First, let's look at why we even use AC motors in aircraft — Page 240, Lesson 257

I want to walk you through the start of Chapter 15, which is all about AC Electrics and specifically AC Motors. This is a fresh topic, so let's begin at the very beginning. First, let's look at why we even use AC motors in aircraft. Alternating current motors can, in most cases, duplicate the operation of DC motors, and they are less troublesome to operate. DC motors have a great deal of trouble with their commutation. High altitude flight causes particular difficulty for DC motors because of the associated arcing that occurs. The brush equipment is another weak link in DC motors. The heat generated at the brushes causes them to stick in the holders, and as a consequence, the resistance between the brushes and the commutator increases. This often increases to the point of becoming an open circuit, and when that happens, the motor will stop. Now, I should clarify that synchronous AC motors do in fact use brush gear. Their rotors are fed by relatively low current DC through slip rings, but these are generally less troublesome than the DC motor's commutator and brushes. AC motors are particularly suited for constant speed applications. This is because their speed is determined by the frequency of the applied power supply. AC motors can be operated from either single-phase or multi-phase power supplies. Let's move to the principle of operation. Whether the AC motor is single-phase or multi-phase, the principle of operation is the same. Alternating current applied to the motor stator generates a rotating magnetic field, and that rotating magnetic field causes the rotor to turn. The majority of AC motors used in aircraft can be divided into two types. The first type is synchronous motors. These are basically alternators operated as motors. Alternating current is applied to the stator, but the rotor has a direct current power source. The second type is induction motors. This type has alternating current applied to the stator, but the rotor has no power source. Now, let's look at the synchronous motor in more detail. The synchronous motor gets its name because the rotation of the rotor is synchronized with the rotating field set up in the stator. That's the core idea — the rotor turns at exactly the same speed as the magnetic field rotating around it. Its construction is what we'll pick up next.