Shaft grounding ring fulfills the promise of sustainable energy savings from variable frequency drives

With the rising cost of energy, the use of variable frequency drives (VFDs), also known as inverters, is growing. By optimizing the electricity flow to AC motors, VFDs provide substantial energy savings. But if this increased efficiency is not sustainable, these savings vanish. Currents induced on motor shafts by VFDs can wreak havoc with motor bearings, causing costly bearing fluting failure and dramatically shortening motor life. To mitigate these destructive currents and realize the full potential of VFDs, a reliable, cost-effective method of shaft grounding is essential. While a number of shaft grounding technologies exist, they vary in effectiveness, and most are not sustainable. Because it is maintenance-free and lasts for the life of the motor, however, one new technology, the AEGIS™ SGR Bearing Protection Ring, qualifies as truly "green."

In the field of flow control, the potential for increased efficiency with VFDs is especially dramatic. Many fans and pumps in HVAC and other systems run continuously, but often at reduced loads. Because the energy consumption of such devices correlates to their flow rate cubed, the motors that drive them use less power if controlled by a VFD. In fact, if a fan's speed is reduced by half, the horsepower needed to run it drops by a factor of eight. In this light, throttling mechanisms that restrict the work of a motor seem old-fashioned and wasteful. In constant-torque applications where the main objective is more accurate process control (reciprocating compressors, conveyors, mixers, etc.), VFDs can be programmed to prevent the motor from exceeding a specific torque limit.

Often overlooked until it is too late to save the motor (even motors marketed as "inverter-ready") is the cumulative bearing damage caused by VFD-induced shaft currents. Hard to predict but easy to prevent, these damaging currents are best anticipated and dealt with in the design stage of a system but can usually be mitigated by retrofitting installed motors. Engineered with special conductive microfibers, the AEGIS™ shaft grounding ring reliably redirects shaft currents from shaft to frame (ground), bypassing the motor's bearings entirely.

A Closer Look at Bearing Damage

Short of dismantling the motor, there are two main ways to check for bearing damage —— measuring vibration and measuring voltage. Neither method is foolproof. By the time vibration tests indicate bearing damage, it is usually far advanced. Likewise, the benefit of shaft voltage tests is a confirmation that destructive voltage and bearing discharges exist.

Shaft currents can be measured by touching an oscilloscope probe to the shaft while the motor is running. These voltages repeatedly build up on the shaft to a certain threshold, then discharge in EDM arcs via the path of least resistance, which all too often runs through the motor's bearings.

Serious, cumulative electrical bearing damage can be attributed to the extremely fast voltage rise times (dV/dt) associated with the insulated gate bipolar transistors (IGBTs) found in today's typical pulse-width-modulated VFDs. The discharge rate tends to increase with the carrier frequency. Discharges through bearings can be so frequent that before long the entire bearing race wall becomes riddled with fusion craters known as frosting. Since many of today's motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in VFD-controlled AC motors.

In the phenomenon known as fluting, the operational frequency of the VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting causes excessive noise and vibration that forewarn of imminent bearing failure.

Fluting of a bearing race wall.

Ideal Bearing Protection for Any VFD-Driven Motor

The AEGIS™ SGR Bearing Protection Ring safely redirects shaft currents along a very-low-impedance path from shaft to ground. Maintenance-free and easily installed at the factory or retrofitted in the field, this shaft grounding ring is a low-cost solution for any AC motor in virtually any VFD application. Unlike conventional shaft grounding brushes, the ring's conductive microfibers work with virtually no friction or wear and are unaffected by dirt, grease, or other contaminants.

The AEGIS™ SGR Split Bearing Protection Ring comes in halves for simplified field installation —— using conductive epoxy —— without having to disconnect coupled equipment.

The AEGIS™ SGR is available in two versions —— a continuous ring and a split ring. For most field installations, the quickest and easiest solution is a split ring kit that comes with conductive epoxy, eliminating the need to disassemble coupled equipment or drill bracket holes in the motor's end bell. The entire installation can be completed in just minutes. After cleaning the motor shaft and end bell to remove any coatings, paint, or other nonconductive material, the installer simply applies the adhesive to the back side of both halves of the SGR, centers them around the motor shaft, and holds them in place to allow the adhesive to set. To ensure that the split ring is centered around the shaft, the installer can make use of small metal spacers included in the kit. These are inserted in the gaps between the two halves and removed after the adhesive has cured.

The Path to Sustainability

For too long, the importance of grounding to protect motor bearings has been underestimated. To minimize harmful currents and realize the full "green" potential of VFDs, an economical, long-term method of shaft grounding is a must. Until all OEM motors marketed for use with VFDs are truly "inverter-ready," retrofitting them with effective, maintenance-free bearing protection is the best approach to ensuring their sustainability and safeguarding energy savings from VFDs.

In summary, the AEGIS™ SGR is the most effective and universally applicable solution to the problem of VFD-caused bearing damage, protecting the system from unplanned repairs and costly downtime. The AEGIS™ SGR provides a sustainable path to energy savings and an exceptionally high return on investment.

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