Motor winding redesign `eliminates bearing currents`

ABB has developed a new type of motor winding that it claims will eliminate the problem of circulating bearing currents that can cause motors to fail after a few months of operation.

Incidents of bearing current damage have been mounting in recent years with the wider use of variable speed controls operating at high switching frequencies. The repeated discharging of high-frequency currents to earth through the motor bearings can score the bearing races and cause them to fail.ABB`s new winding design is the result of three years of testing and development at its Finnish motors operation.

"The experiments showed that the problems were caused by asymmetric distribution of high-frequency currents in the motors," says Tapio Haring, vice-president for technology at ABB Motors. "Once we had established this, finding a solution was relatively straightforward."

The new design involves splitting the windings into two equal parts and wiring these back separately to the drive and the non-drive ends of the motor, before routing them to a centrally placed terminal box.

Haring reports that other motor manufacturers are working along similar lines, but adds that "they seem to be at the stage where we were three years ago, and so are not likely to be able to come up with an alternative solution in the near future".

The first motors to use the new winding arrangement are likely to reach the market by the summer of this year. ABB is still deciding whether to incorporate the new windings in all of its motors or to produce a special range of machines that are immune from the effects of bearing currents. ABB has been granted a US patent for the new winding design and a European patent is pending.