Electrical and power considerations for VFDs and VSDs
Variable frequency drives (VFDs) and variable speed drives (VSDs) depend on having the right electrical and power settings and engineers need to know how to utilize them correctly.
Respondents:
- Richard Barrows, senior electrical engineer, Hitachi Global Air Power, Michigan City, Indiana
- Zack Fowler, product marketing manager, Atlas Copco, Rock Hill, South Carolina
- Eder Matias, director of sales, drives, controls & automation, SEW-EURODRIVE USA, Lyman, South Carolina
- Craig Nelson, senior product manager, Siemens Digital Industries, Alpharetta, Georgia
- Paulo Guedes Pinto, chief technology officer, Infinitum, Round Rock, Texas
- Brandon Teachman, owner, Automation Coffee Break, Appleton, Wisconsin
Question: VFDs generate significant switching frequencies, which can lead to electromagnetic interference (EMI) problems. How can these EMI issues be addressed to comply with regulatory standards?
Zack Fowler: EMI issues generated by VFDs can be effectively addressed by incorporating electromagnetic compatibility (EMC) filters. These filters are designed to mitigate high-frequency noise and ensure that the VFDs operate within the limits set by regulatory standards.

The Infinitum Aircore EC motor system. Courtesy: Infinitum
Craig Nelson: Most industrial plant standards allow for the effects from the switching frequencies of VFD’s. In commercial installations, choosing a VFD with built-in or optional filters to meet the requirement is very common.
Pablo Guedes Pinto: There are several ways to address these EMI issues including EMI filters on the input side of the VFD to prevent high-frequency noise from transmitting further, output filters including common mode filters, metal enclosure of the VFD, use of shielded cables and finally, proper grounding of the VFD and motor.
Question: What are the key differences and trade-offs between using VFDs with synchronous motors versus induction motors? When would you choose one type of motor over the other in a VFD application?
Zack Fowler: Induction motors are a cost-effective option with a simple and robust design that has lower efficiencies and power factors. The synchronous reluctance motor has superior efficiency across a full speed range (better for VSD compressors) and much more efficient at lower speeds. The maximum efficiency for induction motors are usually around 93% where synchronous is about 96%. Although this is only a 4% difference, over a span of years this leads to significant energy savings.
Eder Matias: That’s a common question. There are several factors that need to be considered before deciding on what would be the best motor for your application. Not every VFD is capable of controlling synchronous motors and, the setup tends to be more complex than with a standard asynchronous motor. Another trade-off is the performance at low speed — assuming it is an open loop system.
Theoretically, synchronous motors tend to have a higher power density, wider speed range and a higher energy efficiency rating than asynchronous motors. Also, smaller motors typically have a lower moment of inertia, which has pros and cons, depending on the application.

The MOVI-C automation platform is part of SEW-EURODRIVE’s Industry 4.0 approach and includes VFDs, HMIs, motion controllers, gearboxes, motors, servomotors and encoders, which are designed to work together seamlessly. Courtesy: SEW-EURODRIVE
Craig Nelson: A majority of VFDs will run either induction or synchronous motors. Synchronous motors are typically higher cost but have higher power density and offer better efficiency and dynamics. Induction motors can have the advantage to run further in the field weakening range but upfront cost and encoder-less operation are their big advantages.
Pablo Guedes Pinto: Synchronous motors, particularly permanent magnet synchronous motors (PMSMs), tend to have better efficiency than induction motors with the same rating. On the other hand, synchronous motors tend to be more expensive than induction motors. As the machine size increases, there is an inflexion point where synchronous motors become more economical. This often happens at the point where the rotor of an induction motor has to be made out of segmented laminations instead of a single full-circle lamination (around 30″ diameter).
Question: How do you assess the efficiency and energy-saving potential of a VFD installation in a motor-driven system? Are there any standard methodologies or tools for this evaluation?
Zack Fowler: We have developed a software that calculates and compares energy consumption of different models based on the flow profile. The data will then be compiled in a comprehensive report tailored specifically to your operation, your requirements, and your priorities.
That report includes recommendations on how you can save energy (and reduce costs), reduce your environmental footprint, and which compressor would be the ideal fit.

Atlas Copco’s GA VSD product line brings energy efficiency and reliability for industries worldwide. Courtesy: Atlas Copco
Eder Matias: The assessment starts by understanding the application requirements, how it works, and all drive technology components involved (gearbox, motor and VFD). Our engineers use the appropriate methodology and tools to build a more energy-efficient solution based on the initial assessment. To maximize energy efficiency, consider fine-tuning the VFD, a newer motor technology, energy buffering and management system.
Question: How are VFDs integrated into modern smart grid systems and industrial IoT applications?
Zack Fowler: Through central controllers that provide real time monitoring and control. By analyzing the data and trends the customer can optimize their system for better performance, reliabilities, and energy efficiencies that lead to a quicker return on investment (ROI).
Question: What are the challenges and solutions for using VFDs in renewable energy applications such as wind and solar power?
Richard Barrows: Establish the appropriate environmental conditions. Avoid dirty, wet and hot environments. Also determine and ask if the drives sized properly in order to meet the power needs in order to generate energy.
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