Important advancements in VFD and motor control

Recent advancements in variable frequency drives have transformed these devices to enhance safety features and improve energy efficiency. See three VFD advanced control schemes.

By Dr. Michael Wrinch April 18, 2024
Courtesy: Hedgehog Technologies


Learning Objectives

  • Understand the evolving safety features in modern VFDs.
  • Evaluate energy efficiency upgrades in contemporary VFDs.
  • Explore three advanced control schemes in current VFD technology.


VFD insights

  • Recent advancements in variable frequency drives (VFDs) have significantly improved safety features, energy efficiency and control schemes.
  • Enhanced safety in VFDs includes the integration of network safety protocols like PROFIsafe and CIP safety, as well as the implementation of sophisticated methods for detecting and responding to hazards.
  • Modern VFDs feature advanced control schemes such as vector control, direct torque control and sensorless vector control.

As technologies progress, modern variable frequency drives (VFD) offer enhanced safety, energy efficiency and advanced control schemes. Selecting the right VFD for industrial applications requires additional consideration of potential harmonic issues, manufacturer reputation and cost effectiveness.

Enhanced VFD safety

Recent enhancements to safety features in VFDs are driven by a growing emphasis on safety protocols. One notable development is the integration of network safety protocols, such as PROFIsafe (PI North America) and CIP Safety (ODVA). This integration involves incorporating VFDs into safety networks and protocols.

The key distinction is in the VFD’s capacity to communicate with other safety systems, facilitating a coordinated response to hazardous conditions through real-time safety monitoring and control. This ensures the reliable and integral transmission of safety-related data, including emergency stop signals and light curtain status. To achieve this, techniques like unique identification, sequence counters and cyclic redundancy checks (CRC) are employed, guaranteeing that safety data is not lost, altered or delayed during transmission.

Another recent advancement in safety features is the implementation of safe torque off, commonly known as “STO.” While STO is not a new concept, its application in modern VFDs has become more sophisticated.

Older STO-like features involved a hard stop of the process through direct inhibition of the scan cycle or the use of a contactor with a safety relay to open the circuit and disable the motor. The scan inhibitor was subject to noise and errors while the external contactor simply added additional parts. The contemporary STO features are integrated directly with the drive’s control system to offer a higher level of safety using fewer parts. This ensures an immediate removal of power to prevent unintended motor startup or movement, providing a more efficient and reliable safety mechanism.

The latest VFDs employ sophisticated methods for detecting and responding to overcurrent, ground faults and thermal overload conditions. These methods use advanced algorithms and real-time line monitoring, enabling faster and more accurate protection compared to older systems. These modern additions to VFD safety features reflect a trend toward more integrated, proactive and data-driven approaches to safety. The focus is not just on responding to hazards but on anticipating and preventing them, which aligns with broader trends in industrial safety and automation.

Michael Wrinch is commissioning motor drive control panels for a natural gas plant in Australia. Courtesy: Hedgehog Technologies

Michael Wrinch is commissioning motor drive control panels for a natural gas plant in Australia. Courtesy: Hedgehog Technologies

Modern VFD energy and form efficiency versus soft starters

Modern VFDs exhibit significantly improved energy efficiency when compared to their predecessors. These enhancements are attributed to advancements in transistor technology, such as low-loss surface mount gallium nitride (GaN) and silicon carbide (SiC) power transistors. These technologies contribute to faster switching, higher bus voltage and lower gate voltage drop, resulting in overall higher efficiency.

In contrast to older VFDs with efficiencies below 85%, some manufacturers now boast upper efficiencies of up to 98% at full load. Notably, the limitations of older transistor technologies, which required a trade-off between low voltage fast switching and higher voltage slow and inefficient switching, no longer apply with GaN or SiC technology. Higher currents are attainable in small form factors.

With the advancements in transistor technology, many VFDs have been miniaturized, resulting in a more compact design. This downsizing makes them suitable for a wider range of applications that were previously not feasible. In scenarios where a soft starter was the sole option, a VFD can often substitute for it, offering advanced benefits such as controlled starts, stops and speed adjustments.

Soft starters were traditionally considered a cost-effective alternative to harsh on-off motor starting. With the increasing compactness and versatility of modern VFDs, they are becoming more competitive and viable options in various applications. This shift reflects the broader advantages of VFDs and their ability to handle a diverse range of motor control requirements.

Contemporary VFDs are equipped with sophisticated algorithms that optimize energy use and harmonic generation based on the load requirements. These algorithms dynamically adjust operating parameters, such as voltage and frequency, to align with the load, resulting in reduced energy consumption. This is a shift from older VFDs that operated at fixed or less adaptable parameters.

It is not uncommon for VFDs to include advanced regenerative braking capabilities, which allow them to capture and reuse energy that would otherwise be wasted during deceleration or braking. This feature proves especially effective in applications with frequent stop-start cycles. In addition, VFDs incorporate enhanced designs and components to mitigate electrical harmonics, which can affect power quality and lead to inefficiencies.

Three VFD advanced control schemes

Current VFDs provide enhanced control over motor speed, torque and overall performance, coupled with advanced diagnostics and improved connectivity features. Among the state-of-the-art control schemes used today are vector control, direct torque control and sensorless vector control which are explained in further detail below.

Vector control (field-oriented control – FOC): Vector control, also known as field-oriented Control (FOC), is one of the most advanced control methods. This approach decouples the motor’s torque and flux components, enabling independent control over each. It’s particularly effective in applications demanding precise speed and torque control, such as in robotics and CNC machines.

Direct torque control (DTC): DTC is a method that directly regulates motor torque and flux, providing a rapid and dynamic response. Unlike vector control, DTC operates without the need for position sensors and offers a straightforward implementation with robust performance, making it advantageous in applications with swift and frequent load changes.

Michael Wrinch, with engineer Alia Gola, holding the Allen-Bradley PowerFlex AC Motor Drive from Rockwell automation, to be installed in the panel behind them. Courtesy: Hedgehog Technologies

Michael Wrinch, with engineer Alia Gola, holding the Allen-Bradley PowerFlex AC Motor Drive from Rockwell automation, to be installed in the panel behind them. Courtesy: Hedgehog Technologies

Sensorless vector control: This control scheme delivers performance comparable to FOC but without the need for a rotor position sensor. It estimates the motor’s magnetic flux using mathematical models, which enables precise control even at low speeds. This method is widely used in applications where installing sensors is impractical or excessively costly.

These advanced control schemes significantly enhance the capabilities of VFDs, facilitating precise control, improved efficiency and adaptability to a wide range of industrial and commercial applications. As technology evolves, these control methods are poised to become even more sophisticated, integrating artificial intelligence and machine learning for even greater performance and efficiency.

In the world of variable frequency drives, notable changes have taken place, prompting consideration for upgrades that can enhance overall safety, performance or efficiency within industrial systems.

Michael Wrinch is the founder of Hedgehog Technologies. Edited by Tyler Wall, associate editor, Control Engineering, CFE Media and Technology,


VFD, motor controls, control schemes


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Author Bio: Dr. Michael Wrinch, P.Eng., is the president of Hedgehog Technologies, an electrical engineering consulting firm that specializes in risk management. He is certified through TÜV Rheinland, an international gold standard in safety.