Electric
and magnetic phenomena have been observed for well over 2000 years. In ancient
Greece, it was discovered that rubbing cat fur on amber would allow someone to
pick up feathers without actually contacting them with the piece of amber. As
well, a mineral was discovered in an area of Greece called Magnesia that
contains Fe3O4, magnetite—a
naturally occurring magnetic material. The object of much of our fascination
and discovery for the past few centuries especially, what is now known as electromagnetism has become the
cornerstone of modern civilization. Nearly every facet of our lives is now
dependent upon the perpetual production and transfer of electrical power. TVs,
radios, computers, traffic lights, toasters, microwaves, and all of the other
conveniences we effortlessly plug into an outlet to use function according to a
myriad of insights gained through experimentation conducted mostly in the past
four centuries. Fermat, Franklin, Watson, Lagrange, Cavendish, Coulomb, Volta,
Biot & Savart, Faraday, Gauss, Oersted, Ampere, Maxwell, Tesla, and many
others made incredibly impactful contributions to humankind’s understanding of
this most intriguing phenomenon. However, credit for popularizing induction
motors and the long-distance transfer of power goes to Nikola Tesla.
Nikola Tesla was born in 1856 in
Smiljan, Croatia. After studying physics and math at the Technical University
of Graz and philosophy at the University of Prague, Tesla had a vision in 1882
for a new, brushless design for an AC (alternating current) motor. Over the
course of the next few decades, that vision essentially developed the precursor
to our modern electrical grid. In fact, in 1895—the same year his lab in New
York burnt down—with the financial backing of George Westinghouse, Tesla and
General Electric installed AC generators at Niagara Falls. After many years of
debating Thomas Edison, who championed DC (direct current) transmission as the future, it became apparent that
alternating current was the superior technology for long-distance transmission.
So, in honor of Nikola Tesla, this paper will contrast DC generators with AC
generators. Power factor calculation will also be highlighted.
Electric generators and motors
operate according to principles outlined in Faraday’s Law. Faraday’s Law
summarizes the ways in which an electromotive force (emf) may be generated by
changes in a magnetic environment. That is, Faraday’s Law basically states that
for a coil of wire, any changes in the magnetic environment will induce a
voltage (emf) in that coil.
Essentially, the first form is
called the differential form while
the second is called the integral form.
Faraday’s Law is simplified for understanding generators and motors with
multiple loops by the following equation, where N is the number of turns:
This particular statement of
Faraday’s Law highlights a direct proportionality between the number of turns
of the coil and the magnitude of the induced emf; i.e., the more turns there are in the coil, the higher the
induced emf. There are limitations,
however; some of which will be discussed below. So, what is a generator?
A generator is simply a device that
converts mechanical energy into electrical energy. It consists of a ferrous
frame, called the stator, pole
pieces, wound with wire in opposite directions, and the armature, also known as
the core, which is the rotating part. The armature is made of many thin strips
of an iron-silicon based material, called laminations, that are typically laser
cut or die punched, then stacked together and fixed by a shaft through the
center of the stack. Around the ridges of the laminations, copper wire is wound
in specific patterns a specific number of times. Essentially, the number
depends on the permeability of the stator, i.e. that is, the ease with which a
ferrous metal can be magnetized, as well as the saturation point of the pole
pieces, i.e. the maximum flux density of the pole pieces. At one end of the
armature there is attached to the shaft a piece called the commutator. The
purpose of the commutator is to provide a path to extract the induced voltage
as the armature rotates through the magnetic field. It is made by attaching
evenly alternating conductors and insulators to the shaft and connecting one
end of one of the windings to one of the conductors and connecting the other
end of that same winding to the conductor on the opposite side of the shaft.
This process is repeated until all of the opposite conductors are attached in
the same manner to a single winding.
The pole pieces are also made of
high permeability ferrous material and are wound many times with thin copper
wire. This winding is called the shunt field and produces the main magnetic
field through which the armature spins. If self-excited,
the pole windings are wired in parallel with the carbon-based brushes that
contact the commutator which means that the output will vary greatly with
varying speeds as well as changing load. (The brushes are made of carbon
because it is very slick and resistance decreases as temperature increases.) If
separately-excited, the shunt field
is instead connected to a field supply with a field adjust (typically a
variable resistor) and provides much better control. In essence, the strength
of the shunt field depends upon something called amp-turns. Amp-turns refer to the amount of current put through a
conductor coiled a number of times. Low currents produce low flux density while
high currents produce high flux density. Saturation occurs as the output
voltage tapers off when graphed with field amps on the x-axis and output
voltage on the y-axis. The relationship is linear at first but tapers off as
the pole pieces reach a point where the inherent molecular properties of the
material, along with their limited spatial dimensions, prohibit any more field
lines, and thus flux, from being produced. As loads are added to the generator,
this flux tends to become distorted. This is called armature reaction and is remedied by adding smaller interpoles
between the main poles that are wired in series with the armature circuit.
Now that a basic understanding of
how a DC generator operates has been established, AC generators will now be
examined. The main difference between AC and DC generators is that instead of
implementing the split ring design of the commutator, as
outlined above, AC generators are wired with each end of a loop connected to a separate
slip ring. It is across and because of the separation of these slip rings that
the sinusoidal, alternating voltage, and thus alternating current, are
extracted using carbon-based brushes. Another crucial difference is that as you
add more loops in an AC generator, they must be separated by particular angles.
