AC Electrics -Alternators — Page 194, Lesson 200

I want to walk you through the AC Electrics section, starting with alternators. Most practical alternators you'll find on aircraft are designed with a rotating field and a stationary armature. That means the rotor — the part that spins — carries the field windings. The field can be energized either by a permanent magnet or by DC from a separate source. Let me be very clear on one point: the field MUST be energized by DC to keep the correct polarity in the rotor. That's a hard requirement — if you lose DC to the field, the alternator stops producing output. One big advantage of this rotating field design is that only a low current is fed through slip rings to the field windings. The output is taken from the stationary armature windings, which means problems associated with arcing from the brush gear are greatly reduced. Think about it — the high-power output doesn't have to go through any sliding contacts; it comes straight off the stationary coils. That's a major reliability gain. Now, let's talk about how alternators are rated. The maximum output current from an alternator depends on the amount of heat loss that can be sustained in the armature. This power loss heats up the conductors and can, in extreme cases, destroy the insulation of the windings. So alternators are rated in terms of this armature current as well as by their voltage output. That's why every alternator is rated in Volt Amperes — VA — or Kilovolt Amperes — kVA. That's what we call the Apparent Power. It's the product of voltage and current, without considering the phase angle between them. Moving on to a single phase alternator. A single phase alternator has its stator windings connected in series to supply the output. The stator windings — the coils — are connected so as to be series-aiding, meaning the induced voltages in them are in phase with each other. The rotor consists of two poles of opposite polarity. The output of this type of machine will rise to a maximum in one direction, then fall to zero, rise to a maximum in the other direction, and then fall to zero again. That's one complete cycle of AC. Now, polyphase circuits. Polyphase — or multi-phase — alternators have two or more single phase windings symmetrically spaced around the stator. The number of separate stator windings determines the number of phases present in the supply. The currents and voltages generated in this type of machine will have the same frequency but be out of phase with each other. Corresponding values of voltage or current will be separated by an equal number of degrees. The most common polyphase alternator is the three phase alternator, which has become the standard AC distribution system for aircraft. Note that the phase windings are mechanically arranged to be at 120° to each other in the sequence A, B, C. That means the outputs are electrically separated by 120° as shown in the diagram. You can see that "A" phase reaches a peak going positive before "B" phase reaches a peak going positive before "C" phase reaches a peak going positive. This is called the phase sequence ABC. The peak values of the voltages induced in the three single phase windings of the three phase alternator are 120° displaced from each other. And importantly, the three phases are independent of each other. That's a key point — each phase winding is a separate circuit, and they're only connected at the load.