The additional signal can be obtained
by a suitable estimation of the motor CMcurrent by sensing
the CM voltage at the inverter terminals and applying it to a simple equivalent circuit reproducing the CM high-frequency
motor impedance. In practice, the need for the knowledge of
the CM high-frequency motor model is not a limitation, since
the most relevant CM phenomenon, to be reproduced, is tied to
the front-end capacitance toward ground. Therefore, the motor
model can be approximated by only this capacitance, whose
value is nearly constant for induction motors belonging to a
Fig. 2 shows a block diagram of the proposed active filter,
where F(s) is a low-pass filter, T(s) is a transfer function,
including a linear amplifier, a power amplifier, and a commonmode
transformer (CMT) and G(s) is the transfer function of
the CM current estimator. Each block is described in detail in
the next section.