being told that the change of acceleration of a standard stock train was very noticeable if the flag was lifted whilst the train was under power, but on a 62 TS nothing much happened.

[happybunny]

Aug 22, 2008 15:17:44 GMT neilw said:

Just been in the garden for a look!

You mean... you have an actual tube train... in your garden ??

How ?? Is there tracks in your garden too ?

[Deleted]

Thanks Neil at least thats another curiosity sorted. I wonder if i should model one for my RS 1938 tube stock cab.
Oh! and while were on the subject of cabs. You couldn't possibly take some photos of every inch of your cab Neil and post them on your website which i have already visited to my amazement at seeing the 62 TS in your garden. I would love and i'm sure the community would love a 62 TS cab for RailSimulator and MSTS.
Darren.

[neilw]

The item in my garden is a 62 DM, so I guess one car doesn't technically constitute a "train"!! ;D
I'll take some pictures of the cab for you Darren, when I get a moment, might need to give it a bit of a clean first!

[neilw]

Sorry Happybunny, yes the car is on a 60 ft panel of track so there are 4 ft at each end left over. The panel, quite by chance, is from the Hainault loop, somewhere near Roding Valley. I still need to source some conductor rails to complete the scene, but I'm not sure how to get them in place because of the weight (and how to slide them under the car). I might have to mock them up from another material!!

[District Dave]

It's ok to have one in your garden - but at least the ones I drive actually move !! Sorry Neil couldn't rsist that !

I'll try to remember to have a look in the cabinet on the offside and see if I can get some photos of the set up on a D Stock (unlessanyone beats me to it now I'm on annual leave fpr two weeks !

[Deleted]

ok next silly question how did you get into your back garden?

[100andthirty]

on d stock the flag switches are linked to rotary switches something like these:

www.santonswitchgear.com/

[DWS]

Aug 22, 2008 17:10:24 GMT @aetearlscourt said:

ok next silly question how did you get into your back garden?

It took a wrong Signal ;D

[neilw]

It got into the back garden by low-loader, quite a task as we had to dismantle the front wall, and re-build it afterwards. If you are on myspace, have a look at www.myspace.com/neildwhite if you are on facebook, have a look at www.facebook.com/profile.php?id=680956008 there are some pictures of the event plus some later ones after I cleaned the outside.

[happybunny]

Thanks for explaining neilw... sounds like you are quite a fan of the 62s train. I don't know anyone who has a train in there garden... quite an achievement !