Abstract – An insulation system for a 3 kV, 110 kW, random-wound induction motor stator is presented. The machine is driven by a seven level inverter consisting of 690 V IGBT inverter modules. Due to fast switching times of IGBT modules, the inverter can cause un-even voltage distribution in the motor windings. This means that up to 75 % of a rapid voltage change may exist in the first turn of the coil, and therefore, the potential difference between the first turn and other turns may be high. This may lead to partial discharge activity and in the worst case to direct breakdown.

 A composition of a mica tape and PET film is used as groundwall insulation. PET films are installed on both sides of the mica tape in order to keep the tape in decent form during the curing of the bonding resin of the mica tape. Similar insulation structure was installed also over the first turn of a coil in order to prevent problems caused by un-even voltage distribution.

 After the curing of the bonding resin of the mica tape, the stator was impregnated with a polyester resin using VPI method. Testing according to IEEE standards was performed for the stator winding after a testing drive with the inverter. According to the tests, the insulation system was found to be in very good shape.