US2976476A - Variable voltage transformer - Google Patents

Description

A. E. SNOWDON VARIABLE VOLTAGE TRANSFORMER March 21, 1961 2 Sheets-Sheet 1 Filed April 27, 1956 INVENTOR. A/ZMME (5 /70 Wdon A TTORNE Y6 March 21, 1961 A. E. SNOWDON 2,976,476

VARIABLE VOLTAGE TRANSFORMER Filed April 27, 1956 2 Sheets-Sheet 2 INVENTOR. Ar'zfwr" E. Jnowdon BY WMK ATTORNEYJ VARIABLE VOLTAGE TRANSFORMER Arthur E. Suowdon, Bristol, Conn., assignor to The Superior Electric (30., Bristol, Conn., a corporation of Connecticut Filed Apr. 27, 1956, Ser. No. 581,107

6 Claims. (Cl. '32343.5)

The present invention relates to an alternating current voltage transforming device, and more particularly to a variable or adjustable transformer or potential converter.

In variable voltage transformers, it is common to pro vide acoil on which a sliding contact rides in electrical engagement with the turns thereof for varying the output voltage. To assure electrical connection with the winding through all positions of travel of the brush, a plurality of turns of the winding generally have to be contacted simultaneously by the brush. This creates a condition of short-circuit within the winding causing objectionable heating of the brush and resulting in a limitation on the magnitude of the energy which could be transformed.

"- I The heating at the brush is the product of the circulating current and the voltage across the short-circuited turn. This latter is generally referred to as volts per turn and is found by dividing the voltage across the whole winding by the number of turns. As greater amounts of power are transformed, more current flows through the brush and in order to maintain the heating of the brush to a safe value, the volts per turn should decrease in proportion to the increase in current. However, transforming heavier current requires a large Wire size and to decease the volts per turn requires more'turns in the winding which consequently necessitates a larger core. As a result of such proportionality factors and limitations, the use of variable transformers for translating large amounts of power, i.e. in the magnitude of 10 k.v.-a. or more, has not been practical because of the magnitude of the equipment. Moreover, the present invention is applicable to magnitude ofenergy of less than 10 k.v. a, since it materially reduces the required size of the core and coils witha resulting decrease in the cost of manufacture and the production of a more compact unit. It is, accordingly, an. object of this invention to provide a variable voltage transforming unit for amounts of power having dimensions not heretofore possible in which the volts per turn and the resulting brush heating is maintained at a safe value.

-It is another object of this invention to provide in a voltage trans-forming unit of the type having two voltage transforming devices connected in series, one functioning to divide the initial supply voltage into substantially equal increments and the other device serving to regulate the desired portion of the increment which is included in the output voltage of the unit, in which the switching from one increment to another is independent of the load current so that the load currentis not conducted through the switching elements during the switching operation. [It is a further object of this. invention to provide in a variable voltage transforming unit of the above-disclosed type having a regulating transforming device divided into, at least two portions, in which the switching fromone portion of the regulating device to the other does not re{ quire an instantaneous snap actionand also in which the coils at the time of switching are substantially at the same potential and so that sparking 'at the contacts is obviated. ;;1t is another object of this invention to provide a varial" .1. if, i I r L United States Patent 0 ble voltage unit which is capable of effecting an extremely fine voltage regulation over an extended range with only a unidirectional movement of one control element required to vary the output voltage of the unit from the desired minimum to maximum.

A feature of this invention resides in having the input of a transformer, which may be either an autotransformer or an isolated transformer, connected to the AC. supply source and in which the output of the transformer has a plurality of connections for dividing the output voltage thereof into potential increments. A commutating coil having a winding divided into two distinct coils wound. in the same direction on a common core and the twocoils are selectively energized by switch means from adjacent increments of the output of the transformer. A movable tap rides on the coils in electrical engagement with the convolutions of the winding and forms one line of the output of the variable voltage transforming unit while the other line is a continuation of the supply source if the unit employs an autotransformer, or of the trans.- former output, if an isolated transformer is utilized.

