Motors, drives, and HVAC efficiency

Engineers must understand how the components in the HVAC systems they design use power and how they can be optimized without compromising traditional design values. Motors and drives are shown in relation to the design of HVAC systems.

01/26/2016


This article is peer-reviewed.Learning objectives:

  • Demonstrate how the energy codes directly affect motor and drive design.
  • Compare 6-, 12-, 18-pulse, and active front-end variable frequency drive (VFD) technologies.
  • Understand how VFD selection impacts power quality.
  • Apply products and systems within HVAC design to improve energy efficiency. 

Although variable frequency drives (VFDs) have been used in HVAC applications for a long time, the focus hasn't extended much beyond basic functional and budgetary concerns. It is generally understood that using a VFD can allow for significant energy savings through the affinity laws. It is also understood that with certain types of fan systems like variable air volume (VAV) systems, VFD use is more or less required. However, ongoing changes in applicable codes and standards have effectively eliminated many other HVAC design options and made the use of VFDs a de facto requirement in many situations.

Figure 1: This is an internal view of a standard 12-pulse variable frequency drive (VFD) in a freestanding enclosure. The phase shift transformer typical of this design is clearly visible in the lower left corner of the cabinet. Courtesy: McGuire EngineerCode evolution

Design engineers are often caught off guard by energy-code changes. In many cases, the changes seem to add unjustified expense and complexity to HVAC systems that worked fine before. While engineers usually focus on specific aspects of designs, there is a need to take a step back to get a broader perspective on the external influences that will continue to push the designs toward increased energy efficiency.

A country's energy usage and economy are linked. Although the relationship is changing in highly developed countries where there is a shift away from the industrial sector, a growing economy will generally result in increased electrical usage. Global events like the 1973-74 energy crisis and the two Gulf Wars can destabilize the cost of energy, with major repercussions on a country's economy. The changes that we are seeing now in our energy codes are actually the result of ongoing federal legislation that can be traced back 40 years.

It should come as no surprise that major federal energy legislation (Energy Policy and Conservation Act of 1975, Energy Policy Act of 1992, Energy Independence and Security Act [EISA] of 2007) immediately followed major global events that impacted the cost of energy. That same energy legislation directly resulted in the energy codes and standards that affect our designs. Although projections from the U.S. Energy Information Administration would seem to indicate that the long-term trend is for economic growth to decouple from energy usage, the basic relationship should remain valid through 2040.

So, what exactly is the goal of energy codes and standards? The U.S. Department of Energy (DOE) is responsible for national energy policy and has federal statutory authority for the evaluation of energy codes and standards. They are given this authority through energy codes' ability to affect public health and welfare. Energy codes and standards help ensure the public health and welfare by:

  • Reducing dependence on foreign energy by increasing efficiency and promoting alternate sources of energy
  • Protecting consumers through adoption of consistent standards
  • Providing for a more reliable electrical utility gird
  • Promoting economic development.

Efficiency in motors and VFDs is a prime target for any energy code or standard. The International Energy Agency (IEA) estimates that electric motor-driven systems account for 43% to 46% of global electricity consumption (7,108 terrawatt-hours [TWh]/year). Most of this is in the industrial sector. However, commercial building usage still accounts for 1,412 TWh/year.

These are incredibly big numbers and it can be difficult to truly grasp their magnitude—1 TWh is equal to 1 billion kWh. So, while an increase of a few percentage points in efficiency for an individual motor may not seem significant, it should be clear that small changes in industrywide motor-system efficiency standards can dramatically impact global energy consumption.

Figure 5: Variable torque loads include the centrifugal fans and pumps that are commonplace in HVAC design. While several variables are not taken into account (parasitic loads, minimum pressure requirements, etc.), this graph should provide a general undeDelay in adoption of codes

While both are driven by federal energy legislation, there is a distinction between energy codes and industry standards. Standards typically apply to specific types of equipment like ac induction motors or centrifugal chillers. Energy codes apply to how that equipment is integrated into engineered systems.

While new energy-code requirements may seem overwhelming, what many specifying engineers don't realize is that HVAC equipment manufacturers have been coping with these same problems for a longer period of time. The changes in the minimum federal efficiency standards that are referenced in the energy codes generally take place well in advance of their incorporation into those codes. For example, let's look at the timeline for general-purpose squirrel-cage induction motors that are governed by the NEMA MG 1 industry standard:

  • 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. MG 1-2009 became effective Dec. 19, 2010.
  • ASHRAE Standard 90.1-2013 and International Energy Conservation Code (IECC) 2015 incorporates these MG 1-2009 requirements.
  • DOE gets up to 12 months to determine if these energy codes would effectively improve energy efficiency.
  • Each state is required to certify within 2 years that their commercial energy code is in compliance with the new code.

 

All new motors currently in the marketplace should meet the minimum efficiency standards in MG 1-2009, but most states, as of December 2015, still haven't adopted either ASHRAE 90.1-2013 or IECC 2015, which mandate their use in new construction.

Motors, VFDs, and the energy code

Motors are present in almost every piece of HVAC equipment that we specify and drive their overall energy usage. However, barring unforeseen technological innovations, continuing increases in NEMA MG 1 efficiency requirements are not sustainable. Motors are commodity products, and recent increases in efficiency can be attributed to adopting previously "premium" product requirements and making those the base-level standard. With each increase in motor efficiency, the associated manufacturing cost increases and, at some point, the improved efficiency no longer justifies the additional costs.

Given the diminishing returns associated with further increasing motor efficiency, the next obvious target is to focus how those motors are integrated and controlled within systems. With most HVAC systems, the peak-load/design-day condition represents a small percentage of the overall operating hours for that system. When considered in conjunction with the affinity laws, being able to operate motors at reduced speed/horsepower to meet actual load requirements clearly has the potential for dramatic energy savings.

Figure 6: A small 6-pulse variable frequency drive (VFD) has a lead-lag controller serving a pair of 5 hp fan motors within a makeup air unit. Courtesy: McGuire EngineersThe increasing importance of part-load efficiency is not only reflected in the energy codes, but also in evolving industry standards for several types of HVAC equipment. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the manufacturers' trade association that sets standards for most HVAC, refrigeration, and water-heating equipment. AHRI has set efficiency standards for many types of air conditioning equipment including chillers, condensing units, air-cooled heat pumps, etc. These standards often mandate minimum efficiency requirements for not just full-load but also for part-load conditions.

These part-load efficiency requirements will take a weighted average of the equipment's relative efficiency at multiple different load levels (25%, 50%, 75%, and 100% load). This weighted average is more representative of how the equipment will be used in a real-life application. Case in point: Chillers have different efficiency requirements for full load (FL) versus integrated part load values (IPLV). Large air conditioners have similar requirements where the full load energy-efficiency ratio (ERR) will be different than the integrated energy-efficiency ratio (IEER). Achieving these part-load efficiency requirements is generally accomplished by controlling the motors within that equipment with VFDs.

It is interesting to note that while there are minimum industry efficiency standards for motors per NEMA MG 1, there are no corresponding industry standards for VFDs yet.


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