There are two basic types of electric current, direct
current (DC) and alternating current (AC). A century
ago alternating current beat out direct current for
large scale power systems for one overriding reason.
Alternating current can continuously "induce" currents
in adjacent circuits whereas direct current can do
that only when shut off or turned on.
There are consequently two basic types of electric
transit systems, those based on each type of current.
Now that CTA needs some $7 billion to rebuild its
direct current rapid transit system (per 1999 budget,
allowing $1 billion for a whole new bus operation),
let's take a look at alternating current and what it
can "induce" in public tranportation technology.
The first type of current is direct current, which
flows in one direction only. It was discovered about
two centuries ago in chemical experiments that led to
crude batteries. It has pressure, measured in volts,
and volume, measured in amperes. (Volta and Ampere
were early electric experimenters) Volts times amperes
(or amps) result in watts, the unit of electric power,
named for the developer of the first efficient steam
engine. 746 watts equals one horsepower; 1000 watts or
a kilowatt is 1 1/3 horsepower.
It was discovered in the 1820s that flowing current
will deflect a compass needle. In other words, it will
develop a magnetic field. Also, that reversing the
flow will reverse the field, in other words, transpose
its north and south poles. Likewise, that coiling the
wire will concentrate the magnetic field and wrapping
the wire around an iron bar will concentrate it
further. In the 1830s experimenters discovered that
moving a wire through a magnetic field will generate a
current. They also discovered that a current flowing
through one coiled, insulated wire, when turned on or
off, will induce a current in a nearby but unconnected
coil. A steady current, however, would not induce
another current.
The second type of current is alternating current. It
too has volts and amperes, but constantly rising and
falling as the current continuously reverses
direction. Common house current today reverses
direction 60 times a second, or 60 cycles per second,
or 60 hertz, after a mid-ninteenth century
experimenter. Experimenters quickly discovered that
moving the wire the other way through the field will
reverse the current. Rotating the wire or coil of wire
in a magnetic field causes the direction of the
current to reverse continuously, as the wire cuts the
field one way and then the other, and as metal rings
conduct the current to stationary wires through
sliding contacts. Except for primitive arc lighting
(prior to the light bulb, which generated light with
sparks not unlike arc welding today), alternating
current was pretty much a nuisance in its early years.
To produce direct current, the early rotating coils
needed a wiring arrangement with sliding contacts and
rings divided into segments, known as a commutator, to
reverse the electric contacts as the coils rotated.
With a commutator and mechanical power to rotate the
coils, the rotating coils would generate direct
current. On the flip side, feeding electric power into
those same coils with a commutator would cause them to
rotate and generate mechanical power. Properly
designed, the rotating coils with a commutator could
take mechanical or electric power and convert it into
the other. The same piece of machinery could be a
generator or a motor.
While steady direct current would not induce currents
in neighboring coils, alternating current, however,
constantly rising, falling and reversing, would create
fluctuating magnetic fields that would. Furthermore,
if the secondary coil had more turns than the primary
coil, the voltage would rise in proportion and the
amperage would likewise decrease. If the secondary
coil had fewer turns, the voltage would decrease and
the amperage proportionally increase. The power, volts
times amps, would remain the same, except for losses
to electrical resistance.
In other words, alternating current could be
transformed from low voltage to high and back to low.
By the 1880s practical transformers were available to
the fledgling electric power industry. The high
voltages allow alternating current to be transmitted
over long distances with small losses. The low
voltages allow it to be used safely, except there was
no practial alternating current motor.
It was about this time that the brilliant
mathemitician, engineer and inventor Nikola Tesla took
a distaste to sparking commutators with their sliding
contacts. By the late 1880s he had developed a motor
in which the stationary coils induced currents in the
rotating coils, quite without electrical contact by
moving parts and quite without sparking and its
problems. The induced currents would then create
magnetic fields of their own which opposed those of
the stationary fields and caused their coils to
rotate. Early alternating current motors would run
efficiently at only one speed, however, which was
quite acceptable in many factory applications. Thus
factory machines could be powered independently
instead of linked to a central steam engine with belts
and jackshafts.
