VI Find more information about the topic to prepare a presentation.
VII Using the figures describe a brush-typeac generators. UNIT 8BRUSHLESS-TYPEOF GENERATOR
AIM OF THE UNIT:- to understand the advantage of a brushless generator over a brush-type 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 Speak on the topic. Given schemes will help you to remember and understand the topic. 8Find more information about the topic. Do some research, create slideshow or a project work and present them.
1 What brushless generators are used for? 2 What is the advantage of a brushless generator over a brush-type?
Most naval aircraft areusing brushless generators for voltage generation. Theadvantage of a brushless generator over a brush-type isits increased reliability and greater operating timebetween overhaul. The brushless generator is asalient 8-pole, 6,000-rpm, ac generator. It has a 12-poleac exciter and a three-phase, half-wave diode rectifierrotating with the exciter armature and main generatorfield assembly. The exciter rotor is a hollow frameassembly with the main ac field mounted on the insideand connected to a common drive shaft. A single-phasepermanent magnet generator (PMG) furnishes controlvoltage and power for the voltage regulator. Threehalf-wave rectifiers are on the exciter rotor andconnected to the exciter armature windings. Agenerator shaft shear section prevents possible damageto the engine or drive unit if the generator seizes. A fanat the drive end of the generator provides coolingairflow for the rotor and stator windings and the drivebearings. The end bell not only holds the generatortogether, but also houses the PMG stator and thepermanent magnetic rotor core. The generator to airduct adapter allows a vent tube to be attached to thegenerator so that built-up air and fumes created by thespinning generator can be vented to the outside of theaircraft. Some aircraft have oil-cooled generators. Theaircraft engine or gearbox oil enters the generatorthrough an inlet port and leaves through an exit port inthe mounting flange of the generator. As the oil passesthrough the generator, it absorbs the heat from the rotorand stator. At the same time, it cools the rotating seals,lubricates and cools the bearings, and is used for theconstant speed drive operation.As the generator shaft rotates, the PMG suppliessingle-phase, ac voltage to the voltage regulator andother protective circuits. PMG power isrectified and supplied to the exciter field. Theelectromagnetic field, built by the excitation currentflowing in the exciter, induces current flow in therotating three-phase exciter rotor. This current ishalf-wave rectified by rotating rectifiers. The resultantdc goes to the rotating field winding of the ac generator. The rotating electromagnetic field induces an ACvoltage in the three-phase, WYE-connected, outputwinding of the generator stator. Varying the strength ofthe exciter stationary field regulates voltage. Using anintegral ac exciter eliminates the need for brusheswithin the generator, which minimizes radio noise inother avionics equipment. Two three-phase differential transformers provideprotection against shorts in the feeder lines between the generator and the bus (called feeder fault). Onetransformer is on the generator. Its coils sense the current flow through each of the legs that connectthe ground side of the generator stator to ground. Theother transformer is at the main bus and senses currentflow through the three feeder lines. A short in the feederline would cause the transformers sensing a differencein current to trip the generator off line. A generator mechanical failure warning device isincorporated in the generator. It consists of a softcopper strip embedded in and insulated from thegenerator stator assembly. A bearing beginning to failallows the rotor to rub against the copper strip,completing a warning light circuit to ground. A device, such as an aircraft engine, that providesthe driving force for a generator, is a prime mover. Earlyattempts to control the rotor speed of ac generatorsusing variable-pitch propellers or slipping clutcheswere unsuccessful, and ac generator power was ofvariable frequency. As power requirements grew, itbecame necessary to furnish ac power at a constantfrequency, to save weight and improve performance.The constant frequency is obtained from constant speeddrives such as hydro mechanical constant-speed drive(CSD) units, air or gas turbines, and the constant rpmturboprop engine.The hydro mechanical CSD unit converts variableengine speed to a constant speed output. It holds thefrequency steady to within a few hertz of the desired400 Hz. Load and fault transients limits are within a380- to 420-Hz range. Air or gas turbine drives, whichget the air supply by using bleed air from the jet enginecompressor or from a separate compressor, aresomewhat smoother in operation and hold steady-statefrequencies to within ±10 Hz. The constant rpmcharacteristic of the turboprop engine gives goodfrequency stability to the ac generator output. Thepropeller mechanical governor will hold the generator frequency to 400 ±4 Hz.
EXERCISES FOR BETTER REMEMBERING THE TOPIC
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