Frequency-dependent dynamic, representation of induction-motor loads

Abstract

For comprehensive transient and dynamic stability studies, induction-motor loads have usually been represented by fixed shunt impedances at system nominal frequency. A method is proposed here which allows single induction motors or groups of induction motors to be considered as frequency-dependent and dynamic. If the necessary parameters of the motors are known and if computation effort is not a problem, accurate assessment can always be made for single machines or groups of machines but may involve lengthy calculations. Alternatively, an approximate, rapid method is presented which gives the change in active-power input to an induction motor or group of induction motors due to inertia and frequency-variation effects to within ±2% of the actual change for known power factor, inertia factor and full-load slip; and to within ±5% of the actual change when the assumed parameters are within the following limits: power factor within ±5% inertia factor within ±10% and full-load slip within ±20% (where the actual change in power has been assessed by the use of an equivalent circuit). The change in reactive power, as assessed, is within ±20% of actual change in reactive power. ±5% change in operating frequency has been considered. For simplicity, windage and friction losses have been taken as an integral part of the load supplied by the motor, and core losses have been assumed constant. These methods can be applied to power-system stability studies, as illustrated by an example.