Abstract:
A polyphase generator for medical diathermy apparatus comprises a ferromagnetic ring core bearing two primary windings each of which has two winding portions positioned diametrically opposite one another. The winding portions in each case are connected in series and are wound such that they create opposing magnetic effect which substantially cancel one another. Two primary windings are positioned substantially at right angles to one another. A plurality of secondary windings are arranged on the core to supply output terminals. A direct current supply may be connected via switching means to induce a resonant signal within the primary windings to generate a polyphase output at the terminals for use in application to electrodes of diathermy apparatus operating in excess of 200 kHz.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to polyphase electrical generators and is applicable particularly to apparatus for generating power in the form of alternating electrical voltages between pairs of electrical output terminals, wherein each of said pairs of terminals may be isolated from others of said pairs and wherein the magnitudes and relative phases of the currents flowing between said pairs of output terminals, when loads are applied, are predeterminable. 
     2. Description of the Related Art 
     In the prior art two or more transformers may be driven from a single alternating current power source, which may itself be of a polyphase type, or else from a single phase source through phase shifting components such as capacitors. In either event it is possible for each pair of a plurality of pairs of output terminals to be linked by a plurality of winding turns connected in series, said turns being wound in appropriate number upon each of said transformer cores so that each turn makes a voltage contribution from the particular transformer core upon which it is wound. Between each of said pairs of terminals there will then exist a resultant voltage, formed by the vector addition of voltages from the said individual turns. 
     By choosing for each pair of terminals appropriate numbers of turns, wound upon one or more of said transformer corers, a voltage of any desired magnitude and phase can be obtained. However without the mechanical complication of movable coils, selection or variation of phase is available only in coarse steps corresponding to one winding turn. 
     This type of generator may well be complex and bulky, containing many junctions between individual windings: these are undesirable where a high degree of electrical isolation is called for. A particular example is diathermy used in electro-surgery, where the patient must be well protected from currents flowing to earth and also from any currents derived from low frequency power supplies. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of this invention there is provided an electrical polyphase generator comprising: 
     output means across which polyphase alternating voltages are to be generated; 
     a ferromagnetic ring core; 
     two primary windings on said core and each comprising a pair of substantially diametrically opposed primary sections connected in series and wound substantially to cancel their net magnetic effect around the circumference of said core, the said primary windings mounted with respect to two orthogonal planes so as to possess minimum mutual inductance one to another; 
     means for supplying to said primary windings alternating currents in quadrature; and 
     secondary windings electrically isolated from the primary windings and each mounted on said core and coupled to feed said output means, each secondary winding having an angular position on the core determining the phase of voltage induced in that secondary winding. 
     It is thus possible, in one variation, to provide a single ferromagnetic transformer core with windings in an arrangement which firstly lends itself to the construction of a compact two or more phase resonant power converter and, secondly, can afford a large multiplicity of different output phases using only a single insulated secondary winding for each of a plurality of pairs of output terminals. 
     Preferably each of said pairs of primary coils is coupled to capacitance to form therewith a resonant circuit and said resonant circuits may be independently tuned to a desired operating frequency. 
     This arrangement is possible because magnetic flux will be present in the said ferromagnetic ring core mainly beneath the winding turns which carry current at the time, returning in the form of a stray field which leaves said ring core and flows parallel to the axis of the coil. Thus there will be stored energy associated with each primary coil in the manner of a solenoid, even though neither primary winding creates net magnetic force around the core periphery. Such an arrangement contrasts with a conventional toroidal transformer. 
     One may supply power to the two primary windings so that each maintains as closely as possible a sinusoidal voltage across itself, these two voltages being in quadrature. Given that said primary windings can both resonate at a required working frequency—preferably of 200 kHz or above e.g. about at least 500 kHz, this may be achieved by using semiconductor or vacuum tube switches which connect each of said primary windings across a direct current power supply for a small proportion of the required cycle time, said primary windings being connected alternately and also the polarity of connection being reversed on each occasion. Between such periods of connection a sinusoidal pattern of magnetisation is maintained by current which flows through the capacitance coupled to each secondary winding. 
