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.

By John Yoon, PE, LEED AP ID+C; and Kenneth Lovorn, PE March 1, 2018

Engineers 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.

Q: Are ECMs in the blowers in air conditioning units creating issues?

Yoon: Yes, they have the same THD problems characteristic of any solid-state power conversion device. The primary question is: What percentage of the overall facility load profile is represented by those ECMs? With smaller fractional horsepower ECMs, it’s a contributing factor, but typically not the greatest concern.

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

Lovorn: The percentage of energy cost to the motor lifecycle costs will vary but it could run as much as 96% of the total, according to the amount of time the motor runs and the amount of savings you can achieve.

Q: What are the factors that make ac-ac motors expensive?

Yoon: Assume that this question is about ac-ac matrix-type drives. It’s proprietary technology that has so far only been adopted by a handful of companies. As the technology is adopted by more companies, and more units are shipped to the field, it is expected that the cost will be driven down. Ultimately, it is expected that matrix drives and active front end drives will displace 12- and 18-pulse drives.

Q: On a recent project, we had a VFD put indoors controlling a motor outdoors that was more than 50 feet away. A dv/dt filter was recommended. Is this correct?

Yoon: A generic rule of thumb is that EDM starts to become a concern with the motor-drive distances exceeds 25 feet. For smaller motors that are easily and cheaply replaced, the cost and complexity of adding drive output filters may not be justified. Conversely, large motors that represent greater costs and potential for disrupting the operation of a facility may make their use a requirement. This type of evaluation is project-specific—there’s no one-size-fits-all solution.

Q: Do we have to use harmonic filters in chillers and chilled water pumps with VFDs? If we do not use them, what happens?

Lovorn: Chillers and chilled water pumps are, generally, quite large in comparison to the distribution to which they are connected, so the impact of 6-pulse VFDs without filters could be devastating to other equipment in the system. As a minimum the various VFDs could interact with each other causing operational problems.

Q: Is there a certain horsepower range the energy code requires the use of VFDs?

Yoon: Use of VFDs is not prescriptively mandated by any part of the energy code. The energy code is performance-based and only has a requirement for part load efficiency. It just so happens that VFDs are typically the only solution that meets that part load efficiency requirement.

ASHRAE 90.1-2010 had specific horsepower thresholds that were removed in subsequent versions of the code. The exact requirements change depending on what version of the code is being enforced, but based on ASHRAE 90.1-2013:

  • Any DX or chilled water unit that modulates supply temperature must have two stages of control, 66% speed, drawing 40% of full-speed power.
  • Units that modulate air flow to control temperature (i.e., VAV), must have modulating fan control with a min speed of 50%, drawing 30% of full speed power.
  • Condensers, cooling power, and evaporative condensers also have a speed control requirement, but no corresponding efficiency requirement.
  • Condenser fans serving refrigeration equipment also must meet that 50%/30% rule.

Q: Do you recommend 6-pulse drives for horsepower less than 50 hp? And 18-pulse for applications greater than 50 hp? Or does it depend on the application or equipment?

Yoon: It is application-dependent. The determining factor is the percentage of nonlinear load compared to the rest of the facility. The IEEE 519 standard says 8% THD at the PCC. The question is if motor load represents a disproportional percentage of the overall facility load profile and voltage distortion could impact the operation of other loads in the facility?

Q: Please estimate the shortened life for installing VFDs on older motors.

Lovorn: This could vary greatly, according to how much the speed of the motor is reduced, the age of the motor, and a number of other factors. The best solution would be to use drive-rated motors or have your existing motors rebuilt to drive-rated motor specifications.

Q: Why have a wye and delta winding on the secondary of the 12-pulse VFD, versus the 3 delta windings on the 18-pulse VFD?

Yoon: For effectively cancelation of harmonic orders, a 12-pulse VFD requires 30-deg phase shift transformer (typical delta-wye) and a 18-pulse requires a 20-deg phase.

Q: Has the Trump administration indicated any energy efficiency reductions as a point of policy?

Yoon: I do not know. The amount of information coming out of the U.S. DOE has slowed to a trickle with the change in administration. However, the associated energy code revision process doesn’t appear to have been affected. The federally mandated determination findings for ASHRAE 90.1-2016 were released by the DOE in February 2018.

Q: As a general rule, at what percentage VFD power/total building load will you need to start thinking/implementing harmonic mitigation measures?

Lovorn: Our general rule of thumb is to specify harmonic filters on all motors over 10 hp, no matter what size the distribution system might be. The operational and financial risks are not worth having harmonics running around your distribution system, unchecked.

Q: How can we reduce noise when we use VFDs?

Yoon: Noise is generally caused by resonance in the metal laminations within the motor. Generally, the worst resonance is caused at specific pulse width modulation (PWM) output carrier frequencies. Sometimes, the VFD can be programmed to avoid those specific carrier frequencies, which can reduce, but not eliminate, noise.