Pulse Width Modulated (PWM) power sources, also known as variable frequency drives (VFD), adjustable speedy drives (ASD), “inverters,” or “drives,” cause bearing current problems within electric motor bearings. The most common types of bearing current are capacitively induced shaft voltage discharge, or EDM current, and in larger motors, electromagnetically induced high frequency circulating currents.
The impedance of the bearing to high frequency discharges is a function of the oil film between the rolling element and the bearing race, load and system design variables, the type of bearing, speed of rotation, and other factors. The bottom line is that as the shaft rotates an oil film forms between the rolling elements and the bearing race. This oil film is dielectric and would not normally not allow current to flow. However, because it is very thin, only 5 to 10 microns thick, when the induced shaft voltage is high enough, [10 to 40 volts peak per NEMA MG1 22.214.171.124] the oil film will break down and allow an arc to pit the motor’s bearing.
Bearing current is the current that arcs through the electric motor bearings and leads to mechanical damage. This damage is evident from the pits in the bearing. While the electric motor is in operation, arcing occurs thousands of times per second. In addition to pitting the bearing, arcing also oxidizes the lubrication, which further decreases the bearing life.
Two potential mechanisms for bearing damage when operating on pulse width modulated variable frequency drives are dv/dt (the change in voltage divided by the change in time), which contributes to EDM currents caused by the parasitic capacitive coupling from stator to rotor, and in larger motors (over 100 HP / 75 kW), the high frequency circulating currents are caused by high common mode current levels.
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Electric motors are prone to many sources of damage, some more common, and some better known. An increasingly common, but still not too well known, problem facing these motors is bearing current. Bearing currents damage the ball bearings with electrical discharge machining (EDM) resulting in motor failure. In addition to the repair or replacement costs of the electric motor, there is the possible loss of production caused by that motor failure. Adding it all together results in one expensive failure!
Bearing currents have been a thorn in the side of motor users for years. However, incidents are on the rise for a simple reason: more people are using pulse width modulated variable frequency drives (VFDs). We are going to look at what bearing currents do and how to avoid them.
Bearing Currents Cause Problems
Shaft voltages create electrical bearing damage when they discharge through the bearings in a tiny but destructive electrical arc. Pitting, frosting, and fluting are some of the damages caused by that current. These types of damages lead to premature bearing failure, which then leads to the motor failing as well. In addition, if a conductive coupling is connected to the motor drive end shaft, shaft voltage can discharge through the coupled equipment damaging its bearing with electrical arcing.
Three-phase power is what operates most AC motors. On line power, the three phases always sum to zero volts. The sum of the three phases is also called common mode voltage. The common mode voltage is zero on line power because each phase is a smooth sine wave voltage 120 degrees out of phase with the other two phases. But when a VFD is in control, each phase consists of a series of positive, zero, and negative voltage pulses. The three phases don’t usually sum to zero, so the common mode voltage is continuously changing with the pulses driving the motor. This nonzero common mode voltage creates shaft voltage by parasitic capacitive coupling between stator and rotor.
And when voltage is induced onto the motor’s shaft, that voltage “looks for” a least-resistance path to ground. Usually, the path of least resistance leads through the motor’s bearings. As described above, shaft voltage discharging through the bearing causes EDM and cumulative electrical bearing damage.
Why Test Motors for Shaft Voltage?
If your facility relies on electric motors, include testing for shaft voltage in your preventative maintenance (PM) program. These tests are performed on site and you receive an analysis of any potential problems.
The idea behind an effective PM program is obvious: to stop issues before they become problems. Using an effective testing method for shaft voltage leads to lower cost options than repairing or replacing an expensive motor. Reducing the risk of shaft voltage leads to a reduced risk of bearing failure and presents an opportunity to save substantial expenses.
Testing for shaft voltage is a relatively simple process. At an exposed area of the motor’s shaft, a conductive microfiber probe tip safely touches the motor’s spinning shaft and captures shaft voltage data for analysis. Using that information, recommendations are made for courses of action. Using the AEGIS Shaft Voltage Tester is the simplest and easiest way to take shaft voltage readings. The AEGIS Best Practices Handbook explains the step-by-step procedure as well as many other important topics to protect your motor bearings!
Electric motors make the industrial world go around. Found in most manufacturing facilities, electric motors are responsible for 64% of all industrial electric power used in the United States.
