Using variable frequency drives (VFDs or inverters) to control the speed of motors large and small is not only a flexible, cost-effective means of improving process/system control, it can also greatly reduce energy costs.

But VFDs have their drawbacks.  They can induce voltages on the shafts of the motors they control.  Without an alternate path to ground, these VFD-induced voltages will discharge through motor bearings causing electrical discharge machining (EDM) that blasts pits in bearing and race surfaces.  This process eventually leads to frosting and fluting (washboard-like ridges on the bearing race), and bearing failure — often in as little as 3 months!

While some large motors can be repaired on site, many must be pulled out of service, loaded onto a truck (often requiring a crane), and transported to a motor shop for repairs.  This can be a time-consuming and very expensive process.  And the larger the motor, the greater the cost of replacing damaged bearings.

But the cost of repairing motors often pales in comparison to the staggering costs of downtime and lost production.  Larger motors are typically used in high volume processing applications, where the cost of every minute of lost production is greater.

AEGIS® PRO Series Rings protect medium-voltage motors, generators, and turbines, as well as large-frame low-voltage AC motors (500 HP or greater) and DC motors (300 HP or greater).  PRO Rings are vailable in solid- and split-ring versions for shaft diameters from 2.5″ (50 mm) to 31.5″ (800 mm). They have 6 rows of conductive microfibers that completely encircle the motor shaft, providing millions of contact points that channel harmful shaft voltages away from bearings and safely to ground.

Solid PRO Rings are ideal for installation on new motors before they are put into service or on older motors when they are repaired. Split PRO Rings come in mating halves that slide around the shafts of coupled motors for fast, easy field installation.

There’s even an AEGIS® PRO Ring for protecting and monitoring remotely located or hard-to-access motors. The AEGIS® PROMR combines an AEGIS® PRO Ring with an AEGIS® monitoring ring (separated by an insulated phenolic spacer). It provides bearing protection and remote monitoring in a single, compact unit.  And Universal Mounting Brackets — designed to accommodate shaft shoulders, slingers, or other end bell protrusions — simplify and speed the installation of any size PRO Series Ring.

For horizontally mounted motors with single-row radial ball bearings at both ends of the motor, best practices include:
• At the non-drive end: Isolate the bearings using an insulated sleeve or coating; or install insulated ceramic or hybrid bearings to disrupt circulating currents.
• At the drive end: Install one AEGIS® PRO Bearing Protection Ring either internally (on the back of the bearing cap) or externally (on the motor end bracket).
• Whether the ring is installed internally or externally, be sure to apply AEGIS® Colloidal Silver Coating to the motor shaft where the ring’s conductive microfiber will make contact.
• For low- or medium-voltage motors where both bearings are insulated, install one AEGIS® Ring, preferably on the drive end to protect the bearings of attached equipment (e.g., gearbox, pump, fan bearings, etc.).

For more information on AEGIS® PRO Series Rings, see the PRO Series product page. You can also read an ROI analysis of PRO Rings, or peruse our case studies on how PRO Rings solved real-life problems in a number of applications.


It was a packed house as the editors of Design News presented the 2017 Golden Mousetrap Awards, held in conjunction with Pacific Design & Manufacturing in Anaheim, Calif.

AEGIS Shaft Voltage Tester wins 2017 Golden Mousetrap Award

The Golden Mousetrap Awards were created to acknowledge and recognize American people, companies, and technologies driving innovation in the industry. Last night’s festivities, which also recognized Design News’ Lifetime Achievement Award winner, were a celebration of manufacturing and innovation in North America by honoring the companies and individuals who impact the industry through their hard work and groundbreaking ideas.

“We [at Design News] are honored to recognize those companies and individuals who have demonstrated a drive for excellence in their respective fields,” said Suzanne Deffree, content director and editor-in-chief of Design News . “The future of engineering and manufacturing is truly innovative, as the companies and individuals celebrated at the 2017 Golden Mousetrap Awards have shown the leadership and direction needed to push the industry to new heights.”

In the Electronics & Test: Test & Measurement Category, the AEGIS Shaft Voltage Tester Digital Oscilloscope from Electro Static Technology was chosen as the winner!

The AEGIS® Shaft Voltage Tester™ Digital Oscilloscope is an innovative system used to measure whether destructive shaft voltages exist on the shafts of motors controlled by Variable Frequency Drives.  It is a Digital Oscilloscope  with Conductive Microfiber Probe Tips that can measure the voltage on the spinning shaft of a motor.

It answers the question: Is your motor safe from destructive shaft voltages and electrical bearing damage?

Electrical Bearing Damage: Capacitive coupling between windings and rotor can create voltage on a motor’s shaft — voltage that can discharge through bearings, damaging them and shortening motor life. For motors controlled by variable frequency drives, however, these voltages can be as high as 10-40 volt peak. At these higher levels, voltages can easily cause electric electrical discharge in the motor bearings, causing pitting, fusion craters, and fluting, which eventually lead to premature bearing and motor failure.

Until now, the only way to tell whether a motor was at risk for such bearing damage was to buy, rent, or borrow an oscilloscope and then buy some type of shaft voltage probe. Now, for the first time, there is a total shaft voltage detection system available in one package.

