A McMurray Inverter Controller FREDERICK RODRIGUES AND THOMAS H. BARTON, FELLOW, IEEE Abstract-The specifications of a highly flexible controller for a three-phase McMurray inverter are defined and th e construction of a suitable controller is described. The controller is capable of operation in either the six step or pulsewidth modulation mode and incorporates safe start-up and shut-down procedures. The operation of the controller is illustrated by a series of inverter inverter and controller oscillograms. INTRODUCTION
THE MCMURRAY INVERTER, because of its excellent
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square waveform, its high efficiency, and capability of high-frequency operation is used as the three-phase power source for the experimental investigation of variable-speed, variable-frequency ac drives in the authors' laboratory. The inverter controller described here provides an interface be tween the inverter and other equipment modules such as pulsewidth as pulsewidth modulation (PWM) controllers and drive con trollers. As such it accepts a standardized input pulse train and generates firing pulses for the inverter main and auxiliary thyristors. It also ensures the safe start-up and shut-down of the of the inverter, is self contained with its own power supply, and is shielded against the various and intense noise sources of a power electronics laboratory. As befits a controller designed for use in an experimental laboratory, major design aims were flexibility, simplicity of operation, and reliability. THE McMURRAY INVERTER
The McMurray inverter [1] is widel y known and used and only the briefest description, sufficient to define the the controller specification, will be given here. A three-phase inverter is shown in Fig. 1, 1 , where the components of a phase are identi fied by a final letter, R for the red phase, Y for the yellow phase, yellow phase, and B for the blue phase. Each phase comprises a pair of pair of main thyristors Tl and Tl, a pair of feedback diodes Dl and D2, a pair of auxiliary thyristors TAI and TA2, a com mutating inductor L, and a commutating capacitor C. When Tl is triggered, the output is connected to the the positive de bus. When T 2 is triggered, the output is connected to the negative de bus. Thus the output is a square wave whose peak to peak amplitude is Vnc· The three units are operated sequentially so as to give outputs mutually displaced by 120°, thus pro ducing a three-phase square-wave output. We will commence the analysis of operation by assuming the main thyristor Tl to be conducting, and the commutating Manuscript received November 27, 1978;revised February 24, 1979. F. Rodrigues was with with the University of Calgary, Calgary, Alta, Canada. He is now with the Ballarat College of Advanced Education, Victoria, Australia.
T. H. Barton is with the Faculty of Engineering, The University of Calgary, Calgary, Calgary, Alta., Canada T2N 1N4. y 0
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A three-phase McMurray inverter. The three phases are identi fied as red R , yellow Y, and blue B.
capacitor charges as shown. To tum off Tl, the auxiliary thyristor TAI is fired, thus permitting C to discharge. The combination of L and C is lightly damped so that the dis charge of C is essentially accomplished by a half sinusoid of current. The first portion of this current passes in the reverse direction through Tl Tl , rapidly driving its current to zero. The excess discharge current flows through the inverse parallel connected feedback diode Dl. This situation, Tl carrying zero current and reverse biased by biased by the forward drop of Dl , persists for a few tens of micro seconds, sufficient for Tl to recover its voltage blocking capa bility. capa bility. The other main thyristor, Tl, may then be triggered, thus connecting the output to the de negative bus. The operation of the commutation circuit is such that the capacitor voltage reverses to a value somewhat higher than the de supply voltage so that it is appropriately charged for the next commutation which will tum off T 2 and turn on Tl . A McMurray inverter controller must, therefore, produce an auxiliary thyristor pulse to initiate commutation, followed, with an appropriately adjuste d delay, by a main thyristor pulse. A three-phase unit, as shown in Fig. 1, requires six auxiliary and six main pulses appropriately spaced in time. The matter is somewhat more complicated when the load draws a lagging current. The load current reversal lags the voltage reversal so that the incoming main thyristor T2 only carries current momentarily, the load current almost im mediately transferring to its feedback diode D2. The main thyristor must be ready to carry the current at some later
