Application Date: Aug. 12, 1935. No. 22726/35.
(Patent of Addition to No. 421,546: dated June 16, 1933.)
Complete Specification Accepted: March 12, 1937.
Improvements in or relating to the Supply of Electrical Energy to Varying Loads, for example Thermionic Valve Apparatus
We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a British Company, of Blyth Road, Hayes, Middlesex, and ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following invention:-
The present invention relates to the supply of electrical energy to varying loads, and is particularly concerned with apparatus constituting an improvement in or modification of the invention which forms the subject of Patent No. 421,546.
In the specification of the above-numbered parent Patent, there is described and claimed apparatus for the supply of electrical energy to a varying load, which apparatus comprises a source of direct current and one of more reactive impedance elements either separate from or inherent in the source, means being provided whereby the apparatus as a whole, and including the said means, presents to the load an impedance which is substantially purely resistive over the range of frequency within which energy is to be supplied to the load.
The invention of the parent Patent is described as applied to direct current sources, such as rectifiers associated with an alternating current generator, which feed a load through a smoothing circuit, the latter comprising essentially series inductances and shunt condensers; the invention is carried into effect by building out the smoothing circuit by means of impedance elements which are the inverse of the elements of the smoothing circuit, and therefore function as the mirror image impedances of the smoothing elements, so that the whole apparatus, including the inverse impedances, simulates a pure resistance.
When the source has a D.C. regulation resistance which is so small as to have no appreciable adverse effect upon the constancy of voltage maintained across the load at very low frequencies of load variation and where there are one or more reactive impedance elements which may cause the regulation impedance to rise at certain frequencies to unduly high values, there are provided, according to a feature of the parent Patent, means for preventing the regulation impedance from rising to a value which is more than about three times the D.C. regulation resistance.
It has been found, however, that the values of the impedance elements necessary to build out smoothing circuits such as those described in the specification of the parent Patent are in many cases undesirably high, and it is an object of the present invention to provide improved or modified apparatus according to the parent Patent in which this difficulty is avoided.
In most cases, the smoothing circuit is required to deal more particularly with one frequency or a number of known frequencies; for example, in the case in which the source of direct current in a rectifier, the smoothing circuit is required to prevent current at the ripple frequency or frequencies from reaching the load; the present invention makes use of this fact.
According to the present invention, apparatus adapted for the supply of electrical energy to a varying load comprises a source of direct current and a filter including or constituted by a resonant circuit arranged substantially to prevent currents of an undesired frequency from reaching the load, said resonant circuit being made resonant to said undesired frequency, and means are provided whereby said apparatus, view from said load, has a substantially purely resistive impedance over a predetermined range of load current and over a predetermined working range frequency.
An example of a varying load which the invention may be employed to supply is a thermionic valve amplifier for amplifying television signals have a D.C. component, in such a case, the current drawn varies with variations in the amplified signal.
According to a feature of the present invention, apparatus adapted for the supply of electrical energy to a varying load comprises a source of direct current and a filter including or constituted by a resonant circuit arranged substantially to prevent currents of an undesired frequency from reaching the load, said resonant circuit being made resonant to said undesired frequency, and there are provided means including a further circuit which is resonant to said undesired frequency and is so constructed and arranged as to prevent the regulation impedance of said apparatus as a whole from rising, at said undesired frequency and over a predetermined range of load current, to an unduly high value, that is to say a value exceeding about three times the direct current regulation resistance of said apparatus.
Other features of the invention will appear from the following description and the appended claims.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which
Figs. 1 and 2 show two forms of filter section which are suitable for use in apparatus according to this invention.
Fig. 3 shows filter section suitable for use in apparatus in which either the source of current of the load is floating, that is to say, is unconnected at any point to earth or a like point of fixed potential, and
Fig. 4 shows apparatus according to this invention which includes a full-wave rectifier and a filter comprising circuits resonant to an undesired frequency.
The filter sections shown in Figs. 1 and 2 are suitable for use either alone or in conjunction with additional smoothing means of the kind shown in the specification of the parent Patent. The circuit of Fig. 1 comprises a series arm L1, C1 and a shunt branch R, L2, C2, and that of Fig. 2 has shunt branch L1, C1 followed by series arm L2, C2, R. These references will be taken to signify the magnitudes of the elements to which they refer.
The terminals 1 are in both cases connected either to the source, or to the output terminals of a smoothing circuit associated with the source, and the terminals 2 are connected to the load or to a further stage or stages of smoothing. It should be arranged that the impedance to which terminals 1 are connected, looking towards the left, is equal to R; for example, in the case in which the terminals 1 are connected to the source the value of the direct current regulation resistance of the source is made equal to R and, as will be shown later, it may be necessary in some cases artificially to increase the effective regulation resistance in order to obtain the desired relationship.
Both in Figs. 1 and 2, the circuits comprising inductance L1 and condenser C1 and inductance L2 an condenser C2 respectively are made resonant to an undesired frequency, and it is arranged thatEQUAT. HERE
The impedance looking towards the left from terminals 2 is in both cases equal to R.
