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Transcript
Electrical System Instruments and components Alternating Current An alternating current (AC) is an electric current whose direction reverses cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of an AC power circuit is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves. Phase • Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation: where Vpeak is the peak voltage (unit: volt), ω is the angular frequency (unit: radians per second) – The angular frequency is related to the physical frequency, f, which represents the number of oscillations per second (unit = hertz), by the equation . t is the time (unit: second). Phase A sine wave, over one cycle (360°). The dashed line represents the root mean square (RMS) value at about 0.707 3-Phase One voltage cycle of a 3-phase electrical system, labeled 0 to 360° along the time axis. The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle will repeat 50 or 60 times per second, depending on the power system frequency. Transformers A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing magnetic field. This changing magnetic field induces a changing voltage in the second circuit (the secondary). This effect is called mutual induction. http://en.wikipedia.org/wiki/Transformer 2-phase transformer Oil immersed transformer Distribution Domestic distribution Three-phase distribution Question • Show by sample calculation why smaller cables can be used for long-distance power transmission when very high voltages are used สถาปนิกควรคานึง • Transformer area แรงสู ง • Distance from main line สายส่ ง • MDB Room ไม่ควรอยูใ่ ต้ดิน แรงต่า มี meter, main switch, Zone’s DB 0.6*0.9*2.00 m • LC, PB ,มี Shaft • Floor to floor (height), Plenum space, Attic
