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ORNL inverter may help put electric cars on the road

OAK RIDGE, Tenn., Dec. 6, 1995 — Electric cars, adjustable-speed motors, heat pumps, fans and compressors may benefit from a new electric power inverter developed by researchers at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL).

The Resonant Snubber Inverter (RSI), invented by engineers in the Digital and Power Electronics Group, improves efficiency and reliability, is smaller, lighter, greatly reduces electromagnetic interference (EMI) and potentially lowers the cost of electric power inverters. Inverters, used with many electric devices and motors, convert available power to the type needed - such as direct current to alternating current.

The RSI is about 80 percent efficient at low speeds and 98 percent efficient at high speeds. Conventional inverters are about 60-70 percent efficient at low speeds and 94 percent efficient at high speeds. Inverter efficiency gains of that magnitude - especially at the lower speeds typical of its use in a car - could help electric vehicles become a viable option, researchers say.

To perform their function, inverters employ a series of switches and electronic components. A conventional electric power inverter consists of six power semiconductor switches that turn on or off about 20,000 times per second in different combinations to provide the desired output. The inverter switch turns on and off at full voltage and current, generating a huge, wasteful power spike. This type of "hard switching" is an effective way of obtaining a specific current; however, this circuit design causes many problems.

The conventional inverter is noisy, big, heavy, unreliable and expensive, said ORNL researcher Jason Lai, who worked with ORNL co-inventors Bob Young, Matt Scudiere, John McKeever, George Ott, Cliff White and University of Tennessee co-inventors Daoshen Chen and Fang Z. Peng. All are members of the Engineering Technology Division's Digital and Power Electronics Group, which is led by Don Adams.

In addition to EMI caused by hard switching, conventional inverters put considerable stress on silicon devices and other parts within the inverter, causing reliability problems, Lai said.

While the conventional inverter uses six switches to achieve a desired output, the RSI adds three small auxiliary switches that temporarily - and very briefly - divert current, then route it back to one of the six main switches. This diversion, which lasts only a couple of microseconds, produces a zero voltage across the switch and helps reduce damaging power spikes. It also enables the RSI's soft switching to increase efficiency from four to 15 percentage points compared to a conventional inverter. The efficiency gain is dependent on the speed of the motor connected to the inverter. Greatest efficiency gains occur when the motor is run at less than full speed, typical of an inverter's function in an electric vehicle.

While gains in efficiency are important, Lai also emphasizes that the RSI virtually eliminates EMI. Tests using an oscilloscope show EMI is greatly reduced compared to conventional hard switching inverters and previously developed soft switching inverters. EMI can interfere with functions of appliances, telephones, electronic instruments, television reception and other electronic equipment, such as computer controlled ignition in automobiles.

Another benefit of the RSI is that it reduces voltage and current stress to inverter components. This improves the reliability and allows lower-cost power devices to be used. Because the RSI smoothly, or softly, changes the voltage and current during device switching, it can also help reduce the possibility of motor failure caused by insulation breakdown and bearing overheating. Soft switching also reduces the inverter's operating temperature, reducing the need for large, heavy heat sinks, which are devices to dissipate heat. Instead, the RSI can use smaller, lighter, less expensive heat sinks. Excessive heat degrades electronic equipment and causes failures.

The latest 100-kilowatt, three-phase RSI built by ORNL researchers is compact, measuring 9x12x6 inches and weighing 20 pounds. Hard switching inverters from several years ago were bulky and weighed several hundred pounds. Even newer state-of-the-art inverters weigh two to three times as much as the RSI.

In addition to its use in electric vehicles, another likely application for the RSI is in heat pumps. Using the RSI and fans that run continuously, comfort levels and efficiency levels could be increased, according to ORNL's McKeever.

In a different project led by Adams, the RSI is being incorporated into an advanced air conditioner to be installed on electric buses, including one in Chattanooga in 1997. The unit is the product of ORNL's work in advanced electric motor technology and a cooperative research and development agreement partner's new air conditioner technology. Installation of the unit is expected to eliminate the need for an auxiliary power unit required for the bus's air conditioner. These auxiliary units are currently powered by propane, which results in emissions, noise, added weight and cost.

The research was supported by DOE's Laboratory Directed Research and Development fund.

ORNL, one of DOE's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.