The problem of electric currents passing through the bearings in an ac motor has been recognized since the 1920s. Today, however, adjustable- speed (A-S) drives and plants generating their own power are putting a modern twist to this problem that significantly shortens bearing life.

Determining the problem

There are two ways to determine if bearing currents are the cause of unexpected motor bearing failure: measure the shaft voltage or examine the bearings.

Motors at this paper mill were experiencing unexpected motor bearing failure from harmonics from the mill generating its own power. This is one of the three main causes of bearing currents. The solution was to insulate the bearings. Here, an engineer at the mill checks the installation.

All motors have some level of shaft voltage. Above a certain level, shaft voltage is a failure indicator. Generally endto- end shaft voltage should be less than 0.5 V. Normally, voltage levels below this will not cause harmful bearing currents. Engineers can measure the shaft voltage with any voltmeter that has an impedance of 10,000 ohms per volt or more.

When examining the bearings, look for specific types of damage. Bearing damage results when current is broken at the con- tact surfaces between rolling elements and raceways, also known as arcing, Figure 1. Damage from arcing is in proportion to the number and size of individual damage points. The size of the damage points depends on the magnitude of the induced voltage, the impedance of the current path, and the bearing type. The only observable effects of this damage on the bearings are pitting and fluting.

Figure 2 show a series of electrical pits in a roller and in a raceway of a spherical roller bearing. The pit grows each time the current breaks in its passage between the raceway and roller.

More serious electrical damage occurs when current passes during prolonged periods and the number of individual pits accumulates. The result is fluting, Figure 3. Fluting in anti-friction bearing races specifically indicates the problem is bearing currents. Fluting can occur in ball or roller bearings and develop considerable depth, producing noise and vibration during operation and eventual fatigue from local overstressing. Once fluting is started, it is self-perpetuating until the bearing fails.

Causes

Magnetic imbalances and harmonics causing shaft voltage are the most typical causes of bearing currents. Other causes include improperly grounded electric arc welding or static electricity from any manufacturing process that can develop a static charge, such as pumping or compressor applications.

Magnetic imbalances, which come from the design of the motor or its application, are considered the primary causes of bearing currents.

These imbalances come from:
• Make up of the steel inside the machine.
• Non-uniform magnetic flux path that can be through and between stator and rotor, around the axial vents, and through the shaft.
• Lack of magnetic symmetry in the housing. The frame structure that goes around the core may handle some of the flux, resulting in an imbalance. Some motors are more susceptible to this imbalance than others, depending on the frame and the size of the motor.

These imbalances can even influence one of the secondary causes of bearing currents — inducing shaft voltages to reach damage producing levels.

Shaft voltage is a voltage generated in the rotor. It seeks a complete circuit through its two bearings to ground, or through its outboard bearing and the connected machinery. Unless prevented from reaching high levels, over 0.5 V, it can cause chemical changes in the insulating grease, breaking it down and thereby making the grease act like an electrolytic in a capacitor.

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