An electric motor converts electrical energy into mechanical motion by using electromagnetic forces to produce torque on a rotating component. In a basic motor, electric current flows through a conductor or coil placed within a magnetic field. A current carrying conductor experiences a magnetic force according to the Lorentz force law, and when the conductors are arranged around a rotor, the forces produce a turning effect called torque. The rotor therefore begins to rotate.
A simple direct current motor uses a permanent magnet or electromagnet to provide a magnetic field and a current carrying coil mounted on an armature. The direction of current must reverse at appropriate points in the rotation so that the torque continues to act in the same rotational direction. A mechanical commutator and brushes perform this function in a traditional brushed DC motor. Modern motors can instead use electronic switching, as in brushless motors, or alternating currents and rotating magnetic fields, as in induction and synchronous motors.
The force on a straight current carrying conductor in a magnetic field is proportional to current, conductor length and magnetic field strength, with direction determined by the vector relationship among current and magnetic field. In a rectangular coil, opposite sides experience forces in opposite directions, but because those forces act at different positions, they form a couple that produces torque. The net translational force can be approximately zero while the net torque is nonzero.
This is the essential mechanism by which a motor converts electromagnetic interaction into rotation. The torque depends on the magnetic dipole moment of the coil and its orientation relative to the magnetic field. A current carrying coil behaves as a magnetic dipole, and the field exerts a torque tending to align its magnetic moment with the external field. The motor continuously changes the orientation of the coil or magnetic field so that this torque produces sustained rotation rather than simply stopping when alignment is reached.
In a basic brushed DC motor, the commutator is essential. If the current through a simple coil remained unchanged, the torque would eventually reverse direction after the coil rotated through part of a revolution. The split ring commutator mechanically reverses the current direction in the rotating coil every half turn. This means the magnetic interaction continues to produce torque in the same rotational direction. Brushes provide electrical contact between the stationary supply and the rotating commutator.
Brushed motors are mechanically simple but brushes and commutators wear, generate electrical noise and impose limits on speed and maintenance. Brushless DC motors replace the mechanical commutator with electronic switching. Sensors or sensorless control determine rotor position, and electronics energise the stator windings in an appropriate sequence. This reduces mechanical wear and can provide high efficiency, controllability and reliability.
AC motors operate using different arrangements but rely on the same electromagnetic principles. In an induction motor, alternating current in the stator windings produces a rotating magnetic field. This changing field induces currents in the rotor, and the interaction between the rotor currents and magnetic field produces torque. The rotor generally rotates slightly slower than the synchronous speed of the rotating magnetic field, a difference called slip.
Without slip in a conventional induction motor, there would be no relative motion between the rotor and rotating field and therefore insufficient induced rotor current to sustain torque. In a synchronous motor, the rotor rotates at the synchronous speed and may use permanent magnets, electromagnets or other magnetic structures. Three phase AC systems are particularly useful because they naturally produce smoothly rotating magnetic fields. These motor types are widely used in industrial machinery, pumps, fans, compressors and transport systems.
A motor demonstrates conservation of energy because electrical input is converted into mechanical output along with unavoidable losses. Electrical resistance in windings produces heat, magnetic hysteresis and eddy currents can produce core losses, bearings create friction, and aerodynamic effects produce additional losses. The motor therefore cannot deliver mechanical power greater than the electrical power supplied over sustained operation. When a motor drives a mechanical load, the load produces an opposing torque.
The motor draws sufficient current to generate the electromagnetic torque required to overcome that load and accelerate the system. In many motors, back electromotive force also becomes important. As the motor rotates, its windings experience changing magnetic flux and generate an induced voltage that opposes the applied voltage. At steady operating speed, this back emf helps determine the current drawn by the motor.
Electric motors can be controlled by varying voltage, current, frequency, field strength or electronic switching. In a DC motor, speed can be influenced by applied voltage and load, while torque is strongly related to current. In AC induction motors, variable frequency drives adjust the frequency and voltage of the supply to control rotational speed and torque. Permanent magnet synchronous motors can achieve high efficiency and power density and are used in electric vehicles and industrial applications.
Stepper motors divide rotation into discrete commanded steps and are useful for positioning. Servo systems combine motors with feedback sensors and controllers to achieve precise position, speed or torque control. The motor itself is only one part of a larger electromechanical system. Controllers, sensors, gearboxes, bearings and power electronics can determine how effectively the motor performs its intended task.
The motor effect is historically connected to the development of electromagnetism after the discovery that electric currents influence magnetic needles. Experiments by Oersted, Ampere, Faraday and others established relationships among current, magnetic fields and mechanical force. Faraday demonstrated electromagnetic rotation, providing an important early example of electrical energy producing mechanical motion. The subsequent development of practical motors transformed industry because electrical power could be converted into controllable mechanical work without the need for direct mechanical transmission from a central engine. Motors now operate from microscopic scales in precision equipment to enormous industrial machines. They drive pumps, fans, conveyors, machine tools, refrigerators, computer cooling systems, electric vehicles and countless household appliances. Their versatility follows from the controllability of electrical current and magnetic fields.
An electric motor is closely related to a generator because the two devices use closely related electromagnetic principles in opposite directions of energy conversion. A motor receives electrical energy and produces mechanical output, while a generator receives mechanical energy and produces electrical output through electromagnetic induction. In many physical systems the same machine can operate in either mode depending on how it is driven and connected.
When a motor rotates, it produces back emf through induction, demonstrating that it is simultaneously an electromagnetic generator. When a generator supplies current to a load, electromagnetic forces resist the mechanical motion used to drive it, demonstrating the corresponding motor effect. This reciprocity follows from the unified laws of electromagnetism. The essential operating principle of a motor is therefore that controlled electric currents interact with magnetic fields to create torque, while the resulting rotation simultaneously produces induced electrical effects that influence the motor's current, speed and energy balance.
