US3189816A - Automatic adjustment and compensation of the secondary voltage of a transformer - Google Patents

Description

June 15, 1965 M. BROZEK 3,189,315

AUTOMATIC ADJUSTMENT AND COMPENSATION OF THE SECONDARY VOLTAGE OF A TRANSFORMER Flled May 3, 1961 2 Sheets-Sheet 1 F 16. la

v are) I 4m 8 & a Q A 2 3 a a s s. s 'ji ig i FIG 4 r mar BROZEK AUTOMATIC ADJUSTMEIiT AND COMPENSATION OF THE June 15, 1965 M BROZEK 3,189,816

SECONDARY VOLTAGE OF A TRANSFORMER Filed May 3, 1961 2 Sheets-Sheet 2 OJ X J; 3 o min./(V 3 O A T TORNE Y.

United States Patent 3,189,816 AUTOMATIC ADJUSTMENT AND CQMPENSATL'GN OF THE SEQGNDARY VGLTAGE @F A TRANS- FORMER Mnata Breach, Modrany, near Prague, Czechoslovakia, assignor to Chirana Praha, narodni podnik, Prague, Czechoslovakia Filed May 3, 1961, Ser. No. 107,558 Claims priority, application Czechosiovakia, May 6, Edit, 2,988/69 6 tllaims. (Cl. 323-435) This invention relates to an improved method for automatic adjustment and compensation of the secondary voltage of a transformer, the method being based on the change of the number of turns of the primary winding.

There exists a great number of apparatuses in which the Working voltage has to be changed within a certain range. For example in the case of large X-ray instruments the values of actually used high voltages vary from 40 to 150 kilovolts (peak values). Changes in working voltage are also necessary, in the case of electric filters, electrostatic sorting equipments, supply sources and in many other instances.

There are three generally known methods for changing the value of the secondary voltage. In the first method, an auto transformer is connected across the primary winding. In the second arrangement, the number of turns of the secondary winding is varied. In the third arrangement, the number of turns of the primary winding is varied.

These known arrangements have various disadvantages. In the case of the first arrangement mentioned above, a separate high power auto-transformer must be built into the apparatus, and this involves additional expense. Variations of the number of turns of the secondary winding, while eliminating the necessity for the use of the extra autotransformer, involves direct variation of the relatively very high secondary voltage. In turn, this introduces substantial problems with respect to insulation and to possible arcing and the like. The third arrangement, which is that of varying the number of turns of the primary winding, does not have the disadvantages of the first two arrangements mentioned above.

Neverthless, this third arrangement has not been used up to the present for various reasons. One reason is that it has not been known how to control and stabilize the secondary voltage in such a way that the device could operate fully automatically and with a great degree of accuracy within the whole working range. An other difficulty of this third arrangement resides in the fact that the potentiometer in the automatic controller must have a hyperbolic characteristic and that the sensitivity of the control relay for the servo-system must be a quadratic function of the difference between the maximum and minimum value of the voltage to be regulated.

These (lllllClllllfiS with respect to the third-mentioned control arrangement have prevented such arrangement from being utilized in practical operation.

The method of voltage control, or the secondary voltage control system, to which this invention relates eliminates the afore-mentioned drawbacks in that it suggests an accurately working practicable device based on the third principle mentioned above. A substantial advantage of the invention system consists in that the sensing element need not be calibrated as to its sensitivity since the sensitivity of the sensing element can be constant, thus enabling the system to incorporate electronic components and circuitry. For an understanding of the principles of the present invention, reference is made to the following description of a typical embodiment thereof as illus- 3,189,816 Fatented June 15, 1%65 trated in the accompanying drawings. In the drawings:

FIGS. la, 1b and 1c are schematic wiring diagrams illustrating the three known secondary voltage control systems mentioned above;

FIG. 2 is a somewhat more detailed schematic Wiring diagram of a control system based uponthe principle .of controlling the secondary voltage by varying the number of turns of the primary .winding;

FIG. 3 is a somewhat more detailed schematic wiring diagram illustrating a secondary voltage control system embodying the invention; and

FIG. 4 is a graph illustrating the principles involved in the invention control system.