When the movable tap is operating on the intermediate part of one of the coils then the other coil is switched from one increment to the next progressively alternating increment in order to provide for step by step adjustment of the output voltage of the unit between the desired minimum and maximum voltages. By utilizing such con: struction the volts per turn on the commutating coil is reduced to a value equal to the voltage in each increment divided by the number of turns in each portion of the coil. For example, if there are ten increments, then the voltage per turn on the coil will be only one-tenth of the value encountered in the transformation of a similar amount of electrical energy using only a coil of similar size with the same number of windings.

Other features and advantages of the invention will hereinafter appear.

In the accompanying drawing:

Figure 1 is a view partly in cross-section of the variable transformer comprising the present invention.

Fig. 2 is a partial view of the switching mechanism; and t Fig. 3 is a schematic diagram of the instant invention. The instant invention is shown in connection with'an autotransformer connectible to a source of electrical en: ergy and having its output connected to a commutating coil. A brush rides on the commutating coil for varying the output voltage of the device and switching means are utilized to connect the commutating coil to various por tions of the autotransformer. Though an autotransformer is shownand preferred, an isolated transformer having a primary and a tapped secondary may likewise be employed. i

Referring to the drawing, there is shown in-Fig 1 a variable voltagetransforming unit comprising the invention and generally indicated by the reference numeral 10: A base 11 has uprights 12 attached thereto and a cover plate 13 is supported by the uprights. In order to maintain the coils in spaced relation within the framework} mounting blocks 14 engageable with the exterior surface of the autotransformer 15 are employed andsimilarly, mounting blocks 16 maintain the commutating coil 17 in position. Both the autotransformer 15 and the commutating coil 17 having laminated annular cores of ferromag netic material, generally indicated by the reference numerals 18 and 19 respectively. Toroidally woiundon these cores 18 and 19 are windings 20 and 21,-20;being the autotransformer winding and 21 being the commuthe output voltage of the unit.

i shown, it is within the scope of this invention to employ motor driven means if such were found desirable. A dial 26 is positioned on the top surface of the cover plate 13 for cooperating with a pointer 27 mounted on the shaft 24 for giving a visible indication of the output voltage. The lower surface of the cover plate has a projection 29 which serves as a stop and cooperates with a stop arm 28 carried by the shaft 24 in order to limit the rotational movementof the shaft.

Movement of the handwheel causes the output voltage of the unit to vary and this is accomplished by utilizing a commutating surface 30 on the commutating coil 17. A brush 31 rides on the surface and is attached to an arm 32 which carries a slip ring brush 33 in electrical engagement with a slip ring 34. The arm 32 at its other end is attached to a hub 36 on the shaft 24 through an insulating ring 35. The hub 36 is freely rotatable on the shaft 24 and carries a gear 37 which meshes with a mating gear 38 rotatable on a pivot shaft 39. A small gear 40 is also rotatable on pivot shaft 39 and is attached to gear 38 to cause movement of the latter. A driving gear 40a is fixed to the shaft 24 in order to provide for movement of the arm 32. The ratio of the gears is dependent upon the number of contacts on the output of the autotransformer 15 for reasons hereinafter set forth, and in the instant embodiment with ten contacts the ratio is five to one, though of course with other numbers of contacts the ratio will vary therewith.

A switching mechanism, generally indicated by the reference numeral 41, is located axially of the cores and between them. While other switching mechanisms such as relays, a traveling nut and the like may be used, the specific embodiment of the switching mechanism shown is in the form of two axially aligned circular switch plates, a lower switch plate 42 and an upper switch plate 43. The two plates are maintained in spaced apart relation by spacers 41a equally spaced about the periphery of the plates 42 and 43. Support elements 41b are attached to the base plate 11 and the plate 42 to hold the unit in position. Between the plates 42 and 43, a rotor 44 is located for rotary movement about the adjacent inner surfaces of the plates. The rotor carries a plurality of contacts and is connected to the shaft 24 as by set screw 44a in order to be driven thereby.

Shown in Fig. 2 is a plan view of one of the switch plates and the rotor. Each switch plate has three annular tracks 45. Each track, according to the desired sequence of switching, may have insulating segments 46 and conducting segments 47. The rotor 44 has contacts 48 which move in these tracks to provide the necessary switching at all positions of the handwheel 25 and arm 32. By reason of the foregoing structure, movement of the handwheel 25 through an arc of about nine-tenths of a circle will vary the output voltage between its minimum and maximum output voltage and will move the rotor the same angular distance but will cause rotation of the arm 32 approximately five times as far in rotative travel or about four and one-half turns.

