Black Fives, B1s, Halls: there were literally many hundreds of all of them. I'm yet to be convinced that the Southern had anything similarly versatile.
I agree. What they did have were locos that got used
as if they were that versatile because they didn't have anything that
actually was. After all, the Light Pacifics were technically a "mixed traffic" loco according to the wartime assessment of their driving wheel size... On the other hand the nature of the Southern's traffics and their large electrified area makes it difficult to compare them usefully with the other three.
Maybe, but back-EMF comes into it (in older motors.) You can't put full power through a stationary electric motor without blowing it up. So you can't get 5000 HP, let alone 10, out of an electric or diesel-electric loco from standing. Whereas a steam engine piston will take full boiler pressure when stationary...
...and the steam engine will give you a power output of zero, same as the [diesel-]electric.
Power = (force on piston) * (distance piston moves) / time. If it's stationary, the distance the piston moves is zero, so the power output is zero, even though the force on the piston (and so the engine torque) is at a maximum with full boiler pressure applied.
The current through an electric motor is (applied voltage - back EMF) / (winding resistance) (neglecting effects oif inductance in AC motors and the like). The waste heat generated in the motor is (current squared) / (winding resistance). For a series-wound motor like a traction motor, the torque is also proportional to (current squared). The mechanical power out of the motor is (back EMF) * current.
When the motor is stationary, the back EMF is zero, so the current is simply (applied voltage) / (winding resistance). Since the winding resistance is very small, unless the applied voltage is also very small the current will be very large. Therefore it is necessary when starting to regulate the applied voltage such that the current through the motor does not exceed the allowed maximum value - ie. the value at which heat is being generated faster than it can be removed before the thing cooks itself.
Since the current is
at its maximum value, the motor torque is also at maximum, but since the motor is stationary, the back EMF is zero and the power output is also zero.
Standing further back, as it were, and looking at the loco (steam or [diesel-]electric) as a "black box what moves", the view is the same in both cases: power = speed * torque, so even though the torque in both cases is at a maximum, the speed is zero so the power is zero.
The difference is that while the power
output of the steam engine cylinder is zero, the power
input to it (from the boiler) is also zero, by the nature of the thing; whereas with the traction motor, the nature of the thing is to scarf the maximum amount of input power it is capable of, and turn it all into heat. It will happily (until it melts) eat everything the engine/generator can give it, so the engine power has to be limited so as not to supply more heat than the maximum allowed. The reason this becomes a matter of concern is the need for this active limiting and the potential for overheating the motor if the limiting fails in any way (including driver error), or if the motor cooling is deficient.
(The principle is much the same with a diesel-hydraulic, except that the waste heat is generated in the transmission oil, so it is much easier to remove it than it is to remove it from a solid lump of motor. A hydraulic has a much greater margin of safety against overheating the transmission when starting.)
As the traction motor accelerates, the back EMF increases, so the voltage required to push through it the current necessary to produce a given torque increases (this increase represents the increase in mechanical power output from the motor). But since power = voltage * current, then for a given electrical power, as more voltage is demanded from the generator so the available current falls. Eventually, even at maximum engine power, it will fall below the maximum current that the motor can handle without overheating; above this speed the loco is no longer capable of producing its maximum torque output, and the torque will fall as speed rises, but on the other hand it has now become self-regulating and there is no longer any need to actively limit the engine output.
Note that the effect of back EMF, and its importance, are the same for
all motors, regardless of age.
(Also- it's a very subtle difference, but I suspect as a 'go anywhere, do anything' loco the Grange would pip the Hall. The Didcot fireman H.Gasson was of the opinion that a Grange was the better loco, although his experience was generally more heavy freight haulage than express passenger. They're almost the same locomotive but the slightly smaller driving wheels of the Grange gives it an edge getting a train on the move and Halls generally weren't renowned as high-speed runners anyway, generally struggling to maintain speeds higher than the mid-60s on any sizeable load.)
I agree, and I was going to propose the Grange myself. The smaller wheels match the rest of the engine better. I think they tend to get forgotten about because none of them got preserved, while loads of Halls did.
One thing Bulleid did seem to get right was his boilers, and both MNs and light pacifics would steam very well.
FSVO "get right". They readily produced a lot of steam, but they consumed an inordinate amount of coal to do it; the fireman would have to do a lot more shovelling for the same load and timings than with one of their immediate predecessors.
