When ac motor bearings fail, the culprit is rarely thought to be bearing currents. However, due to technological advances, bearing failures attributable to this phenomenon are increasing. Here’s a look at what bearing currents are, their causes, and how you can prevent them.
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.
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.
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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When a motor operates on sinusoidal power, a safe low level for voltage along the length of the shaft is less than 0.1 V. If a motor operates off an adjustable speed power supply, high frequency transient voltage spikes can cause this voltage to measure appreciably higher.
Harmonics in the voltage supplied to the motor can occur because more facilities are generating their own power, more motors are powered by adjustablefrequency drives, and more drives now use IGBT rather than SCR and GTO technologies in the drive inverter section. (For more on the effects of IGBT technology on motors, see PTD, “Solutions to motor insulation failures,” 7/95). As harmonic content in an adjustable-speed motor-drive system increases, motors that previously had no problem with shaft currents may begin to develop rapid bearing failure.
There are three ways of solving bearing current in any size motor: mechanical shaft grounding, harmonic suppression, and insulating the bearings.
Some third party vendors have attached devices to the shaft with contacting elements that ground it. This is a workable solution, but not the only one or necessarily the best, except for field retrofits.
To control the harmonic content in the power systems, several manufacturers offer line-to-line and line-to-ground sinewave output and common-mode filters for their adjustable-speed drives. These filters reduce harmonics, audible noise, motor temperature, and vibration and increase the life of the motor and its windings. The cost of the filters as well as their tendency to reduce motor-drive efficiency because of extra heat generation determine whether this solution is workable.
Normal production motors of 440 frame and smaller do not have insulated bearings. If bearing currents cause a problem, your motor manufacturer can provide motors with insulated bearings. Insulated bearings are an option on motors of 580 frame and larger, and it is generally accepted, in the U.S. at least, that adjustable-speed drives or motors above NEMA or 440 frames should have insulated bearings to prevent the flow of shaft currents, Figure 4.
It is only necessary to insulate one outboard bearing to open the circuit. Special provisions are necessary in certain cases to maintain an open circuit to the load or another motor, such as insulted drive couplings in large tandem motor arrangements.
Different insulation techniques and materials are effective:
• An insulated film is placed between the bearing mounting ring and the frame of the motor. Any object in contact with the bearing ring — pipes, tubes, washers, and the bolts used to mount the bearing ring to the frame — should also be insulated.
• Use of resin impregnated glass tape rather than film.
• Resin impregnated glass fiber may be applied to the shaft and the bearing.
• Fabric-base impregnated material may be molded to size, then glued to the outer diameter and axial locating surfaces of a bearing or bearing mounting ring.
Insulating the bearing will not eliminate voltage on the shaft, but it will prevent that voltage from using the bearings as a travel path, thus causing bearing damage.
Paper mills solves bearing current problem
In some applications, currents under a pure sine wave may be double or triple that of factory levels. A paper mill, for example, had a problem with machines that were not running on adjustable-speed drives but were located near them. These drives produced harmonic content that fed back into the power system, affecting the surrounding motors and products.
This phenomenon was the result of the facility generating its own power. Their power system was relatively small, so the drives had a disproportionate impact on the motors. (This scenario is unlikely to occur in a plant with a large electrical supply system and one or two adjustable-speed drives in use).
The problem at the mill was solved by insulating the bearings, not putting the plant on the grid. However, plants with adjustable-speed drives and small power systems could benefit from a check of all motors for shaft currents, whether connected to the drives or not.
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Other potential bearing problems
Manufacturers design motor bearings for long life. In an ideal bearing installation, bearings will ultimately fail from fatigue of the bearing metal. Often, though, bearings never attain their calculated life expectancy. When bearings fail prematurely, the blame is often laid on poor lubrication. But by not analyzing the problem further, one can miss the real culprit, which can be:
• Defective bearing seats on shafts and in housings.
• Faulty mounting practice.
• Incorrect shaft and housing fits.
• Ineffective sealing.
• Vibration while the bearing is not rotating.
• Passage of electric current through the bearing.
William R. Finley is manager of engineering and Robert R. Burke is manager of engineering development at the Motors and Drives Div., Siemens Energy & Automation Inc., Norwood, Ohio.