Sim's Electrical Companion
Direct Current D.C.
As the name suggests direct current is on one direction. As we have mentioned conventional flow the current is said to go from positive to negative. Electron flow goes from negative to positive.
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When graphing a DC supply all of the voltage and current is above the Y 0 axis. As shown:
DC is used for all electronic devices like televisions, radios, games consoles, computers & mobile phones etc. Virtually anything that has circuitry components in it will be powered by direct current.
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We use High and Ultra high voltage DC for transmission of electrical energy across national borders and some countries are using the technology to provide the backbone for the national grid for example china and Japan. Our transmission of renewable energy generation is virtually all transmitted as HVDC.
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Sources of Direct Current
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Thermocouple When heat is applied to a thermocouple direct current is produced by a chemical reaction.
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Battery or Cell Produce a steady flow of direct current while their chemical reaction can be maintained. Common types of cell include lithium ion, Zinc carbon, Alkali Zinc manganese, Lead acid etc. A battery is a collection of two or more cells connected together.
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Photovoltaic Cell Photovoltaic cells use photons of light to move electrons across a semiconducting layer. They are connected together in an array to construct Photovoltaic panels.
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Dynamo
A dynamo is a DC electromagnetic generator. You may remember dynamos as a source f DC power for bicycle lights. The dynamo would be turned by the wheel and produce DC energy for the lights. Historically much larger dynamos were used for street lighting when the technology was in its infancy. The dynamo effect was discovered in 1866 by Werner Von Siemens. An example is pictured below which shows a cut away picture of a wind turbine generator.
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Definitions Associated With Direct Current Circuits:
Anode The positive supply from a DC source. Usually coloured red.
Cathode The negative connection to a DC source. Conventionally
coloured Black.
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Extra low voltage 0V - 120V DC
Low voltage 120 - 1500V DC
Medium Voltage ≈1500 - 50kV
High voltage >50kV - 300kV DC
Ultra high voltage >300kV DC
Alternating Current AC
You may have heard of the war of the currents between Thomas Edison and Nicola Tesla. They battled to prove which technology would be the most appropriate for creating the first electrical distribution systems.
Thomas Edison focused on convincing the public that DC was safer than AC and tried to demonstrate this with a series of macabre public experiments which culminated in electrocuting an elephant. The reality is over 50V AC or DC can kill a person if it takes the right path through the body it can fibrillate the heart and stop it. Nicola Tesla was more sensibly looking at the distances that electricity could be transmitted and came up with a grid system that required far fewer generation stations using the AC method of electrical generation.
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Alternating current is not limited to one direction it effectively flows back and forth. The rate of the alternation is counted in frequency per second and is measured in Hertz. A single complete cycle is illustrated below along with a simple resistive AC circuit diagram:
AC is constantly changing from peak to peak so to have a coherent figure for the value of voltage we use the quadratic mean value. This value is an average of the squares of the sine and presents it as a single value. In electrical science we call this the rms (Root Mean Square) or quadratic mean. UK domestic power is supplied at 50Hz 230V rms. The reality is if this were the true rms the peaks would be 325V to -325V 50 times a second and to read the true changing values on a meter would be mind boggling and not of much use to anyone. The value of rms is 0.7071 x the peak voltage or 1/√2 x peak. The average value is rarely used but is quoted as it is the arithmetic mean if all the values used were considered to be positive. The total potential difference between the two peaks is called peak to peak.
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The main reason we use AC for generation, transmission and distribution is for the most part it is very easy to transform. AC transformers are truly very simple to construct alternating current is not used for most electronic devices but the simplicity of transformation is the main reason for its use in transmission and distribution. DC voltage and current converters are very complex and in the case of high voltage applications massive machines.
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The most common means of generating alternating current is by electro magnetic generators. This involves spinning a rotor in a magnetic field to generate an electromotive force (emf). More detail on this is covered later in this chapter, Fleming's right hand rule for electrical generators and Maxwell's screw rule for current carrying conductors.
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As well as single phase AC another method of transmission is poly-phase the most common is to transmit three alternating currents at a phase shift of 120 degrees. The benefits and uses of three phase alternating current supplies are discussed in more detail later in this section. See Star & Delta three phase supplies and three phase motors.
Six phase systems have been employed for motor drives, generation, transmission and to reduce the time landfills take to bed down.
The potential for disturbances, faults and cost implications of an extra three phases is a vastly limiting factor in its adoption so it is mostly experimental with few practical applications.