Your electric motors represent a significant expense. When one fails, costs associated with that motor add up. Those costs include not only the price for repair or replacement of the failed motor but also the cost of lost production time or unplanned downtime. In fact, the cost due to lost production usually dwarfs the motor repair and replacement cost.
As vital as it is to get your motor back up and running, it is as important to find out why it failed in the first place. Discovering the cause of the failure allows you to prevent that problem from causing any future failures. Whether the investigation is done your own maintenance team or by a motor repair contractor, it is essential to learn from failures in order to improve your operations’ reliability.
Bearing failure accounts for over half of all electric motor breakdowns. So when you begin to look into why the motor failed, start by looking at the bearings.
When the motor arrives for repair, cut and inspect every bearing, especially if a VFD controlled it. Inspecting the bearings provides vital information helping make the best recommendation for repair. Knowing why the bearing failed and taking steps that prevent it from happening again improves the machine’s lifetime performance.
Bearing inspection is also required by AR100-2015, the recommended motor repair practices of the Electrical Apparatus Service Association (EASA). Section 2.2 on Bearings begins, “Bearings should be inspected for failure modes such as spalling, contamination, fretting, fluting, frosting, and scoring or other damage.”
We recommend the following procedure to inspect the bearings. These steps are available in the AEGIS® Handbook.
Please Note: Follow established safety precautions and use personal protective equipment including eye protection, hearing protection, face shield, gloves and protective clothing.
1. Inspect the outside and the inside of both bearings. Keep a sample of the lubricant to analyze. When examining the lubricant, look for:
2. Remove seals or shields first, then cut the outer race into halves.
3. Inspect the grease and look for any contamination in the bearing.
New bearing grease is available in many colors. The blue grease is Polyrex EM, commonly found in electric motor bearings.
4. Clean the bearing’s components using a degreaser or solvent.
5. Inspect for evidence of Electrical Discharge Machining (EDM). EDM damage is millions of microscopic electrical pits created when shaft voltage discharges through the motor’s bearings. The electrical voltage overcomes the dielectric of the bearing lubrication and instantaneously arcs through the inner race, through the rolling elements, and to the outer race. The individual pits are usually between 5 and 10 microns in diameter.
6. Frosting: Frosting appears to be a grey discolored line around all, or part, of the bearing race and evident in both the inner and outer race. Both mechanical wear and EDM causes that discoloration. Use a microscope to determine if the line is EDM or of a mechanical nature.
If the motor was operated by a VFD with no bearing protection installed, there is a high likelihood that the frosting is from EDM.
Fluting Damage: Identified by a distinctive washboard pattern. A distinctive washboard pattern identifies fluting damage, seen using the naked eye or with 10x magnification.
Take care to identify the correct electrical fluting damage using the observed damage. Sometimes, fluting gets confused with mechanical bearing damage such as brinelling or false brinelling.
Besides using these recommended practices, please refer to other bearing failure analysis experts to determine the root cause of the failure. Install new AEGIS Ring whenever replacing bearings on an inverter-driven motor.
An article on electrical bearing damage recently appeared in EASA Currents Magazine. While it raised a lot of good points, it also contained information on shaft grounding practices and shaft grounding rings which needs correction or further examination. As we discuss electrical bearing damage, please note that we are referring to motors operated by variable frequency drives (VFD).
The title of the article was “Motor bearings: Electrical damage simplified.” True to this title, the article tries to stay as “nontechnical” as possible. Unfortunately, staying “nontechnical” is not always desirable since a thorough understanding of bearing currents and solutions is critical for successful repairs. As the inventors of conductive microfiber shaft grounding rings – the AEGIS® products – we’ve been discussing electrical bearing damage and its solution for over twelve years. It is my opinion that in trying to keep it simple, the article sometimes oversimplifies the problem.
When talking about shaft grounding, it is a mistake not to differentiate capacitive shaft voltage discharge currents (EDM currents) and circulating currents (be they low or high frequency). The author states that his preferred method of bearing protection is to “break the electrical circuit on the opposite drive end (ODE)” of the motor, and install shaft grounding at the drive side. This mirrors our recommended approach for AC motors over 100 HP and for all medium voltage motors. However, circulating currents are not an issue in small AC motors (under 100 HP), so bearing insulation is unnecessary for them and adding a shaft grounding ring to the DE or NDE is sufficient to protect the motor’s bearings.
But the author then states that “if the [shaft] voltage can be clamped low enough with just a grounding brush, the ODE insulation is not always necessary.” This is false. Shaft grounding cannot prevent circulating currents. So, 100 HP+ and medium voltage motors would still be at risk of circulating currents. These motors will always need both shaft grounding (preferably the drive end), and some type of insulation at the opposite end (preferably the ODE).
