The use of Variable Frequency Drives (VFDs) to control AC motors has increased dramatically in recent years due to their energy savings, low operating cost, and high performance. However, the challenge with VFDs lies in the destructive electrical currents (voltages) that are induced along the rotating motor shaft and discharged to the frame through the bearings, leaving damage to the bearing surfaces known as Electrical Discharge Machining, or EDM.
One of the first “symptoms” of bearing current damage is audible bearing noise. Unfortunately, by the time the bearing noise is noticeable, deterioration due to EDM has already occurred and catastrophic bearing failure is likely not far behind.
This paper will discuss specific types of bearing damage caused by EDM, identifying the root cause of EDM using vibration analysis and shaft-to-ground voltage/current analysis, and factor that contribute to EDM and how to protect against it.
Types of Bearing Damage Caused by EDM
Bearings are designed to operate with a thin, protective layer of oil between the rotating ball and the bearing race. When the voltage accumulating on the motor shaft exceeds the dielectric strength of the lubrication, the voltage seeks the path of least resistance to the ground. That path is the metal component – typically the bearing or seal closest to the shaft. The electrical arcing to the component is called electrostatic discharge.
Frequent electrostatic discharging can cause EDM which creates three types of damage to the raceways and rollers: pitting, frosting, and fluting.
Pits or fusion craters are caused when the electrical current passes from the shaft, through the inner race, through the rolling bearing elements, through the inner race, and then passing from the motor housing to the ground. These are generally not visible to the naked eye and require a microscope for identification. However, audible noises caused by the rolling elements riding over the pitting are often present. See Figure 1 below.
Frosting appears as a grayish discoloration, generated by either electrical or mechanical means like pitting, often requires a microscope to identify. See Figure 2 below.
Fluting refers to concentrated pitting that occurs at regular intervals along the bearing raceway, creating a washboard-like pattern. It’s caused by the arcing of the current between the races and rolling elements, and unlike pitting and frosting, is visible to the naked eye. The presence of fluting is typically indicative of sever bearing damage, eventually leading to complete bearing failure. See Figure 3 below.
Identifying the Root Cause of EDM
Currently, there are two types of non-destructive testing methods commonly used to identify bearing currents and resulting damage: vibration analysis and shaft-to-ground voltage/current analysis. While these methods can be implemented for the specific purpose of confirming the presence of bearing current, there are significant advantages with incorporating these methods into a regular maintenance program. By doing so, a baseline can be established and trends can be closely monitored, thereby providing early detection of any issues.
It is important to note that both vibration analysis and shaft-to-ground voltage/current analysis require highly specialized equipment and should therefore be conducted by an experienced professional familiar with the equipment as well as data analysis. Massive amounts of data are of no use if not interpreted correctly.
Vibration analysis is an effective tool for detecting fluting damage to a bearing caused by EDM. In the early stages of fluting damage, the high frequency, high-resolution spectrum of a bearing will reveal a cluster of energy in the 2-4 kHz frequency range. As the bearing continues to deteriorate, the fault energy spikes will migrate into lower bearing fault frequencies. This is an excellent example of the benefit of having continuous monitoring of vibration levels in place beginning shortly after installation. The appearance of atypical increases or changes in energy spikes has a better chance of being detected early if there is a baseline with which to compare it against.
Figure 4 below is an example of fluting detected from wind turbine remote analysis.
Voltage and Current Analysis
Measuring the actual voltage and current present on a shaft is extremely helpful when attempting to confirm the presence of EDM damage. Shaft voltage testing can clearly determine whether or not capacitive discharge current is flowing through the bearing. This is done using a shaft voltage tester such as the one pictured in Figure 5 below from AEGIS®. The device measures the potential difference between the motor shaft and frame which will be equal to the voltage across the bearing.
To conduct current analysis using an electrical discharge detector, an average of the electrical flux generated around the motor casing is taken in 30-second intervals. Once a value is acquired, a determination can be made as to whether the electric current passing through the bearing is causing the defect. It should be noted that electrical discharge detectors are most reliable when used with high-power motors over 100 HP.
There are three types of electrical currents induced along the motor shaft in a VFD: Capacitive Current, High- Frequency Current, and Rotor Ground Current.
Capacitive Discharge Current
In a capacitator, energy is stored by having two conductive plates with opposite polarization sandwiched around an insulator. In the case of a motor, the rotor and stator generate capacitive current EDM due to their close proximity, polarization, and the air acting as the insulator. See Figure 6 below.
