By Kim Lewis Updated Mar 24, 2022
Electric motors and generators share a common foundation: a rotating current‑carrying loop, or armature, situated within a magnetic field. The iron core of the armature intensifies the internal field, while alternating current flowing through the loop reverses its direction, keeping the armature in perpetual motion. As the loop moves, a changing magnetic flux induces an electromotive force (EMF), the voltage that powers the device.
EMF, short for electromotive force, is not a mechanical force but the potential difference that transforms one energy form into another. Batteries, for instance, convert chemical energy into electrical energy, providing a source of EMF. The magnitude of the induced EMF follows Faraday’s Law of Induction: the faster the magnetic field changes, the larger the voltage produced.
AC generators—often called alternators—convert mechanical energy into alternating electrical power. Rotational motion, typically supplied by steam turbines (coal, oil, or natural gas), nuclear fission, or hydroelectric turbines (e.g., at Niagara Falls), turns the armature. The resulting EMF is a sine wave whose frequency and amplitude depend on the speed of rotation and the strength of the magnetic field.
AC motors perform the reverse: they use alternating electrical current to generate mechanical rotation. Most industrial AC motors rely on induction, where the stator’s magnetic field induces currents in the rotor, creating a torque that turns the shaft. The same voltage that energizes the coils drives the magnetic field, making AC motors efficient and robust for large‑scale applications.
DC versions add a commutator—a split ring connected to brushes—that reverses current direction at the appropriate moment. This ensures continuous rotation whether the device is drawing power (motor) or delivering power (generator). The armature may be driven by a permanent magnet or an electromagnet, and the generated EMF is direct current.
Fundamentally, every motor is a generator in disguise: the rotating armature produces EMF even when consuming electrical power. In a motor, this back EMF opposes the applied voltage, limiting current flow as speed rises. While back EMF reduces power draw under load, it also represents an inherent inefficiency that must be managed in high‑performance systems.
