Pulse width modulation (PWM) AC drives can generate elevated differential mode voltages due to the reflected wave phenomenon, fast rise times and peak voltages. While many users are familiar with differential mode voltages, common mode voltages created by PWM inverters are less familiar. Problems associated with common mode voltages can include erratic behavior of the controller resulting in time out errors, ground faults, and in the most extreme cases, premature failure of the motor bearings, motor windings, and cables.
Many solutions to protect motors are present in the market. This paper presents data showing how three filters: a typical dV/dt filter, a dV/dt filter utilizing a triple defense core, and a common mode choke, perform in providing both common mode and differential mode filtering. The data shows that a typical dV/dt filter does not provide common mode filtering and a common mode choke does not provide differential mode filtering. Only the filter containing the triple defense core provides both common mode and differential mode filtering to protect the motor. This paper will also discuss how the various topologies lend themselves to providing the required protection.