HST power cars fitted with the Brush electronics upgrade (all VTEC, EMT, NR, Grand Central and XC power cars, plus GWR's 43053/056) are, I believe, fitted with slow speed control for that very purpose.The driver controls the speed by varying the engine power. Don't know if any UK rolling stock has speed control other than Slow Speed Control (SSC) which was fitted for unloading at merry-go-round power stations. It makes it very difficult to move a train dead slow when you want to unload ballast or something similar.
Mind you it would have been very helpful for driving passenger stock through carriage washing machines, where precise control of speed is critical.
Does each step of the speed controller give a particular engine speed?
Many thanks for all replies. New question, is there a set number of amps for each notch on a class 43 speed controller??
No, the number of amps is as previous posters have pointed out the amount of current which is determined by many factors.
Starting from 0 mph is the highest demand and will draw the highest current. As the train gets rolling the current draw will decrease somewhat at the same power level as it becomes easier to do its job. at Idle in a station there is effectively zero current (excluding ETS etc) and whilst coasting there is also zero current. Cruising at a regular line speed is by comparison very easy on the motors. Am i correct in thinking that going straight into notch 5 on a class 43 from standstill will overload things and the current will limit, lets ignore the likelyhood of wheelspin.
The class 91 (and Mk4 DVT) i believe has a continuously variable power handle so no notches, just a handle that you can put where you want it from idle to full.
The current is regulated by the load regulator so putting it into notch 5 straight away will rev it up to 1500 rpm and produce around 1500 amps down to the motors.Unless there's a serious problem with it , it will not overload and quite happily do it each time.
I suspect that modern school physics doesn't introduce people to the concept of back EMF - that is the fact that an electric motor also acts as a generator, so if you put a voltage on a motor with a fixed load then the amps drop as it speeds up (because as the speed increases the motor generates more power - "back-emf".)
HST power cars/Class 43 locomotives are not fitted with any form of speed set. The Class 91s/Mk4 DVTs are however. The Class 43s are fitted with a five-notch power controller. If I remember correctly the notches give the following engine speeds in an HST:
0 - Idle - 950 rpm
1 - 1,060 rpm
2 - 1,170 rpm
3 - 1,280 rpm
4 - 1,390 rpm
5 - 1,500 rpm
Class 90, 91, 92 electric locos, Class 175, 180, 185 DMUs, Electrostars, Class 390 Pendolinos and some other EMUs have a speedset device. On most classes a speed is 'dialled up' - i.e. you set a dial to the speed you wish to maintain, however on the Pendolinos, you simply press a button when you reach the speed you wish to maintain. I also believe the speed set in a Pendolino doesn't use the brakes and so cannot control speed on a descent.
A 390 does use the brake in speed set. It will maintain speed on a descent. Going round Northampton being a good example. On the long 75mph stretch the train can run away a bit and it will put brake in to keep at 75mph.