By Mike Thompson, ActelJuly 02, 2007 -- pldesignline.com
Electric motors are used in nearly everything, from elevators to home appliances. In 2005, the United States consumed 4,055 billion kilowatt-hours of electrical power. More than 50 percent of this power was used in electric motors, translating into a staggering 2,000 billion kW-hr bill. Unfortunately, many of the motors in use are inefficient and waste a substantial amount of the power they consume.
Adding electronic control can dramatically improve efficiency, but the high cost of control and power electronics has been a barrier to its implementation. With technology improvements in semiconductor processes and integration, mixed-signal field-programmable gate arrays (FPGAs) are emerging as an important alternative for motor control implementation. These highly integrated, flexible platforms offer the bulk of the resources needed for motor control on a single, low-cost device. Using FPGAs in lieu of fixed logic gives designers the flexibility to implement the most efficient design for their application and the ability to use the same device across a broad range of motor applications.
The efficiency of small ac motors can be as low as 50 percent. While motor efficiency improves to more than 90 percent as motor size increases, there is still opportunity to improve efficiency and reduce energy consumption. By adding intelligent load matching or variable speed control, the power efficiency of electric motors across the full range can be increased. With a mixed-signal FPGA and a soft optimized microprocessor, this can be accomplished with a minimal increase in motor cost. In fact, coupled with best practices, this combination can result in motor efficiencies approaching 95 percent. Implemented broadly, electronic motor control could result in savings of as much as 15 percent of the electric power used in the United States--an annual reduction in energy consumption of as much as 300 billion kW-hr, saving $15 billion and reducing greenhouse gases by more than 180 million metric tons.
The biggest factor impacting ac motor efficiency is loading mismatch. When an ac motor is operated near full load, it can reach efficiencies over 90 percent. Few motors are consistently operated at full load, however, due to natural load variations and oversizing, reducing efficiency by as much as 75 percent and wasting power. It is not unusual in the United States to find motors that are 2x to 3x larger than they need to be for the load that they are driving, which is an expensive mode of operation. Even when correctly sized to meet the maximum load, the motor is typically run at a lower, less efficient loading. For example, an escalator is sized to carry the maximum number of people. Most of the time, however, there are few people on the escalator, causing it to run at a low level of efficiency and waste power.
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