Although variable frequency drives (VFDs) have been used in HVAC applications for years, recent and ongoing changes in applicable codes and standards have effectively eliminated many other HVAC design options. As a result, the use of VFDs stands to become a de facto requirement in many situations.

ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, is a benchmark for U.S. commercial-building energy codes and standards.ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, is a benchmark for U.S. commercial-building energy codes and standards.An article by John Yoon, PE, LEED AP, McGuire Engineers Inc., which appears on the company’s website, addresses the relevant codes and standards, and defines their relationships with motors and VFDs. Yoon says that these are the major pieces of federal energy legislation in play, catalyzing the evolution of HVAC codes and standards:

  • The Energy Policy and Conservation Act of 1975
  • The Energy Policy Act of 1992
  • The Energy Independence and Security Act [EISA] of 2007.

All of these directly resulted in the drive to update HVAC-system designs, says Yoon, which should remain valid through 2040. The overriding goals:

•Reduce dependence on foreign energy, by increasing efficiency and promoting alternate sources of energy.

•Protect consumers through adoption of consistent standards.

•Provide for a more reliable electrical utility gird.

•Promote economic development.

Motor and VFD Efficiency the Prime Target

“Efficiency in motors and VFDs is a prime target for any energy code or standard,” says Yoon. And, small changes in efficiency standards can significantly impact energy consumption. As an example, Yoon looks at how the NEMA MG 1 industry standard impacts use of general-purpose squirrel-cage induction motors.

•EISA 2007 mandates NEMA Premium efficiency levels for motors. Before this, there were both minimum standard-efficiency and premium-efficiency offerings.

•NEMA MG 1-2009 incorporates these as new minimum efficiency levels.

•ASHRAE Standard 90.1-2013 and International Energy Conservation Code (IECC) 2015 incorporate these MG 1-2009 requirements.

•DoE is allowed up to 12 months to determine if these energy codes would effectively improve energy efficiency.

•Each state must certify within 2 years that its commercial energy code complies with the new code.

All new motors in the marketplace should meet the minimum efficiency standards in MG 1-2009, but most states, as of December 2015, had not adopted either ASHRAE 90.1-2013 or IECC 2015.

In explaining how energy codes relate to motor and drive selection and use, Yoon says that ”when considered in conjunction with the affinity laws, being able to operate motors at reduced speed/horsepower to meet actual load requirements has the potential for dramatic energy savings.”

Yoon then turns to how codes deal with the control needed to reduce flow under part-load condition. He references fan systems serving multiple zones; VAV supply fans that control space temperature by airflow; multi-zone systems; pumps (with high- and low-hp motors) in hydronic systems; and heat-rejection equipment fan control.

The reoccurring theme is to have the ability to reduce flow to 50 percent and that the power required at half speed should be no more than 30 percent of that required at full speed. VFDs come into play because many of the traditional ways to adjust flow fail to meet the requirements for 30 percent power consumption at 50 percent flow. These include airfoil and forward-curved fans with discharge dampers or inlet guide vanes, while a vane-axial fan with variable-pitch blades may meet the requirement in some circumstances. Meanwhile, any fan with variable-speed control generally meets the minimum efficiency requirement.

Yoon concludes with discussions of the harmonic distortion that can be created by VFDs, how to quantify the distortion, and where codes and standards help define the allowable distortion level based on the application. The four primary VFD types—6-, 12- and 18-pulse, and active front-end—are described, along with advice to engineers on where and when to use each type.

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