What is "Field Divert"

Johnny_w · 7 Aug 2015

Hi Forum!

Hear a lot of people talking about the field-divert kicking in... Can anyone explain what it is? Google has been less than helpful!

I understand on the old DC Woodhead loco's that you could change
between serial & paralleling the motors - that makes sense to my EE mind.

But Field Divert has me vexed!

any thoughts would be welcome

cheers

JW

RailUK Forums

ac6000cw · 7 Aug 2015

I'll preface this by saying I'm an electronics engineer, not an expert in electrical machines....

Assuming we are talking about series-wound DC traction motors, I've always assumed it means reducing the current in the field (stator) windings by diverting some of the current elsewhere (presumably into a ballast resistor to turn it into heat) - i.e. it's field weakening, which you have to do at higher speeds to reduce the back-EMF and allow more current to flow through the motor (= more power).

Johnny_w · 7 Aug 2015

ac6000cw said:
I'll preface this by saying I'm an electronics engineer, not an expert in electrical machines....

Assuming we are talking about series-wound DC traction motors, I've always assumed it means reducing the current in the field (stator) windings by diverting some of the current elsewhere (presumably into a ballast resistor to turn it into heat) - i.e. it's field weakening, which you have to do at higher speeds to reduce the back-EMF and allow more current to flow through the motor (= more power).

That makes more sense.... So Field Divert is really field dampening then?

My background is National grid electrical engineering so it's not something I come into contact with really.

Thanks for replying - very interesting.

JW

455driver · 7 Aug 2015

That's my understanding of it as well

GM228 · 7 Aug 2015

I have this saved on a word file, but can't remember where got it from:-

"What does the field divert system do.

If you take a motor say out of a model train you have an fixed field magnet which means when it gets to 12 volts it will spin at a set speed with a set torque. Fine for a model train but not the real thing, when a real train moves off at slow speed you have a massive current demand (not uncommon to see 7000+ amps on a 47/3) so you are interested in maximum field strength but since if you had a fixed magnet you wouldnt be able to achieve this but the motors of a locomotive do not have fixed magnets, essentially they are electro magents who's behavour can be altered by changing the voltage being passed through the field windings which alter how the motor behaves. A locomotive developes maximum tractive effort (torque) at around 13mph after which the speed increases and the current demand falls.

All DC traction motors develop back-EMF which is the motor acting as a generator and countering the power going into it, your current falls to a very low point but your engine may be at full power which means you cant spin the generator any faster for more volts, therefore you must change the way your motors behave and get them to spin faster with the availiable voltage you do this by diverting your field voltage through a set of resistors in a process called field diversion the outcome of this process is in principle the same as changing gear on your car."

fishquinn · 7 Aug 2015

Could it also be when a train takes a divert off its main route through a field?

CosherB · 7 Aug 2015

fishquinn said:
Could it also be when a train takes a divert off its main route through a field?

Certainly has happened before ..... a Stobart Class 66 in Scotland springs to mind! :lol:

Jamesb1974 · 7 Aug 2015

Deleted

70014IronDuke · 7 Aug 2015

Jamesb1974 said:
Yes. With a 66 it is quite noticeable. Without getting me manual out, its about 37 mph before there is a lurch (almost like a gear change) and the amps drop.

It's the same with all diesel electrics. With a 45/6/7 it's at about 35 mph and 65 mph, IIRC. The engine winds down and then roars back again.

ac6000cw · 7 Aug 2015

Concerning US-designed diesel-electrics, you might find this interesting - http://www.trainorders.com/discussion/read.php?1,3483774

'Transitioning' is the term they use when the loco changes between a 'more series' motor connection scheme (for lower speeds) and a 'more parallel' connection scheme (for higher speeds, to reduce the total back-EMF). I think EMD normally used series-parallel and full parallel as the two schemes, with field weakening available as well on some locos.

HSTfan!!! · 8 Aug 2015

About 35mph on 66, sure it seems slightly higher on the newer ones mind. Meant to be one hell of a jolt on a 59 but not experienced that.

