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Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

an induction motor test conducted with the shaft held so it cannot rotate. Typically about 25% of rated voltage is applied, often at reduced frequency and the current is measured. The results are used to determine the winding impedances referred to the stator.

cage-rotor induction motor

an induction motor whose rotor is occupied by copper or aluminum bars, known as rotor bars, instead of windings. Also commonly referred to as a squirrel-cage induction motor.

cylndrical-rotor machine

a synchronous machine with a cylindrical rotor containing a distributed field winding and an essentially uniform air-gap. This design is limited to two and four pole machines (3600 and 1800 rpm at 60 Hz) and is usually used in large generators.
See salient-pole rotor machine

dual-cage rotor

a three-phase induction motor rotor with two separate squirrel cage windings, that give the effect of varying rotor resistance. The outer cage has high resistance to obtain high starting torque, while the inner cage has low resistance to reduce losses at full load.

locked-rotor current

the current drawn by an induction motor when the shaft is not moving and rated voltage is applied. The starting current is essentially equal to the locked rotor current and may be as much as eight times the rated current of the machine.

locked-rotor torque

the torque produced in an induction motor when the rotor is locked and rated AC voltage is applied to the stator.

rotor

the rotating part of an electrical machine including the shaft, such as the rotating armature of a DC machine or the field of a synchronous machine.

rotor power developed

the amount of power developed by the rotor. In DC machines, the developed power, frequently denoted by Pd , is calculated as the product of the induced EMF Ea and the armature current Ia. In induction machines, the rotor power developed is obtained by subtracting the rotor copper losses from the air gap power.

rotor power loss

represents the portion of the power transferred across the air gap to the rotor of an induction motor that is lost either through ohmic heating of the rotor windings or due to friction and windage losses in the rotor. The mechanical power available at the motor shaft is the difference between rotor power input and rotor power losses.

rotor speed

quantification of the rotational operation of the moving part of a rotating electrical machine. The rotor speed is measured either in SI units in radians per second (rad/s) or in practical units in revolutions per minute (rev/min).

salient-pole rotormachine

AC motor/generator design in which the rotor is constructed of outward-projecting pole pieces mounted on a shaft-mounted central spider assembly. Spider assemblies are typically spoked. Pole pieces are built up from laminated sheets, which are bolted together between a pole shoe on the outer end and dovetail fixture on the inner end. The dovetails are keyed into slots on the spider to mount the pole pieces to the rotor. Rotor windings are generally constructed from preformed, insulated coils that are fit over the pole pieces during assembly. Salient rotors are typically low-speed designs with short axial length and large diameter.

wound rotor induction motor

an induction motor in which the secondary circuit consists of a polyphase winding or coils connected through a suitable circuit. When provided with slip rings, the term slip-ring induction motor is used.