EP0192915A1

EP0192915A1 - Tansformer with a turn ratio variable while in operation

Info

Publication number: EP0192915A1
Application number: EP85830050A
Authority: EP
Inventors: Giovannini Secondo; Giovannini Gianni
Original assignee: Imec SpA
Current assignee: Imec SpA
Priority date: 1985-02-27
Filing date: 1985-02-27
Publication date: 1986-09-03

Abstract

Transformer with a turn ratio variable while in operation which has at least two wires (20,22) wound together on the same core, exactly interleaved and insulated with respect to each other, and on them there is formed a collector free from insulation, on which a contact metal brush (24) slides, that at every moment, during its movement, touches every wire (20,22) of the winding at the most at only one point. The ends of the wires (20,22) are connected to each other, possibly through impedances and the end inductances counteract the internal circulation current due to the turn short-circuit. At least one turn of at least one wire (20,22) of the winding is only partially linked with the total flux of the magnetic core of the transformer. By means of the use of at least another parted flux turn, the balancing of the voltage differences at the ends of the numerous wires is assured anyway, and the arrangement is such that each wire of the collector has the same voltage difference with respect to each of the two contiguous wires.

Description

The present invention refers to a transformer, as well as an autotransformer with a turn ratio which can be varied (Variao) while in operation, designed to deliver a great power to an electric load with an adjustable voltage.
It is known that the transformers and the autotransformers with a variable turn ratio at the present available on the market have a number of drawbacks, which limit their output power and application possibilities when they are used as voltage regulators and the regulation is to be carried out without the previous break-off from the mains network, due to the internal circulation current which is caused by the brush when it short-circuits two contiguous turns,
The main drawback originates from the fact that, in order to avoid the sudden short-circuit between two contiguous turns, a carbon brush is utilized for the picking-up of the current. Since the carbon has a quite high resistivity, the use thereof on the one hand limits the internal circulation current due to the short-circuit of the turn, but on the other hand it overheats in a geometripal proportion with respect to the magnitude of the current going through it.
All- this drastically limits the use of the variable turn ratio transformers in favour of the electronic regulation devices with silicon controlled diodes (SCR), which, however, although being able to drive strong currents, are very far from an ideal regula- ' tion of the voltage and current supplied to the load. In fact, the SCR devices produce a variable phase shift of the two components of the power, which implies, in certain operation conditions, the presence of very high amounts of apparent power.
However, it has already been suggested that one of the elements of a transformer is wound up as a twin wire winding and that the brush is caused to move on the winding so as it contacts at the most two contiguous turns, which must belong each to one of the two windings
In other words, there are two wires, wound together on the same core, which form a kind of collector, in which the turns of one of the two wires alternate with those of the other one, one by one. These turns are insulated from each other, but have a surface free from insulation where the contact takes place with the brush, which never shorts more than two turns.
However, also with this solution, one cannot avoid the overheating of the machine and the generation of electrical discharges when the brush is moved to change the voltage supplied to the load.
Therefore, object of the present invention is to overcome the above described drawbacks by means of a transformer, as well as an autotransformer which allows the overheating and the generation of electrical discharges to be avoided.
Another object of the present invention is to carry out such a transformer, as well as autotransformer in a simple and cheap way, suitable for mass production.
A transformer with a turn ratio variable while in operation, according to the present invention is characterized in that at least two wires are wound together on the same core, are exactly interleaved and insulated with respect to each other, on them there is formed a collector free from insulation, on which a contact metal brush slides, and the ends of the wires are connected to each other, possibly through impedances, the arrangement being such that at every moment, during its movement, the brush touches every wire of the winding at the most at only one turn.
In such a manner, the internal circulation current flows in all the winding in opposite directions, generating in the self-inductances of the winding itself equal and discordant fluxes, counteracting against each other. These self-inductances can then be increased, by enlarging the windings beyond the extremes of the part concerned with the motion of the brush.
In fact, by using the parallel twin winding, the two wires have each the same voltage values as the other one. In this way, the brush that picks up the voltage, when shorts two contiguous turns, produces one of the two following effects: or a perfect parallel connection, when the two turns have the same potential, or an unbalanced parallel connection, in which an internal circulation current is produced, which is given by the ratio between the voltage of one turn and the impedance of the machine. By doing this, during the switching a voltage step having relation with the potential. of only one turn is obtained.
However this amount, which can seem slight and which is surely acceptable for low and medium powers, becomes greater and greater as long as the power of the transformer grows and less and less acceptable.
In other words, in this way it is possible to limit the internal circulation current to tolerable levels, but it is not possible to overcome the limits imposed by the -presence of the two contiguous windings to the voltage difference between the two subsequent turns of each winding. That is to say, the turns must be double in number t and this implies a double voltage difference between turn and turn under the same physical size of the winding.
According to a further aspect of the present invention, at least one turn of at least one wire of the winding is only partially linked with the total flux of the magnetic core of the transformer, by means of the use of at least another parted flux turn, the balancing of the voltage differences at the ends of the numerous wires being assured anyway, the arrangement being such that each wire of the collector has the same voltage difference with respect to each of the two contiguous wires.
In particular, two wires can be wound in parallel and the end turns of one of the wires of the winding, or the initial turn of. pne of the two wires and the final turn of the other one, can be linked with half of the total magnetic flux of the core.
According to this further aspect of the present invention, the advantage is therefore achieved of being able to finely adjust the absolute amounts of the voltage and current, always keeping them in phase with each other, without there being practically any limitations to the power supplied to the load.
In the following, the present invention will be further clarified from the description of some forms of practical embodiment of a transformer or an autotransformer according to the invention, description made in a purely illustrative and not limitative way, with reference to the accompanying drawings, in which:

