Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.


See high rupturing capacity
acronym for high temperature gas-cooled reactor.
a mechanical oscillation in the speed of a synchronous machine due to changes in the load. Damper windings are used to reduce the hunting by providing a torque that opposes the change in speed.
HVDC transmission

See high-voltage DC transmission
hybrid stepper motor
a stepper motor that combines the rotor design characteristics of variable-reluctance and permanent magnet stepper motors. Hybrid stepper motor rotors consist of an axially magnetized cylindrical permanent magnet capped on each pole by toothed, soft iron caps. Teeth on the caps are displaced with respect to each other to provide stepping control. Hybrid stepper motors combine the higher torque capability of permanent magnet motors with the higher step resolution of variable-reluctance motors.
hydro-thermal coordination
the practice of manipulating water levels in the reservoirs of a power system's hydroelectric plants with respect to the generation levels of the thermal plants in the system with the objective of minimizing generation costs, as well as satisfying waterway considerations such as water conservation, flood control capacity, recreation, and environmental requirements.
hydroelectric generator
large, three-phase synchronous alternator powered by a water-driven turbine. See also generator.
conversion of potential energy of water into electricity using generators coupled to impulse or reaction water turbines.
(1) the phenomenon that the magnetic state of a substance is dependent upon its magnetic history, so that its magnetization for an increasing magnetizing force differs from that for a decreasing magnetizing force.

(2) the characteristic of magnetic materials that causes the trajectory of the flux density vs. field intensity curve as the intensity is increased to be different from that when the intensity is decreased, giving rise to a loss, which is proportional to the area enclosed by the two trajectories.
hysteresis brake
a braking device utilizing hysteresis to provide a constant braking torque irrespective of slip speed.
hysteresis control
a time-optimal feedback control method in which the control variable reaches a reference value in the shortest possible time and then stays within a prescribed hysteresis band around the set point through manipulation of the system state between two configurations. The actual variable is compared with the reference value, and if the error exceeds the hysteresis band, then the control input is changed such that the control variable is forced to decrease. On the other hand, if the actual variable falls below the hysteresis band then the control input is changed such that the control variable increases in magnitude.
hysteresis curve
a graph describing the relationship between the magnetic flux density and the magnetic field intensity in a (usually ferromagnetic) material.
hysteresis drive

See hysteresis torque coupling
hysteresis loss
the energy loss due to hysteresis in a magnetic material subjected to a varying magnetic field.
hysteresis motor
any of a variety of single-phase AC motors that use the hysteresis properties of hard magnetic materials to develop torque. Stator windings of a hysteresis motor can be of any design that produces a rotating flux within the machine. Motion of the rotating flux over the rotor magnetizes the hard magnetic material on the rotor; however, the hysteresis characteristics of the material cause the alignment of magnet flux to lag the rotating stator flux. This misalignment produces rotor torque. Because of the nature of the torque production, hysteresis motors operate at synchronous speed and have a constant torque characteristic, which permits them to synchronize any load that they
can accelerate.
hysteresis torque coupling
a magnetic drive in which the magnetizing stator magnet drives a rotor of hysteresis material through the complete hysteresis cycle once per rotation, resulting in a constant torque characteristic irrespective of relative speed.
See frequency