RUFJAN15 · 8 Aug 2015

I suspect there may be some confusion here between series/parallel connection of DC motors and field weakening.

To avoid any confusion, I'm going to describe a 'traditional' DC motor where the current flowing through the windings in the rotating part of the motor (the rotor winding) also passes through the windings in the fixed part of the motor (the field winding). Some more sophisticated control systems use separate excitation (SEPEX); the Class 60 being an example.

A DC traction drive system will start with motors connected in series (and with additional resistance in circuit to reduce the starting current). As the motors speed up then the resistors will be switched out until just the motors are connected across the DC supply. To increase the speed further the motors are reconfigured so they are connected in parallel to the supply - I suspect that this is the first 'gear change' that posters have referred to. When the parallel connected motors have reached full speed (dictated by electromagnetic forces balancing out in the motor rather than mechanical limitations) then speed can be increased further by maintaining full current through the armature but switching in a 'divert' circuit so that only a proportion of the current flows in the field winding. This is also referred to as 'weak field' operation.

There is a more thorough explanation - with diagrams - at http://www.railway-technical.com/tract-01.shtml

Emblematic · 8 Aug 2015

What you've described isn't wrong, it's the traditional resistor & camshaft switch DC EMU control arrangement fitted to some older stock - Classes 313, 442, 455 for example. However, most of the posts refer to diesel locomotives, which have none of this. The generator is connected straight to the motors, no resistors, no series-parallel switching, current controlled by the driver not applying too much engine power too soon, with little more than an ammeter to help.
They do, however, have field weakening, usually one or two stages, cut in automatically as motor back emf rises. Without it, the generator output voltage would keep rising (the engine power has to be dissipated somewhere) and eventually either generator or motor would flashover. It's the sudden change in voltage and current that causes the gear change effect.
If there are resistor banks fitted to locomotives, they are for rheostatic braking, not dissipating engine power.

Jamesb1974 · 8 Aug 2015

Deleted

RUFJAN15 · 9 Aug 2015

Emblematic said:
What you've described isn't wrong, it's the traditional resistor & camshaft switch DC EMU control arrangement fitted to some older stock - Classes 313, 442, 455 for example. However, most of the posts refer to diesel locomotives, which have none of this. The generator is connected straight to the motors, no resistors, no series-parallel switching, current controlled by the driver not applying too much engine power too soon, with little more than an ammeter to help.
They do, however, have field weakening, usually one or two stages, cut in automatically as motor back emf rises. Without it, the generator output voltage would keep rising (the engine power has to be dissipated somewhere) and eventually either generator or motor would flashover. It's the sudden change in voltage and current that causes the gear change effect.
If there are resistor banks fitted to locomotives, they are for rheostatic braking, not dissipating engine power.

Thanks for correcting my Southern-centred explanation of DC traction systems. I must learn to engage brain before posting!

At the risk of putting my foot in it again, my understanding is that at low speeds the control system has to limit the current drawn by the motors. This is achieved in a different way according to the type of locomotive/unit:

1) In a DC fed system the line voltage cannot be controlled. By initially connecting the motors in series and putting additional resistors in the circuit the voltage applied to each individual motor can be reduced, thus limiting the current drawn. As the motors speed up the applied voltage is increased by switching out the resistors and reconfiguring the motor circuits so that they are connected in parallel.

2) In a diesel electric locomotive the motor current is limited by the characteristics of the generator. Current is controlled by the driver varying the power input to the generator.

3) In the original BR 25kV AC locomotives (with DC motors) the voltage applied to the motors is controlled by tapping the main transformer. This allows the AC voltage fed to the rectifier to be varied whilst the line voltage remains constant.

'Weak Field' or 'Field Divert' works in a similar way for all three types of locomotive and changes the power output/speed characteristic of the motors so that they run faster for a given power level.

Emblematic · 9 Aug 2015

RUFJAN15 said:
Thanks for correcting my Southern-centred explanation of DC traction systems. I must learn to engage brain before posting!

