Chapter10--AC-Power-Generation(民航飞机电气系统10).ppt

Chapter 10 AC Power Generation
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Chapter 10 AC Power Generation
INTRODUCTION: The majority of large modern aircraft now employ three-phase AC generators, because they are more efficient than their DC equivalents. .
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Power Generation
Simple Three Phase Generator
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The rotor carries the electromagnetic field that is driven by the aircraft engine, whilst the stator carries three sets (pairs) of coils (phase windings). These windings are fixed to one another at angles of 120° and the phases are AA1, BB1 and CC1 or coloured Red (RR1), Yellow (YY1), and Blue (BB1) respectively, where the A or Red phase is classified as the reference phase. As the rotor rotates, it induces an EMF in each set of windings in turn and produces a sine wave output from each, as shown in the diagram on the next page.
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At any instant, the sum of the EMFs or the currents in a balanced system will add up to zero.
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Power Generation (continue)
These windings supply the output of the generator and are connected in either a Star or Delta configuration, as shown below.
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Star Connection
Line Voltage (VL) = Phase Voltage (VP)
Line Current (IL) = Phase Current (IP)
If Phase Voltage = 115V , then Line Voltage = 200 V .
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Star Connection (continue)
When a three-phase star-connected generator is feeding a balanced load (ABC phases feeding the same current), the net current of all three phases is zero. In this case, no current flows in the neutral line. When unbalanced currents feed the load, the resultant of these currents will flow in the neutral line.
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Star Connection (continue)
If the currents being used are always balanced, there is no need for a neutral point to be fitted. On aircraft, although desirable, it is not practical for the generator to feed balanced loads all of the time, so it is necessary on most generators to connect the neutral point to earth.
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Delta