February 25, 2013
SUPER-EFFICIENT MOTOR CONTROL FROM NONLINEAR DYNAMICS
Induction motors are relatively efficient when they operate at their rated speed, particularly for NEMA Premium grades. But motors that operate at varying loads and varying speeds — as you might find in an electric vehicle — are another story. The inefficiency under these conditions seems to be the motivation for a new motor control scheme from AC Kinetics Inc., a start-up based in Armonk, N.Y.
AC Kinetics says its drive software incorporates a novel control architecture that takes into consideration nonlinear motor phenomena, including core loss, nonlinear inductances, and saturation in the motor and inverter. The company claims its software is an improvement over vector control methods now employed in variable-speed drives. It also says its software results in less motor energy being lost to heat.
Unfortunately, AC Kinetics gives few details about how it accomplishes a high efficiency under varying loads. It does say that it has conducted comparison tests showing the performance of its software under during th eEPA urban traffic cycle, a standard test regime designed to illuminate motor efficiency during conditions resembling those of stop-start driving in an electric vehicle. It says on the EPA urban traffic cycle it tested, induction motors controlled with its software used half of the energy of other drives, which would equate to increased range in an electric vehicle.
However, the actual increase in range will vary depending on the nature of the driving cycle, the firm points out.
Specifically, to demonstrate the reference tracking capability of each drive, the firm hooked an ac induction motor to a large inertia and commanded the assembly to follow a modified EPA city traffic cycle.
Technicians then measured the velocity tracking error and energy consumption for each drive. To check each drive’s sensitivity to unpredicted load disturbances, technicians applied a sudden full-scale load while the drives were commanded to hold a constant velocity. The load disturbance moved the shaft velocity off its reference value, and the drives attempted to recover to the reference speed. The firm then measured such factors as peak velocity tracking error and time to re-settle while the motor was commanded to maintain 350 and 1,000 rpm. The firm says at both speeds, its drive had the best response to unpredicted load disturbances with the smallest peak velocity tracking error and the quickest time to re-settle. Ditto for the smallest peak overshoot and fastest settling time, while using considerably less energy that other leading commercial drives used for the exact same transition in velocity.
To test steady-state efficiency, the firm operated each drive at 30 different speed and load combinations while measuring energy consumption. It claims that across all operating points, its drive consumed the smallest amount of energy, while maintaining its performance characteristics.
AC Kinetics is showing its new ac motor drive control software for the first time at the 2013 ARPA-E Energy Innovation Summit in Washington, D.C. It will also present its test results there. The software is now available for licensing and is said to be fully compatible with industry standard drive hardware.
Leland E. Teschler
MACHINE DESIGN MAGAZINE