Let me walk through how it works — Page 240, Lesson 259

Let’s start with the name itself. A synchronous motor gets its name because the rotation of the rotor is synchronized with the rotating magnetic field that is set up in the stator. That’s the core idea — the rotor and the field turn together, in lockstep. Now, the construction of a synchronous motor is basically the same as a rotating field alternator. If you’ve already covered alternators, you’ll recognise the layout: a stator with windings, and a rotor that carries a magnetic field. Here, though, we’re using it as a motor — we supply electrical power to get mechanical rotation out. Let me walk through how it works. When you apply a three-phase supply to the stator, it creates a rotating magnetic field around the rotor. I want you to picture a simple experiment: if you suspended a bar magnet inside that rotating field, the magnet would rotate synchronously with it — meaning at exactly the same speed as the field turns. That’s the principle. In a real synchronous motor, the rotor is energised with DC — direct current — so it acts like a magnet. It lines up with the magnetic field created by the stator. If that field rotates, the rotor turns with it. That’s the fundamental operating principle. Now, because the rotor follows the field exactly, a synchronous motor is a single-speed motor. Its speed of rotation depends entirely on the frequency of the supply. In most aircraft applications, the supply frequency is constant — so the motor speed is constant too. A synchronous motor will rotate at the same speed as the alternator that supplies it, provided they have the same number of poles. Let’s put a number on that. The formula is: RPM = Frequency × 60 × (2 / Number of Poles). Let me read that carefully: revolutions per minute equals frequency in hertz, multiplied by 60, multiplied by 2 divided by the number of poles. So if you have a synchronous motor with 4 poles, supplied with a constant 400 Hz supply, it will rotate at a constant 12,000 RPM. Let’s check that: 400 × 60 = 24,000, times 2/4 = 12,000. Yes, that works. There is one important disadvantage: a synchronous motor is not self-starting. It cannot begin rotating on its own from a standstill. To get it started, you have to add some induction windings to the rotor. These help bring the rotor up to synchronous speed before the motor can lock in and run synchronously. So where do we actually use synchronous motors on aircraft? One key application is indicating engine RPM. Here’s how the system works. A small three-phase alternator, called a tacho-generator, is driven directly by the engine. Because it’s an alternator, the frequency of its electrical output is directly proportional to engine speed. That output is connected to a synchronous motor inside the RPM indicator. Inside the indicator, the needle is coupled to the synchronous motor through a permanent magnet and a drag cup. As the synchronous motor rotates, it drags the drag cup around with it. The faster the motor goes, the further the drag cup moves, and the further around the scale the needle moves. So the movement of the needle is directly proportional to engine RPM. That’s the synchronous motor — its principle, its speed relationship, its starting limitation, and its practical use in aircraft for RPM indication.