Driving motor efficiency
Motor-driven equipment accounts for a large percentage of the electricity consumed in the industrial building sector of the U.S. However, that number is decreasing as motors and drives become more efficient. Electrical motor energy consumption is the largest source of electricity usage worldwide, using 69% of the sector energy, per IEA’s 2011 Energy Efficiency Opportunities white paper. Motors used in HVAC systems represent a great deal of untapped, relatively easily obtained energy savings as well. This article will review motors and drives, the codes and standards that dictate their design, and energy efficiency standards.
- Explain the use of motors in today’s industrial building sector and HVAC uses across multiple building types.
- Apply the codes and standards as they relate to specifying motors and drives.
- Compare variable speed drives (VSDs) and variable frequency drives (VFDs) and their uses.
Since the invention of the battery in 1800 and development of dc motors a few decades later, the simple electric motor has evolved into a sophisticated electrical device with electronic control options. The development of motors first focused on the dc type, as batteries were the primary electrical source at the time. Motor development evolved to the use of induction, synchronous, and other alternating current forms. The use of motors and specification options has developed at an ever-increasing pace, as regulatory requirements and efficiency needs have forced the simple electric motor to use less energy and become smarter. Obviously, ac induction motors have not changed significantly from their origins. However, the way we use and control them has evolved as the designs have become optimized for current uses.
The integration of variable speed drives (VSDs) into the industrial sector is based on the need for more precise control of machinery and conveying systems rather than the need for efficiency. This use of drives in the industrial sector has benefitted the HVAC industry in the form of efficiency gains for equipment operation.
The use of the terms "VSD," "variable frequency drive" (VFD), or "adjustable speed drive" (ASD) are used both in jest and as descriptive terminology, because they are used in a similar fashion as the terminology related to facial tissue, more widely known as "Kleenex," in today’s vocabulary. The term VFD as a generic reference to a variable-speed solid-state control device is used herein.
Today’s processes and buildings use VFDs in many locations. Variable volume control of HVAC systems began with the use of inlet guide vanes and variable pitch fans, which remained in widespread use until the early 1990s. Building systems with critical motion-control requirements, such as elevators, did not use VFDs until the 1980s when pulse-width-modulation control came into use. Most anyone should be able to find a VFD in any building of significant size that has been built or upgraded since the early 1990s. There are many uses for VFDs in industrial settings, as well, from process fans or pumps to controlling the speed a product is conveyed or produced (see Figure 1). VFDs can be found in every building type including schools, office buildings, retail, entertainment, housing, and hospitals providing efficient operation of heating, cooling, and ventilation systems.
Historically, the majority of 3-phase ac induction motors driving HVAC systems in commercial buildings were controlled with across-the-line full voltage, nonreversing, magnetic starters. Across the line means that when the pilot device sends a signal to start the motor, full-rated voltage is applied to the motor in one step. The other primary characteristic is that the starter runs the motor at a single speed (100%) and at full torque. Magnetic starters can be outfitted with various pilot lights, push buttons, or auxiliary contacts. They can be configured to be individually mounted or mounted in a motor-control center, but at their core, the starters contain control coils and contacts. These motors are the workhorses of the commercial building industry, but the starters historically used in conjunction with these motors have two main disadvantages. They result in high starting currents—as high as six times the full-load running current—and single, full-speed operation, regardless of the load being served.
To address these disadvantages, other types of starters have been developed and employed for certain applications. Part-winding starters, where starting is achieved by first energizing only a portion of the motor winding, are used to reduce the amount of current inrush on motor startup. These have also been referred to as soft-start controllers. Two-speed starters used in conjunction with specially wound 2-speed motors were employed, which did allow for some adjustment of speed. Different motor applications have also used autotransformer starters, star/wye-delta starters, and solid-state reduced-voltage starters. Depending on the usage, these starters were applied to either manage motor-starting current or control motor speed. A distinct advantage of VFDs is that they can address both of these concerns through the use of a single motor-control device.
Motors have always been one of the largest energy users in commercial buildings. That fact, coupled with a higher level of energy and environmental awareness in recent decades, necessitated that the industry revisit the way in which motors are operated and controlled. VFDs have been used in many applications over a number of years. Technological advances over the last number of decades have reduced VFD costs and sizes to now make them the preferred choice for motor control. As the name implies, a VFD adjusts the frequency at which a motor is operated, thereby adjusting the motor’s speed and torque. Unlike multispeed starters in use prior to this point, VFDs can vary motor speeds from 100% to approaching 0%. The ability to vary and to fine-tune motor speeds using a VFD results in a corresponding reduction in overall energy consumption and energy demand. Energy reduction is calculated through the use of affinity laws, which apply to fans and pumps. These laws result in significant energy savings for constant-torque loads whenever fans may be operated at a lower speed, such as a directly proportional change in flow when the speed is reduced as well as power-reduction changes by the cube of the proportional speed reduction. Additionally, closer coordination of upstream electrical overcurrent protective devices can be achieved, as VFDs will ramp motors up to full load rather than starting them across the line (see Figure 2).
