Protecting bearings against damaging electric currents
09 June 2013
Electrically insulated rolling bearings feature electrical insulation that is built into the bearing, providing reliable protection against the passage of damaging electric currents. Klaus Grissenberger reports.
Under adverse operating conditions, bearings used in electrical machines can be damaged by the passage of electrical currents. Electrical discharges occur in the contact zone between the rolling elements and the inner or outer ring raceway, causing localised melting of the surfaces, which in turn leads to cratering, transfer of material and localised damage to the microstructure as the material is exposed to thermal stress.
At least a very thin layer of the eroded surface undergoes a second hardening process and becomes extremely hard and susceptible to cracking. This process is referred to as electro-corrosion. It also renders the lubricant unusable.
The base oil and the additives contained in the lubricant are oxidised, resulting in a typical black discolouration of the lubricant. This premature ageing permanently impairs the lubricant’s ability to keep the metal surfaces apart. The damage to the functional surfaces of the bearing and the loss of lubrication leads rapidly to failure.
The main reasons for current discharge are known. Asymmetries in the magnetic flux of an electrical machine give rise to a low-frequency voltage between the shaft and the housing, which can result in a flow of current through the rolling bearings. Such a passage of current can also result from the use of unshielded asymmetrical electric cables if the earth connection of the machine is ineffective.
Another cause may be the use of frequency converters. The operating principle of frequency converters is based on pulse width modulation (PWM) and generates high-frequency common mode voltage, which can result in a passage of current through the bearings. Electrostatic charging of the shaft and housing followed by discharge through the rolling bearings is another possible cause.
Symptoms and countermeasures
Typical signs of electrical corrosion include grey, tarnished tracks in the raceways and on the rolling element surfaces. Melt craters or fluting (see Figures 1a and 1b) are also discernible mainly on the raceway surfaces. Damage due to current discharge usually manifests itself in increased running noise.
In order to prevent damage of this kind, it is advisable to insulate the bearing seat in the housing or on the shaft. However, this entails changes to the design of the surrounding parts. A simple, economical solution is to use electrically insulated rolling bearings, as they can replace non-insulated types without change to the overall design.
Electrically insulated bearings are provided with an oxide ceramic insulation layer and offer exactly the same outer dimensions and technical characteristics as corresponding non-insulated types. Moreover, the use of electrically insulated bearings is less expensive than applying insulation to housings or shafts. They are interchangeable with conventional bearings because they have the same key dimensions and technical characteristics.
Electrically insulated bearings comprise cylindrical roller and deep groove ball types, and their applications extend from the traction motors that drive railway locomotives to industrial electric motors and generators – particularly those used in conjunction with fast-switching frequency converters.
The insulation layer
Insulated bearings supplied by the author’s company have a coating that is applied by plasma-spraying, either to the outer ring (see Figure 2a) or to the inner ring (see Figure 2b). The plasma jet serves as a carrier medium for applying aluminium oxide powder either to the outer or inner ring at high velocity. In order to obtain optimum protection the oxide layer also covers the side faces of the sprayed rings. In a following process, the layer is sealed to prevent any moisture penetration.
The physical effect of the insulation layer depends on the frequency of the voltage that causes the harmful currents in the bearings. In the case of dc voltage, the insulated bearing has resistance. The higher this resistance is, the lower the electrical current. The resistance value of the insulated bearings is greater than 50MO, thus limiting the electrical current to a level that cannot cause damage to the bearing.
The capacitive nature of the insulated bearing is advantageous in the case of ac voltage. The bearing then behaves in approximately the same way as a parallel circuit consisting of a resistor and a capacitor with a frequency-dependent resistance (impedance). The impedance determines the magnitude of the alternating current that flows through the bearing for given values of voltage and frequency. Here too, the impedance should be as high as possible in order to reduce the current to a level that is incapable of damaging the bearing.
In order to achieve high impedance values, the resistance of the insulating layer must be high and its capacitance must be low. This can be achieved by making the insulating layer as thick as possible and by reducing the overall insulation surface area. When transferred to the bearings, this means that this layer should preferably be applied to the bore of the inner ring.
However, the coating is usually applied to the outer diameter for reasons of cost and due to the constraints of the manufacturing process. In most cases, this still results in more than adequate protection against damage from electrical corrosion.
Klaus Grissenberger is an application engineer with NKE Austria GmbH
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