Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

breakaway torque
minimum torque needed to begin rotating a stationary load. Breakaway torque represents the absolute
minimum starting torque specification for a motor used to drive the load.
breakdown torque
maximum torque that can be developed by a motor operating at rated voltage and frequency without experiencing a significant and abrupt change in speed. Sometimes also called the stall torque or pull-out torque.
constant-torque drive
a variable-speed drive that is operating in a speed region where it is capable of maintaining rated torque. For DC machines, this region is below base speed and is achieved by reducing the applied armature voltage. For AC induction motors, this region is below rated speed and is achieved by reducing the frequency of the applied voltage.
counter-torque
torque developed in opposition to the rotation of a machine. It is produced as load current flows in the presence of and perpendicular to magnetic flux in a machine that is generating electric power.

electromagnetic torque
the torque produced in a machine by the interaction of the magnetic fields and/or by the varying reluctance principle where the field attempts to maximize its intensity in a machine during electromechanical energy conversion.
full-load torque
the torque of a motor that is producing rated power at rated speed.
gravitational torque
torque that depends upon the position of the robot in the gravitational field.
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.
load torque
the resisting torque applied at the motor shaft by the mechanical load that counterbalances the shaft torque generated by the motor and available at the shaft.
locked-rotor torque
the torque produced in an induction motor when the rotor is locked and rated AC voltage is applied to the stator.
magnetic torque coupling
any device utilizing a magnetic field to transmit torque.
pull-in torque
the amount of torque needed to change a synchronous motor's operation from induction to synchronous when self-started.
pull-out torque
the maximum value of torque that an AC motor can deliver. An induction motor operating at the pull-out torque will operate at maximum slip, and loading it beyond the pull-out torque will cause the motor to stall. Synchronous motors remain at synchronous speed up to the pull-out torque. Exceeding the pull-out torque for a synchronous machine will lead to pole slipping and destruction of the machine.
pull-up torque
the minimum torque generated by an AC motor as the rotor accelerates from rest to the speed of breakdown torque. For an induction motor, this value usually is less than the locked-rotor torque, and thus establishes the maximum load that can be started.
reluctance torque
the type of torque a reluctance machine's operation is based upon.

A reluctance torque is produced in a magnetic material in the presence of an external magnetic field, which makes it to line up with the external magnetic field. An induced field due to fringing flux develops a torque that eventually twists the magnetic material around to align itself with the external field.
starting torque
the torque at zero speed obtained at the very beginning of the starting process of an electrical machine. The condition to obtain the rotation of the rotor is that the starting torque has to be greater than the load torque at zero speed.
torque
the product of a force acting at a total harmonic distortion (THD) an in-distance. The output of an electric motor. dex that quantifies the amount of distortion in the voltage or current waveform with respect torque angle the displacement angle be-to the fundamental component.
torque pulsation
oscillating torque produced by the interaction between the air gap flux, consisting mainly of the fundamental component, and the fluxes produced by harmonics in the rotor. Torque pulsations can stimulate complex mechanical vibrations that can flex and damage rotor and turbine elements.
torque ripple
in variable speed motor drives, refers to the torque not being smooth as the rotor moves from one position to another. Torque ripple may be produced from space harmonics within the machine or time harmonics generated by the supply.
torque servo
a servo where the output torque is the controlled variable and the operating speed depends on the load torque.
See servo
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