The U.S. total annual energy consumption by motor-driven equipment, including uses from the industrial, commercial, residential, and transportation sectors, was 1,400 billion kWh in 2006. This energy consumption equaled approximately 38% of the total U.S. electrical energy use. Further dissecting the data provided by the U.S. Energy Information Administration indicates that 44% of all energy use in the industrial sector was consumed by motor-driven equipment.
Roughly 1.8 million new electric motors are sold each year, which are covered by minimum full-load efficiency requirements of the Energy Independence and Security Act of 2007. In 2006, around 24 motors million were in use in the U.S. infrastructure and large buildings. Building design professions, using best practices of electrical motors efficiency/operation, could reduce energy consumption by 20% to 30% on average. Many improvements have a payback period of 1 year to less than 3 years.
Electrical motor-driven systems consume large amounts of electricity and provide an opportunity for significant energy savings. Electrical energy accounts for more than 97% of total motor operating costs over the motor’s lifetime. The procurement of new motors by owners and contractors is often driven by first cost, not the electrical energy consumed over the motor’s lifetime of operation. A small improvement in energy efficiency could result in significant energy and cost savings. A slightly higher cost on day one for an efficient motor will provide an excellent return on investment for the life of the operating system.
Back to dc?
Options beyond VFDs exist in today’s equipment, such as pumps, fans, conveyance, and production equipment. The technologies that advance ac motor efficiency have slowed to a near standstill, and now motor manufacturers are looking to the original dc motor for the next round of efficiency and control opportunities. An electronically commutated motor (ECM) has become a more economical approach to variable speed for single phase-motors in recent years. By converting ac input power to dc power supply, integrated controllers are able to provide further turndown ratios than potentiometers while providing higher efficiencies and energy savings. Various HVAC-fan manufacturers currently offer these types of motors paired with direct-drive fans and provide various control options. By prepackaging the electronic circuitry with the motor, fan capabilities are expanded and field installation is simplified. Beyond energy efficiency and easier installation, these controllers, integrated into the fan itself, provide a larger opportunity for direct digital control system operation and simpler sequences of operation.
Fan manufacturers are not alone in developing integrated products. Pump manufacturers have also combined VFDs into some of their packaged offerings. One example, self-sensing pumps, uses integrated sensors to automatically adjust pump speed and flow based on measured demand. These integrated products offer distinct advantages over the traditional method of separate field-installed components. When assembled in a factory, the product is able to be pretested and go through other quality control measures. Furthermore, field coordination between multiple contractors is reduced or eliminated.
The industrial building sector uses motors and VFDs in a variety of applications. These applications may include, for example, positive-displacement pumps with constant-torque motors to transfer fluids from one process tank to another in the wastewater or manufacturing sector, small servo-type motors to move materials on conveyor belts in manufacturer environments, or compressors and machine tools.
Proof in print
An excellent example of the evolution of motors and drives may be found in the printing industry (see Figure 3). The world’s second-largest print and media solutions company has experienced an evolution in motor types and controls from the 1980s until today. In the early 1990s, VFDs were coming into use in HVAC systems because of available technological advances. Early adoption by the HVAC market in the 1990s provided an opportunity for other industries to identify issues and resolve problems prior to the use of VFDs on precision control (to the thousandths decimal place) of material movement in the manufacturing sector.
This very large printing and media company was able to begin adopting variable speed ac motors to replace the dc motors that were difficult to precisely control, thus gaining reliability throughout its production processes, specifically movement of paper through printing presses. The earliest motors to be replaced were press-line shaft drives (see Figure 4). These large motors, and sometimes multiple motors on a single press up to 400 hp, are used to control the application of ink and speed of paper through the press. The precision gained with the VFDs increased production by maintaining alignment and better control of paper acceleration and deceleration (see Figure 5). The ability to send the same signal to multiple identical motors, which responded with the same accuracy, and using a "gas pedal" to accelerate and a "brake pedal" to decelerate was a great gain over dc motor control of using "gas" to accelerate and taking your foot off the gas pedal and coasting to decelerate. Each motor could now receive the same signal to provide the same output whether speeding up, maintaining a constant speed, or slowing down. Controlling the rate of change of speed proved invaluable.
