VSD’s Are The Largest Single Source Of Bearing Failures In The World Today.
So what are these high shaft currents and how do they damage bearings? Damaging voltages are induced on the shafts of AC and DC motors controlled by Variable Frequency Drives (VFD). The extremely high on/off switching speeds of the Pulse Width Modulation (PWM), generated by the Insulated Gate Bipolar Transistors (IGBT), induce damaging voltages onto the motor shaft through parasitic capacitive coupling between the stator and rotor. This common mode shaft voltage seeks a path to ground, usually through the motor's bearings.
Damaging currents arc through the di-electric oil film between the rolling elements and the bearing race. This is known as Electrical Discharge Machining (EDM) effect. EDM causes fusion craters, severe pitting, and eventually bearing fluting (a washboard-like pattern in the bearing race) which results in premature bearing failure.
One of the most frequently asked questions is, "Will electrical bearing damage always occur in a VSD application?" The answer is "Yes - with no alternate path to discharge shaft current, pitting of the motor bearing will take place during VSD operation. These discharges will continue and will always seek the path of least resistance - usually through the motor bearings". Yes, many motors still last for some time on a VSD application but bearing life is drastically reduced by up to 50 percent, even when the problem is not critical.
Bearing current problems are nothing new to engineers, but it is the alarming rate of increase in this problem that is becoming a worry to factories and mines. SGS, a company in the USA, undertook a survey on 1000 AC motors on Variable Speed Drives. 250 of the motors monitored had bearing faults appearing by month 18, and of motors averaging 24 months production time, 65% had electrical bearing faults appearing.
Outdated Solutions
As EDM is brought about by high shaft currents, the most effective way is to reduce these shaft currents. This has been assisted by the use of filters for variable speed drives. This has had mixed results and the research in not conclusive, therefore the filter manufacturers will not guarantee the products solves electrical bearing damage.
None have been 100% effective, and have performance (and / or high cost) problems of their own.
The next solution is to make sure that the high electrical currents and voltages that are passing the bearings on their way to ground, are redirected so that they do not pass through the bearings. This is the most effective solution, and there are many systems that have been in place over the years that try to solve this problem (mainly in relation to the old Eddy-current problem). None have been 100% effective, and have performance (and / or high cost) problems of their own.
The most popular solutions companies have used in solving Eddy-current problems, which are similar, but are not the same as VSD capacitive currents are the following:
Insulated bearings: These are effective but are not guaranteed against EDM by the bearing manufacturers. They also do not stop the bearing currents going into the driven equipment machinery (gearboxes, pump casings etc.), which can transfer the problems to other areas in which problems had not previously been experienced. With shaft voltages unable to escape through the motor bearings, there is a massive increase of current through the motor shaft. This can lead to the higher risk of ionization in hazardous areas. The initial, and on-going cost of ownership of insulated bearings globally is also very high.
Insulated bearing housings: Since normally only one insulated end shield is fitted, this solution is not 100% effective when used to protect the motors bearings against VSDs. In addition the capacitive currents differ from Eddy-currents, and the use of only one insulated housing would just increase the shaft currents in the other bearings. This method, of using two insulated housings per motor, is also very expensive as well as increasing the currents diverted to the driven equipment, and the associated risks involved.
Carbon block housings: Conventional shaft grounding brushes need frequent maintenance and become less effective over time. They rely on the spring tension to press the brush against the rotating shaft. This causes the brush material to wear - sometimes in as little as 3 months as it rubs on the shaft. In addition, oil, grease, dirt or oxidation will break the conductive path and reduce the carbon brushes from discharging shaft currents effectively.
Copper or bronze metal brushes: These brushes are very abrasive, wear out faster, and are therefore not maintenance free. These need constant monitoring to prevent the build up of contaminants.