253 – Analysis of an Induction Generator

Overview

Induction machines are based on the double field revolving theory. An induction machine also operates using the principle of slip. Slip is defined as the difference between synchronous speed and operating speed where the synchronous speed is the speed of the rotating field. Thus depending on the slip an induction machine has 2 operating modes. When the slip is positive it acts as an induction motor and when it is negative it acts as an induction generator.
In order to calculate the efficiency of the machine accurately, the machine has to be analyzed using the finite element method (FEM). By performing steady state approximation analysis on the machine and specifying the slip and the speed at which the machine is operating, the machine performance can be analyzed at the various operating points.
In this example, we introduce the case study that evaluates output characteristics of an induction generator.

Machine Characteristics

The Induction generator has specific operating points only at which it is feasible.
The losses also depend on the rotor speed thus if the rotor is rotating too fast the resulting eddy currents might cause more losses and reduce the efficiency.
Fig. 1 shows the variation of output power at different operating speeds. The machine is rated for 1 MW.
As observed in Fig. 1, the maximum output power operating point is different than the maximum efficiency . This shows that there is a trade off between power and efficiency when operating an induction generator. Doing an FEA analysis gives an insight into finding the proper operating point for the machine.
Fig. 2 shows the rotor copper loss distribution at maximum slip and minimum slip. The losses increase as the absolute value of slip increases.

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