Fulfilling the promise of variable frequency drives
By Adam Willwerth
 The AEGIS SGR with NEMA adaptor plate |
With the rising cost of energy, the use of variable frequency drives (VFDs) is growing at an increasing rate. 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. And, these energy savings can be quite substantial—20 percent or more—making VFDs a “green” solution as well as a wise money-saving investment.
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However, in order to be truly “green,” 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.
Already common in heating, ventilation, air conditioning, pumping, and industrial automation systems, VFDs are catching on in many other applications as they become smaller and more powerful, more reliable, easier to program, and less expensive. But to prevent energy savings from being wiped out in a single system failure, VFD/motor systems must be designed for reliability and trouble-free operation.
While VFDs are not without certain drawbacks, these can now be easily overcome. 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 such as motor-shaft grounding rings that protect bearings from VFD-induced damaging shaft currents. Such safeguards will eliminate the need for expensive repairs, and enable VFDs to fulfill their promise of energy and cost savings.
Tremendous Energy-Saving Potential
In today’s typical VFD, a rectifier (thyristor or diode) converts the AC utility feed to direct current (DC). A filter (inductors and capacitors) smoothes the current’s waveform. A pulse-width modulation (PWM) inverter then turns it back to AC in variable form using insulated gate bipolar transistors (IGBTs). Typical output frequency, also called the carrier or switch frequency, is 2 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, such as reciprocating compressors, conveyors, mixers, machine tools, etc., 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. To avoid pitfalls, it should be selected by someone who understands the entire system, including all of its possible current paths. Systems engineers should have the expertise to review all pertinent engineering specifications, operating conditions, and performance curves. Operator training is equally important. With informed decisions from specification all the way through to operation, potential problems can be identified and resolved.
Potential Problems
Because the waveform from a VFD is generated by pulse-width-modulated switching, it has high-frequency components which are capacitively induced onto the motor shaft and discharge 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.
Hard to predict but easier to prevent, 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 (also known as eddy 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. Obviously, the significant cost savings that VFDs deliver in so many applications can all be wiped out if a motor fails because its bearings are vulnerable.
And there is considerable evidence to prove that VFD-induced bearing damage is a large and growing problem. Consider:
- Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month (720 hours).
- An HVAC contractor recently reported that, of the VFD-controlled 30-60 HP vane axial fan motors he installed in a large building project, all failed within a year (two within 6 months). Repair costs totaled more than $110,000.
- Several large pulp and paper companies surveyed noted that the VFD-controlled AC motors used in their plants typically fail due to bearing damage within six months.
- The largest motor manufacturer in the United States has cited eliminating drive-related motor failures as its number-one engineering challenge.
- Almost a dozen Internet blogs focus on problems presented by VFD-induced shaft currents.
- Motor failures caused by VFD-induced shaft currents result in hundreds of thousands of hours of unplanned downtime every year in the United States alone. In addition, these failures affect the performance and mean time between failure (MTBF) of whatever origin