Growing Use of Variable Frequency Drives(SKF)

Growing Use of Variable Frequency Drives(SKF) Phil Burge, Marketing Manager at SKF, discusses how the growing use of variable frequency drives is leading in many cases to bearing damage and failure caused by the affects of electrical currents, and explains how the latest generation of bearing technology has been developed to counter the problem. The increasing popularity of variable speed drives throughout industry is leading to significant improvements in productivity and flexibility for manufacturing, processing and packaging companies alike, offering improved efficiency of motor driven equipment by matching speed to changing load requirements, and enabling accurate and continuous process control over a wide range of speeds. However, this technology, combined with the effects of using larger and more powerful motors and generators, is leading to an increase in the detrimental affects of electrical currents on bearings and lubricants, often leading to complete bearing failure. This issue of electric current passing through rolling bearings and having a detrimental affect on the raceways of the inner and outer rings is not new, but the problem is becoming increasingly widespread. In addition to damage to the bearing elements, damage to lubricants has also been noted in a variety of rotating machines including motors and generators. Traditionally bearing currents have been generated due to asymmetries in a motor's magnetic circuit, with an asymmetric flux distribution inside the motor inducing an axial shaft voltage, and leading to a low frequency circulating current flowing through the bearings. Bearing current has also been seen to be generated by asymmetric, non-shielded motor cabling.Both these conventional causes are a particular problem in large motors with low numbers of pole pairs, which have larger flux asymmetries than small motors or motors with many poles. However, the problem is now arising from another source, where variable frequency motors incorporating pulse width modulation (PWM) converters are used. The high frequency current originating from the common mode voltage of the converter can cause significant damage to bearings, and so too can current originating from the high switching speed of the integrated gate bipolar transistors (IGBTs) used inside the converter. Another source of damage to bearings originates from the way in which the frequency converter tries to simulate a sine wave supply by PWM signals, which have a high switching frequency and very steep-edged pulses, which cause capacitive discharge currents.These currents can severely damage bearings, leading to reduced operating life, lower performance and, in some cases, failure. Heat is generated as a result of the passage of electric current in the contact zone of rolling elements and raceways, causing localised melting on a bearing's surface. Craters form in the contact area and particles of molten material are transferred and become partially detached; therefore when the bearing cools the crater material is re-hardened but is with much more brittle than the original bearing material, a layer of annealed material, which is softer than the surrounding material, being left. With the growing use of variable frequency drives, micro cratering is by far the most common affect of electric current passage. Multiple micro craters, typically between 5 and 8um in diameter, cover the rolling element and raceway surfaces, and although they can only be seen under a microscope at extremely high magnification, result in a dull finish and reduced performance. As well as this problem with rolling elements, fluting or washboard effects can also be seen as patterns of multiple grey lines, which appear shiny or molten, across the raceways. The reason for this fluting is a mechanical resonance vibration caused by the dynamic effect of the rolling elements when they are over-rolling smaller craters.Rather than being caused directly by electric currents upon the bearing, fluting is secondary damage, only visible after craters have been formed. A third result of electric current discharge on bearings is grease blackening, where lubricant in the bearings changes its composition and degrades rapidly.This is due to localised high temperatures that cause additives and the base oil to react, leading to burning or charring of the base oil. In addition, additives are used more quickly, which results in the lubricant becoming almost hard and blackened.A rapid breakdown of grease is a typical result from the passage of electrical current. Although it is possible to combat the effects of electrically generated currents by insulating either equipment housings or each rotating shaft, both of these solutions are relatively expensive and require additional components, leading to increased maintenance costs. By comparison, the latest generation of bearing technology, including SKF's INSOCOAT bearings, make it possible to eliminate the need for additional insulation yet still provide an excellent degree of protection to each bearing. These bearings feature a specially developed surface coating, just 50um in thickness, which acts as an electrical insulator to provide protection against electrically induced flashovers of up to 500V, with thicker coatings also being available to withstand discharges of up to 1,000V. The bearings offer identical performance to conventional devices and can be fitted using normal techniques.The application of this hi-tech coating is made possible through plasma spraying, which applies a coating to the inner or outer ring of a bearing by injecting aluminium oxide powder into a high temperature gas stream. This then heats the powdered coating material to a molten state and sprays it onto the surface of the bearing at high speeds.This process results in a durable and consistent finish, which is unaffected by extremes of temperature or humidity, and can be used with all common lubricants. A further example of this new protective technology are SKF's Hybrid bearings, which incorporate silicon nitride rolling elements to provide excellent insulation properties, making them ideal for isolating housings from shafts in motors and generators. These bearings offer extended service life when used in conjunction with variable frequency drives. Silicon nitride rolling elements also have a lower thermal expansion than steel rolling elements of similar size, which results in less sensitivity to temperature gradients within the bearing, and more accurate preload control. While the use of variable frequency drives offers numerous benefits, including increased control and greater energy efficiency, precautions must be taken in order to avoid premature bearing damage or failure. With the latest generation of bearing technology, protection is now available without the need for costly insulation of equipment housings or rotating shafts.As a result, engineers can now cost effectively take advantage of the benefits of VFDs without sacrificing bearing performance.