AC Electrics -Alternators — Page 194, Lesson 202

I want to walk you through the three-phase alternator connections, because this is the heart of how AC power is distributed in a modern aircraft. Let me start with the advantages of three-phase systems, because they explain why we choose this design. First, three-phase alternators have a greater power-to-weight ratio compared to single-phase systems. That means for the same physical weight, you get more electrical power — critical in aviation where every kilogram matters. Second, they are easier to connect in parallel. When you need to synchronise multiple alternators onto the same bus, three-phase makes that job simpler. Now, the outputs of a three-phase alternator can be connected in one of two ways: the Star connection or the Delta connection. These are shown in Figure 12.4. I'm going to focus on the Star connection first, because that is the type fitted to a typical aircraft distribution system. The Four-Wire Star Connection In a star-connected three-phase alternator, the three phases are joined together at one common end. That common junction forms a fourth connection, called the neutral point. You can see this in Figure 12.5. The neutral point is normally grounded — connected to the aircraft structure — and it is used as the earth return in modern aircraft. What does that mean? In a DC system, current returns through the negative wire. In this AC system, the neutral line serves as the return path. The neutral line will carry any out-of-balance current. If the loads on each phase are perfectly balanced, the neutral current is zero. But if there is an earth fault on one phase — say a wire chafes against the metal airframe — the neutral will carry an exceptionally high load. That is a protective feature; it allows fault currents to flow back to the source and trip protection devices. This is the key reason the star connection is used in aircraft: it can cope with different loads on each bus bar. The delta connection cannot. In a typical aircraft, you might have different electrical buses supplying different systems — some drawing more current than others. The star connection handles that imbalance; the delta connection does not. Phase Voltage vs Line Voltage Now, let's talk about the terminals. The connection at the opposite end of each phase winding from the neutral is called the line connection. So each phase has one end at the neutral point and the other end at a line terminal. That gives us three line terminals, plus the neutral — four wires total. If you put a voltmeter between the neutral and one line lead, you measure phase voltage. That is the voltage across a single phase winding. If you put a voltmeter between two line connections — say line A and line B — you measure line voltage. That is the voltage across two phases. In a star-connected alternator, the phase voltage and line voltage are different. Why? Because phase voltage is measured across one phase, whereas line voltage is measured across two phases and is the vector sum of the two. The two phase voltages are not in phase with each other — they are 120 degrees apart — so you cannot simply add their numerical values. You have to add them as vectors. The Formula Given one value, you can find the other using this formula: Line Voltage = 1.73 × Phase Voltage The number 1.73 is the square root of 3, written as √3. That comes from the geometry of the 120-degree phase separation. So if you know the phase voltage, multiply by 1.73 to get line voltage. If you know the line voltage, divide by 1.73 to get phase voltage. Aircraft Example In a typical aircraft supply system, the line voltage is 200 volts. Using the formula, the phase voltage would be: 200 divided by 1.73, which equals approximately 115 volts. To be more specific, a modern aircraft power supply is described as: 115 V / 200 V / 400 Hz / 3 phase Let me unpack that. 115 V is the phase voltage. 200 V is the line voltage. 400 Hz is the frequency — much higher than the 50 or 60 Hz you see in household mains, because it allows smaller, lighter transformers and motors. And 3 phase tells you it is a three-phase system. So when you hear that specification, you now know exactly what each number means and how they relate through the star connection.