Thomas Edison was not interested in Tesla's induction
motor with its complicated mathematics. Tesla found
backing nevertheless from George Westinghouse,
inventor of the railroad air brake. Thus was set the
titanic battle in the early electric power industy,
Edison with his direct current system versus Tesla and
Westinghouse with their alternating current system.
Edison argued safety, that high alternating currents
were dangerous. His advocates staged numerous
electrocutions of dogs, cats and horses with
alternating current. One even got a law passed by the
New York legislature to replace hanging with the
electric chair, by alternating current, of course.
"Westinghoused" was the term Edison tried to introduce
for electrocution. The first electrocution, however,
of a poor wretch named Thomas Kemmler, was a badly
botched affair that generated a lot of bad publicity.
For safe use in ordinary life, however, 250 volts is
about the maximum allowable. Common house current is
110 volts. Since direct current cannot be transformed,
it had to be generated at the same voltage it was
used. Since low voltage current loses power rapidly
over distance, direct current had to be used close to
its generator. Thus it needed many small generators
instead of one large, much less expensive central
generator. By the early twentieth century direct
current lost to alternating current for the simple
reason that alternating current can be transformed and
thus used at great distances from the generator. All
the brilliance, renown and prestige of Thomas Edison
could not change this simple proposition.
While large electric locomotives generally used
alternating current motors in the early twentieth
century, trolley cars, rapid transit cars, and trolley
buses used direct current motors, sparking commutators
notwithstanding, because direct current motors can be
more easily controlled over wide speed ranges. All the
brilliance, renown and prestige of Nikola Tesla and
George Westinghouse could not change this simple
proposition, even if most power plants generated
alternating current. Direct current transit was the
better mousetrap of its day, better enough that it
rapidly replaced cable cars, a technology only two
decades old. In the early years a large alternating
current motor had to turn a direct current generator,
but practical rectifiers were developed later on,
one-way valves, of sorts, for electricity. Electronic
controls of recent decades allow much wider use of
alternating current motors.
Any rotating motor, of course, has to power the
vehicle through wheels and usually gears of some sort.
The wheels need traction with the road or rail and
complex suspension and braking systems. The mechanical
parts need oiling that picks up dust and creates
sludge. Rail cars need heavy, complex assemblies
called "trucks" (here) or "bogies" (England) to
arrange the wheels, brakes and suspension system, and
motors too in electric vehicles. The motors are under
the car, close to the rail, exposed to dust and snow,
etc. Electric rail cars are about three times as
expensive as buses of the same passenger capacity.
As induction motors developed, simple pieces of metal
replaced expensive rotating wire coils, especially in
low power applications. This we see today in the
common clock or squirrel cage motors. Stationary coils
are still needed to induce currents in the simple
piece of metal, but, once induced, those swirling
"eddy" currents create their own opposing magnetic
fields and cause the simple piece of metal to rotate.
That simple piece of metal does not have to be
magnetic, such as iron or steel. Any good conductor
will do, such as copper or aluminum.
If the fluctuating magnetic fields of alternating
current can make a simple piece of metal go around and
around, they can also make simple pieces of metal go
in a straight line. Thus the linear induction motor,
which is commonly used in industry to move metals,
from powders to ingots, in a straight line without
intervening machinery or even sparking commutators.
Indeed, since different metals form different eddy
currents, linear induction motors can separate powders
of different metals.
Again, on the flip side, it dawned upon some
ingenious souls that maybe the linear induction motor
could move with its vehicle, pushing against a
stationary "reaction rail" with direct magnetic
propulsion. Reversing the motor would brake the
vehicle. Thus expensive, troublesome mechanical
propulsion and braking could be eliminated and it did
not make any difference how slippery it got outside.
No friction between wheel and road or rail is
necessary.