The main issue that occurs with non-precise motor control in printing presses is related to paper-roll tears (web breaks) and requires manual refeeding of paper through the press to begin again. Other benefits to today’s printing systems include using multiple (14, for a specific example) ac motors versus two large dc motors. These smaller motors create an opportunity for one digital control system to provide precise, accurate control. Other benefits to VFD-controlled ac motors in the industrial sector include the ability to avoid use of multispeed gearboxes, easy application of direct-drive variable-speed motors for precise and reliable operation, and flexibility to continuously customize existing processes.
Items to remember when specifying VFDs with motors for industrial applications:
- Large motors, typically categorized as those larger than 250 hp, should be paired with a VFD designed specifically for the motor, due to the different construction standards for motors of that size.
- Typically, small, 2-hp or less, servo-type motors have proprietary logic controls that may only accept or provide limited information, which is acceptable for the limited control requirements.
- Motors from 1 to 200 hp that are invertor-duty-rated (i.e., constructed for voltage spikes and lower operating speeds) may be paired with most VFDs.
Codes and standards
Energy code requirements came into existence with the Energy Policy and Conservation Act (EPCA) of 1975 and the first edition of ASHRAE 90.1, which came out that same year. The EPCA was created as a direct result of the oil embargo in 1972-73. The act provided the Department of Energy with the authority to establish energy efficiency standards for appliances and equipment. The EPCA has evolved over time. Changes in the Energy Policy Act have included the requirement for high-efficiency motors, National Electrical Manufacturers Association (NEMA) MG1 in the 1990s, and motor-efficiency provisions for premium efficiency in 2007. Various editions of the EPCA and the Energy Independence Security Act of 2007 created and adopted international standards for motor ratings. The standard rating system in existence is the International Electrotechnical Commission standard IEC 60034-30. This standard created the categories of IE1 standard efficiency, IE2 high efficiency, IE3 premium efficiency, and IE4 super-premium efficiency for motor-efficiency classifications. This standard applies to motors with 2, 4, 6, or 8 poles, 1 to 500 hp, and less than 1,000 V. The standard does not cover variable speed motors. The use of super-premium efficiency motors in HVAC applications is not common today, but should be expected soon in the industry. These IE4 motors provide up to 14% energy efficiency savings over IE1-rated motors.
Codes and standards have become more stringent over the past 2 decades, with increased requirements for implementing VFDs or other similar methods for reducing motor energy usage. In ASHRAE Standard 90.1, for example, section 184.108.40.206 addresses variable air volume (VAV) fan control. To discourage the use of constant-speed fans with bypass air dampers for part load, the standard sets a minimum motor horsepower that must be provided with either VFDs, variable pitch blades on vane-axial fans, or other similar controls, resulting in decreased power consumption. In 2004, the minimum motor horsepower for this requirement was 15 hp. In 2007 and 2010, the requirement was reduced to 10 hp. Continuing the trend in 2013, the minimum was dropped again to 5 hp. Furthermore, the 2013 version added requirements for single-zone systems above 110,000 Btu to be designed as single-zone VAV systems. The standard also added a new section requiring small-horsepower fans (1/12 through 1 hp) to have an ECM-type motor or be a minimum of 70% efficient if a permanent split capacitor-type.
ASHRAE 90.1-2010 has also added equipment to the Section 6.8.1 Minimum Equipment Efficiency Tables to be more inclusive with today’s HVAC system options. Variable refrigerant flow (VRF) systems were added to provide standards and facilitate compliance with common energy code requirements.
A typical code reference that HVAC designers and engineers must deal with for most projects includes the currently adopted energy efficiency code of authorities having jurisdiction (AHJ). Sometimes, energy codes are set on a municipality level, such as in Texas; other times, the requirements are statewide, creating easier practical use for building design professionals, such as in Illinois. The issue typically occurs when a design professional needs to determine the code requirement, ASHRAE or International Energy Conservation Code (IECC), and the applicable year. The adopted year of one is not the current version of the other. IECC adoption is one cycle behind ASHRAE, resulting in the current version of the IECC (2015) incorporating 2013 ASHRAE 90.1 requirements. The update to IECC 2015 resulted in an aggregate energy savings of 11.5% versus IECC 2012.
The U.S. Environmental Protection Agency (EPA) also sets goals for the reduction of energy use in motors and other operating characteristics related to energy. The EPA uses the Energy Star program, among other resources including the State and Local Climate and Energy Programs, to help owners and designers reduce energy use in buildings.
An enabling technology that takes advantage of variable speed motor applications is VRF. A comparison of a recent design project analyzes the energy use of a VRF system versus a standard air-cooled water-source heat pump system. The savings are extreme with the VRF solution, resulting in more than 66% energy savings by using a system using variable speed compressors and refrigerant. The VRF compressors on this project are inverter-driven (digital) scroll compressors. The compressors used are capable of varying the compressor speed from 15 to 150 Hz.