F IG. la illustrates a system for controlling the secondary voltage of a transformer Tm by the use of an auto transformer AT which is connected across the primary winding of the transformer. KV represents a meter, and the load, such as an X-ray tube, as indicated as L.

PEG. lb illustrates an arrangement for controlling the secondary voltageof the transformer Tm by varying the number of turns of the secondary winding, this being effected by the use of a moveable contact MC, as schematically illustrated in FIG. lb.

FIG. 1c schematically illustrates a secondary voltage control system which is based upon the principle of varying the number of turns of the primary winding. In this arrangement, upon which the present invention is an improvement, the number of turns of the primary winding is varied by means of an adjustable contact MC.

The basic transformer equation suggests that for a-constant number of secondary turns the number of primary turns can be computed for each chosen value of both the main voltage and the secondary voltage.

The basic transformer equation is diagrammatically illustrated in the graph of FIG. 4 covering the usual working ranges. At the same time, this graph also shows the nomograph of the equation expressing the regulating action of the device according to this invention.

Prior to go more deeply into the description of circuits shown in FlGS. 2 and 3 yet us explain the meaning of abbreviations and symbols to be found in these figures:

Referring to FIG. 2, the potentiometer moving contact I is associated with the scale of the control device. The corresponding potentiometer P is linear and a stabilized DC. voltage is applied to its terminals. The control moving contacts 3 which adjusts the number of primary turns, is. coupled to the moving contact J of the hyperbolic potentiometer P This potentiometer is connected across a DC. voltage which is proportional to the voltage of that phase of the mains x, y, z, which is applied across the main transformer TR. Both moving contacts 1 and J are electrically connected through the coil of a polarized relay PR. When this relay is not excited its moving contact is in the intermediate (zero) position between two fixed contacts. This condition exists when the secondary voltage is at the proper value corrresponding to the setting of the contact J with respect to the scale associated therewith. This relay (sensing element, pick-up) can be either electromagnetic (moving-coil system) or electronic or it can be based on a semi-conductor etc. The relay contact is in neutral position when the voltages picked-up by both contacts 1 and 1 are equal. lt-mr instance-the voltage on I is higher than that on J as when the secondary voltage is lower than a desired preset value, the relay contact closes the circuit of the relay n which, in turn, energizes the electro-motor M to rotate in such a direction that the voltage tapped with the contact J begins to rise. As soon as the voltages on 3 and 1 are equal, the contact of the relay PR returns to neutral position, the relay n is de-energized, the electro-motor ceases to rotate and the whole device is again at rest.

Table showing the mains voltage compensation and adjustment of kilovolts:

Z1 {or Zr for Z1 for Sensi Z1 32% Kv. KV.rma 320 v. 380 v. 420 v. Vxeg tivity, 420 v.

28. 3 640 760 840 80 0. 4 200 35. 4 510 605 670 100 0. 625 160 42. 5 422 500 554 120 0. 91 132 49. 5 363 430 476 140 1.24 113 5G. 5 320 380 420 160 l. 6 100 63. 5 288 342 378 180 2 90 70. 6 259 307 340 200 2. 47 81 77. 8 234 278 307 220 3. 02 73 84. G 213 252 279 240 3. 64 66 92 196 233 257 260 4. 6!. 99 182 216 239 280 4. 9 51 106 170 202 223 300 5. 67 53 113 160 190 210 320 G. 4

In the foregoing table Z means the number of primary turns.

The full sensitivity range is 0.4:6.4=1:16.

It is apparent from the above example that, in existing arrangements for varying the secondary voltage by varying the number of turns of the primary winding, of which FIG. 2 illustrates a typical arrangement, the pick-up sensitivity must be changed in the ratio 1:16 if the assumed control range is to be covered. In case of a moving-coil relay this can be achieved with the aid of a variable resistor P connected in parallel to the relay coil while the moving contact 1.; of this resistor is mechanically linked to 5 the moving contact 1 Let us now discuss the problem of the aforementioned voltage control more thoroughly:

The basic transformer equation can be written in the following simple form:

E 2 E2 with Z Z meaning the number of primary and secondary turns and E E being the primary and secondary voltages. Since in X-ray equipment it is usual to consider the peak voltage, the above equation can be re-written as follows where Z =const.=56,500 turns (for E/Z=2) E varies from 40- kv to 160 kv E varies from 320 v. to 420 v.