At the risk of putting my foot in it again, my understanding is that at low speeds the control system has to limit the current drawn by the motors. This is achieved in a different way according to the type of locomotive/unit:

1) In a DC fed system the line voltage cannot be controlled. By initially connecting the motors in series and putting additional resistors in the circuit the voltage applied to each individual motor can be reduced, thus limiting the current drawn. As the motors speed up the applied voltage is increased by switching out the resistors and reconfiguring the motor circuits so that they are connected in parallel.

2) In a diesel electric locomotive the motor current is limited by the characteristics of the generator. Current is controlled by the driver varying the power input to the generator.

3) In the original BR 25kV AC locomotives (with DC motors) the voltage applied to the motors is controlled by tapping the main transformer. This allows the AC voltage fed to the rectifier to be varied whilst the line voltage remains constant.

'Weak Field' or 'Field Divert' works in a similar way for all three types of locomotive and changes the power output/speed characteristic of the motors so that they run faster for a given power level.

Sounds spot on to me!

70014IronDuke · 9 Aug 2015

RUFJAN15 said:
Thanks for correcting my Southern-centred explanation of DC traction systems. I must learn to engage brain before posting!
.......

2) In a diesel electric locomotive the motor current is limited by the characteristics of the generator. Current is controlled by the driver varying the power input to the generator.

...

The driver of a "classic" diesel-electric will watch his ammeter. With low speeds and little or no back EMF generated by the traction motors (which will be stationary, or close to it) - the amps can rise very quickly, potentially leading to overload trip and/or wheelslip. So he will open the controller to 1/4 or so, and be ready to drop it back.

The amps will rise - I forget exactly, I think on a 47 there was a green section to about 2,000 amps, then a yellow section to maybe 3,500 amps, then a red 'danger' section. (someone will surely soon correct me if this is wrong) .

A driver could go into the yellow section, especially on starting, but should not go into the red at all.

As speed increased - and hence back EMF start to reduce the amps through the traction motors - the amps shown on the ammeter would fall, and the driver could then safely notch up the controller steadily to 3/8, 1/2 and so on to full load.

I honestly forget exactly what happened when the field diverts cut in. I think the load regulator would automatically reduce the fuel to the injectors as the field diverts cut in, and the load would fall off - and the process would (more or less) start again. I think that would mean the driver having to notch down manually. (sorry, I should remember, but I don't!). Of course, starting was much more critical than moving along at 35 and 65 mph, when there was already a decent amount of back EMF. Far less danger of an overload trip.

All sorts of assistant controls came in later, like wheelslip detectors, which automatically dtected the sudden drop off in load and cut the output on electric locos and HSTs, IIRC.

cadder toad · 9 Aug 2015

Field divert matches the RPM/power characteristic of the diesel engine to that demanded by the train. Same function as gearbox on a car. It was always automatic on any engine I saw. The Load Regulator would cut-in resistances and reduce the traction motor field strength. The driver couldn't make any decision about how much field diversion to apply. He/she only had a power handle.

Field divert was always done using resistors as far as I knew. Although I can't remember where the resistors actually were in the loco. I do remember cleaning contactors which I think were part of the FD system.

Interestingly not all locos had the Red Zone in the ammeter. I think a 47 didn't.

Johnny_w · 10 Aug 2015

Thank you to all who have replied thus far.

Thoroughly good read and I'm grateful for the knowledge!

Best Regards,

JW

Rugd1022 · 10 Aug 2015

Jamesb1974 said:
Compared to a 60, the field weakening/divert is considerably more noticeable. All you see on a 60 is the ammeter needle flicker. A 66 is a near tea spilling jolt.

I'd agree on that James and in some circumstances it can kick in at well below 37mph on a 66. When we used to work 6059 from Bardon Hill to Acton we'd be on full power going up Desborough Bank with about 2,400 tons on the drawhook, approaching the summit we'd usually be doing around 30mph and the field divert would kick in, by the time the amps needle had settled back down the speed can drop another ten miles an hour - quite some jolt!

cossie4i · 19 Aug 2015

HSTfan!!! said:
About 35mph on 66, sure it seems slightly higher on the newer ones mind. Meant to be one hell of a jolt on a 59 but not experienced that.

