AC Electrics -Alternators — Page 194, Lesson 204

I want to walk you through the delta-connected alternator and then into practical AC generators, starting with the brushed alternator. Let's begin with the delta connection. In a star-connected system, line voltage and phase voltage differ, but line current equals phase current. In the delta-connected alternator, shown in Figure 12.6, the ends of each phase winding are joined together to form a closed mesh — a triangle or delta shape — and the loads are connected in the same delta pattern. Because the voltage measured across a single phase winding is measured directly between two of the three output lines, line voltage equals phase voltage. However, the line current is phase current multiplied by the square root of three — approximately 1.732. So in a delta system: line voltage equals phase voltage, but line current equals phase current times root three. Now, this type of connection will not be used in a practical aircraft distribution system. Why? Because there is no neutral point. Without a neutral, the delta connection cannot cope with unbalanced loads — if the loads on each phase are not equal, the system cannot compensate. However, delta-connected alternators may be used for specific purposes, such as speed sensors or tacho generators, where balanced loads are not a concern. Moving on to practical AC generators. There are two fundamental types: rotating armature alternators and rotating field alternators. Rotating armature alternators suffer from several disadvantages. The rotating coils are heavy, which creates high centrifugal forces. Efficient insulation of those rotating coils is difficult. The resistance across the brushes to the slip rings is high. The rotating coils are difficult to cool. And overall, they have a poor power-to-weight ratio. Because of these drawbacks, rotating field alternators make up the majority in use on aircraft. In this type, the field — the electromagnet that creates the magnetic flux — is in the rotor, and the phase windings that generate the output voltage form the stator, the stationary part. There are two types of rotating field alternator used on aircraft: brushed alternators and brushless alternators. Let's focus on the brushed alternator. The current supply for excitation of the rotor field — the DC current that energises the rotating electromagnet — can be provided initially from the aircraft DC bus bar, which is powered by the battery. Then, once the alternator is running, the excitation current is supplied by rectified AC — AC that has been converted to DC. This DC current is directed through brushes and slip rings to the rotating field winding on the rotor. Control of the excitation current is handled by the voltage regulator. The voltage regulator samples the alternator output — which is 115 volts AC — and adjusts the excitation current to maintain the correct output voltage, regardless of changes in alternator speed or electrical load. In its simplest form, the voltage regulator is a variable resistance connected in series with the field coil. This is the same principle as the carbon pile regulator you would have seen in Chapter 6, page 93. Finally, the output of the brushed alternator feeds the 115-volt AC bus. From there, a Transformer Rectifier Unit — TRU — converts the AC to 28 volts DC to supply the DC bus bar. So the system starts with battery power for initial excitation, then the alternator takes over, regulated to 115 volts AC, and a TRU provides the 28-volt DC supply for the aircraft's DC systems.