Cynthia D. Nyberg
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, Missouri
The paper "Update on the Effects of VFDs on Motor Performance" by Cynthia D. Nyberg, presented at the EASA Convention, explores the impact of variable frequency drives (VFDs) on AC electric motors. VFDs are increasingly used to achieve speed control in various applications, offering significant energy savings, especially in variable torque loads like fans and pumps. However, the use of VFDs introduces new stresses on motors, leading to potential failures if not properly managed.
Nyberg begins by explaining the basic operation of VFDs, which convert incoming AC power to DC and then back to a controllable AC waveform. This process allows for precise speed control, enhancing productivity and energy efficiency. Despite these benefits, VFDs can cause winding failures due to transient voltage surges and bearing failures due to shaft voltages.
The paper is divided into two main sections: Windings and Bearings. In the Windings section, Nyberg discusses the various stresses introduced by VFDs, including continuous voltage spikes that can lead to premature winding insulation failure. The cumulative effect of these spikes can cause the insulation to break down over time, particularly at the end turns where stress is highest. The paper also addresses the issue of reflected wave voltage, which occurs when the impedance of the motor is greater than the transmission line, causing voltage overshoot.
Corona discharge is another concern, where voltage spikes create microscopic holes in the insulation, leading to partial discharges that can eventually cause arcing. Nyberg recommends using vacuum pressure impregnation (VPI) to eliminate voids in the insulation and prevent corona discharge. Harmonics generated by VFDs can cause heat-related failures, nuisance tripping, and interference with electronic equipment. The paper also highlights the importance of managing cable length to reduce the impact of reflected waves and voltage spikes.
In the Bearings section, Nyberg explains that VFDs can cause bearing failures due to shaft currents, which were traditionally associated with larger motors but are now seen in smaller motors as well. The paper describes various methods to protect bearings from shaft currents, including insulating bearing housings, using ceramic bearings, and installing grounding brushes. Insulated bearings are preferred as they interrupt the flow of current through the shaft, preventing damage.
Nyberg concludes by emphasizing the importance of protecting motors from the stresses introduced by VFDs to prolong their life. She suggests using a combination of methods, such as improved insulation, proper winding techniques, and external cooling, to mitigate the effects of VFDs.
Key Points Covered:
- Basic operation and benefits of VFDs
- Stresses introduced by VFDs on windings and bearings
- Continuous voltage spikes and their cumulative effect on winding insulation
- Reflected wave voltage and its impact
- Corona discharge and the use of VPI to prevent it
- Harmonics and their effects on motor performance
- Importance of managing cable length
- Bearing failures due to shaft currents
- Methods to protect bearings, including insulated bearings and grounding brushes
Key Takeaways:
- VFDs offer significant energy savings but introduce new stresses on motors.
- Continuous voltage spikes from VFDs can lead to premature winding insulation failure.
- Reflected wave voltage and corona discharge are critical issues that need to be managed.
- Harmonics generated by VFDs can cause various operational problems.
- Proper management of cable length is essential to reduce voltage spikes.
- Bearing failures due to shaft currents are a significant concern with VFDs.
- A combination of methods, including improved insulation and external cooling, is necessary to protect motors from VFD-induced stresses.
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