White Paper: Protect Critical Motors with Proven Shaft Grounding

Overview

To improve the efficiency and reliability of aging power plants, more and more utilities are replacing aging motor-generator sets with new high-efficiency motors controlled by variable frequency drives (VFDs). While VFDs allow precise control and reduce energy costs for a wide range of motors in electric power plants, they also increase the risk of motor/motor system failures throughout these plants. VFDs induce harmful voltages on the shafts of the motors they control. Without proven, long-term bearing protection, these voltages can discharge through motor bearings, destroying them-often in as little as three months-and jeopardizing the reliability of key power plant systems as well as the stability of the electric grid.

Introduction

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In the United States and many other countries, engineers are looking for ways to extend the life and enhance the control capabilities of aging electric grids. Often this starts with upgrading older equipment in power plants. An obvious opportunity is replacing aging motors with new high-efficiency motors and microprocessor-based variable frequency drives. Not only do VFDs offer greater control and flexibility, but they can also reduce operating costs by precisely matching motor capacity to varying loads.

But VFDs are not without their drawbacks. VFDs induce harmful voltages on the shafts of the motors they control — voltages that — without mitigation -can discharge through the motor bearings causing electrical discharge machining of bearing surfaces and total bearing failure — often in less time than that between scheduled plant maintenance shutdowns. The resulting unscheduled shutdowns can interrupt plant operations, jeopardize service to utility customers, and cost utilities hundreds of thousands of dollars in repairs and lost revenues.

However, such damage can be easily prevented by equipping VFD-driven motors with a simple shaft grounding ring before they are commissioned or during a scheduled maintenance shutdown.

Proven in over 2 million installations worldwide, AEGIS® Rings channel harmful shaft voltages away from the bearings and safely to ground, protecting VFD-driven motors from electrical bearing damage for the full L-10 life of their bearings.

VFD-Induced Currents Destroy Bearings, Kill Motors

Figure 1: Without bearing protection, shaft voltage discharges can cause pitting, frosting, fluting, and complete bearing failure often in as little as three months.

Many power plant 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 will use less power if controlled by a VFD. In fact, reducing a fan’s speed by half cuts the horsepower needed to run it by a factor of eight. In light of this, throttling mechanisms that restrict the work of a motor seem old-fashioned and wasteful.

Although the energy-saving potential of VFDs was never in dispute, for years the true cause of VFD-induced bearing failure was often misdiagnosed. Eventually, repair shops and testing consultants proved that the high peak voltages, fast voltage rise times, non-sinusoidal shaft currents, and parasitic capacitance associated with typical pulse-width-modulated VFDs lead to the cumulative erosion of bearings. 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.

Without mitigation, voltages repeatedly build up on the motor shaft to a certain threshold, then discharge in short bursts along the path of least resistance, which all too often runs through the motor’s bearings. The discharge rate tends to increase with the carrier frequency.

Figure 2: Taken from a failed motor, the fluted bearing (left) resulted from VFD-induced bearing currents. Protected by an AEGIS Bearing Protection Ring, the bearing race on the right is undamaged.

Continued discharges result in pitting [Figure 1] of balls and race walls through electrical discharge machining (EDM). Concentrated pitting at regular intervals along the race wall can cause washboard-like ridges called fluting [Figure 2], a source of vibration and noise that can lead to premature motor failure and high repair costs.

NEMA Standards Call for Shaft Grounding

The standards issued by the National Electrical Manufacturers Association (NEMA) highlight the need for extra bearing protection for VFD-driven motors.

NEMA Standard MG1, Section IV, Part 31, Definite-Purpose Inverter-Fed Polyphase Motors, (to be addressed by Construction Specifications Institute specification 23 05 13 for HVAC motors), recommends bearing insulation at one end of the motor if the NEMA motor frame size is 500 or larger and the peak shaft voltage is greater than 300 millivolts. In these larger motors, bearing damage may be due in part to magnetic dissymmetries that result in circulating end-to-end shaft currents.

For smaller motors, the same standard recommends insulating both bearings with high-impedance insulation or installing shaft grounding brushes to divert damaging currents around the bearings. For these motors, a VFD can generate high-frequency common mode voltage, which shifts the three-phase winding neutral potentials significantly from ground. Because the damaging voltage oscillates at high frequency and is capacitively coupled to the rotor, the current path to ground can run through either one bearing or both.

