AC vs DC Power ǀ Understanding Electric Current
This article is going discuss electricity and how it works. So, when we talk about AC vs DC, we’re not referring to a 1980’s rock band. Electric power comes in two forms, Alternating Current (AC) and Direct Current (DC). I’ll be explaining the difference between AC and DC power, how it is generated, and why we use these different forms of electricity.
In our daily lives we use AC and DC power all the time in many different ways. Generally, we see AC power as the electricity in our homes, the electric sockets that provide 120V or 240V power for appliances. DC power is used for battery-powered equipment – this could be the starter motor for a car, a cell phone, or power tools and cordless appliances. People seldom understand how or why we use these different types of current. Why do we use AC power in our homes? Why is battery power DC? These questions may be far more relevant than you think. If you’re considering whether to buy a battery-powered, cordless, power tool or a conventional corded tool, the debate over AC vs DC power is important. Having the facts to make an informed decision is always a huge benefit.
Direct Current (DC) was discovered first and was popularized by Thomas Edison when he invented the first DC electric light. Later, Nicolas Tesla discovered Alternating Current and invented the AC electric motor. This sparked a debate as to which is better, AC or DC? There can be no correct answer to this question, as it depends on the application.
So, to answer the many questions we have about AC vs DC power, let’s start from the top. What is AC power? What is DC power? How is this electricity generated? Once we understand the basics, we can move to the applications and when one type may be better than the other.
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A DC generator is relatively simple, which is probably why this was the first type of electricity. The name direct current is an apt description of how this form of power works. Electrons flow in a direct, linear path from a point of high charge to a point of low charge. We call this a positive pole and a negative pole.
DC power is the most efficient, but experiences significant power loss through conductive resistance. This means that the energy contained within a volt of DC power will provide more energy, but it loses power when conducted over a long distance. A DC electric motor is more efficient than an AC motor, as long as the motor is close to the power source (the battery or generator).
The greatest benefit to using DC power is that it can be stored in a battery. This is why all portable devices, that are not connected to an electric socket, use DC power.
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How does a DC generator work?
To understand any type mechanical electric generator, we need to have a basic understanding of Faraday’s laws of electromagnetic induction. This may sound way to scientific for most folks, but don’t worry, I’ll keep it simple. If a conductor is placed in a varying magnetic field, an electromotive force (EMF) is induced within that conductor. Simply put, if a conductive metal (like copper wire) is place between two magnets, and these magnets continuously change polarity form North to South, a charge will develop in that conductor.
A simple way of creating EMF induction is to rotate a conductor between two magnets, one with a north polarity facing the conductor and one with the south polarity facing the conductor. This is the basic principle behind a DC generator. A rotating shaft, called a rotor, has copper wire coiled around it. The rotor uses mechanical energy, like a gas engine, to turn it. On either side of the rotor are static magnets, called the stator. As the rotor spins, magnetic forces cause induction in the rotor which energizes the electrons to create electricity.
The electric charge in rotor is transferred to an external conductor using brushes that make constant contact. The brushes are connected to a power outlet. To maintain an even voltage, the current is usually transferred through a voltage regulator before supplying the outlet.
How does a battery work?
Batteries store DC power by means of a chemical reaction. There are three components to a battery, the Cathode, Anode, and electrolyte. Electrolytes are chemical elements that resist and electric charge. The cathode is the positive terminal (+) and the anode the negative terminal (-) of the battery. These are the terminals to which we connect the conductor. Between the cathode and anode, an electrolyte is used to concentrate the electric energy at the anode. This creates a potential difference (voltage). The anode is energized, and the electrons want to move to point of low energy (the cathode), in order to achieve equilibrium.
Because the electrolyte prevents the electric charge from moving directly from the anode to cathode internally. This current can only move when a conductor connects the anode to the cathode. In other words, connecting a conductor to the positive and negative terminals of the battery with enough resistance so as to not cause a short circuit.
Resistance allows the electric energy to be transferred and all electric equipment use some form of resistance to convert the electric energy into motion, heat, or light. Electronic devices control the resistance and manipulate the current, using semi-conductors. Without any resistance, the energy has to go somewhere, and this results in excessive heat being produced within the conductor. At some point, this will cause the conductor to melt and is extremely dangerous.