Z varies from 160 to 840 turns Keeping in mind that Z =const. the former equation can be transformed into:

Substituting 13 and 2 :2. we may draw a graph, as shown in FIG. 4 with three systems of straight lines where Z are straight lines parallel to Y-axis, E are parallel to X-axis and E form a cluster of straight lines, all passing through the origin and having slopes equal to ZZ /Z The graph of PEG. 4 shows how many turns of the primary winding are needed for the full range of control for each secondary voltage and for the corresponding change of mains voltage varying rorn 320 to 420 volts.

It is also possible to introduce a new variable E (i.e. control volts) into the aforementioned graph. Obviously, this new variable should be substituted for Z using the relation Z.const=E,. with the constant equal to the control voltage for Z21. Thus, a constant (C) must be introduced into this equation so as to produce E =CZ Now, we have to substitute E,.:const.Z :CZ into the equation and-in order to secure E,.:const.Z the voltage across the control potentiometer P must be stabilized.

Since Czconst. and Z =const., the substitution leads to the basic equation for the control voltage:

With the sensitivity for the change equal to one turn or one step K (e.g. K=5 turns) being constant, the pick-up sensitivity need not be varied at all. The control voltage E is directly proportional to the pick-up sensitivity and inversely proportional to the secondary voltage.

The equation defining the mains voltage compensation for a constant high-voltage can be written in the form and this is an analytic equation of a straight line.

The equation determining the high-voltage adjustment for constant mains voltage reads K zan,

and this is the well known equation of the hyperbola.

From the last quoted relation, the kilovolt scale can be computed, e.g. for the nominal mains voltage E :38() volts.

This equation also enables the extreme value B to be computed. Thus, another scale can be added to the Z- scale in the graph of FIG. 4 mentioned above. For instance, for 1K:5 turns and for the pick-up sensitivity C:2 volts per one turn, the required control volts can be read from the diagram at a glance.

The actual control circuit based on the above deduced mathematical analysis, is diagrammatically shown in FIG. 3.

Referring to FIG. 3, the control potentiometer P is a linear potentiometer and has applied thereacross a stabilized DC voltage rather than a DC. voltage proportional to the voltage of that phase of the mains which is applied across the transformer 1",. It will thus be noted that the arrangement of FIG. 3 ditters, in this first respect, from that of FIG. 2. The potentiometer P of the invention arrangement, as shown in FIG. 3, has applied thereacross an unstabilized voltage which is a DC. voltage proportional to the voltage of that phase of the mains x, y, z, which is applied across the primary winding of the transformer T The constant I, of the potentiometer P and the contact J of the potentiometer P are interconnected electrically through the differential sensing device such as the polarized relay PR. The potentiometer P can be (1) a linear potentiometer with a hyperbolic scale, (2) a hyperbolic potentiometer with a linear scale, or (.3) a combination of potentiometer and associated scale having a hyperbolic characteristic. It will be noted, from reference to the hyperbolic characteristic indicated at A, and from reference to the scale, that the voltage adiustment increases in a downward direction, across the potentiometer P as compared to the potentiometer P of FIG. 2 wherein the voltage increases in an upward direction. Thus, the arrangement of FIG. 3 corresponds to the aforementioned mathematical analysis, wherein it was pointed out that the control voltage E, is inversely proportional to the secondary voltage of the transformer TR, and wherein it was pointed out that the equation determining the high voltage adjustment for a constant voltage of the mains is an equation for a hyperbola.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

What I claim is:

