Shaft grounding: Fulfilling the promise of variable frequency drives
June 11, 2009
By Adam Willwerth
This online version has been modified from the print version to include a solution to harmful VFD currents: the shaft grounding ring.
With the rising cost of energy, the use of variable frequency drives (VFDs) is growing. By optimizing the frequency of a three-phase alternating-current (AC) induction motor’s voltage supply, a VFD controls the motor’s speed and torque, while providing energy savings. These savings can be quite substantial – 20 percent or more – making VFDs a “green” solution as well as a wise money-saving investment.
To be truly “green,” however, a technology must be sustainable as well as energy efficient. Yet the currents induced on motor shafts by VFDs can wreak havoc with motor bearings, dramatically shortening motor life and severely diminishing the reliability of systems. To mitigate these currents and realize the full potential of VFDs, a cost-effective method of shaft grounding is essential.
Whether used to save energy or increase the accuracy of process control, VFDs only achieve their full potential when carefully matched to the application and installed with appropriate safeguards. Safeguards will eliminate the need for expensive repairs and enable VFDs to fulfill their promise of energy and cost savings.
In today’s typical VFD, a rectifier converts the AC utility feed to direct current (DC), a filter smoothes the current’s waveform and then a pulse-width modulation (PWM) inverter turns it back to AC in variable form using insulated gate bipolar transistors (IGBTs). Typical output frequency is two to 12 kHz, or 2,000 to 12,000 on/off cycles per second. VFDs may be used to directly drive one or more motors in constant-torque applications, to ensure that they do not use any more power than necessary. With encoder feedback, a VFD can also be used to control the speed of a motor by modulating the voltage and frequency of power to the motor according to programmed parameters.
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. With rising energy costs, restricting the work of a motor running at full speed through the use of dampers and other throttling mechanisms seems needlessly wasteful.
In constant-torque applications where the main objective is more accurate process control, a VFD 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. If a machine jams, for instance, the motor that powers it will, without the moderating influence of a VFD, draw excessive current until its overload device shuts it down.
Regardless of the application, the VFD must be compatible, not only with the motor, but also with every other system component.
Because the waveform from a VFD is generated by pulse-width-modulated switching, it has high-frequency components that are capacitively induced onto the motor shaft and discharged through the bearings. These are not pure sine waves; they contain high-frequency currents and voltages called harmonic content, the potential effects of which are many. And even when the motor is designed for inverters, it is vulnerable to bearing failure from VFD-induced currents.
Motor shaft currents induced by VFDs can damage motor bearings. Although best addressed in the design stage of a system, these currents can usually be mitigated by retrofitting previously installed motors. Without some form of mitigation, shaft currents discharge to ground through bearings, causing pitting, fusion craters and “fluting.” This unwanted electrical discharge machining (EDM) leads to excessive bearing noise, premature bearing failure and subsequent motor failure.
Motors are never fully compatible with the VFDs that drive them unless these shaft currents are addressed and mitigated. Various other negative effects can also manifest themselves if the driven motor is not designed for use with a VFD, or if either the motor or the VFD is not rated properly for the application or the load. For example, when required to maintain a constant torque, a motor tends to lose some efficiency, running hotter at lower speeds but hotter still when controlled by a VFD. If such a motor must be operated at less than 30 percent 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 a motor using pure sine wave power. For constant-torque loads, a VFD should be rated for 60 seconds at 150 percent of the load. A VFD’s current rating also limits the load-acceleration rate.
Another rule of thumb is that the cable connecting a VFD with a motor should not be more than 50 feet long, or else two different wave types could meet at the 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 can also be reduced by enclosing motor leads in a rigid conduit. Regardless of its length, the cable between a VFD and the motor or motors it regulates can be enclosed in a corrugated aluminum sheath or another kind of grounded, low-impedance shielding.
VFDs may also not be appropriate for systems that must maintain high pressure. During periods of low flow, a VFD-controlled pump motor may not be able to slow down enough without reducing pressure. For systems that require dynamic braking, VFDs are available with an optional power load resistor that can shunt excess energy from the DC bus.
A closer look at bearing damage
Every VFD-controlled AC motor develops a parasitic capacitance between the stator and rotor. Short of dismantling the motor, there are two main ways to check for bearing damage from induced shaft currents – by measuring vibration and voltage. Both are best used to establish a baseline early on, so that trends can be monitored later. Neither method is foolproof.
By the time vibration tests confirm bearing damage by identifying particular energy spikes in the range of two to four kHz, the damage has usually reached the “fluting” stage. Likewise, the main benefit of voltage tests may be the relief they provide when the results indicate no bearing damage. If a baseline voltage measurement is taken right after a VFD has been installed, successive tests may provide early warning of harmful current loops, but there are many variables.