A 66 is as smooth as silk compared to a 59/0/1, 59/2s are about the same as a 66.
The change over on a 59 varies between engines but it's around 27mph.
I will update the actual speed later and what engine it was.

cadder toad · 19 Aug 2015

Field divert was a feature of dc traction motors of 50's-60's era modernisation plan diesels. From what posters are saying its still used today on 59s and 66s and maybe others too. Are there diesel locos today using different technology? Maybe ac motors? Are load regulators and engine governors still mechanical devices or has the technology moved on?

AM9 · 19 Aug 2015

RUFJAN15 said:
'Weak Field' or 'Field Divert' works in a similar way for all three types of locomotive and changes the power output/speed characteristic of the motors so that they run faster for a given power level.

Surely, don't you mean that the motors have a higher power for the same voltage applied to them. The problem is that when is running at high speed, the back EMF counteracts the applied voltage reducing the current passing through the motor which in turn reduces the power drawn by the motor. Weak field arrangements just absorb some of the back EMF allowing a greater current from the available fixed supply voltage.

Emblematic · 20 Aug 2015

cadder toad said:
Field divert was a feature of dc traction motors of 50's-60's era modernisation plan diesels. From what posters are saying its still used today on 59s and 66s and maybe others too. Are there diesel locos today using different technology? Maybe ac motors? Are load regulators and engine governors still mechanical devices or has the technology moved on?

Field divert is used on all series-wound DC motors, such as those fitted to the classes you mention. The field and armature windings are connected in series, and the same current flows through both. By switching in a resistor across the field windings, the current through them is diverted, reducing the back EMF and hence allowing the motor to draw more power at higher speeds. It's a simple switching process, and noticeable when it cuts in.

An early electronic control system called SEPEX (for separate excitation) became available in the 1980s, and used a separate control circuit for the field coils, which were no longer wired in series with the armature windings. This allowed a much finer control of the motor, and was fitted to classes 58 and 60 (hence the smoothness noted by the posters above - there are no large switching points, but a gradual reduction in field strength controlled electronically.)

Modern locomotive designs such as class 68 have AC motors - which is really a simplified DC motor with the inbuilt mechanical switch (commutator) removed. They rely on the traction package providing a modulated AC current to the motor, most designs having all of the windings in the chassis and a small rotating armature with very few, large conductors producing force by induced current (hence referred to as induction motors.) Very smooth, and very well controlled which gives a useful improvement in adhesion. They are also significantly smaller and lighter for a given power than their DC predecessors.

HSTEd · 20 Aug 2015

SEPEX was used before electronics came along - and all sorts of other variations. Several diesels used various ways of connecting the field and armature coils to try and prevent single wheel wheelslip and other problems.

There were lots of designs tried about before PWM SEPEX became the effective standard in the 80s as mentioned above.

I believe one scheme used at least once was to connect all the field windings in series so the same current flows through all of them, this can be controlled separately as required.

If you then connect all the rotor windings in parallel across the supply then a motor that loses grip and starts to run away will see its back EMF rapidly increase relative to the other motors - the motor would then slow down as the power and thus torque is sent to the other traction motors.

It is supposed to assist in starting heavy trains without wheelslip issues.

Emblematic · 20 Aug 2015

HSTEd said:
SEPEX was used before electronics came along - and all sorts of other variations. Several diesels used various ways of connecting the field and armature coils to try and prevent single wheel wheelslip and other problems.

There were lots of designs tried about before PWM SEPEX became the effective standard in the 80s as mentioned above.

I believe one scheme used at least once was to connect all the field windings in series so the same current flows through all of them, this can be controlled separately as required.

If you then connect all the rotor windings in parallel across the supply then a motor that loses grip and starts to run away will see its back EMF rapidly increase relative to the other motors - the motor would then slow down as the power and thus torque is sent to the other traction motors.

