The operation of split-phase induction motors from pulse width modulated (PWM) voltage source inverters is examined in this paper. Splitting the phase windings leads to reduced voltage ratings for the inverter switches. As compared with seven positions for the space phasor of voltage in three-phase machines, 48 different locations bounded by a 12-sided polygon are possible in the split-phase machine. Based on space phasor modulation, a three-phase inverter can give a maximum peak fundamental of 0.577 VDC. In comparison, the split-phase configuration can provide an equivalent three-phase maximum peak fundamental of 0.643 VDC with a dc bus voltage of 0.5 vDC/cosl~.
Speed control of induction motors requires the accurate estimation of the fluxes in the motor. But the flux estimate, when estimated from the stator circuit variables, is dependent on the stator resistance of the induction motor. As a consequence the flux estimate is prone to errors due to variations in the stator resistance, especially at low stator frequencies. A scheme is presented in this paper for an online estimation of the stator resistance under steady state operating conditions, using variables that can be measured from the terminals of the motor alone. The scheme is based on estimating the steady-state magnitudes of the stator and rotor flux space phasors using the reactive power. An analysis of the effect of the stator resistance variations on the flux estimate is presented. A simulation of a rotor field oriented speed control of a VSI-fed induction motor using stator circuit variables is performed incorporating the online stator resistance estimation strategy.
Split phase induction motor operation from dual PWM voltage source inverters offers the advantage of reduced voltage rating for the inverter devices. In addition, a total of 4 9 different locations are possible for the resultant stator voltage space phasor. The outermost locations form a twelve sided polygon. By space phasor PWM generation based on the vertices of the twelve sided polygon, a higher range of fundamental motor voltage is possible in the modulation range, up to 0.643 VD , where VEC is the DC link voltage for an equivalen$ 3 phase inverter, as compared to 0.577 VDc for the three phase motor. This is brought about by the presence of 5th and 7th harmonics in the average poler voltage waveform.
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