Induced 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 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 to the frame (ground). Serious bearing damage is thought to be more likely in systems that operate with high carrier frequencies, a constant speed or inadequate grounding.
A high carrier frequency means a high discharge rate. For this reason, it is advisable to purchase a VFD that permits fine tuning of the carrier frequency in increments no larger than one kHz.
There is no doubt that inadequate grounding significantly increases the possibility of electrical bearing damage in VFD-driven motors. Without proper grounding, VFD-induced electrical discharges can quickly scar the race wall. During virtually every VFD cycle, these induced currents discharge from the motor shaft to the frame via the bearings, leaving small fusion craters in ball bearings and the bearing race wall. These discharges are so frequent that before long the entire bearing race becomes riddled with pits known as frosting. The damage eventually leads to noisy bearings, but by the time such noise is noticeable, bearing failure is often imminent. 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.
Mitigating bearing damage
Electrical damage to AC motor bearings often begins at startup and grows progressively worse. As a result of this damage, the bearings eventually fail. To guard against such damage and thus extend motor life, the induced current must be diverted from the bearings by means of mitigation technologies, such as insulation, shielding and/or an alternate path to ground.
Insulating motor bearings is a partial solution that more often than not shifts the problem elsewhere. Blocked by insulation, shaft current seeks another path to ground. Attached equipment, such as a pump, often provides this path and it frequently winds up with bearing damage of its own. In addition to being expensive, insulation is subject to contamination. Worse yet, some types of insulation can be totally self-defeating. In certain circumstances, the insulating layer has a capacitive effect on high-frequency VFD-induced currents, allowing them to pass right through to the bearings it was supposed to protect.
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 for the motor, this can block most of the harmful currents that jump across the motor’s air gap. However, this mitigating measure is often expensive and difficult to implement, and attached equipment could still be vulnerable to deflected currents.
Likewise, nonconductive ceramic ball bearings divert currents from the main motor’s bearings but leave attached equipment open to damage of its own. Ceramic bearings can be costly and usually must be resized to handle mechanical static and dynamic loadings.
Yet another mitigation attempt comes in the form of conductive grease, which, in theory, bleeds off harmful currents by providing a lower-impedance path through the bearings. In practice, however, the conductive particles in the grease increase mechanical wear.
Metal grounding brushes certainly help. They contact the motor shaft to provide alternate paths to ground. Unfortunately, they also wear out and corrode, thus requiring regular maintenance.
Alternate discharge paths to ground, when properly implemented, are preferable to insulation because they neutralize shaft current. Techniques range in cost and sometimes can only be applied selectively, depending on motor size or application. The ideal solution would provide an effective, low-cost, very-low-resistance path from shaft to frame, and could be broadly applied across all VFD/AC motor applications, affording the greatest degree of bearing protection and maximum return on investment.
Avoid damage with a shaft grounding ring
A shaft grounding ring (SGR), such as the AEGIS SGR from Electro Static Technology (www.est-aegis.com), meets all these criteria–the company’s patent-pending Electron Transport Technology uses the principles of ionization to boost the electron-transfer rate and promote extremely efficient discharge of the high-frequency shaft currents induced by VFDs. Without some form of mitigation, VFD-induced shaft currents can cause considerable motor/bearing damage.
It’s scalable to any NEMA or IEC motor regardless of shaft size, horsepower or application. The company says these grounding rings have been successfully applied to power generators, gas turbines, AC traction and break motors, cleanrooms, HVAC systems and a long list of other industrial and commercial applications.
For VFD-equipped motors of less than 100 HP (75 kW) with shaft diameters of less than two inches (50 mm), a single SGR on the drive end of the motor shaft is typically sufficient to divert harmful shaft currents.
Large AC motors (100 HP/75 kW or more) and even large DC motors, especially those with shaft diameters of more than two inches (50 mm), 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. Such motors benefit from the combination of an shaft grounding ring on the drive end and insulation on the non-drive end to break the circulating current path.” This may also be the solution in situations where installing an SGR on the non-drive end would be impractical because of encoders, fans, or other special circumstances. For most large motors, the best bearing protection may be obtained by installing an SGR on the drive end of the shaft and insulation on the non-drive end.
This is also a common solution for motors above 500 HP (375 kW), and most manufacturers already take this approach.” However, when insulation on the drive end is not designed into the motor or cannot be easily installed, two SGRs are recommended – one on the drive end (DE) and one on the non-drive end (NDE).
In critical applications where motors with two ceramic bearings are specified, at least one SGR should be used to ensure that shaft voltage does not pass down the line to attached equipment such as gearboxes, pumps, encoders, pillow block bearings or break motors.
The AEGIS SGR is available in two versions–a continuous ring for most NEMA- and IEC-frame motors and a split-ring design, which allows installation in the field, around larger shafts, without the need to disassemble attached equipment.
Adam Willwerth is the development manager for Electro Static Technology.