by Irvine Bell
Peter Rendell wrote:
"Because Dunedin is quite hilly, their buses had 5 positions on the master controller - reverse|unused|off|forward|regenerative braking. Their electrical system was constructed in a manner that allowed the current to be fed back into the overhead. No other NZ system used this option. "
The London pre-war fleet [actually this was most of the fleet, only 127 being purchased after the war] was equipped with regenerative braking.
The regeneration was achieved by using compound wound motors i.e. motors with a main series winding and a secondary shunt winding.
The shunt winding was used during motoring and the some of the higher running notch positions were achieved by weakening and cutting out the shunt field.
When motoring fast [35 mph], notching back a few points on the power pedal from the full on position, reinserted / strengthened the shunt field, causing the motor back emf to rise rise above the line voltage and regeneration and therefore braking to occur. Regeneration was effective down to about 20 mph.
Below that, further electric braking could be achieved down to 2 or 3 mph by rheostatic braking, via two braking notches on the brake [right] pedal.
An old trolley bus electrician I spoke to in 1967 [five years after the London system finally closed] told me that there were a lot of problems with regeneration. It could push the line voltage very high [1000V on a nominal 600V] system. This could cause traction fed lighting in depots to blow bulbs!
A more interesting problem was that all vehicles apparently had an overvoltage relay that was designed to trip out [and stop the bus] in the event of high line voltage.
Sometimes it would seem, this relay could stick. In the event that calls would be received to go out to several stopped buses [with tripped relays], the breakdown crew went looking for the bus that was still running, because that had to be the one with the faulty relay!
My understanding is [or was] that post war [1945], all London trolleybuses had the regeneration capability removed, leaving rheostatic as the only form of electric braking.
However, when the recently restored Leyland London trolleybus 1201 was run at Dudley, she still seemed t have the regenerative braking capability. This caused problems with the substation, because it could not accept regenerated power. The Dudley substation is an electronic [solid state] one: London substations included some rotary [motor generator] sets that could absorb
regenerated power. It was necessary when driving 1201 at Dudley, to lift the
foot off the power [left] pedal sharply, rather than trying to notch back,
to avoid giving regeneration time to build up.
I don't know why 1201 should seem to have the regenerative capability. Perhaps, during restoration, the pre-war control system configuration was restored.
Both regeneration and normal rheostatic braking require[d] the overhead line to function [in rheostatic braking the shunt field is excited from the overhead line]. In the event of dewirement or power failure, both would be unavailable.
London had two very steep hills, and for service on these hills a few vehicles were equipped with additional 'coasting' and 'run back' brakes. Most, if not all Bournmouth, trolleybuses were similarly equipped.
The 'coasting' brake was selected at rest by an additional position on the foward / reverse / off controller. This connected up the motor as a self excited series generator, in a similar manner to a traditional tramcar [drum controller] electric brake.
I have experienced the coasting brake on a Bournemouth trolleybus, down our short, but steep hill, at Dudley. The bus initially runs away freely down the hill, once the handbrake is released, but at around 10 mph. the coasting brake comes in quite sharply [this is a characteristic of motors operating as shunt wound generators], stabilising the speed at about 10 mph regardless of either the gradient or the load on the bus.
I am not quite sure how the 'run back' brake worked, but I believe that it was essentially the same as the coasting brake, but came into action automatically in the event that the vehicle started rolling backward.
I presume that the Dunedin vehicles referred to were probably of British origin and wonder whether their 'regenerative' brakes were actually the same as London hilly route and Bournemouth vehicles?
Post war, most, if not all British [home market] trolleybuses, were equipped with rheostatic electric braking only.
From Irvine Bell irvine_bell@msn.com
Daniel Hammond mentioned " ... Why is it constantly stated that one of the advantages of high priced "chopper" (electronic) motor controllers is that of regenerative braking? ..".
In the traditional designs of British etb equipment [used up to 1972, when
the last system, Bradford closed], which were all based on resistors +
relays [contactors], regeneration was easily achieved on the vehicle. It was
basically a matter of how the motor field windings were arranged. The
problem was having a receptive overhead line, which is why regeneration
dropped out favour after the 1930s as substation designs changed.
