Your questions answered: Energy management with variable speed drives (VFDs)

Reducing fan speed in a variable torque load application by 20% can lead to energy savings of 50%.

By Scott Sullivan November 8, 2022
Courtesy: Advanced Energy

In a Plant Engineering September webcast, Scott Sullivan, a field service engineer with Parsippany, NJ-based Electronic Drives and Controls presented on the topic of how variable speed drives (VFDs) provide effective speed control of AC motors by manipulating voltage and frequency.

As many readers will be aware, controlling the speed of a motor provides users with improved process control, reduced wear on machines, increased power factor and large energy savings.

The most significant energy savings can be achieved in applications with a variable torque load. Reducing a fan’s speed in a variable torque load application by 20% can achieve energy savings of 50%. Therefore, for most motion control applications, reducing motor speed is often the easiest way to get large energy savings.

In the question-and-answer session immediately following the webcast, attendees had many questions for Sullivan. His answers to those we didn’t have time to get to during the webcast are found below.

If a single-phase motor is somehow being run by VFDs, is this a violation of NFPA or NEC codes?

Sullivan: Single phase drives do exist, but typically for only very small horsepower and can only be used on specific types of single-phase motors. Additionally, they tend to cause a lot of overheating in motors, so the motor may need to be de-rated based on the application. In terms of code violations, I would recommend making sure your installation adheres to the guidelines from the manufacturer, and to contact them if you are unsure.

Why is a damper even required? Isn’t an alternative possibility that, if you don’t want the air flow, simply stop the drive.

Sullivan: Dampers are still used to seal off the building when the drive is not running. With a damper that is always open, the building would be “leaking” air and heating and cooling would be affected. Also, in case of a fire, so called “smoke dampers” can be used to control airflow.

Even with an Inverter-Duty motor, is there still a distance limitation from VFD to motor to prevent motor damage? Does VFD cable prevent this?

Sullivan: First, any motor run with a drive should be inverter-duty. Distance limitations vary depending on manufacturer, drive rating and motor rating, but we typically observe 100 feet as a maximum acceptable distance. Beyond this distance, we recommend a load reactor be installed or the drive will need to be mounted closer to the motor. We do not recognize any type of cable that would be exempt from this rule.

What happens to a motor when run at 100% but clutched at 50%?

Sullivan: In this example, the motor will be spinning at 100% and with the clutch at 50% the output shaft would spin at 50%. Assuming the clutch is properly loaded, the motor would be acting as if it’s loaded down 50%.

How about the loss from VFDs?

Sullivan: If you’re asking about the drive’s efficiency, it varies between manufacturers but is usually listed prominently on the drive or the drive’s packaging. If you’re asking about loss due to running a load on a drive versus across the line, the answer would depend on how loaded down the motor is and at the frequency at which you’re operating. Sorry I can’t be more specific.

When applying VFDs, it is typically recommended to change the motor or at least ensure that the motor is VFD compatible. What makes a motor compatible?

Sullivan: Efficiency of a drive is maintained across the entire range of its operating frequencies. The efficiency of the motor is dependent less on the operating frequency and more on the load at that frequency. Typically, any loss in efficiency is offset by the energy savings. Having a motor rated for VFDs is important. Motors running off drives tend to produce more heat and motors need to be insulated to handle this. If a motor is not insulated properly, it can cause the motor to short to ground and may potentially damage the drive. If you are unsure if a motor is rated to work off a drive, the information can usually be found on the motor nameplate. If you are still unsure, contact the motor manufacturer.

Does a motor need to be inverter duty for a variable torque application? Is there any benefit for a motor being inverter duty for a variable-torque, variable-speed application?

A motor should always be inverter-duty rated. One downside of VFDs is that they cause motors to produce more heat than they otherwise would. A motor that is not rated to handle this additional heat will have its insulation between the windings and frame degrade, causing a ground short. When this happens, the drive may detect the ground condition and fault to prevent damage. However, it’s possible that the drive may not catch the problem in time and as a result the drive will short out alongside the motor.

Your graphs appear to show 100% power at 100% speed but wouldn’t there be losses associated with the VSD where the power would be higher with the VSD than without the VSD at 100% speed?

Sullivan: Thank you, that is a good catch. Drives do require power to operate and, while very small, it would be more than a motor without a drive, in the case of both being under 100% load. Drive energy costs versus energy costs of running the motor are typically less than 3% and are more than offset by your energy savings. Even at 100%, drives will still provide some energy savings over a motor starter due to limiting the spike in current caused by the motor inrush.

What is the lowest percentage of full speed that an inverter-rated motor can safely run?

Sullivan: Drives can safely run a motor at any speed. The main issue that arises is in the case of motors cooled by a fan coupled to its shaft. If the shaft isn’t spinning fast enough, the air flow will be insufficient to properly cool the motor. In this case, the drives are usually programmed to not run below some minimum speed, typically somewhere between 25% and 33%. If a motor is cooled by an external blower, any speed is fine.

If a drive is equipped with line reactors, what do these do?

Sullivan: Line reactors act as a filter for incoming power. Harmonics and dirty power can cause a drive, and therefore the motor, to run improperly. These reactors are in place to clean the power and allow the drive/motor to run optimally. While not discussed in the presentation, some drives have what is known as an active front end. When a motor is regenerating (or braking) the motor is acting like a generator and that generated power needs to go somewhere. Active front end drives pass that generated power back onto the line. In this case, the line reactor is used to clean the power so you are not dumping dirty power back onto the line.

Can you explain a little bit about the harmonics associated with VFDs?

Sullivan: Drives, like any non-linear load, will produce harmonics. As discussed in the presentation, the drive’s converter section takes parts of the incoming wave and converts it to positive/negative DC. However, the parts that are rejected will reflect back onto the line and cause distortion. This distortion is represented by harmonics. Very briefly, harmonics cause some percentage of the power you consume to essentially be unusable. This means that you are paying for something you don’t use, and your equipment is running less than optimally.

Please comment on VFD harmonics and reactive power compensation using capacitor banks?

Sullivan: Capacitor banks shouldn’t be used on the line side of a drive. If you are concerned about reactive power, there are LC filters that can correct imbalances. However, the capacitors in those filters are selected to work at very specific frequencies.

How can a standing wave be detected? Is a scope required or can you measure it with conventional tools?

Sullivan: Standing waves are unfortunately usually detected by taking apart a motor that has been damaged by one. The best practice is to always adhere to manufacturer guidelines with regards to distance between the drive and motor, or failing that installing the load reactor in accordance to manufacturer recommendations. We recommend no more than 100 feet between drive and motor unless a load reactor is installed.

Scott Sullivan is a field service engineer at Electronic Drives and Controls (EDC), a CSIA Certified system integrator. Sullivan specializes in the application of variable speed drives (VFDs) technology and on-site field service of AC drives. Since joining EDC in 2016, Sullivan has served on EDC’s field service support team performing repairs, preventive maintenance services, start-ups, training, and much more for AC & DC drives, PLCs and factory automation. Sullivan is a graduate of University of Rhode Island with a bachelor’s degree in electrical and electronics engineering.


Author Bio: Scott Sullivan is a field service engineer at Electronic Drives and Controls (EDC), a CSIA Certified system integrator. Sullivan specializes in the application of variable speed drive (VFD) technology and on-site field service of AC drives. Since joining EDC in 2016, Sullivan has served on EDC’s field service support team performing repairs, preventive maintenance services, start-ups, training, and much more for AC & DC drives, PLCs and factory automation. Sullivan is a graduate of University of Rhode Island with a bachelor’s degree in electrical and electronics engineering.