Avoiding premature bearing failure with inverter fed induction motors

The constant product development associated with variable speed drives (VSDs) has enabled these devices to be applied to almost every type of motion control or variable speed application with advantages over previous systems. Faster switching devices (typically IGBTs) have facilitated these advantages. However, along with the benefits, such advances often bring engineering compromises. In some cases, the power semiconductor issues have been amplified by the use of nondeterministic (non-PWM) switching algorithms such as direct torque injection schemes which may exacerbate any problems. Along with previously covered aspects such as EMC and peak motor voltage issues, there is increasing concern amongst users that modern inverters may, in certain cases, reduce the lifetime of the motor bearing elements due to undue currents which are thought to circulate through the bearings. A considerable number of reports and research papers from North America suggest that bearing current phenomena are the root cause of a considerable number of bearing failures. Some reports indicate that around 8% of bearing failures are due to this issue in VSD fed motors. This is in contrast to Europe where very few incidents have been reported to date. Whilst few failures seem to be confirmed in Europe, the issue warrants further investigation. This paper therefore seeks to highlight any potential areas of concern in premature bearing failure, explain the associated mechanisms, and examine the available preventative measure, with the aim of providing the necessary user guidance