Motor Bearing Current Issues

Part 1 of a 2 Part Series on Motor Bearing Current Failures

More and more motors these days are being controlled by variable frequency drives. Often, the drives result in premature bearing failure due to stray currents. These currents result from the common mode voltage produced by the drive.

As seen in the figure above, current flows from the rotor/shaft through the path of least resistance which is generally the bearing balls and the races. These currents through the bearings ca lead to damage in the form of frosting or fluting. As the condition worsens it will usually result in audible noise coming from the bearings and will eventually manifest itself in bearing failure.

Flolo recommends the installation of a AEGIS Ring to the output shaft of the motor. This ring consists of numerous conductive microfibers which provide an alternate path for current to flow. Microfibers provide full 360 degree of coverage and provide for long bearing life – typically exceeds L10 life of bearing.

An alternative to the AEGIS ring is a Carbon Brush which again provides an alternate path for bearing current. The disadvantages of the carbon brush include that it only has one point of contact and it has a much shorter life expectancy than the AEGIS product.

In addition to the AEGIS ring or the carbon brush most motor manufacturers recommend the use of an insulated bearing on the non-drive end of the motor. Larger motors, over 100HP should utilize insulated bearings on both ends. To provide even better protection in high current applications, the addition of a high frequency connection which utilizes braided straps of copper 2-4 inches wide between motor and ground.

Next issue: Motor Bearing Current Solutions at the VFD.

The in-house technical specialists at Flolo Corporation have extensive knowledge of bearing current issues and solutions. Flolo is a certified AEGIS trained installer. Additional information on motor bearing currents is covered in extensive detail in classes available thru Flolo Tech. Please contact your Flolo sales representative for additional details.