High Performance Control of AC Drives with Matlab / Simulink Models

9.4 Induction Motor Common Mode Circuit

The motor has some parasitic capacitances with small values, in the order of picofarads. The CM voltage on the stator windings creates a shaft voltage by capacitive coupling through the motor air gap. Therefore electrostatic discharges are generated through the bearing lubricating film. Such parasitic currents cause a reduction in the lifetime of the bearings. The motor's main parasitic capacitances are presented in Figure 9.8.

Figure 9.8 Parasitic capacitances in the induction motor

In Figure 9.8, the following motor parasitic capacitances are labeled as:

• C_wf – parasitic capacitance between stator windings and stator frame;
• C_wr – parasitic capacitance between stator winding and rotor winding;
• C_rf – parasitic capacitance between the rotor and frame;
• C_b – equivalent capacitance of motor bearings;
• C_ph – parasitic capacitance between stator windings.

All the parasitic capacitances, except C_ph, are significant for the CM. So in the next explanation, the phase-to-phase capacitance C_ph is omitted.

The C_wf capacitance is dependent on the motor's mechanical size, according to empirical relation [6]:

(9.8)

where C_wf is in [nF] and motor mechanical size H is in [mm].

The bearing capacitance is dependent on the bearing dimensions and mechanical properties. ...