I have volunteered many years at a railroad museum, and have
constructed a lot of overhead wire for trolleys. We wanted to put in
some power switches, and I quickly learned that operating companies
used a number of techniques. My hands-on experience deals with
trolley poles instead of pantographs, but I will try to show how this
would work with pans where applicable. Please also note that I am
using US terminology (motorman, not driver; streetcar, not tram).
Except where noted, I am referring to technology that was developed in
the 1900’s.
For a power switch to work, we have to have a source of trolley power
(typically about 600 volts DC) from the wire down to the switch motor.
Toronto has a really unique system. They use a “NA” (“Necessity
Action”) button in the car. The motorman pushes this button to
operate the switch. A trolley pole has a shoe at the end with a
groove in it that follows the wire. On the side of Toronto shoe is an
extended, insulated knob, connected by a wire to that button inside
the car. There is a flat metal contactor alongside the trolley wire
that is touched by that knob. That contactor then runs to the track
switch. Therefore, when the motorman wishes to diverge from the main
route, he holds down that NA button. When the knob touches the metal
plate, the circuit is completed, and the switch motor operates.
(Incidentally, almost all trolley lines have a preferred direction.
So, after the diverging car proceeds, we have to reset the switch.
Typically, there is another contact plate alongside the trolley wire
that is always bumped by the shoe. That contactor resets the switch
to its normal setting. Note that this idea would also work with pans;
a separate wire alongside the trolley wire will be activated when the
pan touches both wires.)
More common is “power-on, power-off” switching. In this case, we have
an insulated section of trolley wire that has a relay coil between the
power supply and the wire. When a car comes into that section, the
motorman must choose whether he coasts or takes power. No current
means no activation of the relay. If he takes power, then current
will flow through the relay, and activate the switch.
The limits of this system are twofold:
--Even in the old days, if the car must stop in the section, taking
power will throw the switch regardless of the desired direction. Then
the motorman must get out and manually throw the switch the correct
way.
--Secondly, when the cars were being heated in winter, the heater draw
might throw the switch; this was minimized by requiring a certain
current level to actuate the relay. Today with air conditioning and
other loads, this can be a real problem.
Some systems also had a button for “power-on, power-off” switches.
Here it simply placed a load through a resistor so the motorman did
not have to take power. This made for a smoother trip for the
passengers.
In either case, “power-on, power-off” switches work just as well for
pans.
Today “garage-door-openers” are sometimes used. A little radio
transmitter sends a signal to a box that actuates the switch. By
putting the signal on the trolley wire with low power, you can use the
same frequency on every car without accidentally activating the wrong
switch.
You did not ask about trolley coaches (those rubber-tired electric
busses). They need switches in the wire, because the shoe must
swivel. When the bus starts to make a turn, one pole will be sitting
in forward of the other (the wire is running straight, the bus is
turning). Here two contactors are placed in an offset manner. Only
when BOTH contactors are touched at the same time does the trolley
wire switch actuate. |
