High Performance Control of AC Drives with Matlab / Simulink Models

9.5 Bearing Current Types and Reduction Methods

The motor bearing currents are of several types. In Figure 9.11, four classes of bearing current are presented.

Figure 9.11 Bearing current classes [6]

The capacitance bearing current has a small value close to 510 mA at temperature T_b ≈ 25 °C and motor speed n ≈ 100 rpm. For higher temperatures and motor speeds, the current increases but will not exceed 200 mA. Contrary to other bearing current values, the current is negligible for bearing degradation analysis.

The machine discharging current appears when the bearing oil film breaks down. According to the literature, the value of that current peak is up to 3 A [6].

The bearing circulating current i_bcir is induced by a motor shaft voltage u_sh. The u_sh appears due to parasitic motor flux ψ_cir. In [6], the practical condition for i_bcir calculation is given by

(9.9)

where i_g(max) is the maximum value of the measured grounding current.

An in-depth explanation on the particular bearing currents calculation and measurements can also be found in [6].

The rotor grounding current can appear when a galvanic connection between the rotor and ground appears, i.e. caused by a load machine coupled with a motor shaft.

In [6], the types of bearing currents are correlated with motor mechanical size ...