All pump system operators and maintenance personnel must be aware of any side effects that can result from ongoing efforts to improve the energy performance of pump systems. Negative effects can be associated with modern variable frequency drive (VFD) systems, particularly with regard to the motor’s bearing supports. To be clear, this is not about discouraging the continued proliferation of these highly effective methods; rather, it is important to understand the conditions that lead to early bearing failure.
VFD Systems & Bearing Voltages
Many pump operators have achieved significant energy savings by varying the pump speed to match the demands of the attendant system. Although there are many ways to achieve this objective, the most common is the use of electrical control or VFD systems.
A typical VFD system consists of an alternating-current (AC) motor, a controller and an operator interface. VFD controllers are solid-state, electronic power-conversion devices. Most VFD inverter circuits use an insulated gate bipolar transistor (IGBT) to perform the high-speed switching required to control speed.
The high-frequency switching of the IGBT will create parasitic capacitances in the pump motor’s major components, including the rotor and stator. Because the bearing acts as the bridge between these components, it can be exposed to these differences in electrical potential. In many cases, the bearing mitigates this situation because of design features that offer inherent resistance to current.
In some cases, however, the electrical potential across the bearing arcs through the contacting points, resulting in a phenomenon typically known as electrical erosion. This effect can cause the bearing surfaces to deteriorate.
First, a quick primer on basic bearing design, which will focus on ball bearings because they represent the most common type of bearing used in motor applications: Under load, the points of contact between the balls and races will create a pressure ellipse because of the elastic nature of the materials selected. The exact shape of the ellipse depends on several factors, including the following:
- the direction and magnitude of the bearing load
- the number and distribution of the load on the bearing’s rolling elements
- the geometric relationship between the race and the rolling element
- the modulus of elasticity of the bearing materials
Figure 1 shows a typical loading situation on a ball bearing used in an electric motor application. The red areas represent the balls that are loaded during operation.
During normal operation, a properly lubricated bearing will generate an elastohydrodynamic lubrication (EHL) film between the rolling elements and the races. This film prevents surface-to-surface contact and surface damage (see Figure 2). In electric motor applications, the film can also act as an insulating barrier against mild voltage potential across the bearing. In addition to the normal factors influencing the level of protection afforded by the EHL film, there are other considerations including lubrication degradation and contamination.
When a VFD system is introduced, the bearing voltage potential typically increases and can exceed the EHL film’s insulating value. The resulting damage can vary widely. If the voltage is low enough, the bearing can survive and the condition can go unnoticed.
Under certain conditions, serious damage caused by instantaneous electrical arcing (EA) between the rolling element and the race can occur. A single EA event can create small craters as the charge rapidly passes through the lubrication film and actually melts the material at the point of contact.
Once a crater has developed, it can disrupt the EHL film for subsequent rolling elements as they pass over the damaged area. This has the potential to create even more craters. Other resulting debris will cause the lubricant to degrade and produce a stress riser, which is formed by the debris that has found its way between the roller and the race.