RPA Differences

Thanks Tubeprune.
Was the purpose of the dual camshaft system to save the RPA having to notch all the way through the series resistances to get to parallel if the train was already doing a speed above the series threshold. The single camshaft system had to notch up through the whole lot if the driver went from idle to parallel with the train at speed.

I'm wondering, if there were only nine contactors for the dual system they must have had less resistances and less notches for series and parallel.

67, 72 and C stock still have to notch through series to get to parallel - have a careful listen as the train remotors at speed.
When the scheme for rheostatic braking for 67TS was being developed, they used two of the RPA machines to get all the contacts and the overall circuit configuration needed, in conjunction with separate grouping contactors. I think they just used machines available then went with that for production.
For 73TS and D78, GEC had a rethink and redeveloped the RPA into a longer one with more power contacts on it, simplifying the overall equipment count a little.
All the RPA machines have the same number of actual notches.
If you listen carefully you'll find that in Motoring, the Parallel camshaft on C stock takes two notches at a time. I could explain that but this is a bit long already so I'll stop for now...

All RPAs have 10 positions. So you hear 9 clicks as it runs from one end to the other. On D78 stock each of the Series notches cuts out resistance from the motor circuit under control of a Notching Relay. To give a smoother less sudden start, the first 2 or 3 notches build the current up to the current level controlled by the NR. At full Series, grouping contactors reconfigure the power circuit to put the 4 motors into two parallel strings with resistance in circuit again. The RPA then runs back the other way under notching relay control, again cutting out resistance until the motors are in full parallel with 3 notches to go. The last 3 RPA notches actually control weak field contactors. In the actual configuration in use though, none of these close in 'Flag Down' and only the first two in 'Flag up', so you won't feel any increase in traction when an unused notch is taken. It gets to full parallel at about 18 - 20mph from memory.
In rheostatic braking, it can initiate at nominally just over 50mph which is higher than the line speed limit. So it will always take a few notches before the brake current establishes for the speed you happen to be at. Again the Notching relay controls the notches taking a notch as the train speed falls and the current falls. The NR settings vary depending on the brake step selected (1 to 4) by the T/Op and the passenger load. Rheo won't start if you initiate braking from below about 22mph - because by the time it had taken enough notches to establish current you would be almost at last notch, so it would just wear the switchgear out pointlessly. Last notch is roughly 10mph then the friction brake comes back on.

Double notching in parallel on C stock is because the notching relay only detects one of the two motor strings, and it switches in the other one on the first notch and its own one second notch of the pair then the current keeps the relay closed and the notch is held until same thing again. The D78 one switches two contacts in each notch during parallel, one in each motor string so you don't get the double notch effect.

If you travel on D78 you'll notice that the RPA clicks are the quietest on any of the fleets. This seems to be because the equipment case is better sealed and there's more space between it and the vehicle floor. Also the tractive effort swings are less noticeable because the steps are evenly graded and the intercar drawgear is quite stiff longitudinally so you don't get all that longitudinal surging like C stock.
Apologies to anyone who fell asleep unexpectedly here!!!!!

Right, let me get my head round this.
Full parallel is between 18 and 20mph, that's a lot slower than I imagined. I had it about 30mph.
In parallel you would only here six notches before getting to full if the weak field flag is down and eight if up.
The RPA actually puts the resistance back in as the train decelerates. Which I didn't know. Does it notch back up from rest under braking or step back down from parallel or where ever it was last at.

What you said about the C stock double notching sounds very familiar to the Standard Stock when in parallel. That only detected one motor circuit but always switched out the undetected one first.

I need to get one of my friends to take a trip on the D78 to do a RPA sound check I think. From the sound recording I do have I did notice the first 3 clicks to be very rapid.

Thanks for an amazingly informative post. I'd love to get my hands on a circuit diagram for this.

The Notching Relay used with RPAs isn't of this flip-flop type. I see there are pictures of one or two of those in Agnew. I've seen our type pictured many years ago in an Agnew, but it must be in the 38TS volume, which I don't have. It is a single contact type with a knife edge armature.
In rheo brake, the RPA camshaft again switches resistance out as the train speed falls, so as to keep the motor current and hence torque, up to that set by the NR. The circuits are configured so that the camshaft runs up to position 10 then grouping contactors reconfigure things slightly and the cam runs back for the remaining brake notches ending back where it started still cutting out more resistance. The brake energy is dissipated in the resistors of course.
If one of you PM's me, I will try to send you a little more to clear things up and stop us all boring everyone else on here too much!

I recall the 38TS NR has power coil, lift coil and rate coil. This is still the same concept in D78 stock. Other low voltage (-50V fed) circuits modify the current in the lift and rate coils in accordance with passenger load and the T/Op selected motoring rate (rate 1 or 2) acceleration or Service Brake (1 - 4). The effect of current in these control coils is to add to or oppose the effect of the power coil that has the motor current in it. Thus the motor current level at which the relay drops out varies in line with setting selected through the control currents in lift and rate coils. The lift coil is also energised fully, still used in the way used on 38TS to close the relay as a notch is taken ready to hold the next one. But the inter-interlock circuit is 50V not 600V as on 38TS, so that all these other controls can be added too.
Offer of other info via PM still stands.

t697, as far as I remember, the PSG and weak field switches are on separate contactors, not on the camshaft itself. Can you confirm?

I'm not sure if this comes out in what is being discussed but pre-67TS, the single camshaft ran up to full series and ran back to full parallel so it was effectively in the off position at full parallel. For 67/72TS, the series camshaft ran back to its off position as soon at it reached transition and then the parallel camshaft ran back after it reached full parallel. You can hear two camshafts running as the train progresses through the parallel notches, one going fast (the series camshaft running back) and one notching under load (the parallel one). When it gets to full parallel, the parallel camshaft runs back. The sequence is similar for rheo braking.

1. Yes, on all the pre 67TS equipments Series/Parallel transition is on the separate PSG switch (Parallel/Series Grouping switch) which has 1 n/c power contact for Series and 2 n/o power contacts for Parallel. Also weak field is on separate contactors for all the equipments with RPAs right to D78. On D78 and 73TS they are sequenced off control contacts on the RPA.
On 67TS onwards PSG is replaced by individual single pole contactors. There are more too for Braking grouping and Full parallel so the Parallel RPA can run back.
I always thought the PSG switch was the most 'Victorian' looking item in a PCM case!

2. Good description of the camshaft sequences. In rheo on the 2 RPA equipment the Series RPA does the first group of notches then stays put while the Parallel RPA does the rest, until they are both fully notched up. After rheo fade down the last notch the Rheo Proving relay drops out and both RPAs run back fast off load. You can also get them running back in a brake release. They run back inserting resistance and reducing motor current until RPR drops out and then run the rest of the way back.

3. On 73TS and D78 from new, the RPA direction can also be reversed to reduce motor current to correct wheelslip detected by armature voltage imbalance within a motor pair. 67TS and C stock had a mod in the 80's to do the same but can't notch back through parallel/series transition. I don't think we did the mod on 72TS but not sure.

Yup. The worst feature was the RBPR. As you run into a station, one car fails to pick up rheo and the whole train dies while the e.p catches up. Horrible! Still, it doesn't matter too much on the existing Bakerloo, it's so slow!!! But it was dire on the Stanmore line running SB.

Gentlemen - thank you all! Once again our simulaiton efforts will be that much more realistic thanks to your expert input. I'm sure in the future we'll post a video of the Railworks D78 being put through its paces - on some metals we hope you recognize.

Best to all, Jimi