A new surface coating for rolling element bearings has been developed by Schaeffler that insulates the bearings from damage caused by the passage of electrical current in large AC or DC motors.
Schaeffler’s Isotect A coating is an insulating layer for rolling element bearings that can be applied to the outer or inner ring external surfaces. Alternatively, depending on the application, the bearings can be assembled with ceramic rolling elements.
In the past, bearings used in large AC or DC motors with only a few pairs of poles were prone to damage from the passage of electrical current. Today, due to the increasing use of frequency converters, this potential danger is growing. Around 3 per cent of all electrical machines manufactured worldwide are controlled by a frequency converter. This figure is expected to rise to 10 per cent over the next couple of years.
Isotect A acts like a resistor and capacitor connected in parallel. With DC voltage and low frequency AC voltage, the choice depends on the ohmic resistance of the bearing. With higher frequency AC voltage, which is often found in frequency converters, it depends on the capacitive reactance of the bearing. To ensure good insulation, the ohmic resistance should be high and the capacitance should be as low as possible.
Depending on the operating temperature, Isotect A has a high resistance ranging from several Ohm up to 10Gohm. To allow for the capacitive resistance of bearings used with a frequency converter, Schaeffler has selected not only the appropriate material for the insulating layer, but also the most suitable layer thickness, which both play a key role in ensuring optimum protection.
As capacitance is derived from the surface area of the protective layer and its thickness, the coating is applied to either the inner or outer ring external surfaces, depending on the application. If operating conditions require even higher protection, Schaeffler offers rolling element bearings assembled with ceramic rolling elements across most of its bearing range.
Types of bearing damage
There are typically two types of bearing damage caused by the passage of electrical current: “fluting” and “welding beads” (or craters).
Fluting is relatively easy to identify and runs transverse to the raceway. The washboard appearance of the resulting pattern represents the final stage of the destruction of the raceway in a bearing exposed to the passage of electrical current. Fluting can be caused by either a continuous flow of electrical current, EDM (electro-discharge machining) or high frequency electrical current.
Welding beads or craters represent the early onset of raceway destruction and the initial stages of fluting. Identifying craters and welding beads requires detailed examination of the structural changes that have taken place in the bearing rolling surfaces. This is greatly simplified by the use of a scanning electron microscope.
Current-related bearing raceway damage is typically caused by ‘classic bearing current’, in other words, the asymmetrical distribution of magnetic flux in mains-operated AC or DC machines, as well as to converter-induced bearing current (which occurs in the form of circular current, EDM current and rotor earth current).
Classic and converter-induced bearing current can severely damage the raceway structure and the lubricating grease, either through continuous current flow or in the form of voltage flashover and discharge. Capacitive bearing current found in converter operation is considered as harmless.
In electrical terms, a bearing can be thought of as simply a connection between resistance and capacitance of the lubricant. When the elasto-hydrodynamic (EHD) condition (i.e. the complete separation of the rolling elements by the lubricant) is achieved during operation, the bearing behaviour is capacitive. In this case, the lubricant acts as an isolator, preventing the passage of electrical currents up to a specific threshold value.
However, under conditions of mixed or boundary friction, the electrical behaviour of the bearing changes and is considered as an ohmic resistor. Here, the lubricant does not completely separate the rolling elements, resulting in roughness peaks and metal-to-metal contact. This can cause a continuous flow of electrical current through the bearing. During such periods of contact, the rolling elements fuse with the rolling surfaces for a short time and immediately break apart again due to the dynamics of the bearing. During this process, extremely fine metal particles are torn away from the bearing raceway and the rolling element surfaces, mixing with and contaminating the lubricant. These metallic particles are then rolled into the raceway, forming a new layer that does not have the same hardness as the original surface. The problem is exacerbated by system-related vibration and fluting.
Discharge flashes
Even though a protective lubricant layer may prevent a continuous flow of current through the rolling elements, converter operation can cause electrical discharges that penetrate the lubricant film. These “discharge flashes” blast fine metal particles out of the rolling surfaces, producing ‘craters’ with welding beads. The resulting metallic particles add to lubricant contamination. When the number of metal particles reaches a certain level, new contacts and bridges can be formed in the contact zones. Therefore, the process takes the same course as continuous current flow, resulting in fluting.
Experience gained by Schaeffler using its own and independent research has established that current densities of less than 0.1 A/mm2 do not represent a danger to bearings. However, current densities at or above 1 A/mm2 often lead to eventual raceway damage.
Not every bearing in an electrical machine will need to be fitted with one that has current insulating properties. With mains-operated machines, it is often sufficient to replace only one bearing with a current-insulated bearing in order to avoid possible current damage. However, when frequency converters are used, it is normally necessary to fit current-insulated bearings in several bearing locations.
For more information on Schaeffler’s Isotect A coated bearings or ceramic rolling element bearings, visit www.schaeffler.co.uk.
