Shaft Voltage Testing

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.”

https://www.campusservices.harvard.edu/system/files/documents/950/Motor%20Shaft%20Grounding.pdf

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.

Conclusion

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 http://www.est-aegis.com/oems.php

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

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


The AEGIS® Shaft Voltage Tester™ is a CAT II/III digital oscilloscope that includes everything you need to test voltage levels on the spinning shaft of a motor — an oscilloscope with advanced functions, a probe with special conductive microfiber tips for exceptional shaft contact, a probe holder with a magnetic base, and a compact carrying case.

aegis-tester-ic

For some applications, an oscilloscope must be calibrated according to the requirements of a standards organization, like ISO or ANSI.  We now offer an AEGIS Tester™ that is ISO 17025 certified.

Part Number:  AEGIS-OSC-9100MB-W2-IC

For more information see our brochure or visit the AEGIS Tester product page.


Commissioning agents ensure that the plans of the architects and engineers who designed buildings, systems, or processes are executed as intended and that their concerns are addressed before structures or systems are put into use. These professionals spend most of their time reviewing actual structures or systems to verify that they have been built or implemented as designed.  But savvy commissioning agents also take the initiative to point out potential shortcomings in designs and explain the consequences of these shortcomings… before

any oversights or omissions can result in expensive repairs or lost revenue for the building owner.

Many of the recommendations commissioning agents make are obvious — items shown in plans that were not installed or were installed incorrectly.  Others are not so obvious… such as motors controlled by variable frequency drives (VFDs) that have destructive shaft voltages but are not protected against electrical bearing damage.

VFDs create harmful voltages on the shafts of the motors they control — voltages that can discharge through motor bearings, destroying them and costing owners huge sums in repairs and lost production or revenue.

Testing for Shaft Voltage at Commissioning

A commissioning agent tests a VFD-fed motor for the presence of shaft voltage.

The best time to check motors to see if they are vulnerable to electric bearing damage is during the commissioning process.  By taking shaft voltage readings from the spinning shafts of VFD-controlled motors, a commissioning agent would see immediately whether the motor is protected with shaft grounding or not.

Motors with effective shaft grounding — such as ones with AEGIS® Shaft Grounding Rings installed — exhibit a virtually flat waveform, indicating a continual discharge of shaft voltage at levels too low to cause bearing damage.  Unprotected motors, on the other hand, exhibit a build up of voltage on the motor shaft to high levels, often 10-40 volts peak, followed by sudden violent discharges.  These discharges blast fusion craters or pits into the metal bearing surfaces.  Left unchecked, these discharges can lead to pitting, frosting (widespread pitting), fluting (washboard-like ridges on the bearing race), bearing grease degradation, and complete bearing failure — often in as few as 3 months!

The savings from preventing such damage can more than offset the cost of hiring a contractor to retrofit AEGIS® Bearing Protection Rings onto motors. Consider the following:

  • Most motor bearings are designed to last for 100,000 hours, but motors controlled by VFDs can fail in as few as 720 hours of operation.
  • When a large HVAC contractor ignored a specification that called for the installation of AEGIS® Bearing Protection Rings on all motors, the commissioning agent pointed out the omission to the building owner.  The job involved 300 motors, and the contractor was forced to remedy the problem at a cost of $200,000!
  • Motor failures caused by VFD-produced shaft voltage result in hundreds of thousands of hours of unplanned downtime each year in the US alone.  In addition, these failures reduce the performance and mean time between failure of the OEM systems in which they are used.
  • An HVAC contractor recently reported that, of the VFD-controlled 30-60 HP vane axial fan motors he installed in a large building project, all failed within a year (two within 6 months).  Repair costs totaled more than $110,000.
  • Several large pulp and paper companies surveyed noted that the VFD-controlled AC motors used in their plants typically fail within 6 months due to bearing damage.
  • After testing AEGIS® Rings on a VFD-controlled 50 HP “problem” fan motor — a motor that had suffered bearing failure 3 times — the engineering firm hired to test and tune the VFDs at the Time & Life Building in New York recommended installing AEGIS® Rings on all 240 HVAC system motors in the building.
The oscilloscope shows a bearing discharge on this motor without shaft grounding.

On this motor without shaft grounding, the AEGIS® Shaft Voltage Tester™ detects shaft voltage discharging through a bearing.

The best way to check for VFD-produced shaft voltages is to use an oscilloscope specifically designed to take such readings. The AEGIS® Shaft Voltage Tester™ Digital Oscilloscope is specially designed and configured to take accurate voltage readings from the spinning shafts of motors. It comes complete with a special conductive microfiber-tipped probe, a probe holder with magnetic base, and a compact carrying case.  The conductive microfiber probe tip provides exceptional electrical contact with the motor shaft.  It is made from the same highly conductive microfibers as the brushes of AEGIS® Shaft Grounding Rings.

If the motor specifications call for shaft grounding, a visual inspection of the motor may not be enough to verify that AEGIS® Shaft Grounding Rings were installed.  Since a large and growing number of motor manufacturers now offer motors with AEGIS® Rings factory-installed inside the motor, shaft voltage testing is needed to verify that motors are protected – especially if the AEGIS Installed label is not visible.  If readings indicate continuous low-level shaft voltage discharges, then the commissioning agent can feel confident that the AEGIS® Rings were, in fact, installed.

If the motor specifications call for shaft grounding, a visual inspection of the motor may not be enough to verify that AEGIS® Shaft Grounding Rings were installed.  Since a large and growing number of motor manufacturers now offer motors with AEGIS® Rings factory-installed inside the motor, shaft voltage testing is needed to verify that motors are protected – especially if the AEGIS Installed label is not visible.  If readings indicate continuous low-level shaft voltage discharges, then the commissioning agent can feel confident that the AEGIS® Rings were, in fact, installed.

now

Here is the same motor from above, now with an AEGIS® uKIT installed.  The shaft voltage has dropped to near zero.

If design specifications do not call for shaft grounding on motors, the commissioning agent can prove his value by recommending that motors be retrofitted with uKITs — AEGIS® Rings with universl mounting brackets.  AEGIS® uKITs were designed for fast, easy field installation around the shafts of coupled motors, and can accommodate shaft shoulders, slingers, and other end bell protrusions.

By making such a recommendation, the commissioning agent can help the building owner save the time, hassle, and expense of trying to fix a building or process problem… before it becomes a problem.

For more information on specifying bearing protection, click here.

For more information on testing motors to ensure they are protected against damaging VFD-induced bearing currents, click here.

For more information on the AEGIS® Shaft Voltage Tester, click here.