A comparison of another recent design project further analyzes the use of variable volume single-zone air handling equipment versus a constant constant-volume air handling solution. While the results indicate a savings using a variable speed fan, the savings are not as significant as one might expect. The result is only 8% in energy-use savings. It is important to acknowledge that with an early result indicating small single-digit savings, more effort should be put into the final design solution. Is this a situation where the air handling unit is operating more than necessary during off-hours for maintenance of the space-heating setpoint? Is there a constant load that is not scheduled off in the model, or a real-life situation that is affecting operation? Is something about the envelope load causing issues? Are there many hours at full-speed operation when the VFD will actually be penalizing energy use due to the additional consumption of a drive at full load? These are important questions to consider, although more should be analyzed for a given project to evaluate the need or control strategy for a variable speed fan or pump.
A final comparison of constant versus variable speed motors presents interesting findings. In this situation, a 200,000-sq-ft, 4-story office building with a central air handling unit using chilled water and air-cooled chillers was evaluated with both constant chilled-water flow and variable speed pumping. The findings were interesting in that the results indicated a minimal energy cost and dollar savings due to the situation of a single cooling-coil load served by the central plant. This situation indicates that additional analysis is required to determine the most cost-effective solution for pumping, both in first cost and operational cost.
- Designers and engineers using motors on projects should remember that just because it is always done this way, or it has been done that way in the past, then it is the best option. The best solution for a project must be analyzed/calculated, not assumed. Proudly make the typical engineer’s response, "it depends," and spend some time analyzing.
- Be careful when applying VFDs to existing motors. Consideration should be made for the motor rating, inverter duty or not, and does that even matter in the specific operational case. Make sure the application of the VFD will result in savings, do not make assumptions, and use VFD savings calculators for a unique motor/drive application, such as a constant-torque motor.
- Consider using VFDs for air and water balancing applications, even in near-constant load systems.
- To squeeze out another few percentage points of energy savings, consider new technology solutions including variable speed VRF systems for cooling applications, ac motors with integral VFD control, or ECM motors whenever using small single-phase motors or direct drive at every opportunity.
Jason Gerke is the mechanical and plumbing group leader at GRAEF. He has designed mechanical systems for a variety of project types including industrial, commercial, education, and resort entertainment facilities. He is a member of the Consulting-Specifying Engineer editorial advisory board.
Robert R. Jeffers is an electrical consulting engineer at GRAEF with more than 30 years of experience with various electrical systems in many building types. His experience includes designing low- and medium-voltage electrical distribution, standby emergency generation, and fire alarm systems for commercial, industrial, institutional, laboratory, and sports venues.
Motor/VFD best practices
Best practices when specifying features for a motor controlled by a variable frequency drive (VFD):
- Include consideration of the motor’s service factor required and classification required—Class B for general-purpose use (275°F), Class F for industrial use (311°F), Class H for heavy-duty use (356°F)
- Know your speed range—10:1 or 20:1—and if it requires a general induction motor or a basic inverter duty motor.
- Select the operating point at 75% of rated motor load for peak VFD efficiency and avoid operating at below 20% load due to the efficiency penalty. Investigate using multiple smaller motors if the load will be below the 20% level on a regular basis. Selecting a motor at full load may be a realistic choice depending on the operating situation.
- Watch for VFD high output frequencies when looking for better control at low speeds; these higher frequencies will reduce insulation life. Avoid connecting multiple motors to a single VFD unless they are identical to avoid mismatched motor impedance. Take care when a fan/pump selection uses a VFD to overspeed the equipment to meet load; this may be acceptable in some situations, but should be avoided in others. Watch for a reduction in the motor’s useful service life and loss of torque over 60 Hz.
- Identify the need for constant- or variable-torque motor performance, VFD turndown in a constant-torque application may be 2:1 or, at best, 4:1 for process loads where the compressor or positive-displacement pump requires a constant torque.
- Direct drive fan motors will result in a base savings of approximately 5% on average as compared with a belt-drive application, but could vary wildly depending on the belt type, alignment, and tension. It is important to understand the system operating parameters and control requirements.
A consideration for use of variable speed drives involves HVAC applications at a basic level and some process applications in potentially more complicated situations. A VFD is an excellent method to help test-and-balance contractors achieve the design water and airflows in HVAC systems. A typical test-and-balance specification may require the contractor to trim an impeller if valve throttling exceeds 5% of rated motor horsepower. Maybe this is a case of an oversized pump, but if a VFD is able to balance the necessary flow without creating artificial pressure drop, what could be a better energy-saving solution (besides trimming the impeller or selecting a right-sized pump)?