Why AC, instead of DC?
Being in the middle of a strenuous afternoon, I was wondering how the idea of Alternating Current(AC) came this far after all the years. Why not people use Direct Current(DC) which is more straightforward to understand. The thought brought me all the way here to blog on it and break it down in an easy way for you laymen in Electrical Engineering.
What is AC or Alternating Current?
Alternating Current is a charge flow that alternates its direction periodically. If the flow of electricity is thought of in an analogy with the flow of water, it would suggest us something similar to the following animation.
Alternating current waveforms cross zero marks periodically and need not be a sine function as we all recall when thinking about AC. The following waveforms suggest how voltage/current varies with time in Alternating Current.
Alternating current is what we generate in power stations using the basic principle of moving a current-carrying conductor in a magnetic field. This is also known as an alternator. Keep this fact in mind because it also backs up the reason why we use AC but not DC in powering up households and other end users.
What is DC or Direct Current?
Direct Current is the flow of charges unidirectionally. DC provides a constant value of voltage or current and thus pretty easy to understand.
With the water analogy we took earlier it would be just a one-way water flow from a tank.
The waveform of Direct Current is simple as follows.
DC is produced by batteries, fuel cells, rectifiers, and generators with commutators.
So with the fundamentals lined up let us move on to the question.
Why do we use AC instead of DC to power up our houses?
The answer needs to be backed by how power is transmitted from the point of generation to the point of end-users/consumers.The general equation of electrical power is
P=V⋅I
So to transmit a greater power from the generation plants, the multiplied value of the transmission voltage and the current needs to be high.When transmitting power through the power lines, obviously we encounter some losses due to resistance and other factors too.
Power loss due to resistance of lines is,
Power Loss =I^2.R
So higher the current, the higher the loss since its square is proportional to the loss. So to reduce power losses in transmission we need to have low current values, but still, the generated power should be transmitted, and therefore according to the previous equation, we will have to acquire higher voltage values in transmission.
The households won’t need higher voltages when consuming therefore voltage needs to be stepped down in substations when distributing. The process of stepping up and stepping down voltages is fairly easily done in transformers with the use of Electromagnetism. The governing equations of this process require a change of flux which is only possible to develop using AC. The process of stepping up/down voltage if it’s DC is fairly expensive, complex, and not easily feasible. This is the main reason why we use AC instead of DC for transmitting power.
Brief facts from History! (The Current-War)
In 1886, Ganz Works, an electric company located in Budapest, electrified all of Rome with AC. Thomas Edison, on the other hand, had constructed 121 DC power stations in the United States by 1887. A turning point in the battle came when George Westinghouse, a famous industrialist from Pittsburgh, purchased Nikola Tesla’s patents for AC motors and transmission the next year.
In the late 1800s, DC could not be easily converted to high voltages. As a result, Edison proposed a system of small, local power plants that would power individual neighborhoods or city sections. Power was distributed using three wires from the power plant: +110 volts, 0 volts, and -110 volts. Lights and motors could be connected between either the +110V or 110V socket and 0V (neutral). 110V allowed for some voltage drop between the plant and the load (home, office, etc.).
Even though the voltage drop across the power lines was accounted for, power plants needed to be located within 1 mile of the end-user. This limitation made power distribution in rural areas extremely difficult, if not impossible.
With Tesla’s patents, Westinghouse worked to perfect the AC distribution system. Transformers provided an inexpensive method to step up the voltage of AC to several thousand volts and back down to usable levels. At higher voltages, the same power could be transmitted at a much lower current, which meant less power lost due to resistance in the wires. As a result, large power plants could be located many miles away and service a greater number of people and buildings.
Modern approaches where DC is emerging outcasting the issues.
China’s Three Gorges Dam, DC transmission lines transport power to people with fewer losses of energy than AC, displaying that household use of DC power is becoming more conventional. Engineering company Siemens has even installed a 65-mile high-voltage direct current (HVDC) line that stretches from the Pennsylvania/New Jersey power grid to Long Island. Such projects could cause an unprecedentedly high usage of renewable energy. Still, while these higher DC voltages usually lead to riskier power deliveries and DC grids can be difficult to monitor, high AC voltages can be reduced to a safer level when they’re transmitted from a power station.
So that’s where we wrap up!
Good day fellas!
References
https://www.britannica.com/science/direct-current
https://learn.sparkfun.com/tutorials/alternating-current-ac-vs-direct-current-dc/all
