Should you specify an “inverter duty,” “inverter ready,” “inverter rated,” or a “VFD compatible” motor for your fans and pumps in HVAC applications? Is there a difference in these terms? Don’t answer too quickly without doing a little research.

Now, without taking time to research any industry definitions of these terms, go look at your company fan, pump, and motor specifications and see if there is any clarification on specifically what type of motor would be acceptable to you if you were reviewing a submittal for a motor for a fan or pump on a VFD. Would you need a specific type of insulation? Would you need a premium efficiency motor, and, if so, what level of efficiency would make the motor acceptable for a VFD drive, or does motor efficiency matter when a VFD is used? Would either an ODP or TEFC motor be acceptable with a VFD? Can you use a brand “B” motor with a brand “M” VFD? How would you enforce this compatibility based on your specification criteria?

To make it even more interesting, would you be able to defend your decisions based on your specification and the definition of whatever term your specifications use in a discussion with an attorney in a construction mediation or deposition for a lawsuit where a supplier or contractor provided a motor you claimed didn’t comply with your design intent? What if the motor failed prematurely? What would you consider a premature failure of a motor, and do you specify the motor life in your contract documents, when VFDs are used? Does the motor manufacturer give a warranty for any type of motor used with a VFD, and, if so, under what “limited” warranty conditions?

If you subscribe to one of the “master” specification products does it cover motor and VFD drive compatibility issues in a manner that is appropriate and defendable? Do the companies that write these master specifications clearly define the terms or provide industry standard references? If not, you may want to consider how you would be able to review a submittal from a motor and drive manufacturer that uses different terms other than what your specifications use. If your specifications are not clear in terms, then you have increased the risk of your client not getting what they thought they were going to get when you discussed this issue in a design-phase meeting prior to writing the specifications.

It might be good to take some time at the AHR Expo in Chicago in January to stop by the motor and drive manufacturers’ booths to discuss this topic with them. It may be like asking 10 engineers their opinion: you could get 10 different opinions on how to best write a spec that will ensure a motor is compatible with a VFD drive and what the correct terms and definitions are. Be sure to discuss warranty issues and be sure to not get caught up in marketing hype or proprietary product criteria if your desire is to have a truly competitive spec. Of course, be cautious of marketing hype at the booths also. Your goal is to get the engineering truth, recognizing that there are multiple ways to accomplish the same goal.


It is enough to make a mechanical engineer’s head spin out of control when trying to keep up with the changing technology of motors and VFDs and the compatibility of the two when specified for HVAC equipment such as fans and pumps. One quote from an internet blog states, “Be careful. The marketing people are running amok on these terms.”

I have often warned manufacturers’ marketing people to say whatever they want to say to professional engineers as long as it can be defended in court and they are willing to put it in writing with their name on it. Amazing how this warning will slow down non-engineering, non-technical marketing people.

Engineers, likewise, need to have clear specifications that are not only defendable but that also allow for appropriate competitive bidding of products that are truly equal in performance to meet the specified functional criteria.

Where is the integrity of the motor and VFD industry manufacturers if they can’t agree on proper use of terms for specification purposes? Is this a “buyer beware” concern for facility owners? How much fingerpointing will occur if there is such confusion on how to specify motors that can work with a VFD/inverter drive? Is NEMA the entity that should step up and provide better clarity and a specification and motor/VFD design selection guide matrix? Should the AMCA be involved in clarifying motor applications for fans under varying speed conditions, or do the fan manufacturers not concern themselves about how long a motor will last if they don’t build the motor and simply provide what is specified?

Maybe ASHRAE should get together with CSI in a collaborative effort to educate manufacturers, suppliers, contractors, and engineers about proper specification writing. Manufacturers’ guide specs with selective verbiage can make the spec a proprietary specification. Engineers need to clarify the design intent when using manufacturers’ guide specs. Basis-of-design products do not intend to instruct one manufacturer to build a product the same way another one does, since in general a consulting engineer is not a product engineer taking the liability for a product design.


Engineers are at risk when they use marketing terms in motor and VFD specifications that are not clearly defined by industry standards or within the contract documents. Per CSI ( contract documents should adhere to the four Cs: clear, concise, complete, and correct. Without clear, concise, complete, and correct contract documents, the bids will not be competitive, and the enduser will likely not be getting appropriate products if there is confusion in the process.

