The module opened with electricity doing work through heat and light; it closes with electricity doing work through motion. The electrical motor drives the aircraft's pumps, valves, actuators, fans and, above all, the starters that wake the engines. This lesson covers what a motor is and what its parts are called, why it is a generator's twin, the energy arithmetic of engine starting, how torque is made, and the two ways a motor can fail: seized, and running away.
Purpose and parts
The purpose of an electrical motor is to convert electrical energy into mechanical energy: current in, torque and rotation out, the exact reverse of the generator. Its anatomy carries two names the syllabus asks for:
Rotor — the rotating part of the machine.
Stator — the stationary part around it.
One of the two carries the field windings, the other carries the current that pushes against that field; which does which varies by design, and the names never change.
A generator and a motor are so alike that one machine can be both: feed it torque and it delivers current, feed it current and it delivers torque. That symmetry is the starter generator of the generation lesson: the same machine on the engine's gearbox motors the engine up to starting speed on battery power, then spends the rest of the flight being driven as the generator.
The energy of starting
How much energy a start needs is set by the size of the engine: more mass to spin, more compression to overcome, more energy per start. That single fact divides the fleet:
Small turbine engines — may be started from the aircraft battery alone, but only for a very limited number of attempts: each start is a deep, hot withdrawal, and a battery asked for a third or fourth try in succession is both emptying and overheating.
Large turbine engines — require one or more power sources beyond the battery: external power on the ground, or an on-board source such as the APU's generator. The battery's role shrinks to starting the APU, which then starts everything else.
Torque
A motor's torque comes from two magnetic fields pushing on each other: the supplied voltage drives a current through the motor's windings, the current creates magnetic fields in stator and rotor, and the fields' interaction across the gap drags the rotor around. The chain sets the scaling: more voltage drives more current, more current makes stronger fields, and stronger fields make more torque. It is why a start on a weak, cold battery is audibly lazy: less voltage, less current, less torque, and the engine turns over slowly or not at all.
Motors can be either AC or DC. DC motors run wherever the battery and the DC buses are the natural supply, and they start and control simply. AC motors, especially three-phase ones, carry the big steady loads: the fundamentals lesson's rotating succession of three-phase peaks drags the rotor around smoothly, with no brushes or commutator to wear.
Seizure and runaway
A motor can fail in two opposite directions:
Rotor seizure — a motor that cannot turn: a jammed valve, a frozen bearing, a fouled mechanism. A stalled motor does not politely stop drawing power; it draws its maximum current continuously, and with no rotation being produced, every watt of it becomes heat in the windings. The consequences are the short-circuit story of the protection lesson in slow motion: overheating windings, melting insulation, and a fire risk that the circuit protection exists to interrupt, which is why a motor's breaker that trips and trips again is telling the crew of a jam or a fault, and either forbids the reset.
Rotor runaway — the opposite failure: a motor that has lost its load or its control accelerates unchecked, because the motor types most prone to it have no built-in speed cap; with nothing holding it back, the speed climbs until something limits it, and if nothing does, the rotor bursts under its own centrifugal load. The consequences are overspeed, vibration and mechanical disintegration, and the protection is the same pair as ever: monitoring, and disconnection.