Alexander Rocks

Prediction of common mode ground current in motors of inverter-based drive systems

A ladder parameter model to predict the common mode ground current in motors of inverter-based drive systems is analyzed. The comparison of the calculated and measured common mode current at IGBT inverter supply shows a good agreement for a 110 kW induction motor. The parameter calculus for the winding has been done with analytical methods and was checked with FEM.

Application of varistors for overvoltage protection of machine windings in inverter-fed drives

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltage in combination with very low rise times impose extreme dielectric stress on the insulation of electrical machines. Here a novel approach of overvoltage protection in inverter-fed drives is presented. After a discussion of the origins of the overvoltages and of overvoltage limitation by varistors in general, the dimensioning for the particular application, which is quite unusual for varistors, will be presented. Two special cases of actual overvoltages are addressed. Oscillograms are analyzed, and comparisons of measurements and calculations are made as well. Finally, possible applications for certain cases are discussed.

Modeling of asymmetrical cables for an accurate calculation of common mode ground currents

The paper deals with modeling methods for asymmetrical cables of speed variable AC motor drives to calculate precisely the common mode ground stator currents. The analytical and numerical calculation of the cable parameters is presented and the results are compared to measurements. An impulse run-time method for the cable parameter evaluation by measurements is proposed.

A network model for inverter-fed induction-motor drives

A network model is presented for inverter-fed induction motors operated at steady state. The model is based on the transmission line matrix (TLM) method considering the cable impedance L, capacitance C, conductivity G and resistance R described by 3times3 matrices combined with a 3 phase (abc)-model for the induction machine. For steady-state operation, the induction-motor model is reduced to a 3times3 frequency- dependent matrix which can be described in a very simple way. The 3-phase cable is modelled using the Transmission-Line (TLM) Theory where cable impedance L, capacitance C, conductivity G and resistance R are described by 3times3 matrices. The induction-motor is represented by a 3-phase (abc)-approach. In the steady state case this approach is simplified to a 3times3 impedance matrix which can be coupled with the TLM-model. The result is a transfer function which couples the input voltage with the output voltage at the motor. To calculate the voltage at the motor, the voltages pulses from the inverter are transformed into the frequency domain via FFT. In the frequency domain the voltage at the motor is easy computed by multiplication with the transfer function. The simulation results are compared to measurements for the voltage output of a drive with an inverter, cable and induction motor for which the parameters are well known.

Overvoltage protection of inverter-fed drives with the help of energy varistors - Dimensioning rules for consideration of different cable types

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltage in combination with very low rise times impose extreme dielectric stress on the insulation of electrical machines. With varistors it is possible to decrease these overvoltages in combination with an extended rise time of the impulses occurring at the machine terminal. During this service the varistor has to withstand the power losses and has to dissipate heat into the surrounding medium. The operation mode of the varistor strongly depends on the basic parameters like pulse frequency, rise time of the impulses at the inverter, applied cable, size of the motor etc. Here correlations between the voltage-current characteristic (U-I characteristic) of the varistor, the maximum occurring voltage steepness and the applied cable type are given in order to be able to carry out detailed dimensioning for different application cases.

Effect of varistors for overvoltage protection in inverter-fed drives on the voltage distribution in the stator winding

The use of varistors for overvoltage protection in inverter-fed drives can be a promising alternative to common solutions like filters. In this contribution the varistors influence on the voltage distribution in the stator winding is presented under certain aspects such as cable type and varistor type. The results of different measurements are shown and the main aspects are discussed. It is shown voltage amplitudes and voltage distribution can be affected positively to increase life time of the winding insulation of the machine.

Overvoltage protection of inverter-fed drives with the help of energy varistors — dimensioning and lifetime considerations

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltages in combination with very short rise times impose extreme dielectric stress on the insulation of electrical machines. With varistor discs it is possible to limit these overvoltages in combination with an increased rise time of the impulses occurring at the machine terminal. During this service the varistor has to withstand the power losses and to dissipate the resulting heat to the surrounding medium. Here, currents and power losses are focused in order to get better understanding of the mechanisms that lead to a reduction of the overvoltage at the machine terminal. Finally, a proposal for an accelerated aging procedure will be made that takes the given service conditions into account. First results are presented, and the chances of this application for overvoltage protection will be discussed as well.