Used by Plant Maintenance, Facility Managers and Commissioning Agents:

With the AEGIS® Shaft Voltage Tester™ Digital Oscilloscope, plant maintenance personnel, facility managers and also new equipment commissioning agents can take shaft voltage readings from motor shafts quickly and easily – readings that confirm or deny the presence of shaft voltage discharges that can damage motor bearings.  Users who already have a 10X oscilloscope with bandwidth at least 100 MHz may be able to measure shaft voltage with it and an appropriate set of our Shaft Voltage Probe™ Tips.

meter readings

Shaft voltages on a motor before (left) and after (right) installation of an AEGIS® Shaft Grounding Ring.

High amplitude EDM discharge pattern – Evidence of a Bearing Discharge
Typically EDM discharges can occur from 6 volts peak to 80 volts peak depending on the motor, the type of bearing, the age of the bearing, and other factors. The waveform image shows an increase in voltage on the shaft and then a sharp vertical line indicating a voltage discharge. This can occur thousands of times in a second, based on the carrier frequency of the drive. The sharp vertical discharge at the trailing edge of the voltage is an ultra-high frequency dv/dt with a typical “discharge frequency” of 1 to 125 MHz (based on testing results in many applications).

Damage caused by Bearing Discharges – Lead to Critical Motor Failure

Because of the high-speed switching frequencies in PWM inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor. These voltages, which can register 10-40 volts peak, are easily measured by touching an oscilloscope probe to the shaft while the motor is running.

Reference: NEMA MG1 Section 31.4.4.3

Once these voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance — typically the motor bearings — to the motor housing. During virtually every VFD switching cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a small fusion crater (fret) in the bearing race. When this event happens, temperatures are hot enough to melt bearing steel and severely damage the bearing lubrication.

These discharges are so frequent (millions per hour) that before long the entire bearing race becomes marked with countless pits known as frosting. A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race. Fluting causes excessive noise and vibration, and in heating, ventilation, and air-conditioning systems, it is magnified and transmitted by the ducting. Regardless of the type of bearing or race damage that occurs, the resulting motor failure often costs many thousands or even tens of thousands of dollars in downtime and lost production.

Failure rates vary widely depending on many factors, but evidence suggests that a significant portion of failures occur only 3 to 12 months after system startup. Because many of today’s AC motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in AC motors with VFDs.


Common mode choke (CMC) manufacturers now go to great effort to market their products as a preventative measure against bearing failure. A recent paper presented by H. William Oh at the Motor and Drive Systems Conference in Orlando, FL explains why CMCs are not an effective solution. VFD-driven motors exhibit two types of bearing currents: inductively-coupled and capacitively-coupled bearing current. CMCs are indeed an effective tool at minimizing high frequency transients in the common mode current as demonstrated through numerous tests.  They do not, however, address or eliminate the capacitively-coupled bearing current, also known as EDM bearing current, which caused fluting damage in electric motors of all sizes.

Common mode voltage, which is capacitively coupled from stator to rotor, does not influence circulating bearing current, and it remains the primary source of bearing currents in VFD driven motors up to 100 HP and is present in all larger motors as well.

Shaft Voltage Discharge

 

 

 

 

 

 

 

 

Motors up to 100 HP

 

 

 

 

 

 

 

 

 

Inductively-coupled bearing current, or circulating bearing current, appears as motor frame size surpasses 100 HP.  Research has demonstrated that circulating bearing current is influenced by the inductance of the motor and common mode current variations.  Circulating bearing current will thus only present itself when end-to-end shaft voltage is present, a condition found only in large frame motors. CMCs can reduce the risk of damage through circulating bearing current by regulating the amplitude of high frequency (HF) common mode current. In a small frame motor, the lack of circulating bearing current renders CMCs unnecessary.

Motor with circulating currents

Motors over 100 HP

CMC’s which are sometimes also referred to as “Indutive Absorbers” only attenuate common mode current; it has no effect on common mode voltage. As a result, use of a CMC will not address the risk of damage from EDM bearing current. Indeed, quite the opposite may be the case: an improperly-selected CMC may result in resonance, amplifying EDM potential.

This fact can often be overlooked as a result of the way the industry defines shaft voltage, neglecting to separate the two types which separately drive EDM bearing and circulating bearing current.

The best solution to this problem in a large frame motor involves pairing the AEGIS Ring on the drive side with an insulated bearing on the non-drive side of the motor.  This insulation method interrupts and stops the high frequency circulating current and is recommended in our best practices handbook.

Common Mode Chokes on the other hand, when properly matched to the motor,  could reduce the high frequency circulating current, but an AEGIS shaft grounding ring is still needed to discharge the capacitive EDM currents.

For more information on shaft grounding, visit Electro-Static Technology, designers of the AEGIS Bearing Protection Ring. EST presented Mr. Oh’s paper, “Common Mode Choke Cores (CMCs) Cannot Prevent Bearing Failure in All Motors,” at the 2017 Motor and Drive Systems Conference in Orlando, February 8-9.

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