Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Patent
1996-08-06
1998-05-26
Sterrett, Jeffrey L.
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
363132, 363 95, H02M 7797
Patent
active
057576360
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to the generation of pulse width modulating waveforms, and is concerned more particularly, but not exclusively, with the reduction in switching losses in inverter circuits employing the pulse width modulating principle.
BACKGROUND ART
Inverter circuits employing high frequency switching devices are used in conjunction with the pulse width modulating principle to provide high quality variable frequency and variable voltage outputs. The waveforms used to modulate the pulse width of the high frequency carrier, so as to produce fundamental frequency output voltages, are typically continuous and of a sinusoidal nature such that every switching device in the inverter be switched continuously over a fundamental cycle. Discontinuous waveforms may also be used providing that the resultant output voltages are sinusoidal. The discontinuous waveforms have dead-bands where high frequency switching in the phase legs is suppressed for sections of the fundamental period and the phase connection point of the leg clamped to one of the direct current input rails. In one fundamental cycle, the total number of switching device commutations is decreased thus reducing associated switching losses. The switching loss reduction is, however, dependant upon the phase leg current flowing during the dead-band.
DISCLOSURE OF INVENTION
It is an object of the invention to provide a method of linking the dead-band regions of the discontinuous modulating waveforms to the peaks of the associated phase leg currents.
According to the invention there is provided a drive circuit for a load, the circuit comprising connection means for connection to a direct current supply, respective switching means associated with each phase of the load and connected to the connection means for supplying current to the load, sensing means for sensing the current supplied by way of the connection means and voltage control means for switching the switching means so as to supply voltage, in accordance with the required voltage, to each phase of the load, the voltage control means being operatively coupled to the current sensing means.
In one possible embodiment of the invention the switching means associated with each phase of the load comprises two switching elements connected in series, as a phase leg, and connectable at their common point to the associated load phase. The voltage control means is arranged to control the switching means such that the phase leg carrying either the largest or second largest sensed current magnitude is clamped to one of the d.c. rails and the other phase legs are pulse width modulated so as to supply the required phase voltages.
BRIEF DESCRIPTION OF DRAWINGS
In order that the invention be more fully understood, reference will now be made to the accompanying drawings, in which:
FIGS. 1 and 1a are a circuit diagram of a preferred embodiment of drive circuit in accordance with the invention for a three phase (3.phi.) induction motor drive.
FIG. 2 shows explanatory waveforms indicating the manner of operation of the circuit shown in FIG. 1 using prior art.
FIGS. 3, 4 and 5 show explanatory waveforms indicating, in accordance with the invention, the manner of operation of the circuit shown in FIG. 1.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring to the circuit diagram of FIG. 1 the circuit comprises an a.c. voltage supply 1, a full wave bridge rectifier 2 and a smoothing capacitor 3 which converts the a.c. voltage from the supply 1 into a d.c. voltage rail. An inverter bridge 4 consisting of six bridge switches T1 to T6 which are commutated by the pulse width modulator 5, thereby supplying the desired phase voltages, Va, Vb and Vc to the induction motor 7. The signals SA to SC control the pulse width of the high frequency carrier produced by the pulse width modulator 5. These signals, furnished by the modulation generator 6, are dependant both on the line currents, Ia, Ib and Ic sensed by 8, 9 and 10 and the desired phase voltages, Va, Vb and Vc. Each phase leg comprises a pair of
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PWM Drives Limited
Sterrett Jeffrey L.
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