Electric Motor

Electric motors are machines designed to convert electrical energy into mechanical energy, providing controlled motion for a vast range of industrial, commercial and domestic applications. Their operation is founded on the interaction between electric current and magnetic fields, producing torque through electromagnetic forces. Because electric motors and electric generators share the same fundamental structure, a motor can function as a generator when driven mechanically, a principle widely applied in regenerative braking systems.
Electric motors can operate using direct current supplied by batteries or rectified sources, or alternating current supplied by utility grids, inverters or dedicated electrical generators. With their high efficiency, reliability and adaptability, they form a critical technological foundation of modern mechanical systems.

Classification and Applications

Electric motors may be classified in numerous ways depending on their power source, construction and output characteristics:

• By power source: DC motors, AC single-phase, two-phase and three-phase motors.
• By construction: brushed or brushless motors, axial-flux or radial-flux designs.
• By cooling method: air-cooled and liquid-cooled systems.
• By motion output: rotary motors and linear motors.

Standardised industrial motors support heavy-duty applications such as marine propulsion, pipeline compression and pumped-storage hydroelectric installations, often exceeding several megawatts of output. Other widespread uses include machine tools, fans, pumps, blowers, household appliances, power tools, computer disk drives and electric vehicles. Extremely small motors are incorporated into watches and miniaturised instruments.
In traction systems, motors can regenerate electrical energy by operating in reverse as generators during braking, enhancing overall system efficiency. As actuators, motors deliver continuous rotary or linear motion suited to driving external mechanisms, whereas solenoids, although electrically actuated, produce only short-range linear displacement.

Components and Construction

Electric motors comprise two principal mechanical parts:

• Rotor: the rotating part that delivers mechanical output.
• Stator: the stationary part surrounding the rotor.

Electrically, motors contain:

• Field magnets: producing the magnetic field.
• Armature windings: carrying current and generating torque.

Field magnets may be permanent magnets or electromagnets. The typical arrangement places field magnets on the stator and the armature on the rotor, although certain machines employ the reverse configuration. Together, they form a magnetic circuit in which magnetic flux interacts with current-carrying conductors to produce torque.

Rotor

The rotor contains conductors through which current flows. The stator’s magnetic field exerts force on these conductors, producing rotation. Rotor design varies widely among motor types, ranging from wound rotors to squirrel-cage configurations.

Stator

The stator supports either the field coils or permanent magnets. Its core normally consists of thin, insulated laminations of electrical steel. These laminations reduce eddy current losses, improving efficiency and limiting heat generation. To increase reliability, AC motor windings are often varnish-impregnated under vacuum to prevent vibration and insulation damage. In hostile or submerged environments, stators may be encapsulated in resin to protect against corrosion and reduce electrical noise.

Air Gap

The narrow air gap between rotor and stator is critical to performance. A smaller gap enhances magnetic coupling and efficiency, while an excessively small gap risks mechanical contact and noise.

Armature

The armature consists of windings on a ferromagnetic core. When current flows through these windings, the magnetic field exerts torque via the Lorentz force. Motors may feature:

• Salient-pole construction, with protruding magnetic poles each wound with coils.
• Non-salient or round-rotor construction, with evenly distributed windings in slots around a smooth cylindrical core.

Supplying AC to such distributed windings produces continuously rotating magnetic fields.

Commutator

In brushed DC and universal motors, the commutator provides mechanical switching of current direction within the rotor windings. It comprises a segmented cylindrical assembly mounted on the armature, in contact with carbon brushes. As the rotor turns, brushes slide over successive segments, reversing current flow every half rotation. This ensures torque remains in a constant direction. Modern motor designs have largely replaced commutated systems with brushless DC motors and induction motors due to reduced maintenance and higher durability.

Shaft and Bearings

The rotor’s mechanical output is delivered through a shaft which often supports overhung loads. Bearings transfer radial and axial forces to the motor housing while allowing smooth rotation and long-term operational stability.

Historical Development

Early attempts to convert electrical energy into motion involved electrostatic motors in the eighteenth century. Demonstrated by researchers such as Andrew Gordon and Benjamin Franklin, these devices relied on Coulombic attraction and repulsion. Although theoretically significant, they were impractical due to their reliance on high voltages and inability to produce useful power.
The invention of the voltaic cell in 1799 enabled sustained electric currents and paved the way for the development of electromagnetic motors. In 1820 Hans Christian Ørsted discovered that an electric current produces a magnetic field, and shortly afterwards André-Marie Ampère formulated laws describing the mechanical forces arising from current interactions.
In 1821 Michael Faraday demonstrated the first electromagnetic rotation using a free-hanging wire rotating around a magnet immersed in conductive fluid. This experiment revealed that current-carrying conductors experience circular magnetic forces, a principle that remains fundamental to electric motor theory. The demonstration inspired refinements such as Barlow’s wheel, a homopolar motor illustrating the same phenomenon, although early designs lacked practicality.
Progress accelerated in the mid-nineteenth century. Ányos Jedlik’s work from 1827 produced one of the earliest electromagnetic self-rotors, including devices with both stationary and rotating electromagnetic coils. His realisation of continuous self-rotation anticipated later motor developments.
Further contributions came from James Prescott Joule in the 1840s and from numerous mechanical and electrical engineers who refined materials, windings and magnetic circuits. Advances in commutation, iron core design and insulation ultimately produced motors suitable for industrial use.
By the late nineteenth and early twentieth centuries, the emergence of AC power systems led to the development of induction motors and synchronous motors, which became widely adopted due to their robustness, efficiency and compatibility with modern electrical grids.