figure 1 is a schematic circuit and shows a first embodiment of an autotransformer according to the present invention;
figure 2 is a schematic circuit and shows an embodiment of a transformer according to the present invention;
figure 3 is a schematic circuit and shows a second embodiment of an autotransformer according to the present invention;
figire 4 is a schematic circuit and shows a third embodiment thereof; and
figure 5 is a schematic circuit and shows a voltage stabilizer making use of an autotransformer according to the present invention.

With reference to the accompanying drawings, in which the windings are slown in a schemetic way and with an extremely reduced number of turns, and particularly to the figure 1 thereof, it is seen that an autotransformer according to the present invention comprises a nagnetic core 10, on a column 12 of which there is a winding, sche- natically shown at 14, in which the machine current flows. The winding 14 is provided with two posts 16 and 18, connected to the mains network, and is wound as a twin-wire winding. In other words, the winding 14 is formed with two wires 20 and 22, which are carefully offset, exactly interleaved and insulated with respect to each other in the working area, and their ends are short-circuited with each other at the posts. A metal brush 24, generally of copper, is connected to a load 26 and can move between two extremes 28 and 30 of the working area leaning at the most on two, never on three, contiguous wires. At one of the two extremes of the working area, in this case at 28, a tap is provided on only one of the two wires, in this case the wire 22. To such a tap the other end of the load is connected.
Each of the two wires has to be sized for the nominal current of the transformer.
The internal circulation current due to the turn short-circuit, which is found when the brush 24 shorts two contiguous turns, is therefore limited by the self-inductances formed at the ends of the winding and which are wound in the same direction, so that the mutual inductance also opposes the circulation of the turn shorts circuit current.
Furthermore, the winding 14 can be carried out with more than two parallel wires if the brush 14 has to be so large to short-circuit more than two contiguous wires. In such a case, the homonymous extremes of all the wires will be connected together, so that the winding results comprised of n wires all connected in parallel, the number n being computed so that at every moment, during its movement, the brush touches every wire at the most at only one turn.
With reference now to the figure 2, it is seen that a transformer according to the present invention differs from the autotransformer of figure 1 in that the two extremes 16 and 18 of the winding 14 are no longer connected to the mains network. On a column 36 of the core 10 a primary winding is now wound, the extremes of which are connected to the network that supplies the electric power.
With the embodiment of figure 2 a physical separation is obtained between the load and the mains and one can carry out a maximum voltage for driving the load indipendent from the mains voltage.
In the case of a transformer, the voltage change can be carried out both at the secondary or primary winding; in this latter case, it is obvious that, by varying the supply, one varies the induction of the machine and consequently also the secondary voltage.
With reference now to the figure 3, it is seen that there the column 12 is crosswise drilled at 32 and 34 and the extreme turn of each of the two windings 20 and 22, at their opposite ends, is carried out by threading the wire into one of the holes, so that the extreme turn itself is linked with only half of the magnetic flux of the column 12. In such a manner, the turns which are located under the brush 24 have no longer two by two the same voltage, but every turn has, with respect to the contiguous turn, which is part of the other winding, a voltage difference equal to half of the turn voltage.
The internal circulation current due to the turn short, which is generated when the brush shorts two contiguous turns, is therefore half in amount with respect to that of an autotransformer of the kind shown in figure 1.
Then the fact that between turn and turn only and ever half of the turn voltage is present allows a finer regulation of the load voltage to be achieved under the same physical size of the column 12, the construction principle and the sizing besides, being those of a usual transformer or autotransformer.
With reference now to the figure 4, it is seen that in this embodiment one of the two parallel windings, particularly the winding 20, has been carried out with turns that are all linked with the whole flux of the column 12.
The other winding 22 conversely has its extreme turns with the wires threaded into the column 12 so as to embrace only half of the magnetic flux. Obviously the number of the winding turns that embrace the whole flux is equal to the number of the turns of the winding 20 minus one, and this in order to keep the voltage and current balance between the two windings.
