Tenants of the Time & Life Building in New York’s Rockefeller Center include Time Inc., publisher of more than 100 magazines. In an effort to “green” the building, shaft grounding rings have been installed to protect the bearings of HVAC fan motors.
Proper tuning of a variable frequency drive’s output range and proper grounding of a VFD-controlled motor’s frame are paramount. Only recently has it become clear that without an effective motor- shaft grounding device as well, stray currents induced by a VFD can wreak havoc with bearings, causing premature motor failure. Ironically, some products designed to protect bearings, such as conventional metal grounding brushes, require extensive maintenance themselves. Others, such as insulation, can shift damage to connected equipment.
The Importance of Shaft Grounding
One of the newest and most promising bearing-damage mitigation devices uses patented advanced electron transport technology to safely bleed off these damaging currents to ground. Engineered with special conductive microfibers, the AEGIS™ SGR Bearing Protection Ring™ safely discharges VFD-induced shaft voltages by providing a very-low-impedance path from shaft to frame, bypassing the motor’s bearings entirely.
A preventive maintenance plan that reduces the total life-cycle cost of operations in the Time & Life Building at the heart of New York City serves as a good example of how the push for more sustainable (“green”) building management has led to a growing awareness of a chronic, widespread problem with heating, ventilation, and air-conditioning motors — electrical bearing damage.
With the rising cost of energy, the use of variable frequency drives, also known as inverters, is growing. By optimizing electrical flow to an alternating-current (AC) motor, a VFD can provide substantial energy savings. But if the increased efficiency is not sustainable, those savings vanish. Currents induced on motor shafts by VFDs can severely damage motor bearings, dramatically shortening motor life and causing costly repairs. To mitigate these currents and realize the full potential of VFDs, a reliable method of shaft grounding is essential.
Figure 1. Voltage peaks on the shaft of a motor can damage bearings.
In the field of flow control, the potential for increased efficiency with VFDs is especially dramatic. Many centrifugal fans and pumps 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, 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, and so forth), a VFD can be programmed to prevent the motor from exceeding a specific torque limit.
Hard to predict, VFD-induced shaft currents cause cumulative damage to a motor’s bearings, even in many motors marketed as “inverter-ready.” Because the problem is best addressed in the design stage of a system, the best solution arguably would be a motor with built-in bearing protection, available at a reasonable cost. Minimal, voluntary standards issued by the National Electrical Manufacturers Association (NEMA) for IGBT-inverter-controlled motors rated for 600 volts or less state that such motors should be designed to withstand repeated surges of 1600 volts (or 3.1 times the motor’s rated voltage) and rise times of 0.1 microsecond. However, since no one enforces these standards and testing is problematic, motor manufacturers are free to make whatever claims they like, and most models boasting extra protection from VFD currents have beefed-up insulation for the windings, not for the bearings.
Fortunately, the problem usually can be mitigated by retrofitting previously installed motors. Whether a VFD-controlled motor is being used to run an air-conditioning fan in a “green” building or to run a conveyor on an energy-efficient assembly line, shaft grounding is a cost-effective way to achieve sustainability.
Some of the Many Ways Inverters (VFDs) Can Cause Motor Failure
Typically, the most vulnerable parts of motors controlled by VFDs are the windings and the bearings. The cause of their damage is repetitive electromagnetic interference (EMI, although it often goes by other names) arising from the non-sinusoidal current produced by a VFD’s power-switching circuitry. Through wiring, it has been called high-frequency line noise, harmonic content, eddy currents, parasitic capacitance, capacitive coupling, magnetic dissymmetry, electrostatic buildup, high-voltage ringing, reflective voltage, overshoot, steep voltage wavefronts, and common mode voltage. Through radiated waves, it is known as radio frequency interference (RFI). These unwanted currents can cause degradation of insulation, bearings, coil varnish, and so forth, which is cumulative and can lead eventually to motor failure. More specifically, causes of such failure include but are not limited to high peak voltages, fast voltage rise times, the corona effect, and induced shaft currents.
High peak voltages arising from the high switching frequencies of modern VFDs are a major concern, especially if a single VFD is used to control multiple motors or if the line connecting a VFD with a motor is more than 50 feet long. As a rule, the longer the cable, the lower its impedance. If the load impedance is higher than the line impedance, current is reflected back toward the VFD, creating voltage spikes at the motor terminal that can be twice as high as the DC bus voltage.
Often overlooked until it is too late to save the motor is the cumulative bearing damage caused by VFD-induced shaft currents. Hard to predict but easier to prevent, these currents are best addressed in the design stages of a system. Without some form of mitigation, shaft currents discharge to ground through bearings, causing unwanted electrical discharge machining (EDM) that erodes the bearing race walls and leads to excessive bearing noise, premature bearing failure, and subsequent motor failure.
A Closer Look at Bearing Damage
Shaft currents can be measured by touching an oscilloscope probe to the shaft while the motor is running (figure 1). These voltages repeatedly build up on the rotor 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.
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 VFD. 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 (photo 1), the operational frequency of the VFD causes concentrated pitting at regular intervals along the race wall, forming washboard-like ridges. Fluting can cause excessive noise and vibration that forewarn of imminent bearing failure.
The Search for a Solution
Motor failures caused by VFD-induced shaft currents can result in significant unplanned downtime. In addition, these failures affect the performance and mean time between failure (MTBF) of the original equipment systems in which the motors are used. In some production applications, even a momentary stoppage due to motor failure can cost more than $250,000, excluding the cost of repairing/replacing the motor. Clearly, there is a need for a device that mitigates bearing damage from VFD-induced shaft currents.
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 is quick to point 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 can go through a pump, gearbox, tachometer, encoder, or brake motor, which consequently can wind up with bearing damage of its own.
The ideal solution would be a low-cost, maintenance-free device that safely redirects shaft currents along a very-low-impedance path from shaft to ground, protecting connected equipment as well as bearings. This device could be installed by the motor manufacturer or retrofitted in the field in virtually any VFD application. One product that meets all these criteria is the bearing protection ring (photo 2), a relatively recent invention that overcomes the dirt-collection, corrosion, and wear problems of conventional grounding brushes. A few manufacturers have introduced motors with bearing protection rings already installed, but at this writing such motors are the exception, not the rule.
Regardless of the application, the success of an automated control system depends upon its design. If in-house engineers lack the special expertise required, they should enlist the services of a qualified systems integrator who understands the engineering specifications, operating conditions, and performance curves for the whole system. With informed decisions at every stage from specification to commissioning of the system, potential problems can be anticipated and resolved.
Especially important is the selection of VFDs and motors for such systems. They should be rated for compatibility, not only with each other but also with other components of the system. A savvy specifier will choose a motor that is truly equipped for use with today’s fast-switching VFDs — one with adequate protection against bearing damage as well as winding damage.
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 shaft grounding is the best approach.