Modulation method and apparatus for static power frequency chang

Electric power conversion systems – Frequency conversion without intermediate conversion to d.c. – By semiconductor converter

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363164, H02M 5257

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active

047775815

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates principally to cycloconverters but also to other static power frequency changers, and, in particular, to specific methods of and apparatus for switching input waveforms from an AC supply of one or more input phases to achieve approximations to desired output waveforms.


NOMENCLATURE
a given trigger period. period. trigger period. start of a trigger period.


PRIOR ART

A static frequency changer is essentially a device for synthesizing an approximation to a desired output waveform by means of switching one portion of one or more input waveforms consecutively to the output of the device. The input waveforms used are either the input phase voltage waveforms or the inversion of these or both. The number of input waveforms used is called the pulse number of the frequency changer, so called because this is usually (although not necessarily) the average number of portions of the input waveforms switched to the output over one input cycle. The desired output waveform will typically have a frequency of less than half that of any input waveform. Switching of the input waveform is typically done at the input waveform frequency.
Cycloconverters can be defined as static frequency changers which use thyristors that are naturally commutated. Cycloconverters may be either of the circulating current type or the non-circulating current type. All other types of static frequency changers presently use switches that either have the intrinsic ability to turn off or use thyristors which are turned off with forced commutation.
Known input waveform switching strategies include "cosine crossing control" and "integral control".
Cosine crossing control uses a switching criteria based upon the intersection of selected portions of a phase shifted input waveform (typically 90.degree.) and the desired output reference waveform. The integral method is based on the selection of input waveform triggering instants determined when the integral of the difference between the output voltage waveform and the desired reference voltage waveform (determined in real time) is equal to zero. The limitations of both these methods as applied to the cycloconverter are discussed in U.S. Pat. No. 3,585,485 to Gyugyi, Rosa and Pelly. In U.S. Pat. No. 3,585,485 a particular solution to an inherent problem in applying the integral method to approximate a non DC output waveform is disclosed. The solution involves injecting an offset component into the next integral calculation, the offset component being proportional to the DC component of the calculated ripple integrals of the integral method. U.S. Pat. No. 3,585,485 is concerned with the application of the integral method to a circulating current type cycloconverter. U.S. Pat. No. 3,585,486 is a concurrent patent to the same inventors concerned with applying the integral method to the non-circulating current cycloconverter.


PROBLEMS SOUGHT TO BE OVERCOME AND ADVANTAGES OF PREFERRED EMBODIMENTS

The method of the present invention has particular applicability to but is not solely limited to non-circulating current cycloconverters.
The non-circulating current cycloconverter has many advantages over other forms of A.C. variable speed drives: its maximum power output is virtually unlimited; its power circuit is very simple, consisting of only phase-controlled thyristors and their associated snubbers; it is very efficient; and it is naturally regenerative. With the present modulation methods in use, however, (cosine crossing control/integral control-with or without feedback) it suffers from some severe disadvantages. It has a low maximum output frequency (of about 25 Hz for a 6-pulse system) due to subharmonics appearing on the output. It suffers from voltage distortion and the associated torque pulsations due to the uncertainty of the current cross-over points and the inability of the prior art modulation methods to compensate for discontinuous currents in the thyristors. Also, it has a poor input power factor, particularly at low output voltages.
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REFERENCES:
patent: 3585485 (1971-06-01), Gyugyi et al.
patent: 3585486 (1971-06-01), Gyugyi
patent: 3593106 (1971-07-01), Lafuze
patent: 3636423 (1972-01-01), Jenkins
patent: 3832624 (1974-08-01), Gilmore
patent: 4013937 (1977-03-01), Pelly
patent: 4307444 (1981-12-01), Stacey et al.
"Static Power Frequency Changers", by L. Gyugyi and Pelly 1976 Excerpts: Title pg, Preface, Table of Contents, Nomenclature and pp. 298-308.
"Line-Commutated Frequency Changers for Speed Control of Electrical Machines", 1983 Excerpts: Title pg, p. 6; pp. 88-101 and Bibliography (p. 117).
"Microprocessor Control of a Cycloconverter", Robert E. Betz and Robin J. Evans, IEEE Transactions on Industrial Electronics, vol. 32, No. 2, May 1985, pp. 120-129.

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