Naturally, if one does not wish to drill the iron, which forms the magnetic core, it is possible to reach the same result by inserting through the magnetic sheets at half of the column a flat shaped, but big enough in cross-section wire, which is linked with half of the magnetic flux. Obviously both the wire which goes through the hole, and the flat shaped wire must be well insulated from the surrounding magnetic sheet.
This principle of parting the flux linked with one turn can further be extended to enlarge as much as one likes the sharpness in the adjustment, by decreasing the voltage present between the contiguous wires in contact with the metal brush.
In fact it is possible to carry out a winding with parallel wires in any number n, the initial turn of each of which is linked with 1 to n n-th of the magnetic flux of the core, respectively, with the balance at the respective final turn, so that, by locating the n windings exactly offset in the working area,' a voltage difference equal to 1n of the turn voltage is obtained between two contiguous wires and at the same time the internal circulation current generated by the short-circuit of two contiguous wires is reduced by1n
The internal circulation current can be controlled in a way different from that shown in the figure 1. There, the impedance of the extreme parts is utilized, leaving to the brush 24 the possibility of moving only on the central part of the two windings.
In such a case, the extreme parts of the two windings, not utilized for varying the voltage of the load, can also be wound in a multilayer fashion and located under the central part, restraining in such a manner the longitudinal size of the column. At the intermediate tap 28, as already stated, the zero voltage for the load is obtained.
Bearing in mind that the internal circulation current can be considered acceptable till a maximum equal to the nominal machine current, in the figure 4 there is shown another embodiment of a device for controlling, the internal circulation current itself.
In figure 4, an input impedance 40 is connected between the input posts 16 and 18 and the extremes 20a and 22a, on the one hand, and 20b and 22b, on the other hand, of the two windings 20 and 22, respectively.
The impedance 40 is comprised of two symmetrical windings 42 and 44. with central tap, to the ends of which the ends of the windings 20 and 22 are connected, and the central taps of which are connected to the input posts 16 and 18.
The impedance 40 must be sized for the power relating to half of a turn and furthermore the two windings 42 and 44 must be connected in such a manner that the internal circulation currents in the impedance are opposite in phase, as better shown in figure 4.
In figure 5 there is shown the use of an autotransformer according to the present invention for carrying out a voltage stabilizer. In this case, the extreme 16 of the winding 14 and the brush 24 are connected to the two wires of the mains power, respectively, whereas the load 26 is connected between the same extreme 16 and an intermediate tap 46, provided on only one of the wires of the winding 14, in this case the wire 22.
It is provided that the working area for the brush 24, going from 28 to 30, extends between about +10+15% and -10+15% of the nominal value of the mains voltage, nominal value at which the tap 46 for the load 26 has been carried out.
In such a manner, only the part of the winding 14 near to the extreme 18 is concerned with the movement of the brush 24, which can never go close to the extreme 16. The end inductance near such an extreme can therefore be omitted and the extreme 16 can in this way be connected with a post both of the mains and load.
Furthermore, it is to be noted that connecting the brush 24 to the mains instead of the load allows the magnetic flux in the iron to be kept constant while the mains voltage changes.
Naturally, the moving of the brush can be carried out by a servo- mechanism connected to a drive system, besides well known to the skilled in the art.
The exactness of the voltage distribution in the machines carried out according to the present invention is such that it is possible to make equipotential connections of same voltage turns belonging to different transformers having equal structure and power and connected to the same phase, for instance in order to make a parallel connection of some machines to obtain high powers without increasing the turn voltage and therefore keeping the adjustment sharpness.
Furthermore, it is to be noted that the ends of the wires which form the winding on which the brush operates,'besides being shorted with one another, can be connected by interposing impedances of any kind, such as inductances, resistances or capacitances, and combinations thereof. In this way it is possible to expand the part of the winding utilized as a working area, as besides already shown and described with reference to the figure 4.
Although reference has always been made to a column core, it is obvious that, in order to carry out a machine according to the present invention, cores of any shape can be used, e. g. annular cores.
It is obvious that other numerous and different changes and modifications can be performed by the skilled in the art on the embodiments of the present invention hereinbefore described, without departing from its scope. It is intended therefore that all these changes and modifications are encompassed in the field of this invention.