     Preferably, the supplying means comprises switching means arranged to operate at 4 n times (where n is an integer&gt;0) said operating frequency to connect the extremities of said primary windings sequentially to an electrical power source so that substantially equal sinusoidal currents will flow through said primary windings in quadrature relationship, and substantially sinusoidal voltages will be induced in the secondary windings. 
     The superposition of the two patterns of magnetic flux generated respectively by said two primary windings will constitute a non-uniform magnetisation of the said ferromagnetic ring core which is equivalent to the magnetisation which would be produced were a single pair of primary windings to be energised with direct current and their mounting plane physically rotated about the axis of said ferromagnetic ring core at the rate of one revolution per cycle of alternating current generated. Such physical rotation would of course be impracticable at the frequency required for applications such as surgical diathermy and so it is useful that the same result can be achieved by the described construction. 
     Given the set of primary windings already described, a secondary conductor may be threaded through the said ferromagnetic core and if it is formed into a complete turn wrapped around any particular location on the circumference of said core, that secondary conductor will be incompletely coupled to both of said primary windings. It will have generated in it an induced voltage which varies in phase according to its angular position around the circumference of said ferromagnetic core with respect to said primary windings and hence according to the proportions of the magnetic fields created by each of said primary windings which are intercepted. 
     By providing secondary coils, each of a required number of turns and positioned suitably around the circumference of a ring-shaped ferromagnetic core and each wired between a pair of output terminals, it becomes possible to accommodate almost any number of output terminals and phase differences. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a first embodiment of polyphase generator; 
     FIG. 2 is a circuit diagram of a second embodiment of polyphase generator; 
     FIG. 3 is a circuit diagram illustrating a star configured diathermy apparatus; 
     FIG. 4 is a vector diagram corresponding to FIG. 3; and 
     FIG. 5 is a circuit diagram of a delta configured diathermy apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment is illustrated by FIG. 1, wherein a ferromagnetic ring core  1  is provided with four equally spaced substantially identical windings numbered  2  to  5 , of which windings  2  and  4  are connected directly in series opposition so as to constitute one primary winding producing no net magneto-motive force round the circumference of the ring core  1 . A capacitor  6  is connected across windings  2  and  4  in series, the aggregate inductance of which windings resonates with capacitor  6  at a desired operating frequency. Windings  3  and  5  are similarly connected so as to resonate with capacitor  7 . 
     Electronic switches  8  to  15  connect the terminals of capacitors  6  and  7  to the opposing poles  16  and  17  of a direct current supply  18 , control electrodes of said electronic switches being linked in pairs to control inputs  19  to  22  inclusive so that no one control input can cause a short circuit of supply  18 . Driving circuits  18   a  cause the control inputs  19  to  22  to be energised in the set sequence  19 ,  20 ,  21 ,  22  with intervening “off” periods, which repeats so that the time for a whole cycle corresponds with the nominally equal resonant frequencies of the windings  2  and  4  with capacitor  6 , and of windings  3  and  5  with capacitor  7 . “The action of the driving circuit at  18 A is to be understood by reference to the corresponding box  43  of FIG. 2 with its corresponding control inputs  48 ,  50 ,  52 ,  54 .” 
     Insulated secondary windings  23  (e.g. 4 or 8 windings) placed in desired angular positions on the ferromagnetic ring core  1  generate across pairs of output terminals voltages at the operating frequency which differ in phase. In practice a desirable arrangement comprises more than two windings, which may be symmetrically placed on the ring core  1 . The secondary windings  23  may be mounted on formers slidable on the core  1  so that the output phases can be adjusted during assembly or during use. They may be secured in place after adjustment by any suitable means. 
