Report gives advice on bearing currents

Two UK trade bodies have drawn up a technical report on the problems of shaft voltages and bearing currents in motors controlled by PWM (pulse width modulation) inverters. The report*, which is available as a download from the Internet, is designed to act as an authoritative guide to good practice to minimise these problems.

The guidance has been prepared jointly by Rema (the Rotating Electrical Machines Association) and Gambica, the trade body representing the UK`s automation and controls sector. It supplements the IEC 60034-17 (1998) standard and an earlier user guide on the problem produced by the same bodies.

The new document covers the basic operating principles of PWM inverters with an emphasis on those using IGBT (insulated gate bipolar transistor) switching devices. It looks at the various mechanisms whereby voltages can be induced in motor shafts and their effects, and gives advice on measures to mitigate these effects to ensure long-term motor operation.

Although the recommendations can be applied to all motors and inverters linked by a separate cabling system, it makes specific references to products sold by Rema and Gambica member companies.

* The report, Motor shaft voltages and bearing currents under PWM inverter operation, can be downloaded from Rema`s Web site.