The Influence of Volts/Hertz Control on Filter Impedance Effects in VSI-Fed Induction Machines

The use of a transient volts/hertz controller can produce great improvement in the dynamic behavior of a voltage source inverter (VSI)-fed induction machine with a dc link filter. The effects of transient volts/hertz control are investigated and methods for selecting the proper controller gain GW for a wide range of systems are given. The information is presented in terms of 1) the upper and lower critical gains (GW
CU
and GW
CL
) needed to ensure stability, 2) a form of optimum gain (GW
B
) which gives the best dynamic performance for a specific machine and operating point, and 3) an acceptable gain (GW
A
) which gives acceptable dynamic performance over a range of operating conditions. Utilizing nondimensional parameters, eigen-values, and root locus plots, the values of the gains of the volts/hertz control are studied for a wide range of parameters. Graphs of the critical gains and methods of choosing GW
B
and GW
A
are presented. The results indicate that volts/hertz control can always be used to improve system dynamics, and with properly chosen controller gain the dynamic performance of the inverter-filter-motor system is typically better than the motor alone for a wide range of motor sizes and operating conditions.