Sources of Power
600-750 volts DC
Trolleys traditionally operate on 600 volts direct current,
drawn from an overhead wire (by means of a trolley pole) and returned through
the rail (by the wheels). Modern light rail lines typically use somewhat higher
voltage (750 or 1000 volts) to power the higher performance and heavier light
rail equipment. Thus if a streetcar operation plans to use the track of a
light rail system, and if historic cars or replica cars with traditional
equipment are to be used, then the cars may have to be modified to operate at
this higher voltage. Dedicated heritage trolley lines can simply be built to use
600 volt power. Modern streetcar lines are typically built to use 750 volts.
As the power to operate all the cars on a line must pass
through a single small trolley wire, the capacity of that wire will be exceeded
if many cars operate simultaneously. Thus trolley power systems are normally
broken into separate "sections," each fed by a different generating source, and
high capacity feeder cables are strung along the line (on poles or below ground)
and connected to the trolley wire at intervals to increase capacity.
Unfortunately, utility companies are seldom able to supply
600 or 750 volt DC power, as virtually all utility customers today use alternating
current (AC) at higher or lower voltages. Thus streetcar operators need to
provide means of adjusting commercial AC voltage and converting it to direct
current. Several options exist for obtaining the needed DC power.
During the streetcar era, the means for producing direct current from
commercial AC electricity was by use of a motor generator set. Such a set
consists of a large AC electric motor connected, either by the armature shaft or
by gears, to a rotary generator that creates the DC power. Second hand generator
sets of this type may be available at low costs (some trolley museums use such
equipment), but operating and maintenance costs for these units is greater than
for the more modern equipment described below.
Solid state rectifier sets
electronic equipment can perform the same function with no moving parts. These
units are readily available and physically small (a cube 6 feet per side or
smaller) and can be placed indoors or outdoor easily. Most modern streetcar and heritage systems use
Alternate sources of power
The needed DC current can also be generated
from other sources of energy such a petroleum based fuel, water power, solar
power, or wind power. Some heritage operators (such as Charlotte) have used
small, mobile generators towed or pushed by a car until trolley wire and a
central generating plant can be installed. Though a practical interim solution,
this approach means each car operating simultaneously must have a generator and
it also detracts from the authentic historical ambience of the trolley.
Streetcars without Ovehead Wire
As described above, streetcars traditionally have drawn power from overhead wire. However, there have been a few examples of independently powered streetcars and there are new developments in delivering electricity to cars without overhead wire:
- In Galveston, Texas the replica streetcars are powered by on-board diesel engines which generate electricity to power the car.
- In Savannah, Georgia the first replica streetcar uses an innovative on board generator powered by biodiesel fuel which generates electricity for a 600-volt modern propulsion system.
- in Bordeaux, France a system now marketed by Alstom called APS (Alimentation par Sol, French for Ground Contact System) delivers power to streetcars from a central rail which is energized only when the car passes over it. This system is not offered in North America, but has been adopted by several other French cities and a system in Dubai. Alstom also offers a battery system that can propel cars for short distances (such as through a historic square) where installation of overhead wire would be objectionable.
- The Canadian manufacturer Bombardier has developed a system called Primove, which uses induction from coils placed between the rails to deliver power to a car with no contact between the power source and the car. This system has been demonstrated but not yet installed on a streetcar system.
- The Japanese manufacturer Kawasaki has developed a battery system called Swimo designed to power cars through short sections without overhead wire and to allow wider substation spacing. This system (which was presented to the Subcommitee in New Orleans in December 2008) has been demonstrated but not yet installed on a streetcar system.