There is another confusing or confused statement about circulating currents in the same paragraph: “This looping is also the reason [not to] install shaft grounding on the ODE as the loop can extend into the driven machine” and thereby damage its bearings. This is easily misconstrued to mean that shaft grounding should never be installed at the ODE. But it is important to know that this applies only when circulating currents are a potential problem, i.e. motor over 100 HP and medium voltage motors.
When discussing circulating currents potentially extending “into the driven machine” we advise referring to the IEEE 112 which recommends insulating the NDE to avoid a circulating current path in the driven machine. Last but certainly not least, it is important to remember that installing a shaft grounding ring on the DE is still necessary to prevent capacitive voltages from causing EDM damage to the steel (non-insulated) bearing and/or going down the shaft to the driven equipment.
Now for a discussion on shaft voltages: The author gives a maximum safe shaft voltage level of 5 V peak to peak, and correctly points out that there is no universally applicable “safe” level of shaft voltage. In fact, it the peak voltages which harm the bearings so discussing “peak” voltage is key. The NEMA MG1 document gives a range of 10 to 40 volts peak (20 to 80 volts peak to peak) as the shaft voltage level where bearing discharge can occur. But every system is different; every motor in every state of operation will have its own bearing breakdown voltage where discharge through the bearing occurs. With the lack of a consistent safe level across systems, neither peak voltage nor peak-to-peak measurements are a reliable measure of risk.
Therefore, we recommend that machine owners or motor repair technicians take shaft voltage readings with a high-speed oscilloscope (at least 100 MHz bandwidth). With this equipment, you can unambiguously tell whether discharge is occurring in the bearings. The smoking gun is a discharge pattern in the shaft voltage waveform (a slow voltage buildup followed by a rapid transition down).
While the article recommends shaft voltage testing, it only recommends measuring peak-to-peak voltage, even though it is not the most important measure. The sample shaft voltage reading in the article has a timescale of 2 ms/div, which is far too long to detect discharges, which occur on a timescale of microseconds. More on this in the Bearing Protection Handbook.
The shaft grounding ring section starts off well, but goes awry in the second sentence by referring to “the standard single row ring.” This refers to the AEGIS® SGR, for low voltage motors, but SGRs have two rows of conductive microfibers, not one. Next the article states that the standard SGRs are “usually” sufficient for smaller motors, but “may not always provide a low enough resistance circuit [sic] to ground.” An SGR with two fiber rows is extremely effective and is proven in millions of applications worldwide on low voltage motors under 500 HP (and as noted earlier, motors over 100 HP also need bearing insulation in addition to the SGR ring).
The article does better when talking about the six fiber-row AEGIS® PRO Rings, saying “These rings are the best option for the least amount of maintenance.” AEGIS® PRO rings are in fact the best and recommended option for medium voltage motors and LV motors over 500 HP.
The author does seem confused, though, about the purpose of the extra rows of conductive microfibers in the PRO ring. He seems to think the extra rows are to decrease the PRO’s resistance relative to an SGR. But in fact, the extra rows serve to increase current carrying capacity, not resistance/impedance per se. This extra capacity is needed for medium voltage and large low voltage motors (500 HP+) because they have higher shaft currents and the PRO ring’s extra rows accommodate that increased current.
The article later claims that “they can be susceptible to wear, heat, and grease contamination.” It is unclear whether “they” refers to PRO rings or SGRs, but the claim is inaccurate for either product. Any properly installed AEGIS® ring is not susceptible to wear. It is designed as a “wear-to-fit” device; the fibers slowly wear until they just-touch the shaft surface, a process which can take 200,000 hours or more. Electrical contact is maintained, by physical contact and near-contact electron transfer, so the ring does not stop working. As for heat tolerance, the manufacturer spec states that AEGIS® rings can withstand temperatures up to 410° F (210° C), and we know of at least one customer who uses AEGIS® rings in an oven at 400 degrees.
As for grease, the AEGIS® fibers cut through and tolerate light grease, but this is a moot point. As the article mentions, AEGIS® rings can be mounted inside motors, away from external grease and grime, and this is where motor repair shops should install them. One bit of information the article left out is that there is even a UL-approved process for installing AEGIS® rings inside explosion proof motors, which is unique among shaft grounding technologies.
