Your questions answered: Critical power: Motors, variable frequency drives, and variable speed drives

The Feb. 22, 2018, “Critical power: Motors, variable frequency drives (VFDs), and variable speed drives” webcast presenters addressed questions not covered during the live event.


John Yoon (left), PE, LEED AP ID+C, McGuire Engineers; Kenneth Lovorn, PE, Lovorn Engineering Associates. Courtesy: McGuire Engineers and Lovorn Engineering AssociatesEngineers must understand how the components in the systems they design use power and how they can be optimized without compromising traditional design values. Standard induction motors use (and waste) electricity. Total motor energy usage for the industrial sector outstrips commercial usage by roughly 3:1. To reduce operational costs across all nonresidential buildings, variable frequency drives (VFDs) and variable speed drives (VSDs) are frequently used with ac induction motors that operate pumps, fans, compressors, or similar equipment with variable load profiles.

Although engineers have little control over the applicable efficiency standards and codes they are mandated to follow, they are still tasked with designing appropriately sized and functional systems. The adoption of more stringent energy codes and standards has put greater emphasis on energy efficiency in engineering designs. Efficiency requirements will only become more stringent. This webcast addresses the efficiency standards that apply to ac induction motors and the systems in which they operate, load issues, applications, and harmonic mitigation.

Presenters John Yoon, PE, LEED AP ID+C, McGuire Engineers, Chicago; and Kenneth Lovorn, PE, Lovorn Engineering Associates, Pittsburgh, respond to questions not answered during the live Critical power webcast: Motors, variable frequency drives (VFDs), and variable speed drives on Feb. 22, 2018.

Question: IEEE 519-2014: IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems references more about TDD, than THD. Can you explain the differences, and what factor line reactors reduce?

John Yoon: TDD is "total demand distortion." THD is "total harmonic distortion." Both are expressed as percentages of a basis. Although both are calculated at the point of common coupling (PCC), the time basis for each is different.

Per the IEEE-519-2014 definitions: For TDD, basis is the maximum demand load current. The demand load current is defined as the sum of the maximum demand currents during each of the previous 12 months divided by 12. It is an evaluation of harmonic distortion over a period of time.

For THD, basis is a singular fundamental current, not an extended weighted time average like TDD. IEEE 519 typically references THD, not TDD.

So, while line reactors when installed in series add a characteristic impedance to a circuit, the voltage drop across that reactor and associated impedance changes in direct proportion to the amount of current flowing through it. With a variable load where TDD is much different than THD, it may be desirable to specify a different impedance reactor to address the expected average operating conditions.

Q: If an air handling unit (AHU) is running at 100% speed, what is the VFD percentage losses?

Yoon: Most manufacturers will quote low to mid 90% range efficiencies for standard 6-pulse drives. Inefficiencies generally are attributed to switching losses in the rectifier and inverter sections. And 12- and 18-pulse drives will have even lower efficiencies due to their phase shift transformers. ac-ac matrix-type drives typically have higher efficiency than any traditional double-conversion drives when operating at 100% speed.

In all of this, please note that there isn't a nationally recognized IEEE or NEMA standard for measuring VFD efficiencies. The AHRI standard doesn't seem to have been embraced either by the U.S. Department of Energy or ASHRAE/ICC.

Q: Where do you apply line reactors when the distance between the VFD and the motor exceeds 25 feet?

Kenneth Lovorn: I apply the line reactors adjacent to the VFD to provide the best protection of the motor and downstream branch circuit conductors.

Q: Do you foresee electronically commutated motors (ECMs) replacing all induction motors in the near future? Can you vary the speed of these ECMs? If so, what products are used to vary the speed of ECMs?

Yoon: Most traditional permanent split capacitor (PSC) motors can't meet the 70% efficiency requirements of the current energy codes. It is inevitable that ECM will represent an ever-increasing percentage of the market share for fractional horsepower motors because of that. For large polyphase induction motors, it's unlikely that we'll see that same market shift in the near future. The efficiency gap isn't nearly as large and the available types and sizes of ECM and PMAC motors doesn't come close to accommodating the wide variety of induction motor applications.

The type of control associated with ECMs varies, but most can at least accommodate 0 to 10 Vdc control signals.

Q: Is it necessary to have special VFD-prepared motors when implementing speed controllers?

Yoon: I've personally seen a VFD destroy a generic induction motor in less than a month. I've also seen other retrofit VFD installations where the original motor has operated without issues for years.

Ultimately, the question is if motor service life is a concern. Adverse operating conditions (high ambient temperatures, long drive-motor feeder lengths, etc.) will reduce service life and may make the investment in a NEMA MG-1 part 31 motor cheap insurance.

Q: Please compare 16-pulse motor controllers versus other advanced designs to minimize harmonics. Please discuss issues for standby power of total load comprised of 90%+ process motors on variable frequency/speed controllers.

Lovorn: There are no 16-pulse VFDs. Perhaps you mean 18-pulse? With 18-pulse VFDs, it is very rare for harmonic filters to be required. However, for smaller motors (under 200 hp) you may find that the use of passive harmonic filters is a more economical alternative. VFDs applied to standby generators is always a problem due to the harmonics generated by the VFD. The solution is to use 5% THD harmonic filters on all VF drives and always use PMG generators sized at least 150% of the total running motor load.

Q: What is the benefit of having a dc bus?

Yoon: The dc bus is an integral component of any standard VFD. The only exception that I'm aware of is an ac-ac matrix-type drive.

Q: What percent of the lifecycle costs is the energy cost?

Yoon: You can't make a blanket statement in response to this question. Energy cost depends on the hours of operating at any given load factor.

Q: With regard to 18-pulse VFDs versus active harmonic filters, what are the pros and cons?

Lovorn: From my perspective, the use of 18-pulse drives or harmonic filters (either active or passive) is purely economic. The bottom line is to eliminate the harmonics in the distribution system so you do not have the interaction with other loads that are harmonic sensitive.

Q: Does a 3% line reactor actually require a larger motor cable, or does one make this voltage drop up by setting the VFD voltage output to compensate?

Yoon: For 6-pulse VFDs, you can't put out more voltage than what you get in. Upsizing feeder conductors isn't an effective way to counter voltage drop associated with line reactors. The primary disadvantage of line reactors is their characteristic voltage drop. That's one of the reasons why using line reactors with greater than a 5% impedance isn't recommended.

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