Although we usually call a device that generates an AC current a generator, the electricity is actually produced by an alternator. The term alternating current is used to describe the oscillating flow of electric energy. Unlike DC power that follows a direct path from negative to positive, AC power continuously changes direction, moving back and forth between a point live charge to a point of neutral charge.
Alternating current pulses at a controlled rate or frequency (Hz). So when we talk about 60Hz AC power, this an alternating current that moves between live and neutral 60 times per second. The oscillating movement of an AC current is measured in a wave form, much like sound. Conventional AC power, like the grid supply to our homes, uses a sine wave. This is a smooth oscillating curve with a height depicting the amplitude of the wave and the length represents time. So, the height of a single wave indicates the amount of power it produces and the width, the time it takes for the current to oscillate (frequency).
AC power can also be in the form of a square wave or a triangular wave. These are unnatural sine waves that have been electronically manipulated. The curved sine wave is known as a true or pure sine wave.
How does an alternator work?
An alternator works on the same basic principle as a generator, using Faraday’s principle of EMF. The only real difference between an alternator and a generator is how the transfer of the magnetic field is achieved.
A generator transfers magnetic energy from the stator to the rotor, the electricity is generated in the rotor. An alternator does the opposite, the magnetic field is created in the rotor and the energy is transferred to the stators. More than one stator can be used, creating phases. A single stator will generate single phase 120V AC power, a second stator produces 240V 2-phase AC power, and a third stator will generate 3-phase 360V AC power. This allows us to use different voltages, depending how we connect our conductors to the stator circuits.
The power will continuously alternate between the stator (or stators) and the grounded casing of the alternator. Ground is a state of neutral charge and is connected to the neutral wire. The stator is in a charged state and is connected to the hot wire or wires for more than phase.
An alternator is able to conduct the power more effectively as the load is not transferred through brushes from a moving rotor, but rather a stationary stator. This means a fixed connection directly to the stator. Brushes experience friction, generating heat and the load places extra force on the rotor bearings. For these reasons, a generator experiences a loss of energy transfer and requires more maintenance.
Because an alternator is more efficient and reliable, modern cars no longer use a generator to provide the DC electricity. Instead, an alternator produces an AC current that is converted into DC using a rectifier. If we want to do the opposite, convert DC current into AC, we use an inverter. Inverters are used when a DC battery is used to supply AC power. Inverters are also used to provide a more stable sine wave for portable generators. Because inverters are able to electronically monitor the output current, the AC current produced by an alternator is converted into DC, using a rectifier. The inverter then converts the power back into AC current in a process known as double conversion. The inverter uses a microprocessor to measure the voltage and frequency. Any changes are corrected using transistors and capacitors, maintaining an almost perfect sine wave. This eliminates the possibility of Harmonic Distortion.
What is Harmonic Distortion?
Harmonic Distortion (HD) is an irregular sine wave. Ideally, an AC sine wave has to be uniform, each wave has an identical width and amplitude to the preceding wave. Factors, like increased load or non-linier load, will cause the wave to change. If the voltage or frequency changes, the wave will become distorted. We measure Total Harmonic Distortion (THD) by using the first sine wave as a base point. Each subsequent sine wave is measured against this initial wave. Any changes in amplitude and width are compared and represented as a percentage.
You will often see inverters and portable generator specifications for THD. Generally, anything below 5% THD is deemed safe or “clean” power. High THD can cause electronic equipment to behave erratically. This is often observed as lines moving across a TV screen or computer monitor, the image may also be distorted. Stereos can produce a distorted sound when the THD is excessive.
Along with the noticeable effects of THD, it also generates excessive heat in electronic circuits and electric motors. Over time, this heat may cause damage to electric equipment, or complete failure.
There are pros and cons to AC vs DC power. For bulk electricity supply, over long distance, AC power has been selected as the preferred method. It is relatively easy to increase and decrease AC voltage. A high voltage reduces the amperage, thereby reducing heat and energy loss. So high-power, long distance power lines use a high-volt AC current to eliminate power loss. The voltage is then reduced at the point of use, using an AC transformer.
Because DC electricity can be stored in a battery, it is mostly used for portable cordless equipment. Even though DC power is actually more efficient, AC power is more commonly used. This is simply because it is more readily available through mass grid power supplies.