The bridge-type section shown in Fig. 3 is suitable for use is cases in which the source or the load has neither side earthed; the relationship between the values of the several components is that given in equation 1; that is, the same relationship holds as for Figs. 1 and 2.
The attenuation introduced by the filter sections shown in the three Figures so far considered is very high at the frequency at which L1 and C1 are resonant, and is dependent on the resistances of these components. Several sections of the kinds shown in Figs. 1 to 3 may be employed in association with the same source, the sections being made to provide their maximum attenuation at the same frequency or at a number of different frequencies, such as a series of ripple frequencies.
Reference will now be made to Fig. 4, in which a full-wave rectifier comprising two hot-cathode mercury vapour rectifiers 4, fed with alternating current from supply mains through mains transformer 3, is arranged to feed direct current to a load (not shown) connected between terminals 2. A complex filter circuit 5 is connected between the rectifiers 4 and terminals 2, and the regulation of the apparatus depends on the regulation of the mains, the characteristics of the mains transformer and the rectifiers 4 and upon the nature of the filter.
The resistance R1 is provided to build out the direct current regulation of the part of the apparatus to the left of the filter 5 to a value equal to the input impedance of the filter; this value is made high (about 100 ohms) so as to avoid the necessity for the use of condensers of very large capacity in the filter.
The first series arm of the filter, comprising inductances L1 and L3, and condenser C1, is required to introduce a large attenuation at all ripple frequencies. Inductance L1 and condenser C1 are chosen to resonate at the main ripple frequency – that is to say, with two well-balanced rectifiers, twice the supply mains frequency – and the inductance L3 is arranged to have a large impedance at the remaining ripple frequencies; it will be observed that on account of the provision of resonant circuit L1 C1, the inductance of L3 can be made less than the value which would be necessary if circuit L1 C1 were omitted, and inductance L3 were required to deal with the whole series of ripple frequencies. It will be understood that the amplitude of the electromotive force at the lowest ripple frequency is greater than the amplitudes of the remaining ripple frequency electromotive forces.
It must be arranged, however, that the capacitive reactance of circuit L1 C1 at the second ripple frequency (which is approximately the second harmonic of the first ripple frequency) is less that the reactance of inductance; that is to say, the resonant frequency of L3 and L1 C1 in series must be below the second ripple for this resonant frequency is 1.25 times the first ripple frequency, or less.
The first shunt branch of the filter, comprising a resistance of value R, inductance L2 and condensers C2 and C3, is arranged to build out the filter to a resistance equal to R; for this purpose the following relationship must be satisfied:EQUAT. HERE
A further stage of smoothing comprising series inductance L4, condenser C4 and resistance R4, the condenser C4 being the inverse of inductance L4 with respect to a resistance of magnitude R1 and R4 being made equal to R; henceEQUAT. HERE
so that the whole apparatus, viewed from terminals 2, simulates a pure resistance of value R.
For a mains supply frequency of 500 cycles per second (c.p.s.), giving a first ripple at about 1000 c.p.s., the following values have been found suitable for the various components of the apparatus of Fig. 4:
Inductance L1, 0.1 Henry Condenser C1, 0.25 microfarads
" L2, 0.0025 " " C2, 10.0 microfarads
" L3, 0.2 " " C3, 20.0 "
" L4, 0.15 " " C4, 15.0 "
Resistance R1, 100 ohms Resistance R1, 80 ohms
With these values and with rectifiers having a d.c. regulation resistance of 20 ohms, it was found that the regulation resistance of the apparatus was constant for load currents of from 0.2 to 1.0 amp., but that for currents of less than 0.2 amp., the regulation increased rapidly. Accordingly, if the load is likely to require steady currents of less than 0.2 amps, it is desirable to connect across the terminals 2 of a dead load such as a resistance which will ensure that the current taken never falls below 0.2 amp.
The resonant frequency of circuit L1 C1 may be made slightly higher than the first ripple frequency so as to ensure that the first ripple frequency does not become too close to the resonate frequency of L1 C1 with inductance L3 if the supply mains frequency wanders.
If the leakage impedances between the two halves of the secondary winding and the primary winding of the transformer 3 are small, and if the mains regulation for voltage and frequency is also small, then changes in load current (or changes above the value below which the regulation of the apparatus rises) introduce substantially no changes in the amplitude of the ripple current, and the smoothing obtained is thus constant in spite of variations in the load.
In setting up the apparatus, resistance R1 is not inserted until a preliminary test for D.C. regulation has been carried out; for this test, the whole of filter 5, with the exception if desired of inductance L4 and the shunt branch R4, C4, should be in circuit. From this test, the desired value for R1 can be readily determined. Further measurements of impedance may be carried out in the neighbourhood of a frequency f, whereEQUAT. HERE
in order to ascertain whether allowance should be made for the inductance of the rectifiers in fixing the value of L3; any necessary allowance may be made by increasing C3.
The invention is of course not limited to the arrangements described above by way of example; many modifications of the invention within the scope of the appended claims will occur to those versed in the art.
Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-
Dated this 12th day of August, 1935.
REDDIE & GROSE,
Agents for the Applicants,
6, Bream’s Buildings, London, E.C.4.
Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1937.