Variable Frequency Drives

Insulating motor bearings is a solution that tends to shift the problem elsewhere as shaft current looks for another path to ground. Sometimes, because of the capacitive effect of the ceramic insulation, high frequency variable frequency drive (see http://www.vfds.org ) induced currents actually pass through the insulating layer and cause bearing failure. If attached equipment, such as a pump, provides this path, the other equipment often winds up with bearing damage of its own. Insulation and other bearing-isolation strategies can be costly to implement. IGBT VFDs have output risetimes from 50 ns to 400 ns. Maximum cable distances that limit motor terminal voltages to 480 V motor vendor capabilities of NEMA MG1 Part 30 of 1,000 Vpk (1.55 pu) or typical 1,200 Vpk (1.85 pu) can be determined from Fig. 2 depending on VFD output risetime. For example, a 400 ns IGBT VFD with a 1,200 Vpk motor can have a 150 ft cable length before external protection is required. The intersection of the motor capability line and the VFD pu overvoltage vs cable length curve in gives a maximum allowable cable distance. It shows a typical 480 V VFD output voltage vs. cable length intersecting a 1,000 Vpk and 1,200 Vpk motor capability line. Some VFD manufacturers have given maximum cable distances for each VFD hp using 1,000 or 1,200 Vpk motors, since output risetimes and cable / motor surge impedance mismatch changes at each VFD hp size. In a phenomenon known as fluting, a variable frequency drives operational frequency causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting can cause excessive noise and vibration that forewarn of imminent bearing failure.