Efficiency on solid ground

With growing energy concerns (and prices) it's no wonder that efficiency-boosting components are on the rise in industry. Variable frequency drives (VFDs) are no exception. The components are already common in HVAC and industrial automation systems, and now less expensive, more powerful VFD models are catching on elsewhere.

By optimizing the voltage supply frequency of three-phase ac induction motors, VFDs control motor speed and torque to boost control accuracy and efficiency. But there is a down side: VFDs induce damaging currents on motor shafts that can wreak havoc on motor bearings, dramatically shortening motor life and diminishing system reliability. So, special precautions must be taken with VFD-driven systems, because even one failure can easily erase any energy savings gained from VFDs.

The problem at hand

Every VFD-controlled ac motor develops parasitic capacitance between the stator and rotor. Because the waveform from a VFD is generated by PWM switching, it has high-frequency components capacitively induced on the motor shaft. These are not pure sine waves; they contain high-frequency currents and voltages called harmonic content that have several negative effects.

Without some form of mitigation, shaft currents (also known as eddy currents) often discharge to ground through bearings, causing unwanted electrical discharge machining damage, premature bearing failure, and subsequent motor failure.

Make no mistake: Even motors designed for use with inverters are vulnerable to bearing failure from VFD-induced currents. Motors are never fully compatible with the VFDs that drive them unless shaft currents are mitigated.

In fact, VFD-induced bearing damage is a large and growing problem. Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month. Large pulp and paper companies report that VFD-controlled ac motors used in some plants typically fail due to bearing damage within six months. With recent copper (and motor) price surges, this problem is increasingly costly. In fact, the largest motor manufacturer in the U.S. cites eliminating drive-related motor failures as its number-one engineering challenge. Almost a dozen Internet blogs focus on VFD-induced current problems.

Short of dismantling the motor, there are two main ways to check for bearing damage from induced shaft currents: measuring vibration and measuring voltage. Both require equipment and experienced personnel, and both are best used to establish a baseline early on, so that trends can be monitored later. Using these checks provides little preventative protection. Vibration tests confirm bearing damage by identifying energy spikes of 2 to 4 kHz; usually at those levels, bearing damage has already reached the fluting stage.

More telling is tracking induced shaft currents due to common-mode voltages. These can be measured by touching an oscilloscope probe to the shaft while the motor is running. The voltages repeatedly accumulate and then discharge along the path of least resistance — which all too often, runs through bearings. During nearly every VFD cycle, these discharges (from the motor shaft to the frame via bearings) leave small fusion craters on bearing surfaces. The discharges are so frequent that before long, the entire bearing race is riddled with pits known as frosting.

In another phenomenon called fluting, the operational frequency of VFDs causes concentrated pitting at regular intervals along bearing race walls, forming washboard-like ridges. Fluting can cause excessive noise and vibration, and in HVAC systems, the noise is often magnified and transmitted via ductwork.

Reducing harmful VFD effects

To guard against incidental VFD damage and extend system life, several approaches exist.

1. Motors should be specially designed for use with VFDs, and should be rated for the application and load at hand. For example, when required to maintain a constant torque, a motor tends to lose some efficiency, running hotter at lower speeds and hotter still when controlled by a VFD. If such a motor must be operated at less than 30% of its maximum speed, it may need extra cooling or thermal protection. Similarly, a VFD-controlled motor's capability to produce torque drops more quickly at lower motor speeds than would that of a motor using pure sine wave power. For constant-torque loads, a VFD should be rated for 60 seconds at 150% of the load. (A VFD's current rating also limits the load-acceleration rate.)

2. The cable connecting a VFD with a motor should be no longer than what the motor manufacturer specifies. Otherwise, two different wave types could meet at motor terminals and in effect double the voltage received by the motor. If a longer cable is required, extra line filtering is recommended to protect the motor and other sensitive equipment nearby from harmonic content and radio frequency interference RFI. (RFI is also reduced by enclosing motor leads in rigid conduit.) Regardless of length, cable between a VFD and the motor or motors it regulates can be enclosed in a corrugated aluminum sheath or other grounded low-impedance shielding.

3. If VFDs are not appropriate for an application, they should not be used. For example, VFDs may be unsuitable for systems that must maintain high pressure. During periods of low flow, a VFD-controlled pump motor may not be able to slow down sufficiently without reducing pressure.

