A second insulating layer, on the inboard bearing, can prevent the noncirculating currents from flowing through the bearings. The voltage on the shaft remains, however, to seek another path to ground. The path it finds can lead through the bearings of the driven equipment or through the bearings of a shaft-mounted tachometer. Von Jouanne said the motor buyer can request insulated bearings from a manufacturer, although she thought the cost of this might be prohibitive for motors under 200 horsepower. A fluted race warns of bearing currents at work in motors under the control of PWM drives.
Fluting is especially likely when the motor runs at a constant speed.
Ceramic bearings are yet a third way to address the problem of bearing currents. The nonconductive balls block the electrical path from shaft to frame. As with insulated bearings, however, shaft voltage remains to seek another route to ground. Another detriment, said von Jouanne, is that ceramic bearings add cost and manufacturing time to a motor order. A suitably conductive grease could provide another low-impedance path for shaft voltage, she said.