Thyristor control

Are the SCR and the thyristor the same device? I thought the SCR predated the thyristor and was a cruder device. The thyristor also has voltage-control characteristics which enabled it to be used to replace heat-generating resistors so in fact has two functions...

They did. The floors rusted and they all had to be re-floored.

There was a later trial in the early 1980's with a new train, which had to operate under engineer's possessions. I used to work at Westinghouse at the time, and I worked on the monitoring unit for that, with the intentio of letting it loose over Woodford Hainault.

The buzzing noise came from a large aircored inductor, which carried the traction current, and buzzed when it was chopped. Oscilloscopes and test gear would actually pick this up, and there was some comment as to whether it would affect punters with heart pacemakers - although the frequency was 384Hz as I recall. Probably it would have been best to provide those punters with a hamster wheel to run off the effects of excessive heart stimulation- or even let them power the train Fred Flintstone style, legs on the track.

The buzzing corresponded to the period for which resistance would be cut into the circuit of a conventional drive. Therein lay one of the technical problems. The switching frequency was chosen to avoid interference with track circuits at 125 Hz. So we had to prove that if the drive was running, it was running at the right frequency. But when the drive was full on, the thyristors were on for 100% of the time, and there was no signal, which was indistinguishble from the sensor wires falling off.

We solved this by injecting a pilot signal, giving 2 channels to monitor, with the additional requirement that if the drive was switching, it had to obliterate the pilot signal, to suit the logic requirement that a lack of pilot signal meant it was switching, in which case the switching frequency had to be detected at 384Hz, otherwise the monitoring system was broken, in which case, failsafe required a shutdown.

With me so far? After that it got complicated. Someone said "What if the drive goes at 384Hz, but only switches on 2 pulses out of 3?" Actually, this is a very similar case to doing 1 out of 3, which is effectively 128Hz, within the acceptance of a track circuit, but which could still satisfy a test for 384Hz. We then designed a further channel to detect this phenomenon, biting our tongues against cursing the choice of 384Hz, although on reflection, whatever frequency was chosen, you could always say 'n out of m' and get 125Hz.

I lost track of it all after that, moving onto other projects.

These were they, thyristor controlled. On a point of strict accuracy, the Westinghouse unit is better attrbuted to Brush, who were also part of the Hawker Siddely group, as were Westinghouse. Westinghouse only did a pair of Monitor units. We had, IIRC a prototype of a Blackpool Tram drive for our lab work, because this was variable frequency, whereas the Underground stuff was fixed frequency - we needed to be able to vary from 384Hz.

We did some night tests from Northfields, some of which involved laying Teepol for wheel spin. One of the concerns was about noise from the line breakers disturbing the nearby sleeping population. Never did figure out what the line breakers were doing exactly...

Our habit was to eat out and eat well, which raised some eyebrows when we put in our expenses. One night 2 managers turned up from our outfit, and proceeded to brag about having eaten on the M4 services, and about how _cheap_ and _good_ it was. We just smiled and carried on, having eaten really well ourselves, just waiting. Come about 0330, we were still bright and bushy tailed, on the back of our meal, but our mangers began to look very tired, deflated and dare I say it, insignificant. We then tucked into some sandwiches and asked them if they had any. Needless to say, they hadn't. But our expenses were not challenged again!

There was also a 38 unit converted to act as a pilot when moving around the system.

One of the cars (01306?) has the trial high windows and the oval casement windows.

10306! The only car number I have ever remembered! Converted 1948?? into the "Sunshine Car" as we heard it called. It lived with its then sister cars in Northfields for a time and I used to see it regulalrly when I passed by on the Picc to work...was it used as part of a test train?

TP, do you agree that there was a moot afoot to preserve 10306?

The time when it was at Northfields would coincide with me commuting to Piccadily Circus from Hounslow West as I went to Bristol Poly Feb 1979.

And for our youngers viewers..... (10306 in service)