US2247079A - Variable ratio transformer - Google Patents

Description

June 24, 1941. Y BQLTE 2,247,079

VARIABLE RATIO TRANS FORMER Filed July 10. 1939 Inverwtor: Karl Blte,

His Attorney.

Patented June 24, 1941 2,247,079 VARIABLE RATIO TRANSFORMER Karl Bolte, Berlin-Karlshorst, Germany, assignor to General Electric Company, a corporation of New York Application July 10, 1939, Serial No. 283,696 In Germany July 14, 1938 1 Claim.

This invention relates to variable ratio transformers and more particularly to improvements in the connections between the phase windings of a variable ratio polyphase transformer system.

One known way of adjusting the ratio of a polyphase transformer is to provide means for changing the connections between the phase windings from star to mesh or vice versa, either gradually or in a single step. This has been done by connecting one terminal of each of the phase windings to a different conductor of a polyphase circuit and connecting the remaining terminal of each of the phase windings to a ratio adjuster or tap changing switch or other suitable means which connects this remaining terminal to either end or to intermediate points on one of the other windings. The other winding is usually an electrically adjacent winding as determined by the phase rotation of the circuit. These phase windings may constitute either the primary, the secondary or a tertiary Winding of a transformer. In either case, if the system is a three-phase system, the ratio of the maximum to the minimum voltage obtainable by such connection change is equal to the /3 because it is the ratio of the line-to-line voltage to the line-to-neut-ral voltage.

I have discovered that by merely making a simple single change in the connections, which does not involve the addition of any new parts or the increase or decrease of the turns of any of the windings, I can obtain much greater ratios between the maximum and minimum voltage than are obtainable with the above described previously-used connection. This change consists of connecting the ratio adjusters or tap changers on each winding to an intermediate point, instead of to the free terminal, on the electrically adjacent winding. Furthermore, the ratio of maximum to minimum voltage, for a given range of the ratio adjusters or tap switches, can be adjusted by the adjustment of the location of the intermediate point.

An object of the invention is to provide a new and improved transformer system.

Another object of the invention is to provide a new and improved connection for increasing the voltage range of a variable ratio polyphase transformer.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claim.

In the drawing Fig. 1 illustrates diagrammatically an embodiment of my invention with the ratio adjusters in the maximum voltage position (if the illustrated winding is considered the primary winding), Fig. 2 is a diagram illustrating vectorily the voltage relations in the circuit shown in Fig. 1, Fig. 3 is similar to Fig. 1 except that the ratio adjusters are in their outer extreme position corresponding to minimum voltage, and Fig. 4 is a diagram illustrating vectorily the voltage relations existing in Fig. 3.

Referring now to the drawing and more particularly to Fig. 1, there are shown therein three windings I, 2 and 3 which are the phase windings of what may be considered the primary winding or the secondary winding or a tertiary winding, etc. of a three-phase transformer. One terminal of each of these windings is connected respectively to the conductors A, B and C of a threephase circuit. A correspondingly located intermediate point P on each of the phase windings is connected to a ratio adjuster or tap switch 4 of any suitable type operating on an electrically adjacent winding. The particular adjacent winding may be determined consistently from the phase rotation of the circuit. Thus, if the phase rotation is ABC, point P1 is connected to the ratio adjuster on winding 2 which is connected to phase B, while point P2 is connected to the ratio adjuster on winding 3 which is connected to phase C etc. The ratio adjusters are shown in one extreme position where they contact directly with the terminals of their associated windings which are connected to the conductors of the polyphase circuit.

In Fig. 2 the points A, B and C may be taken to represent the conductors A, B and C of the three-phase circuit and they are equally spaced from each other to form the usual equilateral triangle constituting the line-to-line voltage of a three-phase circuit. N is the neutral point of the system so that the voltage to neutral is represented by AN, BN or CN. The ratio of one of the line-to-line voltages to one of the line-toneutral voltages is equal to the /3 or roughly 1.73 and this corresponds to the maximum range of voltage variation obtainable by a change of connections from Y to delta.

As will be seen the point P3 corresponds with point A, the point P1 corresponds with the point B and point P2 corresponds with the point C. The voltage of winding l in proportion to the other voltages then becomes V1 and the voltage of winding 2 becomes V2 and the voltage of winding 3 then becomes V3. As shown in Fig. 1 the number of turns of each winding between the terminal which is connected to one of the line consumed as unity and P stands for the fraction:

of the total number of turns of each phase winding included between point P on each Winding and the terminal thereof which is permanently connected to one of the line conductors, then the maximum voltage V1, V2, V3, etc., shown in Fig. 2, is equal to an P In Fig. 3 the ratio adjusters 4 have been moved to the opposite extremities of their associated windings. This produces a decrease in the voltages across the windings I, 2 and 3, as shown by the voltages V1, V2 and V3 in Fig. 4 wherein points A, B and C correspond to the similar points in Fig. 2 and are correspondingly spaced. Using the conventions adopted above for expressing the maximum voltage the magnitude of the minimum Voltage V1, V2 and V3 as shown in Fig. 4 may be derived from the properties of triangles and equals The range of voltage variation may be expressed as the ratio of the maximum voltage to the minimum voltage and if the above given expressionfor the maximum voltage is divided by the expression for the minimum voltage the resulting ratio may be reduced to 1 1 vfi'ip-tl These expressions are general and hold for V the straight Y-delta connection. In that connection P=1 and it will readily be seen by substituting 1. for P in the three expressions given above that the maximum (Y) voltage will be 1.73, the minimum (delta) voltage will be 1 and the ratio of maximum to minimum voltage or the range of voltage adjustment will be 1.73. It will also be apparent that whenever P is less than 1, that is to say, whenever Pv corresponds to an intermediate point, that the range of voltage adjustment is greater than 1.73 becoming infinitely great when P becomes zero.

When the ratio adjusters 4 make connection with intermediate points on the windings, intermediate values of voltage will be obtained. If

we let it stand for the fractional part of each winding included between the connection point of the ratio adjuster 4 and the terminal of the winding permanently connected to a line conductor, that is to say, if we define a: in the same manner as We define the location of point P, then it can be shown that the voltage of each phase winding is equal to Thus, byproperly selecting P and a: any desired voltage range at any particular voltage may be obtained. a

If Figs. 1 and 2 are considered the secondary winding of a transformer which is excited by a conventional primary winding, then Fig, 1 will of course correspond to the minimum voltage position and Fig. 3 will correspond to the maximum voltage position.

While the invention has been described particularly in connection with the usual threephase circuit, it will of course be understood that the invention is not limited to any particular number of phases.

While there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claim to cover all such changes and modifications as fall within' the true spirit and scope of the invention. i

What I claim as new and desire to secure by Letters Patent of the United States is:

A polyphase transformer including, in a com-' bination, a plurality of 'equal'windings each having a terminal which is normally connected to a different phase conductor of a polyphase circuit, separate ratioadjusting switches associated respectively with each ofsaid windings for selectively making connection to. correspondingly located points on their associated windings, one set of said points comprising the terminals of said windings which are normally connected to said circuit, another of saidsets of points comprising the other terminals of said windings, an intermediate fixed point on each of said windings, said fixed points being correspondingly located thereon, and a permanent connection between each fixed point and the ratio adjuster for the electrically succeeding winding as deter- V mined by a given order of phase rotation, said ratio adjusters being operative to change connections from one set of corresponding points to another such set in the manner of conventional polyphase transformer ratio adjusters.

' KARL BOLTE.

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