There are so many options, so many considerations, so many ways to improperly specify motors and drives that can not only cause incompatibility issues but that may also cause confusion for bidders trying to determine if the spec can be trusted.

VFD, ASD, VSD — no matter what you call them, the intent is generally the same: to vary a motor speed by varying the frequency of the voltage seen by the motor. Varying the motor speed serves the purpose of varying the capacity of the piece of equipment for either energy or flow control to satisfy some design criteria. But this is a case where design intent can have a huge impact on interpretation if more definition is not provided in a set of contract document specifications.


One challenge for the engineer is to understand just how low the motor speed needs to go. This requires a good understanding of the part-load operating condition of the application. How much “engineering” is really done when it comes to analyzing a building fan or pump application to identify how much time a motor will be at various frequencies and the impact of this on the life of the motor? Also, is there a way to perform a life cycle cost analysis on this? If a system will be at very low part-load conditions where the motor is running hotter longer, is it worth spending more money on a better motor than when a system is very rarely at low part-load conditions? Also, is the VFD being used as a way to balance a system in lieu of properly selecting a fan or pump and as an alternate less expensive means of capacity control?

Just how long is a motor expected to last when used with a VFD? One way to look at this issue is noted in a quote overheard several years ago, “Any motor will work with a VFD drive, but some will last longer than others.”

One brave VFD manufacturer has come forward to admit that VFDs can adversely affect motors and suggests the use of protection devices, based on a few variables. These devices, known as load reactors and output filters (also dv/dt or dual element filters), protect motor insulation from damage due to the voltage spikes and harmonics caused by the switching of transistors on the output side of VFDs. These spikes are of very high frequency and voltage compared to the power frequency seen by the motor; they can break down the motor winding insulation, and the harmonics can cause heating of the windings and motor bearings.

As pulse-width modulation (PWM) has become the standard technique of variance in VFDs and transistors are used in that technique, every VFD manufacturer must choose how to address the issue of voltage spikes. Most ignore it; however, some have partnered with manufacturers of reactors and filters to properly protect motors based on two factors, voltage and distance.

Voltage, of course, refers to the input line voltage of the motor, while distance is measured between the VFD and the motor. Simply put, the higher the voltage (with corresponding higher voltage spikes) and longer the distance, the more possible damage to the motor. Figure 1 offers suggestions about which type of protection to be used based on those factors. These suggestions come from an international leader in reactor manufacturing and have been proven over several years to be accurate (as opposed to a marketing ploy to sell more devices).

Do any specifications really tell the supplier how long the motor for a fan or pump is supposed to last when used with a VFD drive? What about other solutions to the effects of VFDs on motor life, such as shaft grounding kits? Do the manufacturers of grounding kits, reactors, and filters offer warranties on the motors their products protect? Is getting past the one-year product warranty acceptable to the engineer? Does every motor manufacturer have a warranty on the life of a motor used with a VFD? Is this acceptable to the facility owner maintenance manager?

Before you answer these questions, give some thought to how an expected motor life could be determined. Is it a function of the operating conditions and installation? Are the full- and part-load operating conditions accurately calculated by an engineer such that it could even be determined how many hours a fan or pump motor would be run at 100%, 80%, 50%, 20%, or as low as even 10% of operating speed? If this calculation is not done, how is a motor selected?

It is a common disclaimer that the motor runs hotter at lower speeds, which affects the winding insulation life, so wouldn’t it make sense that some sort of statistical data should be provided by the motor manufacturers to help understand how long their specific motor would last under varying speed conditions? Depending on the size and importance of the motor and the process it supports, should external forced draft cooling of the motor be considered?


There is a concern that VFDs can cause standard motors to malfunction and fail for several reasons. Changes in motor torque may not match the load requirements as the motor speed varies. The cooling airflow around the motor will be reduced as the motor speed is reduced, causing the motor to run hotter. Harmonics from the controller may overheat the motor. Voltage spikes generated by the output section of the variable-frequency controller may cause the motor insulation to fail. Can these issues be totally prevented with the proper specifications for motors and VFDs? How are the specifications enforced through the bidding and submittal process to ensure the products actually provided and installed are indeed compatible?

Most motor manufacturers have motors designed to tolerate the effects of VFDs on motors. The terms used for these motors are not necessarily the same so a specification needs to essentially be written in a “Performance” specification method or a “Reference Standard” method instead of a “Descriptive” specification method. Care must be taken not to write a “Closed Proprietary” specification, based on only one manufacturer’s product (unless that is what the facility owner wants). Note: if you are not familiar with these specification terms, it is recommended that you consider joining CSI and becoming a CDT/CCS. Look at for more information.