For the overhead line to be able to receive regenerated power, there, as has
been pointed out by Jim, another vehicle to be taking power at exactly the
right moment [and in the same electrical section].
If this is not the case, then the substation feeding the section, needs to be able to absorb power e.g. by putting power back into the [public] supply network. Since motor generator type substations [which could put power back into the public supply] went out of fashion in the 1930s, in favour of mercury arc rectifiers and more recently solid state [electronic / silicon] substations, substations have not been inherently able to absorb power.
My understanding is that it is possible to design substations that can absorb power, either by dissipating it in resistor banks, or converting it back to ac and putting it back into the public supply. However, this increases the cost and complexity of substations.
The economics of regeneration would seem to be very weak. I can't lay my
hand readily on any figures, but my understanding is that energy savings
from regeneration in British practice were only at best a few percent and it
is difficult to make an economic case, given the higher costs required in
substations, to achieve only modest savings.
[Of course in a battery powered vehicle, regeneration into the battery is virtually essential, to maximise range - but that is another issue].
Daniel Hammond mentions the possibility of using batteries to achieve rheostatic braking in the event of dewirement.
With traditional British controller designs this was not possible. The traction batteries could provide a maximum of 60 Volts. British etb rheostatic braking depended on excitation of the motor shunt field, which needed the 600 Volts, or so, of the overhead line.
One thing I wonder about is how rheostatic braking is, or has been, achieved on other controller designs than British?
British etbs had compound wound motors i.e. a shunt field winding in addition to the main series winding.
The motors could have been arranged, as in traditional tramcar control systems, as series wound self exciting generators, not needing the overhead for excitation, for braking.
Unfortunately series wound generators develop very different braking efforts
[including none at all] at different speeds. In the case of traditional
tramcar [drum] control systems, this requires the driver to move
progressively through the braking notches as the vehicle slows down to try
to achieve an [approximately] constant braking rate.
British control system designers considered such an arrangement for an etb to be unacceptable [at least if there were to be only two pedals, one for power and the other to be a combined electric and mechanical brakes pedal].
Instead, they went for a combined self excited series field with a shunt field powered from the line arrangement. This does gives a braking effect that is substantially independent of speed. British etbs typically had two notches of rheostatic braking on the [air, etc,] brake pedal.
[Actually, the self excited series field only braking arrangement was used occasionally for so called 'coasting' brakes and 'run back' brakes on hilly routes like London's Highgate Hill route 611.].
But that was with two pedals. If there is a third pedal, as I understand there is / was in [some] French, Swiss and Russian practice, how was the rheostatic braking done? My guess would be that it might have been done like a traditional tramcar, presumably requiring the driver to change pedal position / pressure as the vehicle slowed down, to keep braking effort approximately constant.
Sorry, this is getting bit long again!
But going back to electronic controllers, I find it very hard to see how it could be worth paying as much for an etb electronic control system as perhaps two or three complete equivalent diesel buses can be brought for!
In the heyday of the British trolleybus, diesel buses and trolleybuses cost much the same.
However, attempts to reintroduce etbs in Britain in the 1980s failed on economic grounds [mainly] because of the [quoted] cost of the vehicle [several times the cost of the diesel designs they were derived from]. The main factor in the excessive cost seems to have been the electronic controllers.
[Deregulation of the British bus operating industry in 1986 was the other major factor for etbs not [yet] returning to Britain.]
It seems to me that a very good case should still be able to be made for simple and cheap [dc] resistor + relay systems on new etbs. Such systems ought to be very cheap and still highly effective. I presume that this is what San Francisco has done.
At least in British practice, such [low] levels of maintenance and [high] levels of reliability were being achieved by dc equipment, that it would be very difficult to improve on them and certainly not worth spending a lot of money to gain improvement.
To take one example. AC motors are often stated to be very much lower
maintenance the DC motors, because they do not need a commutator and
brushes. But I know that the motors in London trolleybuses were achieving
mileages of 250,000 [say five years running] between needing attention to
brushes. How much better could a brush less AC motor really be than that!
