Shunt-wound DC generatorDC Voltage Regulator UNIT 12 AC POWER GENERATION
AIM OF THE UNIT:- to understand ac power generation
TASKS 1 Do your best to answer the brainstorming question. 2 Read the text for general understanding. 3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian equivalents of the words. 5 Write sentences with the new vocabulary. 6 Make up exercises as in the UNIT 2 (Master`s degree students individual work with the teacher) page 210 (exercises for better remembering the topic). 7 Speak on the topic. Given schemes will help you to remember and understand the topic. 8 Find more information about the topic and make up a project work on the topic. An AC system uses a generator to generate a sine wave of a given voltage and, in most cases, of a constant frequency. The construction of the alternator is simpler than that of the DC generator in that no commutator is required. Early AC generators used slip rings to pass current to/from the rotor windings; however these suffered from abrasion and pitting, especially when passing high currents at altitude.. This AC generator may be regarded as several machines sharing the same shaft. From left to right as viewed on the diagram they comprise: Ø A Permanent Magnet Generator (PMG) Ø Exciter Generator-An excitation stator and an excitation rotor containing rotating diodes Ø Main Generator -A Power rotor encompassed by a power stator Main Generator The flow of power through this generator is highlighted by the dashed line. The PMG generates ‘raw’ (variable frequency, variable voltage) power sensed by the control and regulation section that is part of the generator controller. This modulates the flow of DC current into the excitation stator windings and therefore controls the voltage generated by the excitation rotor. The rotation of the excitation rotor within the field produced by the excitation stator windings is rectified by means of diodes contained within the rotor and supplies a regulated and controlled DC voltage to excite the power rotor windings. The rotating field generated by the power rotor induces an AC voltage in the power stator that may be protected and supplied to the aircraft systems. Most AC systems used on aircraft use a three-phase system, that is the alternator generates three sine waves; each phase positioned 120 degrees out of phase with the others. These phases are most often connected in a star configuration with one end of each of the phases connected to a neutral point. In this layout the phase voltage of a standard aircraft system is 115 VAC, whereas the line voltage measured between lines is 200 VAC. The standard for aircraft frequency-controlled systems is 400 Hz. The descriptions given above outline the two primary methods of power generation used on aircraft for many years. The main advantage of AC power is that it operates at a higher voltage; 115 VAC rather than 28 VDC for the DC system. The use of a higher voltage is not an advantage in itself; in fact higher voltages require better standards of insulation. It is in the transmission of power that the advantage of higher voltage is most apparent. For a given amount of power transmission, a higher voltage relates to an equivalent lower current. The lower the current the lower are losses such as voltage drops (proportional to current) and power losses (proportional to current squared). Also as current conductors are generally heavy is can be seen that the reduction in current also saves weight; a very important consideration for aircraft systems.
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