Design and development of an inverter-drive system for IPM-motor-driven air-conditioners
Interior permanent-magnet (IPM) motor’s increasing popular use in air-conditioners is owed to its high efficiency – highest among ac and dc motors, and 30%-50% more efficient than common induction motors are. Air-conditioners guzzle power so the consequently reduced power consumption and running costs are welcome.
IPM motor’s rotor is unique; made from high-energy neodymium-iron-boron (NdFeB) magnet, robust, and able to run at very high speeds, its interior construction creates flux distribution superior to that of surface-mounted permanent-magnet motors (90% efficiency against 80% of surface-mounted permanent-magnet motor’s), and the motor’s overall construction provides superior power density and torque-to-inertia ratio and efficiency.
Starting torque can be either line-start or inverter-fed. Line-start IPM motors are used in fixed-speed or synchronous-speed applications. Air-conditioner systems need variable speed to maintain the desired temperature, thus an inverter is used for starting and for speed variation.
Inverters convert to application-specific ac voltages and frequencies, the dc voltage rectified from ac line voltage. For advantageous use, they need advanced control methods. Speed control of IPM motor is complex as its permanent-magnet rotor construction provides fixed magnetic-field excitation, making rotor-speed control difficult. Conventional-motor speed control is through variation in input voltage and in supply frequency but this limits the speed range. Integration of current vector control with flux weakening control expands the speed range but the technique needs DSP, complicating control. The most advanced IPM motor control developed is the sensor-less drive; without speed-determining position sensors, the system is more robust, and, without feedback signal, more reliable. Speed approximation is through DSP-based calculation.
Inverters raise efficiency levels yet most air-conditioning systems are non-inverter types. Highly developed countries have started replacing conventional motors with IPM ones, and the rest of the world will, too, as energy resources dwindle. The only stumble to IPM motor’s out-phasing conventional ones in air-conditioners is that reliable, high-performing inverters are still in the works.
The researchers, Prof. Dr. Nasrudin Abd. Rahim and Assoc. Prof. Dr. Hew Wooi Ping, are with UM’s Centre of Research in Power Electronics, Drives, Automation and Control, UMPEDAC.
For more information on the research, Please contact: PROF. DR. NASRUDIN BIN ABD RAHIM