Influence of the earth connections of the PWM drive on the voltage constraints endured by the motor insulation

The high electrical stresses in the Electrical Insulation Systems of machines fed by pulse width modulation (PWM) inverters remain a limitation of the lifetime. The stress is caused by repetitive over voltages caused by the short switching times of the PWM inverter power electronic circuit. This paper provides measurements made on an industrial drive connected to the grid with standard cables that include a ground connection wire, following the current standards. The phase-to-phase, phase-to-neutral, and neutral-to-ground voltages are measured focusing on short and repetitive voltage spikes. The causes of these voltage spikes are analyzed using a high frequency equivalent circuit of the whole drive, including the earthing system. The simulations are made with the well-known SPICE circuit simulator. This study focuses on the common-mode currents flowing in the earthing connections and its influence on voltage spikes. A solution that can reduce significantly the phase-to-neutral voltage spikes is proposed.