Regenerative Braking

[philspace]

Evening all,

Discussion of the Woodhead route has reminded me of a question which has puzzled me for a long time.

I understand the principle of regenerative braking - the traction motors being reversed to act as generators - but there will be no braking effect if there is nothing to absorb the power being generated.

Presumably the 1500V D.C. supply on the Woodhead route came from the 50Hz National Grid via rectifiers. In those days, there would have been no way of converting the regenerated D.C. power back into A.C. for the grid, so if there was no other train under power at the time, there would be nothing to absorb the power and no braking effect available.

How did the driver know whether there was likely to be any regenerative braking available, or was it just suck it and see? Was the regenerative braking regarded as a bonus, the conventional brakes being sufficient? What happened to the braking train if the under-power train suddenly cut off demand?

I've seen resistive braking in operation, where the regenerated power is dissipated as heat in resistors, usually on the train roof, but it is not a particularly green solution - just a different way of expending (wasting) energy.

Best wishes

Phil

[AndyUK]

The designers of the scheme catered for the case where the line would otherwise have been non-receptive. Some of the substations were fitted with resistance banks to absorb the power from locos regenerating when there were no other trains taking power. The resistances were switched in automatically when a rise in the line voltage was detected.

Later in their lives the Class 76s were fitted with rheostatic braking so that electric braking could be continued down to very low speeds, 3-4 mph, and drivers were recommended to switch from regen to rheostatic braking below 20 mph. Presumably the regen brake became much less effective below that speed.

All in all the 76s must have been quite a challenge to learn, a lot different to today where there's only one handle to pull and push!

Andy L

[philspace]

Hello Andy

Many thanks for your reply - clever stuff!

Best wishes

Phil