摘要:

Describing functions at subharmonic frequencies are established for a range of pulse modulators. These allow an analysis to be made of the phenomenon of ripple instability in closed-loop systems containing such modulators, wherein the loop may oscillate at a submultiple of the fundamental frequency of the ripple in the modulator output. This basically occurs because the gain of the modulator at these subharmonic frequencies can be greater than its d.c. value. The modulation processes studied are pulse-length, pulse-position, pulse-width and pulse-amplitude modulation. It is shown for these cases that the gain of the modulator is enhanced most at the ½-order subharmonic, which, as a result, tends to be the dominant type of ripple instability. A particular application of these modulators in closed-loop systems which is of current interest is the thyristor invertor. In the second part of the paper the describing-function approach is developed to analyse the particular case of a d.c.-link invertor in a closed-loop system, e.g. a wide-bandwidth current-limit control in an invertor drive. It is shown to explain and predict the ripple-instability behaviour of the system, and good agreement is reached with analogue-computer results. The cases of fixed- and variable-frequency operation are investigated, together with the effects of utilising a thyristor amplifier in the loop to provide the d.c.-link voltage.

DOI:10.1049/piee.1968.0036

出版商:IEE    

年代:1968

数据来源: IET

摘要:

A describing-function analysis, taking into account d.c. conditions, of a feedback structure containing a single asymmetrical nonlinearity representative of those occurring in solid-state and thermionic feedback amplifiers is used to calculate the closed-loop frequency response of such amplifiers. From this analysis, general conditions governing the shape of the open-loop frequency response for the avoidance of jump resonance are derived. It is shown that, when such shaping is not possible, a critical input amplitude exists below which jump resonance is absent, and a general expression for this amplitude is developed. Experimental work on a solid-state integrated-circuit amplifier having inherent jump resonance confirms the accuracy of the analysis and the conditions for jump avoidance.

DOI:10.1049/piee.1968.0037

出版商:IEE    

年代:1968

数据来源: IET

摘要:

A nonlinear reactor will produce from several input frequencies all possible harmonics and intermodulation products. If every single one of these input and output frequencies is fed through an ideal rotating electromechanical transducer a mechanism is created which has as many degrees of freedom as there are independent frequencies. Equilibrium requires that for any virtual angular displacement of the shafts the work done by the applied torques shall equal the associated change of stored energy. For noncommensurate independent frequencies the time average of this change is zero; i.e. the virtual work of the mean applied torques is zero. The Manley-Rowe relations as published are a specialised form of this statement, requiring that for each independent shaft the resultant of all mean torques (referred to it in accordance with the relevant gear ratios) should be zero. These relations cease to be generally valid if the independent frequencies are commensurate. Then there is synchronism between the harmonic of one frequency and a harmonic or fundamental of the other. The change in stored energy is a measure of the reaction produced if one attempts to change the relative angular positions of the two synchronous shafts. Its time average is not necessarily zero, and if it is not we have a mean power flow between these shafts. Exactly the same situation is encountered in the case of two synchronous generators working in parallel. Analogues can be classified according to the extent of simulation achieved: constraints only, mean energy storage or full energy storage. One of the simplest given is the horizontal lamina (‘teatray’), floating on a needle point and loaded in positions specified by the harmonic index numbers with weights proportional to the associated mean torques. Its equilibrium is described by the Manley-Rowe equations. This analogue covers the commensurate case if there is an elastic tie between the principal ‘synchronous point’ and ‘earth’. The associated diagram also draws attention to the existence of near-synchronous conditions, in which case pulling effects between the independent frequencies may be expected.

DOI:10.1049/piee.1968.0038

出版商:IEE    

年代:1968

数据来源: IET

DOI:10.1049/piee.1968.0039

出版商:IEE    

年代:1968

数据来源: IET