Analysis of the Overvoltages in PWM-Inverter fed Induction Motors

An increasing part of low voltage standard induction motors operates with PWM voltage source inverters using IGBTs and they are applied to a wide range of power 0.1 kW up to some MW. The PWM (Pulse Width Modulation) inverter mode of control generates a large frequency spectrum of overvoltages. The resulting undesired overvoltages at the motor terminals stress the motor insulation. Usual standards applied to the induction motors recommend to limit the value of rate of rise of voltage to be less than (dv/dt) < 500-600V/mu s for the motor insulation to be healthy condition. This value is suitable for a motor fed directly by the AC power supply, but when the motor is fed by an inverter without any filtering device, the dv/dt may reach up to 6000-7000V/mu s or even more depending on the installation. This paper presents the simulation and experimental investigations of transient overvoltages on ac induction motors when connected through a cable to IGBT-PWM inverters. Considering classical transmission line and travelling-wave analysis with zero initial electric charge on cable, these transient over voltages are about twice the dc link voltage with a short rise time of about 200-300ns. Nevertheless for long cables, these transient overvoltages may reach 3 or 4 pu. In this paper, by using the system representation consisting of inverter-cable-motor, simulation is carried out to assess the motor terminal overvoltages. Experimental investigations on a low voltage induction motor in laboratory are carried out. The simulation results are compared with experimental results to validate the modelling of the system. The validated model of the system is used to study the influence of the cable length, rise time and motor rating on the overvoltages produced at motor terminal.