1. in a control system for automatically controlling the secondary voltage of a transformer by correctively adjusting the number of primary Winding turns of the transformer responsive to deviation of the secondary voltage from a preset value as a result of variation in the supply voltage impressed across the transformer primary winding, the improvement comprising a first contact adjustable along said primary Winding to vary the number of turns thereof effectively in circuit; a first linear potentiometer including a second contact adjustable therealong and mechanically coupled to said first contact; means for applying a stabilized DC. voltage across said first potentiometer; scale means; second potentiometer means; a third contact adjustable along said second potentiometer means with reference to said scale means to preset said voltage value; a servo-system for conjointly adjusting said first and second contacts; means for impressing, across said second potentiometer means, a DC. voltage proportional to the AC. voltage impressed across the primary winding of said transformer, to provide a control voltage; and differential responsive control means for correctively operating said servo-system responsive to a difference in the adjusted DC. voltages represented by said second contact and said third contact; at least one of said second potentiometer means and said scale means being hyperbolic.

2. In a control system improvement, as claimed in claim 1, said control voltage impressed across said second potentiometer means being inversely proportional to the secondary voltage of said transformer.

3. In the control system improvement, as claimed in claim 1, the sensitivity of said differential responsive control element being constant over the complete regulating range of the system.

4. In a control system improvement, as claimed in claim 1, said second potentiometer means comprising a linear potentiometer and said scale means comprising a hyperbolic scale.

5. in a control system improvement, as claimed in claim 1, said second potentiometer means comprising a hyperbolic potentiometer and said scale means comprising a linear scale.

6. In a control system improvement, as claimed in laim l, the combination of said second potentiometer means and said scale means having a hyperbolic characteristic.

No references cited.

LLOYD MCCOLLUM, Primary Examiner.

MELTON O. HERSHFIELD, Examiner.

Claims (1)

1. IN A CONTROL SYSTEM FOR AUTOMATICALLY CONTROLLING THE SECONDARY VOLTAGE OF A TRANSFORMER BY CORRECTIVELY ADJUSTING THE NUMBER OF PRIMARY WINDING TURNS OF THE TRANSFORMER RESPONSIVE TO DEVIATION OF THE SECONDARY VOLTAGE FROM A PRESET VALUE AS A RESULT OF VARIATION IN THE SUPPLY VOLTAGE IMPRESSED ACROSS THE TRANSFORMER PRIMARY WINDING, THE IMPROVEMENT COMPRISING A FIRST CONTACT ADJUSTABLE ALONG SAID PRIMARY WINDING TO VARY THE NUMBER OF TURNS THEREOF EFFECTIVELY IN CIRCUIT; A FIRST LINEAR POTENTIOMETER INCLUDING A SECOND CONTACT ADJUSTABLE THEREALONG AND MECHANICALLY COUPLED TO SAID FIRST CONTACT; MEANS FOR APPLYING A STABILIZED D.C. VOLTAGE ACROSS SAID FIRST POTENTIOMETER; SCALE MEANS; SECOND POTENTIOMETER MEANS; A THIRD CONTACT ADJUSTABLE ALONG SAID SECOND POTENTIOMETER MEANS WITH REFERENCE TO SAID SCALE MEANS TO PRESET AND VOLTAGE VALUE; A SERVO-SYSTEM FOR CONJOINTLY ADJUSTING SAID FIRST AND SECOND CONTACTS; MEANS FOR IMPRESSING, ACROSS SAID SECOND POTENTIOMETER MEANS, A D.C VOLTAGE PROPORTIONAL TO THE A.C. VOLTAGE IMPRESSED ACROSS THE PRIMARY WINDING OF SAID TRANSFORMER, TO PROVIDE A CONTROL VOLTAGE; AND DIFFERENTIAL RESPONSIVE CONTROL MEANS FOR CORRECTIVELY OPERATING SAID SERVO-SYSTEM RESPONSIVE TO A DIFFERENCE IN THE ADJUSTED D.C. VOLTAGES REPRESENTED BY SAID SECOND CONTACT AND SAID THIRD CONTACT; AT LEAST ONE OF SAID SECOND POTENTIOMETER MEANS AND SAID SCALE MEANS BEING HYPERBOLIC.

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