The NEMA standard points out, however, that bearing insulation will not prevent damage to other connected equipment. When the path to the bearings is blocked, the damaging current seeks another path to ground. That other path is often through a fan, pump, compressor, air handler, gearbox, encoder, brake motor, or another piece of connected equipment, which can consequently wind up with bearing damage of its own.

AEGIS Rings Protect Motor Bearings From Damaging Currents

Ironically, some products designed to protect bearings from electrical damage, such as conventional spring-loaded grounding brushes, require extensive maintenance themselves. Others, such as ceramic bearings, can shift damage to connected equipment. Many so-called “inverter-duty” motors offer beefed-up winding insulation, but this insulation does nothing to guard against bearing damage. If they are to be truly ready for use with VFDs, these motors also need bearing protection.

AEGIS Bearing Protection Rings use the principles of electron tunneling, field emission of electrons, and ionization to safely and efficiently bleed off damaging currents. Installed around a motor shaft, maintenance-free AEGIS Rings provide a very-low-impedance path from shaft to ground, bypassing motor bearings.

The following two images represent shaft voltage readings before and after installation of an AEGIS Bearing Protection Ring.

At 28.6 V, shaft voltage levels on a VFD-driven motor without an AEGIS Ring are high enough to damage the bearings

At 3 V, shaft voltages from a motor protected with an AEGIS ring are too low to cause bearing damage.

As preventive maintenance for motors already in service, these rings can be quickly and easily installed on any NEMA or IEC motor regardless of shaft size, horsepower, or end-bell protrusion using conductive epoxy and/or a universal mounting kit (uKIT) or universal mounting brackets. The conductive microfibers that line the entire inner circumference in two rows, completely surrounding the motor shaft, boost the ring’s electron transfer rate. The rings require no maintenance and last for the life of the motor, regardless of RPM. When installed on VFD-controlled motors, they qualify as sustainable technology under the Federal Energy Management Program.

AEGIS Bearing Protection Rings have proven effective in VFD-driven motors in conventional power plants; whether coal, natural gas, biofuel-powered, or hydroelectric.

AEGIS Rings have been proven in millions of installations worldwide, including the processing of aggregates, chemicals, cement, pulp and paper, water and wastewater, as well as HVAC, materials handling, marine, mining, food, light rail vehicles, electric vehicles, and other applications.

Protect Critical Power Plant Motors, Avoid Unplanned Downtime

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Virtually everywhere you look in today’s power plants, motors, drive critical equipment and processes, from fuel preparation and feeding to steam generation and cooling to the scrubbing of exhaust gases and the management of fly ash. Following is a partial list of critical operations in the generation of electric power that relies on motors. The failure of any of these motors could necessitate the shutdown of a plant, the interruption of power to utility customers, and the loss of millions of dollars in revenue.

Fuel Preparation and Feeding

• Coal mills/pulverizers
• Conveyors
• Coal feeders
• Fuel gas booster compressors
• Primary air fans
• Air preheaters
• Forced draft fans
• Oxygen and carbon dioxide compressors (for Oxy-Coal plants)
• Plant air compressors (for gas turbines)

Combustion

• Oxygen compressors
• Carbon dioxide compressors

Steam Generation

• Boiler feedwater pumps
• Primary air and secondary air fans (forced draft, induced draft)

Cooling Systems

• Cooling water pumps Systems
• Recirculating pumps
• Boiler fans
• Condenser fans

Scrubber Systems

• Limestone slurry pumps
• Gypsum slurry extraction pumps
• Recirculation water pumps
• Thickner pumps
• Filtrate pumps
• Wastewater sump pumps
• Process water pumps
• Oxidation air compressors
• Induced draft booster fan
• Absorber recirculation pumps

Cogeneration

• District heating circulation pumps

Renewable Energy

• Drives for solar thermal power plants, geothermal power plants, and soft starters for synchronous condensers

Conclusion

To be available, electric power plants must be reliable. Today, the unplanned shutdown of one plant can have a ripple effect throughout the electric grid.

Adam Willwerth

As power plants age, many are replacing older motor controllers with variable frequency drives. While VFDs offer improved control and greater flexibility, they can damage motor bearings, shutting down key plant systems and curtailing plant output. By installing AEGIS Shaft Grounding Rings to protect VFD-driven motors from electrical bearing damage, power producers can improve plant performance, flexibility, and efficiency without sacrificing reliability.

This paper is written by Adam Willwerth. Adam is the Development Manager for Electro Static Technology.

[Ed. Note: we are grateful to Electro Static Technology for allowing us to reproduce Adam’s white paper.]

Originally published at L&S Electric.