Most low-voltage, general-purpose motors are suitable for operation with variable-frequency controllers for typical HVAC applications, such as pumps and fans, which present the motors with variable torque loads that reduce as speed is lowered. Many energy- and premium-efficient motors have windings with an insulation system that is specifically designed to withstand the voltage spikes from PWM drives as defined by NEMA MG-1. These motors are frequently called inverter-ready. For constant torque loads with a constant torque speed range of greater than 10:1, using an inverter-duty motor with internal thermal protection and externally operated cooling fan should be considered.

Would below be a good specification for motors used with VFD drives for HVAC fans and pumps? If not, why not? (Send in your comments and suggestions to me at

  • All three phase motors (ODP or TEFC) used with VFDs for variable torque applications shall be constructed in conformance with NEMA Standard MG1, Part This means 230 and 460 volt motors meet NEMA’s corona inception voltage requirements, under MEMA MG1 Standardand can withstand peak voltages of up to 1,600 volts, switching frequencies up to “xxx” ( “xxx” depends on the specified VFD).
  • Turndown: Motors shall be capable of a turndown ratio of 10:1.
  • Windings: Copper magnet wire with moisture-resistant insulation varnish, designed, and tested to resist transient spikes, high frequencies, and short time rise pulses produced by PWM inverters.
  • Class: Class F temperature rise; Class H insulation.
  • Motors shall be compatible with VFDs using either power bipolar junction transistor (BJTs) or insulated gate bipolar transistors (IGBTs).
  • Thermal Protection: Comply with NEMA MG 1 Part requirements for thermally protected motors.
  • Provide a full product replacement and labor guarantee and warranty ensuring complete compatibility of the motor with the VFD drive such that the service life of the motor and VFD shall be a minimum of one year under installed operating conditions.


Motor efficiency is a measure of the ability of a motor to convert electrical energy into mechanical work. Electric motors have a significant impact on overall energy use in the building industry. Is there a correlation of motor energy efficiency and the energy savings provided when VFDs are used? Does the use of a VFD on a motor affect the energy efficiency of a motor in a way that would cause the motor to be less efficient when not running at full speed, and is this taken into account in any sort of true life cycle cost analysis in a fan or pump selection based on a full- and part-load analysis of the building loads? Does more research need to be done in the industry before VFDs are so quickly used for convenience of TAB purposes if the energy efficiency of the motor is lessened when used with a VFD?

The Energy Policy Act (EPAct) was signed into law on October 24, 1992 with an effective date of October 24, 1997. EPAct and was aimed at conserving energy by increasing motor efficiencies for those electric motors responsible for the largest portion of the electricity consumption.

The Energy Independence and Security Act (EISA) was signed into law December 19, 2007 with an effective date of December 19, 2010. EISA further raised the minimum efficiency levels mandated by EPAct and requires motors previously covered by EPAct to now meet the premium efficient (PE) levels defined in NEMA MG-1 (2006), Table 12-12. It also added additional motor types that must meet the Energy Efficient limits defined in NEMA MG-1 (2006), Table 12-11.

One question to be considered now is are all PE motors VFD compatible if they meet NEMA MG-1, or is there more to consider in the electrical system design?


It would be impossible for one article to cover all that is needed to know when discussing motors and VFDs. It would probably be wise for most mechanical engineers to consult with an electrical engineer who is knowledgeable about motors, VFDs, and the entire electrical system in a building when selecting and specifying electric motors and drives.


In preparation for writing this article, the author solicited opinions on what issues needed to be addressed. As you can imagine, there were many more issues than space available, so this article was written in part as food for thought for specifying engineers who deal with this challenge on a daily basis. Several sources have been used to write this article, too many to document. Suffice it to say that there was not one single expert resource found that really addressed all the concerns of engineers trying to specify motors and VFDs.

What has become obvious is that there is a huge need for the motor and VFD industry to develop a “Designer’s Guide For Engineers” in the HVAC Industry from a solid engineering perspective. One last comment — this author readily admits to not being an expert in the field of motors and drives and has written this article in a manner to stimulate discussion in the industry to help clear up some obvious confusion and marketing that is clearly meant to generate proprietary specifications.