According to the U.S. Environmental Protection Agency, electric motors account for over 60 percent of the electrical power consumed in the U.S., equal to over 1.7 trillion kWh per year, worth over $100 billion per year (at 6 cents per kWh). More than 80 percent of this power is consumed by motors larger than 20 HP, which are less than 1 percent of the country’s installed motor population. Each 1 percent improvement in motor efficiency results in a savings of over $1 billion in electrical energy costs, 6 to 10 million tons less per year of combusted coal, and approximately 15 to 20 million tons less per year of carbon dioxide released into the atmosphere.

The two ways most commonly used today to obtain motor energy savings is through the use of high- and/or premium-efficiency motors or the use of adjustable-speed drive systems, when the application’s load requirement varies over time.

While offering the potential of significant operating energy savings, the extensive use of premium-efficiency or even better efficiency AC motors has been inhibited by higher initial prices that increase with improved efficiency level. This is because most of the improved efficiency is achieved either by increasing the amount of the active material (copper, aluminum, electrical sheet steel) used in the motor and/or by using higher quality active material, both of which incur additional cost. This process is subject to the law of diminishing returns, and therefore the cost of achieving super premium efficiency levels through these methods is quite costly.

Fig. 1. Electromagnetic modeling (left) and copper rotor core cross-section (right).

New, sophisticated electromagnetic modeling and design technology, such as that offered by Raser Technologies, promises to at least partly alleviate this problem by enabling efficiency improvement above NEMA-premium levels for line-driven AC motors, with little or no increase in their manufacturing cost. (See Fig.1.)

Potential levels of improvement expected for a 10 HP, 4-pole AC motor of various efficiency classifications, depending on the motor manufacturer, are shown in Table 1.

By enabling lower cost NEMA-premium or above NEMA-premium AC motors, this new design technology will allow government motor energy-efficiency regulators a freer hand in raising today’s relatively low EPACT level motor efficiency standards.

Enlarge this picture Table 1. 10 HP, 4-pole AC motor efficiency improvement.

Prior to the introduction of cost-effective, electric adjustable-speed drives (ASDs), control of motor-driven devices such as pumps, fans and compressors were almost always achieved by valves, vanes, dampers and other mechanical devices, which are inherently inefficient. The use of AC variable-frequency drives (AC VFDs) allows the speed of commonly-used AC induction motors to be varied to match a load’s speed and torque requirements, providing a significant savings in energy for motors driving centrifugal pumps, fans and compressors which spend part of their duty cycle operating at less than rated speed and torque, such as those used in the heating, ventilation, and air-conditioning and refrigeration (HVAC/R) industries.

Conventional ASDs and AC VFDs, however, do not optimally minimize motor input power on the basis of energy efficiency at all motor speeds and load torques. Likewise, almost all AC motors used in the HVAC/R industries are designed as constant speed machines, and are not optimized to work with VFD synthesized current and voltage waveforms. The efficiency of an AC induction motor being fed constant-frequency sinusoidal power can drop drastically under reduced loads, especially loads below 50 percent of the rated torque. (See Fig. 2).

Its efficiency can drop even more rapidly at reduced loads when fed pulse-width modulated (PWM), non-sinusoidal voltage waveforms.

An adjustable-speed drive system with a power electronic rectifier, inverter, microprocessor and electric motor is a highly non-linear drive and control system. The complexity of its system model and its varying, uncertain parametric relationships due to the inductance and temperature changes inherent in an AC induction motor, mean that an accurate system model for simulation, performance prediction, and control is very difficult to achieve. This is especially true considering the accuracy and precision needed to obtain the last few percent of efficiency for a VFD-driven motor system.

Enlarge this picture Fig. 2. AC motor efficiency vs. rated load when fed with sinusoidal constant-frequency voltage.

Most AC drive systems used in HVAC/R applications today use a constant volts/hertz control method, where the motor is operated with its airgap flux maintained at rated value, and where the motor’s actual shaft speed is not measured, that is, open-loop speed control. This allows speed control with the best transient response. However, in lightly loaded operation, this method causes the core (iron) losses to be excessive compared to the stator (copper) loss, giving non-optimum efficiency performance.

In 2005, the U.S. Department of Energy funded a study administered through the State Technology Advancement Cooperative (STAC) to investigate ways to reduce the cost of high-efficiency HVAC systems. In partnership with Advanced Energy of Raleigh, N.C., a leading independent motor testing lab, and Washington State University Energy Program, Raser Technologies was tasked with developing an efficient, modular AC drive and controller for such applications.

Under this project, Raser Technologies developed the FlexMod AC drive controller using Symetron™ drive-control technology. The FlexMod drive controller design uses adaptive tuning to optimize performance based upon unique motor characteristics. The prototype drive can use various power modules and different IGBT designs and uses a modular construction method to reduce manufacturing costs.

Enlarge this picture Fig. 3. 5 HP FlexMod efficiency improvement.

Advanced Energy tested the FlexMod drive controller and a comparable, commercially available, leading HVAC AC drive on 5 HP and 20 HP AC commercially-available AC induction motors. The tests produced system efficiency maps from 300 RPM to 1,800 RPM and 25 percent to 125 percent of rated torque. The tests demonstrated that the FlexMod design was 2 percent to 10 percent more energy efficient on average. Fig. 3 and Fig. 4 show the efficiency improvement for Symetron-enhanced 5 HP and 20 HP, 4-pole AC induction motor-drive systems over the speed range from 300 RPM to 1,800 RPM at 25 percent, 50 percent, 75 percent, and 100 percent of rated full-load torque (and 125 percent for the 5 HP motor).

Based on typical duty cycles encountered in HVAC/R applications, Raser estimates median savings for a typical 20 HP V/Hz open-loop AC VFD used in HVAC/R equipment could conservatively be expected to be about 2 percent, or roughly 2,400 kWh/yr, for an electrical energy cost savings around $156 per year, based on a power cost of 6.5 cents per kWh; and for a 100 HP AC VFD motor-drive system, a 2.0 percent energy savings, or roughly 12,000 kWh/yr, for an electrical energy cost savings of around $781 per year.

This energy cost savings is in addition to that normally obtained from the use of an adjustable-speed drive system versus a constant-speed system mechanically controlled. For the AC VFD manufacturer, the cost of implementing such technology is relatively minor, involving either a change in the drive’s operating software, or, in the case of retrofits, the addition of an application-specific integrated circuit (ASIC).

Enlarge this picture Fig. 4. 20 HP FlexMod efficiency improvement.

As such, it is expected the HVAC/R equipment industry may soon begin to see motor efficiency optimization software specified more and more on the AC VFDs it buys for use in its equipment, and perhaps even government mandates for such efficiency optimization software in HVAC/R VFDs in the not so distant future.

For more information, email: david.west@rasertech.com