THREE PHASE INVERTER FOR ULTRA HIGH SPEED MOTOR DRIVE

SEVERAL OF THE LONG-LIFETIME CRYOCOOLERS BEING DEVELOPED FOR SPACE SENSOR APPLICATIONS ARE BASED ON THE REVERSE-BRAYTON CYCLE. AN ATTRACTIVE IMPLEMENTATION OF THE CYCLE WOULD USE HIGH-SPEED, MINIATURE CENTRIFUGAL COMPRESSORS AND TURBOEXPANDERS RUNNING ON GAS BEARINGS TO PROVIDE RELIABLE AND VIBRATION-FREE OPERATION. HARDWARE DEVELOPMENT AND DEMONSTRATION PROGRAMS ARE PRESENTLY UNDERWAY FOR THREE OF THE FOUR KEY CRYOCOOLER COMPONENTS: THE TURBOCOMPRESSOR, THE TURBOEXPANDER, AND THE HEAT EXCHANGER. THIS PROPOSAL ADDRESSES THE DEVELOPMENT OF THE REMAINING KEY COMPONENT: THE VARIABLE FREQUENCY, THREE-PHASEINVERTER FOR DRIVING THE HIGH SPEED COMPRESSOR MOTOR. THE MOTOR DRIVE REQUIREMENTS FOR THIS APPLICATION ARE VERY DIFFERENT FROM THOSE OF EXISTING HIGH SPEED MOTORS. THE DESIGN SPEED IS ABOUT FIVE TIMES HIGHER (500,000 RPM) AND THE STATOR IMPEDANCE IS SEVERAL TIMES LOWER AND HENCE DRAWS VERY HIGH CURRENTS (UP TO 40 AMPS DURING STARTUP). A NOVEL THREE-PHASE INVERTER CONCEPT IS PROPOSED WHICH HAS THEPOTENTIAL TO GENERATE EXTREMELY LOW HARMONIC CONTENT WAVEFORMS AT FREQUENCIES OF UP TO 10 KHZ OR MORE USING VERY SIMPLE AND RELIABLE ELECTRONICS. PHASE I OF THIS PROJECT WILL ASSESS THE FEASIBILITY OF THE PROPOSED CONCEPT BY BUILDING AND TESTING A BREADBOARD MODEL. IF PROVEN FEASIBLE, A FLIGHT PROTOTYPE INVERTER WOULD BE DEVELOPED ANDTESTED IN PHASE II.