In Fig. 3 is shown the schematic diagram of the instart invention and contains three components; the tapped autotransformer 15, the switching means 41 and the commutating coil -17. The tapped transformer 15 is utilized to subdivide the input voltage into voltage increments and the switching means 41 selectively connects each winding of the commutating coil 17 to alternating increments so that only the voltage of one increment is impressed across each winding. The values of the voltage increments and the number of turns in each winding are such as to have a safe value of volts per turn on the winding. Thus if, for example, the number of turns in each winding is substantially different, then the values of voltage increments would likewise have to be substantially different in order to maintain a safe value of volts per turn. While the commutating coil 17 may have more than two windings, the specific embodiment of the commutating coil 17 has two windings having an equal number of turns. Accordingly, in order to maintain the same value of volts per turn on each winding, the autotransformer 15 has ten taps; 15-1, 152, 15-3, etc. for subdividing the input voltage into substantially equal increments. The number of taps employed involves a matter of choice depending on the value of the input voltage, the current and the size of the coil 17. Though the taps have been shown as dividing the total input voltage from zero to 12.5% above input voltage maximum, it is within the scope of the present invention to provide taps only on the portions of the winding of the autotransformer 15 which correspond to the desired range of the output voltage of the unit '10. The input of the autotransformer 15 is through the leads 15a and 15b.

The winding of the commutating coil 17 is broken so as to provide two, independent yet substantially equal, coils 17a and 17b. Both of these coils are wound in the same direction on the core 19 and since the voltage increments are equal they have substantially the same number of turns. The winding 21 encompasses substantially completely the annular core 19, i.e. throughout 360, so that there are no portions without turns and so that the brush 31 will at all times engage convolutions of the winding. The coil 17a has ends designated by a; and a while the ends of the coil 17b are b and b As previously set forth, the switching mechanism in the specific embodiment disclosed is provided with tracks having insulating segments 46 and conducting segments 47 For illustration purposes in Fig. 3, in order to aid in the explanation of the device, the annular tracks are schematically shown as being straight and having insulating segments indicated by the reference numeral 46 and conducting segments shown symbolically by full line rectangles. Each conducting segment is connected to one of the taps, 15-1, 15-2, 15-3, etc., and accordingly, are indicated by the reference numerals 47--1, 47-2, 473, etc. to show to which tap they are connected. The rotor contacts 48 are shown as shaded squares and since they are connected to the ends a a b and b reference characters 48-a 48-11 48-b and 48-]1 respectively are employed to indicate the contact which is connected to the appropriate end.

With such a switching mechanism, the location of the insulating segments, the conducting segments and the moving contacts are such as to cause connections between the taps and the windings in the following manner. Initially, with the pointer 27 indicating the output voltage of the unit set at zero, the coil 17a is connected between taps 15-1 and 15-2 by segments 47-1 and 47-2 being in engagement with contacts 48-a and 48-11 to ends a and a respectively. Fig. 3 illustrates this relation of the elements after the handwheel has been slightly moved from its zero position. Further rotation of the handwheel 25 causes the contacts to move upwardly on the tracks such that as the brush 31 approaches the ends a b the contacts 48-b and 48-11 will be in engagement with conducting segments 47-2 and 47-3 respectively so that the winding 17b will be energized by the voltage increment between the taps 15-2 and 15-3. Both coils 17a and 17b are energized while the brush 31 is in the immediate vicinity of the ends of the coils 17a and 17b and since the contacts 48-11 and 48b are on the same segment 47-2, these ends of the coils are at the same potential. Further clockwise movement of the handwheel 25 positions the brush 31 on the coil 17b,

, reason of contacts 48-a and segment 47-3 and contact 48-a and segment 47-4 being inengagement. With thebrush 31 adjacent the junction of the ends a and b, the coils are energized, 17a by taps 15-3 and 15-4 and 17b by taps 15-2 and 15-3. For the output voltage varying from zero to maximum, the coil 17a will be successively energized by the voltage increments between the taps 15-1 and 15-2, 15-3 and 15-4, 15-5 and 15-6, 15-7 and 15-8, and 15-9 and 15-10 while the coil 17b will be energized by the voltage increments between the taps 15-2 and 15-3, 15-4 and 15-5, 15-6 and 15-7, 15-8 and 15-9. The lower tap numbet is always connected to the end a when the coil 17a is energized and to the end b of the coil 17b when it is energized. Though the above structure increases the output voltage of the transforming device from zero to maximum as the brush 31 is rotated clockwise, it

, will be apparent that rotation in the opposite direction would decrease the output voltage.

