UNIT 5AC GENERATORS
AIMS OF THE UNIT:- to identify alternating current (ac) generators - to describe ac power systems - to have a basicunderstanding of the components of these electrical systems and power distribution systems - to understand the grammar point ‘ The categories of adjectives`
TASKS 1 Do your best to answer the brainstorming questions. 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 Do the given exercisesfor better remembering the tеxt.7 Study grammar point `Category comparison of adjectives ` find sentences with the comparison categories of adjectives in the text. 8 Speak on the topic. Given schemes will help you to remember and understand the topic. 9 Find more information about the topic. Do some research, create slideshow or a project work and present them.
1 What is AC? 2 What is the function of AC? 3 What is the difference of ac and dc generators?
Alternating current (ac) generators supply theelectrical energy for operating aircraft avionicsequipment. A generator is a machine that convertsmechanical energy into electrical energy by electromagneticinduction. Ac power systems result in better design and use ofequipment than older electronic equipment powered bydirect current (dc), which have inverters for ac powerand dynamotors for supplying higher voltage dc power.These components are very heavy compared to theirrelative power outputs. They are not reliable andincrease maintenance costs and time. In today’saircraft, the same ac-powered equipment obtainsvarious ac voltages and dc power by using simpletransformers and transformer-rectifiers. These componentsare lightweight, simple, and reliable. Modern naval aircraft use the three-phase,120-/208-volt, 400-hertz (Hz) ac power system. Thenumber of magnetic poles and rotor revolutions perminute (rpm) determine the voltage frequency of thegenerator. With a fixed number of poles, constant frequencyrequires constant rotor rpm.An ac generator-rotating field has 12 poles withadjacent poles of opposite polarity. Each pair of polesproduces one cycle per revolution; therefore, eachrevolution produces six cycles. The output frequency ofthe generator varies in direct proportion to the enginedrive speed. A generator operating at 6,000 rpm is operating at 100 revolutions per second or at 600 Hz. The 120-/208-volt, 400-Hz, three-phase ac powersystemhas many advantages over the 28-volt dcsystem. It requires less current than the 28-volt dcsystem because of higher voltage and a ground neutralsystem. The current required is a fraction of thatrequired for the same power in a 28-volt dc system.This permits the use of smaller aircraft wiring, savingweight. The ac generator and many of the system’scontrol and protection components are lighter. Twelvekilowatts is the practical limit to the size of an aircraftdc generator. Aircraft now have ac generators withratings up to 90 kilovolt-ampere (kVA).
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