4. Purchase VFDs that permit fine-tuning of the carrier frequency in increments no larger than 1 kHz. Serious bearing damage is more likely in systems operating with high in systems operating with high carrier frequencies at constant speed. (High carrier frequencies have high discharge rates.) Try to keep this frequency as low as possible.

5. Properly ground all motors. Inadequate grounding significantly increases the possibility of electrical bearing damage in VFD-driven motors. With no electrical discharge, bearing walls are marked by nothing but mechanical wear.

6. For systems that require dynamic braking, some VFDs are available with power load resistors that can shunt excess energy from the dc bus.

7. When addressing electrical discharge, insulating motor bearings is one solution. Proceed with caution here: Often insulation simply shifts problems elsewhere. Blocked shaft current seeks other paths to ground. Attached equipment often provides this path, and frequently exhibits bearing damage of its own. In addition, insulation is subject to contamination — and some types can actually have a capacitive effect on high-frequency VFD-induced currents, allowing these currents to pass right through to bearings it is designed to protect.

   Nonconductive ceramic ball bearings divert currents from the main motor's bearings but leave other attached equipment exposed to damage. Too, ceramic bearings can be costly and must usually be resized to handle mechanical static and dynamic loading.

8. Alternate discharge paths to ground, when properly implemented, are preferable to insulation because they neutralize shaft current. A Faraday shield can be created by installing grounded conductive material such as copper foil or paint between the stator and rotor. If built to the proper specifications, this can block most harmful currents that jump across a motor's air gap. However, this mitigating measure can be expensive and difficult to implement, and attached equipment can still be vulnerable to deflected currents.

   Another option is conductive grease, which bleeds off harmful currents by providing a lower-impedance path through the bearings. Sometimes, however, the conductive particles in the grease increase mechanical wear. Metal grounding brushes are often a better solution; they wear and corrode, but contact the motor shaft to provide effective alternate paths to ground. to ground.

9. Another way to address electrical discharge problems is to use bearing protection rings. These rings are engineered with conductive microfibers to provide a low-impedance path from shaft to frame, redirect shaft currents, and bypass motor bearings entirely. The protection rings include electron transport technology that leverages principles of ionization to boost electron-transfer rates and promote efficient discharge of the high-frequency shaft currents induced by VFDs. Cost is relatively low: Typically, an ac motor coupled with a VFD costs from $2,400 to $100,000 or more and may be part of a manufacturing process that generates revenues from $10,000 to $1,000,000 or more per hour. In comparison, the cost of installing a bearing protection ring is usually less than 1% of equipment cost.

How the rings work

By preventing electrical damage to bearings, bearing protection rings protect the VFD system from the costly downtime of unplanned maintenance. In all but the smallest applications, these rings provide high return on investment; for many production applications, even a momentary stoppage due to motor failure can cost more than $250,000, excluding the cost of motor repair.

Unlike conventional shaft grounding brushes, which call for occasional maintenance or replacement, the microfibers of a bearing protection ring work with no friction or wear — less than 0.001 in. per 10,000 hours of continuous operation. In addition, the ring's fibers do not break, even under millions of direction reversals. The rings are unaffected by dirt, grease, and other contaminants, so they last for the life of the motor regardless of rpm.

One protection ring manufacturer guarantees that new motors (usually to 100 hp) on which rings are properly installed will not fail from electrical fluting bearing damage. The guarantee is useful because it's often difficult to make warranty claims against motor and VFD manufacturers, as many factors promote bearing damage, and even perfectly matched, nondefective VFDs and motors can create discharge problems.

Protection ring application

Bearing protection rings are scalable to any NEMA or IEC motor — and are successfully applied to power generators, gas turbines, ac traction and break motors, cleanrooms, HVAC systems, and other industrial and commercial applications. Some rings are available in two versions — the continuous ring for NEMA and IEC-frame motors, and a split-ring design for in-field installation, and around larger shafts, without requiring disassembly of attached equipment. Although best addressed in the design stage of a system, the rings can also be retrofitted on previously installed motors. Mounting adaptors included with some NEMA or IEC protection ring models facilitate installation on motors with shaft shoulders, slingers, bearing caps, or end-bell protrusions.