Now we come to the author’s preferred shaft grounding method, the copper bristle brush. These devices have a few shortcomings that went unmentioned. The brush’s springs may clog and jam, copper is easily oxidized to its nonconductive green/blue oxide, and the shaft can glaze. Also, we are rather skeptical about the use of grease on the brush.
The article only mentions common mode chokes (CMC) briefly, but manages to get a few errors into those three sentences. The most important one to correct is the misleading statement that they “do not always reduce the shaft voltage to a low enough level to eliminate bearing damage.” In fact, common mode chokes barely affect shaft voltage at all and do nothing to eliminate EDM bearing discharges from the capacitive induced shaft voltage. CMCs are just not useful against the shaft voltage discharge currents that shaft grounding protects against. They may be able to reduce high-frequency circulating currents, but CMCs are not as effective as insulating one bearing, as described above. At any rate, common mode chokes cannot take the place of shaft grounding.
All that said, the article ends with some good preventive maintenance advice, including making regular shaft voltage measurements, and advising specifying engineers to call for motors with preventive measures installed. It ends, “Good service centers will identify electrically damaged bearings and strive to recommend the best method to prevent further issues.” We agree wholeheartedly. Bearing inspection is required by the ANSI/EASA Standard AR100-2015, and by the EASA warranty checklist, and is also one of the processes required of accredited EASA motor repair shops.
Ultimately, this article was written to educate motor repair shops and more importantly, to help them improve their processes and so improve their business. We agree with these goals and hope that the original article succeeds in encouraging an enlightened discussion and seeking the most accurate information when considering bearing protection. And we hope that this article, too, will help advance those goals.
AEGIS Rings have worked for years to improve reliability and sustainable operatuion of HVAC and Pump motors. Case in point: Harvard University’s Campus services Facility Maintenance Operations (FMO) “offers Motor Shaft Grounding, a maintenance program that prevents bearing failure and significantly extends motor life.”
Variable frequency drives (VFDs) present a compelling option for energy savings in sustainable building projects. Power usage in continuously running centrifugal pumps and fans decreases notably with input modulation by a VFD. For example, a 20% reduction in fan speed can reduce energy consumption by nearly 50%. VFDs also introduce their own sustainability problems, however. VFD-induced shaft currents can damage bearings, leading to shorter motor life and costly repairs. Shaft grounding technology such as the AEGIS Shaft Grounding Ring offers a sustainable solution to this problem. Unfortunately, most new construction projects including VFD-driven motors do not utilize this effective option.
The university maintenance department has spearheaded a campus-wide sustainability drive starting with their own Platinum LEED certified headquarters. A major component of this has been a testing program for VFD-driven HVAC motors. Maintenance technicians employ oscilloscopes and voltage measuring probes to ascertain the presence of shaft voltage. When harmful voltage levels are detected, the maintenance department may recommend the installation of an AEGIS ring. Already successfully employed in multiple new buildings on campus, expanding the AEGIS ring to existing motor setups that require bearing protection continues to increase campus-wide sustainability.
In December 2009, the ring’s manufacturers installed their product on two VFD-controlled HVAC motors in the maintenance headquarters building as a demonstration of the new program. The identical three year old Baldor 7.5HP motors respectively powered a chilled water pump and an air supply fan. With the VFD set at 60HZ, the first motor was running at 1,776rpm. The oscilloscope measured peak-to-peak discharges of 61 volts. Results showed rapid voltage collapses at the trailing edge of the waveform, typical of the electrical discharges that damage bearings. After the reading, technicians cleaned the shaft and installed a split AEGIS ring. Follow-up test results displayed the discharge plot as a straight line, indicating that the AEGIS ring diverted shaft voltage discharges.
The second motor was tested under identical conditions and measured 50.8 volts peak-to-peak shaft discharge. Due to the limited accessibility of this motor, application of a hand-held heater sped epoxy curing in the AEGIS ring installation. After the complete installation, a new test read only 380 millivolts peak-to-peak, again indicating the AEGIS ring successfully diverted shaft voltage discharges to ground.
VFDs provide a compelling option for energy savings in sustainability-minded design. However, unless a product such as the AEGIS Shaft Grounding Ring mitigates the risk of bearing damage, repair costs could outweigh any savings. While this problem remains best addressed in the design stage of the system, the university case study effectively demonstrates the potential to retrofit previously installed motors with shaft grounding technology. Once installed, an AEGIS ring requires no maintenance and lasts for the life of the motor, providing effective protection against shaft voltage.
More information about the university case study can be found here.