AC Electrics - AC Motors — Page 249, Lesson 261

I want to walk you through the induction motor, which is the most common type of AC motor you'll encounter in aviation. Let's start with why it's called an induction motor in the first place. The name comes from the fact that an alternating current is induced in the rotor by the rotating magnetic field in the stator. So the rotor doesn't get electrical power directly — it gets it through electromagnetic induction, just like a transformer. That's the core principle. Now, why is this motor so widely used? It's because of three things: its simplicity, its robustness, and the fact that it's relatively cheap to produce. And the main reason for that low cost is that the rotor is a self-contained unit — it has no electrical connections to the power supply. No brushes, no slip rings, no external wiring going to the rotor. That makes it mechanically simple and very reliable. Let's look at the rotor first. It's called a squirrel cage rotor, and you can see it in . The rotor consists of a cylindrical laminated iron core. Laminated means it's made of thin sheets of iron stacked together, which helps reduce eddy current losses. Around the circumference of this core, evenly spaced, there are a number of longitudinal bars made of copper. These bars run the length of the rotor. At either end, those bars are joined together by rings made of the same material — copper. This whole assembly — bars plus end rings — forms a composite structure called a squirrel cage. The name comes from the visual resemblance to those old exercise wheels for pet hamsters or squirrels. Now, an important detail: the rotor bars are made of a very low resistance material. Why? So that a large current can flow through them. We'll see why that matters in a moment. Now let's move to the stator. The stator contains windings. The number of windings is related to two things: the number of poles the motor has, and the number of phases of the power supply. So for a three-phase motor, the stator windings are arranged accordingly. Here's how it all works together. The stator produces a rotating magnetic field. That rotating field cuts through the copper bars of the rotor. Since the rotor is basically a closed circuit of low resistance — thanks to those bars and end rings — the induced voltage creates a relatively large current flow in the squirrel cage. That current flow then sets up its own magnetic field around the rotor bars. That rotor magnetic field interacts with the rotating magnetic field of the stator, and the result is a torque — a twisting force that makes the rotor turn. One more practical point: if you have a three-phase induction motor and you reverse any two of its supply phases, the direction of rotation will reverse as well. That's a useful way to control which way the motor spins.