It also dawned upon some ingenious souls that the
linear induction motor exerted magnetic forces in two
directions. It exerted not only a force along the line
of travel, but about ten times as much force
perpendicular to the line of travel, if the "reaction
rail" were steel instead of copper or aluminum.
(Standing passengers can only accelerate about
one-tenth gravity anyway.)
In other words, the linear induction motor could
suspend the vehicle, while also propelling it. It is a
"levitation machine." All those expensive, troublesome
wheels and suspension systems could be eliminated too.
It was not until recent decades, again, that
electronic controls became sophisticated enough to
fully exploit these possibilities.
To suspend the vehicle, the linear induction motor
has to be under the reaction rail, in this case a
steel beam. It only exerts attractive force toward the
steel beam, not the repulsive forces in some proposed
"maglev" schemes. Not that this creates any problems,
but rather solves quite a few.
Take this one step further and put the car under the
motor. The steel beam thus has to be elevated over the
car, and while we are elevating it, we might as well
make it high enough to clear traffic. Thus, instead of
a noisy, bulky, light-blocking, complicated, expensive
railroad structure, we only need a simple, standard
steel beam supported by heavy duty light poles. It
blocks very little light, makes almost no noise, and
costs about one-tenth as much as standard elevated
structure. While a linear induction motor with its
sophisticated controls is rather an expensive piece of
equipment, still a car for such a system costs only
about as much as a bus of similar capacity, again,
about one-third as much as a rail car.
Alternating current can even eliminate sliding,
sparking electric contacts. The electrical pick-up
need only be a transformer, with a moving coil on the
car and stationary wires. Attached to the beam with
small brackets, it does not need a heavy, exposed,
notoriously dangerous third rail or the expensive,
complicated bracing that trolley wires do. Without
wheels and a bouncy, springy suspension system, the
two parts are close enough for efficient operation.
The small amount of heat generated will melt ice, and
water will not disrupt electrical contact. The power
wires are well removed from any possible passenger
contact in the first place and remain insulated in the
second. It would take a determined vandal with cutting
tools to get himself electrocuted by this system.
Edison's argument of safety is completely turned
around by alternating current transit.
In the 1860s James Clerk Maxwell wrote four famous
equations which still form the basis of electronics.
He reasoned that if changing magnetic fields induce
changing currents which in turn induce more changing
fields, then waves will be created that will propagate
with the speed of light. Maxwell's predictions were
confirmed by Hertz, for whom the frequencies of
changing currents or waves are named.
Alternating current still forms the basis of
electronics. Instead of simple cycles per second,
however, AM radio is based on kilohertz frequencies,
or thousands of cycles per second, FM and TV on
megahertz, or millions of cycles per second, and
microwaves on gigahertz, or billions of cycles per
second. Microwaves can be projected down a metal tube
called a waveguide, perhaps one with a slot for a
moving antenna. Thus control signals can be easily and
reliably sent and recieved.
In a moving vehicle with a bouncy mechanical
suspension such a system would not be practical due to
close clearances needed and conflicts with other
functions. On an elevated steel beam with a
magnetically suspended vehicle, it is not only quite
possible but practical. Thus the system can be easily
automated, much more so than one with wheeled
vehicles.
A century after alternating current won the battle
for major power systems, direct current still
dominates transit systems, even when they require
billions for rebuilding. It dominates transit systems
so completely that precious little mention is made of
alternating current's advantages. There is no George
Westinghouse with the funding to build a better
mousetrap. The federal capital funding that commonly
funds public transportation is generally the captive
of pork barrel politics. We joke today about the
patent clerk a century ago who quit because he thought
everything had been invented, but that is the attitude
today, in the transit bureaucracy and among the
traveling public both.
P.S. The current CTA president, Frank Kreusi, is a
great-grandson of John Kreusi, on of Edison's top
assistants. When shown a diagram of alternating
current transit at the 1998 CTA budget hearing and
reminded of the AC-DC war of a century ago, he said
his grandfather, Frank Kreusi, was instrumental in
setting up the AC network. He was not particularly
interested in AC transit, however.