High-Frequency Circulating Current
All VFD-involved bearing currents involve high frequencies. Whereas the capacitive current discharges in one direction – from shaft to frame or frame to shaft, the high frequency circulating current travels in opposite directions across each bearing simultaneously; from frame to shaft at one bearing and shaft to frame at the other. Charge flows onto the shaft at one end and off of the shaft at the other, in a roughly circular pattern. See Figure 7 below.
Rotor Ground Current
In short, this type of current is a lot like the capacitive current. However, it is only a problem when the motor frame is poorly grounded.
It should be noted that since all motors have some level of shaft voltage, it is critical to determine at what level, once reached, would be cause for alarm. The difficulty lies in the fact that predicting EDM damage is not an exact science. There is no “industry standard value” that can be applied to all machines to determine whether a particular machine is “safe” from EDM damage.
Additionally, there are many variables that must be taken into consideration when determining an alarm level including but not limited to running speed, bearing lubrication, measurement equipment, and method.
To repeat, there is great value in trending the correct measurement values amidst so many other indicators. Measurement and trending of shaft voltage/current data can lead to early identification of EDM damage. Early identification means planning the right maintenance on the right location at the right time.
Factors That Contribute to EDM Damage
There are certain situations that can contribute to the occurrence of EDM damage including:
· High carrier frequency
Generally speaking, the higher the carrier frequency, the quieter a system will run but the onset of damage caused by EDM will occur more rapidly. That’s why most experts recommend determining the safest frequency for your system by adjusting it as low as possible without creating unacceptable audible noise and avoiding frequencies above 6kHz altogether. It can be helpful to install a VFD with a small increment (i.e. by 1 kHz) adjustable carrier frequency to allow for fine-tuning to the lowest possible level.
· Inadequate grounding
It is critical, especially when dealing with the high frequencies of VFDs, to provide a low-impedance path for the electric current to flow to the ground so that it doesn’t get there via the motor bearings. While bearing current is unavoidable to some degree, maintaining a low-impedance path to grounding will help to prevent damage caused by EDM.
· Constant speed operation
There is debate as to whether constant-speed operation makes VFD-controlled motors more at risk for developing bearing current damage. However, since VFDs are rarely used in applications that require constant speed, the question is moot.
Protecting Against EDM Damage
While there are many options currently available to protect bearings from EDM damage, we will focus on the most frequently used methods:
· Shaft grounding/protection ring
For motors under 100 HP where Capacitive Current will likely be the failure mode, the “fix” is to install a Bearing Protection Ring on the motor’s DE. Doing so would in effect “short circuit” the current, creating a simpler path, straight to the ground with less resistance than passing through the bearings.
· Insulated bearings
For motors over 100 HP, both the Capacitive Current and the High-Frequency Circulating Current can be the cause of damage to bearings. Therefore, not only does the Bearing Protection Ring on the
motor’s DE need to stay in place, but also, an additional layer of protection (e.g. an isolating or
insulating bearing) needs to be installed on the motor’s NDE to protect the NDE bearing from the High Frequency Circulating Current.
· Ceramic bearings
Ceramic bearings generally consist of ceramic rolling balls with an inner and outer steel race. The conductive properties of the ceramic balls prevent the discharge of shaft voltage through this type of bearing, thereby forcing the current to seek an alternate path to the ground.
Protection Starts at Installation
Because deterioration caused by electrical currents can be a complex issue, various precautions can be taken at the time of installation including:
- The use of shielding cables such as continuous corrugated aluminum sheath cables to improve high- frequency
- Reducing the switching frequency, as higher switching current frequencies can lead to the current having more opportunities to
- Installing inductive line filters (outside of the drive) for long cables
- The use of insulated
EDM damage can be catastrophic if left unaddressed. Yet, spending money on measures to protect against damage that may or may not happen can be a tough pill to swallow. However, the price paid for unplanned downtime often far exceeds the cost of implementing protective measures and continuous monitoring that can catch issues early on.
About IVC Technologies
IVC Technologies is dedicated to helping our customers achieve optimal efficiencies through condition-based monitoring (CBM) utilizing our highly experienced and certified CBM analysts, cutting-edge PdM technologies, and equipment with unsurpassed analytic capabilities. Our Advanced Testing Group (ATG) is comprised of the foremost leading experts in the diagnosis of the most complex problems plaguing the industry today.