Author:

Keywords:

0906 Electrical and Electronic Engineering, Artificial Intelligence & Image Processing

Abstract:

Generally, three-phase induction motors with star-connected windings are used in AC servo-drives. Consequently, the dynamics of the machine is completely determined by the two-phase equivalent model, generally used in the synthesis of machine control. The necessary condition for using two-phase models of the induction motor in the so-called flux oriented control synthesis (especially when using the rotor flux orientation) is the precise knowledge of model parameters. However, when the variation of parameters is not taken into account, the detuning of orientation and control is possible. The parameters can be measured by conventional measurements, but some clearly defined measurement conditions, as for instance the reducing of voltage and frequency, are very hard to realize. Similarly, it is possible to measure the value of the current dependent mutual inductance with measurements at various stator current values at no-load, but it is hardly possible, or rather impossible, to split the rotor and the stator leakage defined by the locked rotor test. It is also difficult to define the value of the rotor resistance. The determination of induction machine parameters described in the paper is based on the idea of measurement imitations. The computation of magnetic fields in electromagnetic devices using finite element (FE) methods offers an ideal opportunity for simulating measurements that cannot be performed in reality for various reasons. In the proposed procedure a FE field solution is used to determine the flux distribution set up by defined set of currents in a machine with a rotor at standstill in different positions. Self and mutual inductances are defined in terms of flux linkages, whereas the rotor resistance is determined by calculating the power loss in the rotor cage. The determination of parameters described in the paper differs from the one presented in the very recent works because of the used two-axis excitations. The choice of magnetic excitations in two separate axes is confirmed by the derivation of a corresponding two-axis equivalent circuit model in the general reference frame that considers the saturation. The chosen excitation mode makes it possible to determine the magnetizing inductances as well as the real values of stator and rotor leakage inductances (even the saturated ones) which cannot be separated arbitrarily with classic measurements. The comparison of the calculated and the measured inductances as a function of the magnetizing current is shown in the figure 7. The comparison of calculated machine parameters determined by the proposed FE method and the parameters measured or calculated in the conventional way is given in Table 2. The calculated values of mutual inductance and rotor resistance in the non-saturated state at Im = 1.4, obtained with the proposed method, agree almost perfectly with the measured ones. In the calculated case the stator leakage inductance is greater than the rotor's, but the sum of boath calculated leakage inductances is relatively close to the sum of the measured reactances. The absolute high value of the stator inductance, as well as when compared with the rotor inductance, can be explained by the fact that the stator end winding coils are very high due to the machine design. In the nominal operating point the rotor resistance decreases because of the changed current distribution. In addition, the rotor leakage increases relatively more than the stator leakage. From the comparison of results presented in Table 2 we can see that three different ways of determining the parameters of the induction machine give very similar results. However, the parameters obtained from the analysis of the calculated magnetic field, given in the first culumn of Table 2, show certain advantages over the remaining two ways. The stator and the rotor leakage are separeted, and the real distribution of the rotor currents is included in the calculation of the rotor resistance. The seemingly irrelevant differences between the calculated values of both leakages and the rotor resistances shown in Table 2 have made it possible to realize the high performance nonlinear state-feedback input-output linearizing control of an induction motor. The proposed method for calculating the machine parameters by using the FE field solution is not completly new, but it might be a useful alternative to the exsisting numerical and experimental methods because it eliminates some of their drawbacks.