Electric power conversion systems – Cascaded or combined – diverse conversions in which the... – Current and phase
Reexamination Certificate
1997-09-23
2001-07-31
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Cascaded or combined, diverse conversions in which the...
Current and phase
C363S064000
Reexamination Certificate
active
06269009
ABSTRACT:
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>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.
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