It is supposed to assist in starting heavy trains without wheelslip issues.

The main reason to connect the field windings in series and the rotor in parallel would be to avoid having the massive field windings typically seen in shunt-wired motors. In a series motor the current is fixed, but most of the voltage is taken across the armature windings. In a shunt motor the windings face equal voltages, so are similar in size (number of turns) making the overall motor quite large. The defining characteristic of the shunt motor is an extreme rise in back EMF as speed rises (both windings contributing) so much so that they are effectively constant-speed machines, and that is their typical application in industry - varying power delivered across a narrow speed range.
By connecting the field windings in series as described, you then make the field windings 1/2 or 1/4 of their previous size, and the characteristics of the motor would be intermediate between series and shunt designs. It could have limited wheel slip by limiting the motor speed in shunt-fashion, then by switching resistances in series to reduce the field current further, allow the motor speed to gradually rise.
However, the series-connected field windings wouldn't be effective at transferring load between motors, as most of the back EMF is created in the rotor windings. The field windings would all carry the same current, so every motor would have the same field strength, whatever speed each was doing. Indeed the typical arrangement for DC powered, DC motored EMUs is to connect the motors in series at low speeds, and that's not effective at transferring power in wheelslip conditions. Rather the opposite in fact, the current through the motor pairs drops as one axle slips, so the non-slipping motor has it's power reduced when that's not what you want to happen, resulting in one axle spinning and the other contributing little tractive effort.

HSTEd · 20 Aug 2015

Emblematic said:
The main reason to connect the field windings in series and the rotor in parallel would be to avoid having the massive field windings typically seen in shunt-wired motors. In a series motor the current is fixed, but most of the voltage is taken across the armature windings. In a shunt motor the windings face equal voltages, so are similar in size (number of turns) making the overall motor quite large. The defining characteristic of the shunt motor is an extreme rise in back EMF as speed rises (both windings contributing) so much so that they are effectively constant-speed machines, and that is their typical application in industry - varying power delivered across a narrow speed range.
By connecting the field windings in series as described, you then make the field windings 1/2 or 1/4 of their previous size, and the characteristics of the motor would be intermediate between series and shunt designs. It could have limited wheel slip by limiting the motor speed in shunt-fashion, then by switching resistances in series to reduce the field current further, allow the motor speed to gradually rise.
However, the series-connected field windings wouldn't be effective at transferring load between motors, as most of the back EMF is created in the rotor windings. The field windings would all carry the same current, so every motor would have the same field strength, whatever speed each was doing. Indeed the typical arrangement for DC powered, DC motored EMUs is to connect the motors in series at low speeds, and that's not effective at transferring power in wheelslip conditions. Rather the opposite in fact, the current through the motor pairs drops as one axle slips, so the non-slipping motor has it's power reduced when that's not what you want to happen, resulting in one axle spinning and the other contributing little tractive effort.

I believe the objective in that system is to abandon any attempt to switch the rotor windings at all - all of them are always connecting in parallel across the full supply voltage.

All control of speed is done by controlling the field strength of the motor array - as as you say all of them have the same field strength, and could be 'shimmed' to ensure all had exactly the same field strength despite small tolerances in the manufacture of the motors.

cossie4i · 20 Aug 2015

Findings from yesterday when the field divert changes

59002, 25mph
59201, 29mph
59202, 30mph
66200, 36mph

3141 · 20 Aug 2015

On older London tube stock, e.g. 1931 stock, I recollect that the term "weak field" was used and also "shunt", which both (I think) had to do with what happened as the train went faster - or to enable it to go faster.

455driver · 20 Aug 2015

3141 said:
On older London tube stock, e.g. 1931 stock, I recollect that the term "weak field" was used and also "shunt", which both (I think) had to do with what happened as the train went faster - or to enable it to go faster.

455s have a camshaft setup and they have Shunt, Series, Parallel and Weak Field

What is "Field Divert"

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