     These secondary windings are independent and isolated from one another and from the primary windings. It is further possible, instead of the said pairs of output terminals being independent, to connect said secondary windings together in a star or delta arrangement in order to constitute a single polyphase power source. 
     FIG. 1 also shows various resistors and capacitors as an indication of means whereby, in a practical situation, deviations from a substantially sinusoidal waveform and attendant peak currents might be minimised by one skilled in the art of transformer construction. 
     In an alternative arrangement of windings, each primary winding is replaced by a pair of closely coupled bifilar windings to provide a balanced arrangement with a centre tap. Thereby, the two centre taps can remain permanently connected to one pole of a DC power supply, the four free ends of the said c(composite) primary windings being connected in rotation to the opposite supply pole. Here four electronic switches will suffice to achieve the required pattern of energisation with time. 
     FIG. 2 depicts this alternative arrangement of windings and semiconductor switches. The various resistors and capacitors such as shown in FIG. 1 would be equally applicable here and are thus not shown for clarity. A ferromagnetic ring core  25  is provided with four bifilar windings  26  to  29  such that the two primary windings created by series connection of diametrically opposed winding sections, as in FIG. 1, may be centre-tapped while the two halves of each primary winding so created remain closely coupled. This would not be the case if the individual windings of FIG. 1 were to be separated by a centre tap. 
     Capacitors  30  and  31  are connected to form a resonant circuit with their corresponding primary windings, created by series connection as shown. The centre tap of each primary winding is connected to one pole  32  of a direct current supply  33 , the opposing pole  34  of which is connected to semiconductor switches  35  to  38  whose control inputs  39  to  42  fulfill the same function as control inputs  19  to  22  in FIG.  1 . 
     The control inputs can be driven by an electronic counter to provide the required pattern of power levels and magnetisation, noting that, without a bifilar winding arrangement, eight electronic switches would be required to be connected in pairs to said electronic counter. 
     In a preferred arrangement using bifilar primary windings, a master oscillator  45  operates at eight times the required operating frequency and drives a binary ring counter  43 . Four semiconductor switches are actuated respectively by outputs  1 ,  3 ,  5  and  7  of the said counter and link the four ends of said primary windings intermittently to one pole of a DC supply. The two centre taps of said primary windings are connected together to the opposite pole of said supply. The design of a commercially available 8-way binary ring electronic counter  43  such as the industry standard type 4022 is such that at no time can two different outputs become energised simultaneously. 
     FIG. 2 shows how switches  35  to  38  are controlled by the outputs of the 8-way binary ring counter  43 . Said ring counter has a reset input  44  which forces a count of zero, whereby a voltage output exists at the “zero” output  47  alone. 
     Hence, on applying a “reset” voltage to the counter its “zero” output alone is energised and no power is drawn from the supply. With the counter running, the two resonant circuits are energised with a quadrature phase difference so that outputs of any required magnitude and phase are obtainable from secondary windings positioned as already described. 
     When said reset input voltage is removed, an alternating voltage applied by generator  45  to count input  46  causes sequential voltage outputs to occur at each of the counter outputs  47  to  54  each lasting for one eighth the period of a complete cycle. The counter outputs  48 ,  50 ,  52  and  54  when they occur each switch on one of the switches  35  to  38  for a period corresponding to 45 degrees of a 360 degree cycle. 
     With reference to radio frequency amplifier technology known in the prior art, it will be recognised that the arrangement described above constitutes a pair of class C amplifiers whose conduction angle is fixed at 45 degrees. An improved arrangement provides that at least four of the said semiconductor switches, for example MOSFET transistors, are controlled via an AND gate, the second input of which is controlled by a common pulse of variable length synchronised to the said master oscillator. This arrangement allows the proportion of the cycle during which each of the said semiconductor switches conducts to be variable and thus not only is it independently controllable but also it is adaptable to offset the effects of variable load upon the system. 
