Having now had a chance to think some more and discuss with Reuben, we have come to the conclusion that-
1. The HW1000 may well have actually predated the competitor’s 63 machine, so presumably was designed in the late 1950s, probably in anticipation of all the re-signalling work expected after the 1955 modernisation strategy, almost everywhere then being mechanically signalled and steam traction.
2. The subject of materials science was in its infancy and whereas there had evident;y been some recent advances in the quality of magnetic material, quite possibly as a result of the war, the production of permanent magnets was definitely still very expensive. It is also understood that in the early days of AWS around that time, it was found that magnets did sometimes weaken quite quickly. I don’t believe this is still much of a problem nowadays. It is notable how much the size of magnets of equivalent strength have reduced over the last 20-30 years; in the railway sphere note the new Vortek AWS for example.
3. Therefore it was no doubt considerably cheaper to produce motors where the magnetic field was produced via current flowing in static coils (wired in series with the rotating windings of the armature) rather than use permanent magnets.
4. Note that if the polarity of the external voltage applied to a series wound motor is reversed, then the motor still rotates in the same direction; to change the direction of rotation needs either the magnetic field reversed or the current through the armature windings reversed but not both. To avoid complications of switching the polarity of current through the coil windings, the HW1000 motor design actually consists of two separate groups of static windings, one being utilised for one direction and the oppositely wound set providing a reversed magnetic field that is utilised for the other direction of drive- this is what is meant by “split field” motor.
5. Whereas false operation of a motor from dc would require conduction due to contact with a power source (typically one direct physical contact, one indirect contact via an earth fault), this is not true for ac interference which can also be induced, for example from parallel wires which are correctly insulated. A series wound motor could rotate and indeed both cycles of the ac would cause rotation in the same direction and hence the points could unlock and then the closed switch spring open and the points move across slightly. Therefore the HW1000 is non-immune and thus cannot be used in an area where ac interference is a possibility.
6. It is quite possible that when first designed, the HW was only intended as a non-immune machine. The need for an ac immune machine may have arisen later (e.g. West Coast electrification in the mid 1960s). The HW2000 certainly needs additional contacts (see7) and by the look of the design there is definitely a feeling that they are squeezed in, as would be the case if having to modify an already extant machine as the starting basis.
7. There wouldn’t have been any sense in using HW2000 where no requirement for ac immunity was anticipated. The motors would have been more expensive and probably would not have lasted anything like as long, the associated internal wiring of the machine was more complicated and also needing more contacts (for swapping the armature winding polarity, undertaking snubbing) that could be a source of unreliability and certainly need regular cleaning and occasional replacement due to wear or corrosion. A further advantage of the series wound motor is that it has a very high starting torque when power is first applied; when the armature is stationary or very slowly rotating it is not generating a significant back emf and therefore a very large current is flowing through both the static and armature windings thus producing the greatest force- as the speed of rotation increases, the potential difference applied to the static windings decreases accordingly. Hence if a set of points is stiff to unlock, a series wound motor might succeed where a permanent magnet motor would probably fail, particularly if the magnet had weakened over its lifetime. Hence where the lack of immunity inherent with use of a HW1000 wouldn’t cause a problem, then clearly it was the preferable choice.
8. I presume the situation changed considerably once magnet technology improved; I guess that the cost equation has now swapped (as winding of individual coils can’t be cheap even highly automated whereas I imagine that forming individual magnets would be much easier, given the availability of suitable material.
9. In addition as the need for ac immunity spreads wider geographically, then it is the non-immune variety which becomes the exception and the benefits of standardisation to the more capable version makes sense.
10. The 3rd rail dc lines are generally concentrated in the area south of the Thames on the lines from London to Kent and the south coast. There are actually now plans to start extending ac overhead electrification onto some such lines, but even ignoring this then there can be a need for ac immunity. The dc traction supply has always had a 50Hz (and harmonics) ac component since it is derived from the national grid; historically though the trains had dc motors and were controlled through resistors. This is not the case nowadays; modern trains are actually very similar to those operating on the 25kV ac network and in effect just use the dc traction to power an on board switch mode power supply, with speed control effected by controlling the frequency and waveform.
Hence in summary we have come to the conclusion that in the past the rationale for using HW1000 rather than HW2000 was that it had several advantages, but primarily economic. In the early days use of the HW2000 was restricted to those places where its ac immunity was really needed. The various changes discussed above which have occurred over the years now means that there is no longer any significant advantage of using a HW1000 and thus HW2121s (updated version of HW2000 having magnetic rather than friction clutch) is the NR standard (where traditional machines rather than other point mechanisms are appropriate) as that both standardises and gives future-proofing.
The perception that the HW1000 is somehow more immune to dc interference than the HW2000 seems just to be a fallacious- one of the “red herring” / "old wives tales" which exist in the folklore of railway signalling.
We have also realised from looking at internal wiring drawings of the HW1000 and W2121 that the difference between the “red” and the “beige” diode blocks is that the
orientation of the diodes within the package is different in the two cases- look at the terminals of the two diodes which are connected to the common terminal.
Hence whereas we have not completely established the above is the precise truth and much is history before even our time in the industry, we feel that the story presented above is pretty plausible; it seems consistent with what we know / suspect and the resulting explanation seems convincing to us. Happy to be corrected if someone knows better!
Attached here are some other diagrams of the wiring of the HW1000 / 2000s for comparison. The use of circuit controller-like symbols for the operating and snubbing contacts certainly makes things clearer than on the earlier diagrams where the "N" and "R" designation tends to confuse.
(07-07-2013, 08:47 AM)reuben Wrote: Have just located a comparative diagram of the two types side by side.
Notably, the curly inductor symbols in the 1000 are the field windings, and the "MOTOR" is the rotor. because of this, if you reverse the polarity of the feed, the magnetic fields of both static field and moving rotor reverse, so the reversals cancel each other out and the machine turns in the same direction, so the machine will turn if fed with AC.
