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.