There are a number of factors to consider when deciding whether to repair or replace a damaged motor.  These include (but are not limited to) the age and general condition of the motor, the availability of a suitable replacement (including the size and horsepower), and the energy efficiency of the motor versus a newer motor.

But if you decide that a motor is worth repairing, protecting it from electrical bearing damage makes good sense from both an operational and a financial standpoint.  And installing an AEGIS® Bearing Protection Ring — or having your motor repair shop install one — is one of the least expensive “insurance policies” you can buy…especially if the motor is controlled by a variable frequency drive (VFD).

By controlling the speed of motors, VFDs can improve process control and reduce energy usage/costs by 30% or more. But VFDs induce voltages on the Pitting, frosting, & fluting bearing damage from shaft voltage dischargeshafts of the motors they control — voltages that can damage bearings. And even motors not controlled by VFDs can sustain bearing damage due to stray currents.

Without bearing protection, shaft voltages can discharge through bearings, causing pitting (tiny fusion craters in metal bearing surfaces), frosting (widespread pitting), or fluting (washboard like ridges in the bearing race due to the operational frequency of the drive) and complete bearing failure — sometimes in as little as 3 months!

To protect VFD-driven motors from electrical bearing damage, long-term bearing protection is needed.  As proven in millions of installations worldwide, AEGIS® Shaft Grounding Rings channel harmful voltage discharges away from bearings and safely to ground, protecting motor bearings for their full L10 life.

AEGIS® SGR Rings are available for low-voltage, low power motors, while AEGIS® PRO Series Rings are designed for medium-voltage motors, generators, and turbines, as well as large low-voltage AC motors (500 HP or greater) and DC motors (300 HP or greater).

AEGIS® Rings are available in solid rings (for installation before motors are put into service) and split rings (for fast, easy installation on coupled or in-service motors).  AEGIS® uKITs include an AEGIS® Ring (solid or split) and a set of mounting brackets that speed and simplify installation on motors with shaft shoulders, slingers, or other end bell protrusions.

Most motor repair shops can install AEGIS® Rings in-house or in the field, using best practices to protect new, in-service, or refurbished motors.

For motors up to 100 HP, an AEGIS® Ring is typically all you need to protect motor bearings and coupled equipment from electrical bearing damage.

Motors greater than 100 HP and medium-voltage motors, however, are subject to high-frequency circulating currents as well as capacitive shaft voltage dischargeAEGIS PRO Ring installed with brackets on a large motor bearing current.  So to adequately protect these motors, install an AEGIS® Ring on the drive end of the motor and an insulated bearing on the non-drive end.  For low-voltage motors less than 500 HP, use an AEGIS® SGR.  For low-voltage motors greater than 500 HP or medium-voltage motors, use an AEGIS® PRO Series Ring.

Regardless of the type of AEGIS® Ring you install, be sure to thoroughly clean the motor shaft and apply AEGIS® Colloidal Silver to the shaft in the area where the ring’s microfiber brushes will contact it.  Or ask your local motor shop to install AEGIS® Rings on any motors they repair for you!

To download an AEGIS® Bearing Protection Handbook which details best practices for diagnosing failed motors, testing in-service motors for damaging shaft currents, and protecting at-risk motors from electrical bearing damage, click here.

And if you decide that a motor is not worth fixing, choose a new replacement motor with an AEGIS® Ring factory-installed either internally or externally.  For a list of manufacturers that offer motors with AEGIS® Bearing Protection Rings factory-installed, click here.

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.

The University Test Case

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.

Pump Motor with AEGIS Ring installed with EP2400 Conductive Epoxy

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.

An array of motors with AEGIS factory-installed

Most motor manufacturers have product lines with AEGIS Rings factory installed.

More information about the university case study can be found here.

Unlike asynchronous AC induction motors in which the speed of rotation is slower than the speed of the magnetic field, permanent magnet AC (PMAC) motors are synchronous machines that rotate at the same speed as the magnetic field produced by the stator windings. In doing so, they eliminate rotor conductor losses, offering significantly higher efficiencies and power factors, as well as improved performance and speed control.  In fact, PMAC motor losses are typically 15-20% less than those of equivalent size NEMA premium induction motors.  And they provide higher flux density than comparably-sized induction motors.  Simply put, they deliver more power (torque) within a given physical envelope, or equal torque in a smaller package.  They also have a wider speed range than AC induction motors.

PMAC motors are also known as Permanent Magnet Synchronous Motors (PMSMs) and Brushless AC Motors. These motors use rare earth permanent magnets, such as neodymium or samarium-cobalt, that have very strong magnetic properties and produce high flux levels in the motor.  They are so strong, in fact, they can interfere with pacemakers, hearing aids, and other implanted medical devices, as well as electronic devices (cell phones, tablets, watches, etc.) and even credit cards.  But once the rotor has been enclosed within the motor housing, radiated magnetic energy is no greater than that of an induction motor.

Virtually all true permanent magnet motors are operated by variable frequency drives (VFDs).  Consequently, they are at risk of electrical bearing damage from VFD-induced shaft voltages, just like AC induction motors.  And because they typically cost more than AC induction motors, they are more expensive to replace.

Lab testing of two PMAC motors (a 45 kW motor and a 300 kW motor) operated by pulse width modulated (PWM) VFDs illustrates that permanent magnet motors are not immune to electrical bearing damage caused by

Bearing current without shaft grounding

Shaft voltage on a 300 kW PMAC motor without shaft grounding, showing a discharge through the bearing.

VFDs.  Both motors were tested at 5000 RPM.  The shaft voltage reading from the 45 kW motor with no bearing protection showed peak-to-peak voltage spikes of 21.8V.  The waveform illustrates common mode voltage from the PWM drive capacitively coupled to motor shaft. The second reading was taken after installation of an AEGIS® Bearing Protection Ring on the motor.  The smaller wave form demonstrates that the AEGIS® Ring dramatically reduced shaft voltages, channeling them away from bearings and safely to ground.  In fact, the AEGIS® Ring reduced damaging shaft voltages by more than 70% (from 21.8V to 6.6V peak-to-peak) and eliminated EDM discharges through motor bearings.

When the same testing was repeated on a 300 kW motor, the results were even more impressive.  The AEGIS® Ring reduced shaft voltage levels from 23.4V peak-to-peak to 4.8V peak-to-peak, a reduction of 80%.  And the AEGIS® Ring also eliminated EDM discharges through motor bearings.

Shaft voltage on the same motor after installation of AEGIS® shaft grounding.

Field testing also demonstrated that shaft voltages and bearing currents were present on a new PMAC motor at levels sufficient to damage motor bearings. Readings taken from the shaft of an unprotected 5 HP PMAC motor operated by a PWM drive (designed for a

PM motor) showed common mode shaft voltage spikes (typical of bearing discharges) of 14.8V peak-to-peak as well as actual dv/dt bearing discharges of 12.1 volts. Once an AEGIS® Bearing Protection Ring was installed on the motor, voltage levels dropped to 3.28 volts peak-to-peak — a 78% reduction!

So, while PMAC motors offer many advantages over AC induction motors (higher flux density, more torque, greater energy efficiency, and wider speed range), they are every bit as prone to EDM bearing damage from VFD-induced shaft voltage discharges as AC induction motors.  The good news, however, is that AEGIS® Shaft Grounding Rings are equally effective in preventing VFD-induced EDM bearing damage to these high-performance motors as they are in preventing such damage to AC induction motors.

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