For example, in a typical single phase generator, there is a single sinusoidal voltage
and current, i.e. positive to negative to positive to negative and so on, because
there is a single coil of wire. Two phase generators have two coils that are
perpendicular to each other which means two sinusoidal voltages/currents will
be 90 degrees out of phase when graphed. Although many more
types of polyphase generators exist, such as six phase, perhaps the most common
are three phase generators.
Three phase generators have a high
efficiency in changing mechanical energy into electrical energy due to a
continuous field flux. This is because of the Y-configuration of the coils in
the generator. These coils are 120 degrees apart from each other. Each sinusoidal wave
will be
120 degrees out of phase and generated at a rate of 60
Hz. As a result, the voltage is never 0. Hence, current is never 0. Therefore,
there is relatively little energy lost through the process of converting the
mechanical energy to electrical energy. As long as the load is balanced, each
phase will deliver the same power. With the Y-configuration, if a voltmeter is
placed across any phase to neutral, it will read 120V.
Once the alternating voltage is
produced with the AC generator, transformers are used to step up and step down the
voltage for long distance transmission. Sending large currents over long
distance is highly impractical because of the size of wire required. By
stepping up the voltage, current is reduced in order to maintain the same power
consequently requiring smaller gauge wire:
P = IV
Typically, from the power station,
the voltage is stepped up to around 345,000 V; sometimes to over 700,000 V.
Once reaching the substation near the destination city or town, the voltage is
stepped down to around 7,200 V and sent on to houses and businesses. Still
other transformers, the ones at the top of a lot of the poles along almost
every street around, step the voltage down once more to, 240 V, 120 V, or some
combination of both, and then connect to residences and commercial buildings.
Alas! A switch is flipped and lights come on. Well, it’s not quite that simple!
When power companies transmit energy
over long distances, there are many varying loads along the way which, as
alluded to above, will affect the output voltage of the generators. So, how do they maintain proper voltage
output and thus keep the lights on without noticeably flickering? Well, the
voltage output from the power company is essentially constant, so it’s really
up to the customers drawing large amounts of power, such as commercial
facilities that have many electric motors running to not waste power. The short
answer is that phasors are used. No,
not like the ones from Star Trek; those are phasers.
Phasor diagrams are graphical representations of impedance caused by capacitors and inductors in a circuit.
Impedance, Z, is anything that resists or impedes
current.
If a graph is made with the x-axis
labeled real resistance and the
y-axis labeled as an imaginary axis, +j and –j, the above equation will become
much clearer. Essentially, this equation is saying that impedance is equal to
the algebraic sum of the resistance, R, the
capacitive reactance, XC,
and the inductive reactance, XL.
Inductive reactance is situated in the first quadrant while capacitive
reactance is in the fourth quadrant. There is an interesting interplay between
all of these quantities that ultimately are calculated as real or true power, i.e.
the power across the resistor that is the only kind doing work, apparent power, i.e. the power across
the inductor or inductors, and reactive
power, i.e. the power across a capacitor bank. It is on the careful,
dynamic balance of these three quantities that the companies rely to maximize
the real power extracted. The goal is
to maintain a power factor at or at least close to 1. In other words, as loads
change in the facility, capacitor banks are used to balance the reactive and apparent power along the real
power line. Since resistive power is the only power doing work, it is the
only type that is useful; the others are basically a waste. Therefore, it is
important for high energy consumers to constantly monitor the changes in
capacitive and inductive reactance in order to reduce waste of the transmitted
power; something that would surely make Tesla proud.
Nikola Tesla lived to see a large
portion of his work implemented in society. But, he didn’t do any of it for the
money. In fact, he died alone and broke in his suite at the New Yorker Hotel in
January of 1943. Instead, Nikola Tesla wanted to power the world for free. He even designed the system
that could do it. The breadth of his accomplishments stands as a testament to
the power of creative, visionary minds the world over. Tesla’s dedication in
the face of adversity and unparalleled innovative skills have inspired
countless scientists and engineers since his death. And, it is doubtless that
he will continue to inspire generations to come.
In a society that operates almost entirely
dependent upon science and technology, it is up to the scientists and engineers
to recognize the necessity to examine other methods of social organization and
not just focus on churning out more and more advanced technology that fewer and
fewer people understand. Why should the idea of allowing those in society that
have no clue how a certain piece of technology even works and that typically
only have financial motivations dictate the development of such technology? It
seems inexcusable to have virtually no one in the scientific community step up
and ask the question, “Do we have the resources
and the technical know-how?”, instead of, “Do we have the money?” And, it’s a bit strange that a lot of
scientists seem to have no problem at all working on weapons of death and
destruction. For humans, it’s totally
reasonable to, as if by some banking miracle, find the money to bring
together 130,000 scientists to develop one of the most unimaginably destructive
devices known. But, try to find the money to bring together 130,000 scientists
to develop a globally synergistic production and distribution system to provide
the basic necessities for every human on the planet and prepare to revel in the
glory of being called a socialist, a quack, an idealist, a utopian, or any
number of other disparaging, rather unscientific pejoratives. What an absolute
travesty! That mindset flies in the face of Tesla and the others that have contributed
massively to scientific revolutions in our society by stepping outside of the
boundaries of the norm. It turns out
that just because a lot of people agree that a certain idea is infallible does
not automatically make that idea correct or accurate. In the words of
industrial designer and social engineer, Jacque Fresco, “Everyone once believed
Earth was flat, but that didn’t make it
so.”
No comments:
Post a Comment