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Question

Answer

Geoff Brown, Drive Applications Consultant, ABB Limited 

Bearing currents are the common name for currents induced in the motor rotor and discharged to earth through the bearings. They can cause premature bearing failure in drive systems.

Bearing life in an incorrectly installed new drive system can be extremely short, sometimes only a few months. When nearly new bearings fail, this is likely to be due to high frequency common mode voltages, generated in the drive system and allowing a current to circulate through the bearings. Bearing currents are quite normal in motors and have always existed; they were first documented around 100 years ago. But while modern design and manufacturing practices have nearly eliminated bearing failures from such currents under normal circumstances, modern drives which have high switching frequency and fast rising voltage pulses can induce relatively high voltages in the rotor. The result can be that the repeated discharging gradually erodes the bearing races, giving a characteristic fluted appearance to the races.

Historically, motor manufacturers have been well aware of the possibility of bearing current problems within some types of networks and have developed solutions intended for large fixed speed drives.

While circulating currents have been around for as long as there have been electric motors, it is only with the widespread application of drives to larger motors that the problem has become more well known. The problem is related to the physical size of the motor, and occurs very infrequently with motors of less than 90 kW or in frames below an IEC 280 size.
The factors that affect the formation of bearing currents in VSD applications will also vary with the type of installation, the supply voltage, the motor impedance, the switching rate of the inverter and even the type of semiconductors used.

Good cabling and earthing practice will minimise the possibility of problems for low power drives, insulated bearings breaking the current path in larger drives generally prove highly effective,. It is therefore important to look carefully at the motor before installing a new drive, and seeking the manufacturers' advice regarding the need for insulated bearings.  Insulated bearings are widely available in the same sizes as conventional types.

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