GegenEMK

GegenEMK, short for Gegen electromotorische Kraft, is the back electromotive force that arises in electrical machines when electrical energy is converted to mechanical energy or when moving conductors cut magnetic fields. In technical usage, GegenEMK describes the voltage generated within a winding due to changing magnetic flux as a rotor turns in a motor or as a generator armature moves relative to a magnetic field. The induced voltage opposes the applied supply voltage in accordance with Lenz’s law, thereby limiting current and shaping the machine’s dynamic behavior.

The physical basis is Faraday’s law of induction. In an inductive circuit, the back EMF is related to the rate of change of current by E = -L di/dt. In a brushed direct-current motor, the back EMF is typically modeled as proportional to the rotor speed: E_back = k_e ω, where ω is angular velocity and k_e is a machine-dependent constant. The exact relation depends on winding geometry, magnetic characteristics, and electrical loading.

Practical effects include a high inrush current at startup when back EMF is small, followed by a reduction in current as speed increases and back EMF grows. This natural current limitation influences torque and efficiency and enables speed sensing by measuring the generated voltage during operation. Back EMF is also exploited in braking and energy recovery in some systems, where the motor operates as a generator to dissipate or feed energy back to the supply.

See also: electromotive force, Faraday’s law, motor control, regenerative braking.