EE 2145230 Chapter 10 Alternators, Inverters and Related Controls 10.1 Alternators (AC Generators) • Large aircraft typically employ AC generators (alternators). Light aircraft generally uses DC generators. • Concept: In a simple AC generator a loop of wire rotates inside the magnetic field produced by two opposite magnetic poles. Contact is made to the loop as it rotates by means of slip‐rings and brushes. The brushes are spring loaded and held against the rotating slip‐rings so that, at any time, there is a path for current to flow from the loop to the load to which it is connected. • The voltage is at a zero value when the loop is in a vertical position, and then it climbs to a maximum value when the loop is in the horizontal position. This is indicated on the sine curve from 0 to 90°. As the loop continues to turn, we find that the voltage is maximum at 90°, zero at 180°, maximum at 270°, and zero again at 360°. 1 • However, in a practical AC generator the magnetic field is rotated rather than the conductors from which the output is taken. Furthermore, the magnetic field is usually produced by a rotating electromagnet (the rotor) rather than a permanent magnet. • The significant advantage of the stationary armature and rotating field construction are: Increased armature tooth strength, reduced armature reactance, improved insulation, more rigid construction, reduced number of insulated slip rings, reduced rotor weight and inertia, ventilation and improved heat dissipation. • Single‐phase AC generator • Three‐phase AC generator 2 • Brushless generator was developed for the purpose of eliminating some of the problems of alternators which employ slip rings and brushes to carry exciter current to the rotating field. • A brushless alternator consists of rotor which includes the rotating armature of the exciter generator, a full‐wave, three‐phase rectifier, and the rotating field for the main generator (alternator); the stationary field of the exciter generator and the stationary armature (stator) of the main alternator; and necessary framework and bearings to make the unit functional. • The exciter generator for the alternator is a three‐phase alternator, the output of which is converted to d‐
c by means of the rectifier. The direct current from the rectifier is fed to the windings of the alternator rotor. Note that the main rotating field (rotor) and the exciter armature are mounted on the same hollow shaft. The exciter armature output passes through conductors to the inside of the shaft and to the three‐
phase rectifier. The d‐c output from the rectifier is carried outside the shaft to the main rotor windings. Since the exciter armature, the rectifier, and the main rotor are all mounted on or in the same shaft, there is no need for slip rings, brushes, or a commutator. Advantages 1 . Lower maintenance cost since there is no brush or slip‐ring wear. 2. High stability and consistency of output because variations of resistance and conductivity at the brushes and slip rings are eliminated. 3. Better performance at high altitudes because arcing at the brushes is eliminated. 10.2 Inverters • An inverter is a device for converting direct current into alternating current at the frequency and voltage required for particular purposes. • Inverters are typically used on large aircraft for emergency situations only. The aircraft employs engine‐
driven generators (alternators) to supply needed AC power during normal operating conditions. If all AC generators should fail, the inverter would then be used to convert battery DC power into AC power available for essential AC loads. • There are two basic types of inverters: rotary and static. Modern aircraft employs static inverters because of their reliability, efficiency and weight saving over rotary inverters. 3 10.3 Three‐phase Generation and Distribution • When three-phase supplies are distributed there are two basic methods of connection: Star and Delta • A complete star-connected three-phase distribution system • A complete delta-connected three-phase distribution system • Powers in a three-phase system: The total power in a three-phase system is the sum of the power present
in each of the three phases. [
References: Textbooks # 1 and # 2 EE 2145230 Aircraft Electricity and Electronics Asst. Prof. Thavatchai Tayjasanant, Ph.D. 4