Claims

1. A transformer with a turn ratio variable while in operation, characterized in that at least two wires (20, 22) are wound together on the same core (10), are exactly interleaved and insulated with respect to each other, on them there is formed a collector free from insulation, on which a contact metal brush (24) slides, and the ends of the wires are connected to each other, possibly through impedances, the arrangement being such that at every moment, during its movement, the brush touches every wire of the winding (14) at the most at only one turn.

2. A transformer according to claim 1, characterized in that only a central part (from 28 to 30) of the whole winding is utilized as a working area for the brush, the two end parts forming self-inductances that limit the internal circulation current due to the turn short-circuit.

3. A transformer according to claim 2, characterized in that at an extreme (28) of the working area for the brush an intermediate tap is provided on only one of the wires of the winding, said intermediate tap defining the zero voltage for the load.

4. A transformer according to any one of t'he preceding claims, characterized in that at least one turn of at least one wire of the winding (14) is only partially linked with the total flux of the magnetic core (10) of the transformer, by means of the use of at least another parted flux turn, the balancing of the voltage differences at the ends of the numerous wires being assured anyway, the arrangement being such that each wire of the collector has the same voltage difference with respect to each of the two contiguous wires.

5. A transformer according to claim 4, characterized in that two wires are wound in parallel and the end turns of one of the wires of the winding are linked with only half of the total magnetic flux of the core.

6. A transformer according to claim 4, characterized in that two wires are wound in parallel and the initial turn of one of the two wires of the winding and the final turn of the other one are linked with only half of the total magnetic flux of the core.

7. A transformer according to any one of the preceding claims, characterized in that two wires form the winding (14) and the extremes thereof are connected to the extreme of two symmetrical windings (42, 44) with central taps connected to the power supply, the two symmetrical windings being located opposed in phase on a magnetic core (40) so as to form an input impedance that limits the internal circulation current of the machine.

8. A transformer according to any one of the preceding claims, characterized in that turns, having the same voltage, belonging to different transformers having equal structure and power and connected to the same phase are connected together for making a parallel connection of some machines and increasing the output power under the same adjustment sharpness.

9. A transformer according to any-one of the preceding claims, characterized in that it is provided with a column core.

10. A transformer according to any one of the claims 1 to 8, characterized in that it is provided with an annular core.