When installing one ring, the motor's drive end is the preferred location. Large ac motors (of 100 hp or more) and even large dc motors, especially those with shaft diameters of more than 2 in., are more likely to have high-frequency circulating currents (as well as EDM-type discharges) that can damage bearings. Motors with roller bearings are also more vulnerable to damaging circulating currents because roller bearings have a greater surface area and their lubricant layer is usually thinner. All these motors benefit from the combination of a bearing protection ring on the drive end and insulation at the non-drive end, to break the circulating current path. This setup may also be helpful where installing a bearing protection ring on the non-drive end is impractical because of encoders, fans, or other special circumstances.

For larger motors, the best protection is often obtained by installing a bearing protection ring on the drive end of the shaft and insulation on its non-drive end. This is common for motors above 500 hp; most manufacturers already take this approach. However, where drive-end insulation is not designed into the motor or cannot be easily installed, two rings are recommended — one on each end.

In critical applications running motors with two ceramic bearings, at least one bearing protection ring should be used to ensure that shaft voltage does not pass down the line to attached gearboxes, pumps, encoders, pillow block bearings, or brake motors.

For more information on protection and grounding solutions, call (207) 998-5140 or visit www.est-aegis.com.

VFD benefits

The potential for increased efficiency with VFDs is most dramatic in flow control. Many centrifugal fans and pumps run continuously, but often at reduced loads; because energy consumption of such devices correlates to flow rate cubed, if a fan's speed is halved, the horsepower needed to run it drops by a factor of eight. With rising energy costs, restricting the work of a motor running at full speed through the use of dampers and throttling mechanisms is needlessly wasteful. Motors use less power if controlled by a VFD.

In constant-torque applications where the main objective is more accurate process control, as in reciprocating compressors, conveyors, and machine tools, VFDs can be programmed to prevent the motor from exceeding a specific torque limit. This protects the motor, and in some cases associated machinery and products, from stress and damage. Otherwise, if a machine jams without the moderating influence of a VFD, its driving motor draws excessive current until an overload device shuts it down.

Regardless of the application, a VFD must be compatible with both the motor and other system components, and must be selected by someone who understands the entire system, including all possible current paths. With informed decisions from specification all the way to operation, any potential problems can be identified and resolved. Systems engineers should have the expertise to review all pertinent engineering specifications, operating conditions, and performance curves. Operator training is equally important.

VFD basics

In typical VFDs, a rectifier (thyristor or diode) converts ac utility feed to direct current. Then a filter (consisting of inductors and capacitors) smoothes current waveforms. Next, a pulse-width modulation (PWM) inverter using insulated gate bipolar transistors or IGBTs turns it back into ac — this time, in variable form. Typical output frequency, also called carrier or switch frequency, is 2 to 12 kHz, or 2,000 to 12,000 on/ off cycles per second. In some systems, VFDs are used to directly drive one or more motors in constant torque, to ensure they use no more power than necessary; in other systems with encoder feedback, VFDs can also control motor speed by modulating power voltage and frequency to the motor according to programmed parameters.

Bad buildup

Tracking common-mode voltages is revealing. These voltages repeatedly accumulate on the rotor to a certain threshold, and then discharge in bursts — often through motor bearings to the grounded frame.

Normal race wall wear

Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface. As the motor runs, tracks form where ball bearings contact the wall. Without electrical discharge, the wall is marked by nothing but mechanical wear.

Over and over

Because most newer ac motors have sealed bearings to keep out contaminants, electrical damage is now the most common cause of bearing failure in VFD-controlled motors. Induced currents discharge via bearings, leaving small fusion craters on their balls and races. The pits make for noisy operation and imminent failure.

Damage done

By the time vibration tests confirm bearing damage by identifying energy spikes, damage has usually reached the fluting stage. Here, the VFD's operational frequency causes concentrated pitting at regular intervals along the bearing race wall, forming washboard-like ridges.

Problem solved

Without mitigation, VFD-induced shaft currents (top) can cause considerable damage. Bottom shows how effectively bearing protection rings reduce these currents by channeling them to ground.

Location, location

A bearing protection ring is installed with a mounting plate and standoff posts or spacers.

Protection ring arrangements

For VFD-equipped motors of less than 100 hp with shaft diameters of less than 2 in., a single bearing protection ring on the drive end of the motor shaft is typically sufficient to divert harmful shaft currents. Two rings are recommended for some larger motors.