Monthly Archives: February 2017

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

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.

Motor with circulating currents

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 common mode current. In a small frame motor, the lack of circulating bearing current renders CMCs unnecessary.

CMC’s, which are sometimes also referred to as “Inductive Absorbers,” only attenuate common mode current; they have 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 because of the way the industry defines shaft voltage.  That is, neglecting to separate the two types of shaft voltage, which separately drive EDM bearing and circulating bearing current.

Motors over 100 HP

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.

Variable frequency drives (VFDs) are increasingly the norm in heating, ventilation, and air conditioning (HVAC) applications. These inverters reduce energy consumption by allowing motors to run at lower speeds. While yielding potential energy savings of 30%, VFDs introduce electrical currents which can shorten motor life if not properly addressed. VFDs provide such savings because HVAC fans and pumps run continuously at reduced loads. Reducing a fan’s speed by half cuts horsepower requirements to one-eighth. For this reason, VFDs represent a superior solution to the traditional throttling mechanisms used in such applications.

Most of today’s HVAC motors are built with sealed bearings. As a result, the primary cause of bearing failure in VFD-controlled motors is electrical damage. This damage occurs because voltages build up on the shaft and then discharge in short bursts along the path of least resistance–through the bearings. Over time, this discharge can lead to pitting and fluting. The National Electrical Manufacturers Association (NEMA) officially recommends bearing insulation to address these problems.  AEGIS Bearing Protection Rings provide a maintenance-free solution to bearing insulation in HVAC applications. Two rows of conductive microfibers lining the entire inner circumference of the ring provide safe avenues for discharge. Additionally, AEGIS rings qualify as sustainable technology under the Federal Energy Management Program.

AEGIS Rings are already widely used in a variety of HVAC installations. Electro Static Technology (EST), the manufacturers of AEGIS rings, recently ran a test on a rooftop air conditioning unit to demonstrate their effectiveness. Running the unit at the same conditions, peak-to-peak discharges from the shaft totaled 44.8 volts without an AEGIS ring. AEGIS ring installation reduced this reading to 3.76 volts peak-to-peak, well below levels that would damage bearings. A Chicago hospital recently observed the same results after introducing AEGIS rings to an air handling unit for operating rooms.

All VFD-driven motors are vulnerable to electrical damage. As VFD-driven motors continue to grow in HVAC applications, the AEGIS Bearing Protection Ring provides a compelling solution to mitigate the risk of bearing damage through electrical discharge. Motors protected by AEGIS rings will both enjoy reduced energy consumption and reduced maintenance costs. For more information about this sustainable, cost-saving solution, including the two effectiveness tests mentioned above, consult the following white paper.