UNIT 21INVERTERS
AIM OF THE UNIT: - to describe inverters
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 topic.7 Grammar point: find the passive and perfect sentences in the text. 8 Speak on the topic. Given schemes and questions will help you to remember and understand the topic. 9 Find more information about the topic and prepare a presentation or a project work.
1 What are inverters? 2 In an inverter, what converts electrical energyto mechanical energy? 3 On what does the rating of an aircraft inverterdepend? 4 What is the purpose of a transformer? 5 What is the function of a transformerrectifier? 6 What is the only moving part in a transformerrectifier? 7 What is the difference between an autotransformerand an ordinary transformer? 8 Autotransformers offer savings in both sizeand costs over conventional units. When arethese savings greatest? 9 What are the two general types of instrumenttransformers?
Inverters are used to convert direct current into alternating current. The input is typically from the battery; the output can be a low voltage (26 V AC) for use in instruments, or high voltage (115 V AC single or three phase) for driving loads such as pumps. Older rotary inverter technology uses a DC motor to drive an AC generator. A typical rotary inverter has a four-pole compound DC motor driving a star-wound AC generator. The outputs can be single- or three phase; 26 V AC, or 115 V AC. The desired output frequency of 400 Hz is determined by the DC input voltage. Various regulation methods are employed, e.g. a trimming resistor (R v) connected in series with the DC motor field sets the correct speed when connected to the 14 or 28 V DC supply. The desired output frequency of a rotary inverter is determined by the DC input voltage. Modern aircraft equipment is based on the static inverter; it is solid state, i.e. it has no moving.The DC power supply is connected to an oscillator; this produces a low-voltage 400 Hz output. This output isstepped up to the desired AC output voltage via a transformer. The static inverter can either be used as the sole source of AC power or to supply specific equipment in the event that the main generator has failed. Alternatively they are used to provide power for passenger use, e.g. lap-top computers. The DC input voltage is applied to an oscillator that produces a sinusoidal output voltage. This output is connected to a transformer that provides the required output voltage. Frequency and voltage controls are usually integrated within the static inverter; it therefore has no external means of adjustment. A typical inverter used on a large commercial aircraft can produce 1 kVA. Static inverters are located in an electrical equipment bay; a remote on/off switch in the flight compartment is used to isolate the inverter if required. Inverters convert 28 VDC power into 115 VAC single-phase electrical power. This is usually required in a civil application to supply Captains or First Officers instruments following an AC failure. Alternatively, under certain specific flight conditions, such as autoland, the inverter may be required to provide an alternative source of power to the flight instruments in the event of a power failure occurring during the critical autoland phase. Some years ago the inverter would have been a rotary machine with a DC motor harnessed in tandem with an AC generator. More recently the power conversion is likely to be accomplished by means of a static inverter where the use of high-power, rapid-switching, Silicon-Controlled Rectifiers (SCRs) will synthesize the AC waveform from the DC input. Inverters are therefore a minor, though essential part of many aircraft electrical systems. EXERCISES FOR BETTER REMEMBERING THE TOPIC
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