     It will be seen that reset input  44  of ring counter  43  functions as a power on-off switch. Thus in a further improvement to this invention, a low frequency square wave is continually applied to the reset input of the said binary counter. This causes the power generation to be periodically interrupted so that measurements can be interposed which might be affected by the presence of radio frequency power, such as temperature checks during the progress of surgical diathermy. 
     In a preferred arrangement, the said low frequency square wave is applied via a monostable trigger circuit which after delivering an energy pulse of known length, requires to be returned to a starting condition by the output of a temperature sensor, said output being absent if said temperature exceeds a predetermined limit. 
     FIG. 2 illustrates how the output  55  of a conditionally monostable circuit built using 2-input NOR gates  56  and  57  may be connected to reset input  44  so that, in the stable condition with input  58  at logic high, no power output results. The effect of maintaining input  58  at logic low is for repetitive pulses of power generation to take place (much greater than the cycle time of the ring counter  43  and typically of the order of one second as determined by resistor  60  and capacitor  59 ) with intervening periods of interruption. Input  58  is shown to be at a voltage controlled by a temperature sensor  61  to give a form of thermostatic control. Safety conditions, such as the presence of electrical power at an undesired location, may also be arranged to cause inhibition by taking input  58  to logic high via diode  62 . 
     FIG. 3 diagrammatically represents medical equipment in this case in the form of apparatus for surgical diathermy, being apparatus for causing high frequency electric currents to pass through living tissue in order to generate heat. This may be for the purpose of cutting or destroying it, or for causing a local temperature rise the effect of which will be selectively to degrade infections or tumour cells. Such apparatus might be generally as disclosed in UK Patent Specification No. GB-A-2281863, the content of which is hereby incorporated by reference. In such an example, the frequency of operation will be in excess of 200 kHz in order to cause only heating. 
     If a patient on an operating table is connected by means of an extensive conducting (normally metal) pad to one pole of a high frequency current generator operating at a frequency in excess of 200 kHz, and typically at least 500 kHz, the other pole of the generator can be connected to a cylindrical or needle-type electrode applied to an area of tissue which requires to be heated. This process is known as diathermy and more details may be ascertained from GB-A-2281863. Such heating tends however to be localised along restricted paths or in the neighbourhood of the electrodes. When it is required to effect the relatively uniform heating of a volume of tissue, a plurality of electrodes may be implanted with a view to electric currents flowing along multiple paths between them, these electrodes being desirably energised by alternating voltages which differ from one another in phase. 
     FIG. 3 shows at  71  a ring of 8 such electrodes coupled to respective output terminals  72  of a polyphase generator  70  in accordance with FIG. 1 or FIG.  2 . The electrodes are held in a holder, shown by dotted lines, which maintains a desired physical relationship between the electrodes in use. In this example the polyphase generator is represented by its secondary windings  23 . In this embodiment a star-configuration is used; thus each winding is coupled between the associated terminal  72  and a common centre point  73 , while capacitance is being used to DC-isolate the terminal  72  from the secondary windings  23 . 
     FIG. 4 shows a vector diagram of the voltages induced in the windings  23  and in this example it is assumed that they are substantially regular and that the ends of the vectors lie approximately on a circle. This is not necessary but is illustrated in this figure for simplicity. The intention is that the free ends of the electrodes  71  will be placed so as to form a configuration corresponding substantially to the configuration represented by the dots drawn at the ends of the vectors of FIG.  4 . 
     FIG. 5 shows an alternative form of the diathermy apparatus  70  in which the windings  23  are connected in series and the terminals  72  are coupled by capacitance to the junctions between the secondary windings, thus providing a delta arrangement. The vector diagram of FIG. 4 applies equally to the voltage differences between the terminals  72 . The electrodes  71 , coupled with respective electrodes  72 , will be arranged according to the pattern determined by the ends of the vectors, as shown in FIG.  4 .