The output of the variable voltage unit 10 is positioned between lines 53 and 54, the line 53 being connected to the tap -1 of the autotransformer 15, shown in the present embodiment, which is in actuality one line of the input to the unit, while the line 54 is connected to the arm 32 which carries the brush 31 riding on the commutating coil 17. If an isolated transformer is used the line 53. would be connectible to one output lead of the transformer; With the structure shown, the coil 17 and the brush arm 32 carry the output current of the unit and this output would create difliculties at the switching mechanism 41 when changing from one coil to the other. In order to obviate the problems which arise when the coil being switched from one set of taps to the next carries the output current, the present invention provides for switching of one coil when the other coil is in the output line. The switching is thus independent of the load, that is, the output does not pass through the switching means 41 at the time of switching the coils from one set of taps to another. Additionally, since the coils are wound in the same direction on the core 19 and since one coil is energized while the other is being switched there is no sparking at the switch contacts. This is so by reason of having an induced in the coil 17b, for example, when the arm 32 is operating on the coil 17a, which opposes the voltage in the increment and is substantially equal thereto with the consequent result that there is substantially no voltage difference therebetween and hence no sparking occurs at the contacts of the switching mechanism 41. The same effect occurs in the coil 17a when the arm is operating on the coil 17b.

As shown in Fig. 3, the turns of the coils 17a and 17b are substantially adjacent each other, thus at certain positions of the arm 32, the brush 31 may be in contact with one or more turns of the coil 17a and one or more turns of the coil 17b. By the present invention of having both coils energized there is no break or interruption in the output of the unit when the brush 31 changes from one coil to the other. Moreover, the present invention obviates the necessity of having instantaneous switching when changing from one coil to the other. With instantaneous switching, it is apparent that if the desired voltage output is of such a value as to require the brush to be located at either of these two junction points on the winding 17, there would be a resulting constant switching back and forth or hunting from one coil to the other with consequent detrimental result thereto. According to the present invention, when the brush 31 is operating on one coil and as it approaches the junction of the ends of the coils, the other coil is energized so that both windings are energized when the brush is in the immediate vicinity of the junctions. This is accomplished by having both coils 17a and 17b wound on the same core and in the same direction and having the cur rent (i.e., instantaneous values thereof being considered) 6 flow in the coil in the same direction so that the magnetic flux produced thereby is additive. The effect of such construction is to make a single autotransformer of the commutating coil 17 and to impress on it the voltage between two adjacent taps. For example, with the brush 31 on the coil 17a and adjacent the end b of the coil 17b, both a and b are connected to the same tap on the autotransformer 15.

From the foregoing, it will be appreciated that there has been disclosed a variable voltage transforming unit capable of translating electrical energy which is of a compact, practical and economical size as compared to similar units for transforming the same amount of energy and yet which overcomes heating of the movable brush element above a safe value. A first transforming device breaks down the output voltage into substantially equal increments and then a second transforming device having two substantially similar coils enables step by step energization with progressive alternate increments to provide for the variation in the magnitude of the output voltage. The use of two similar coils permits the switching of one coil when it is not being utilized in the circuit with the advantages of no sparking at the contacts, independence from the load current and the obviating of a complex instantaneous switching arrangement.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. A variable voltage transforming unit comprising a first voltage transforming device having an output and an input connectible to a source of alternating current; means for providing voltage increments from the output; a second-voltage transforming device having a winding formed into at least two separate coils wound in the same direction on a closed magnetic core; switch means for electrically connecting at least one of the coils to an increment of the output of the first voltage transforming device; a contactor for electrical engagement with a se-- lective portion of the electrically connected coil; and, means, forming an output of the unit, connected to the: contactor and the first voltage transforming device.

2. A variable voltage transforming unit comprising a. first voltage transforming device having an output and an! input connectible to a source of alternating current; means for providing substantially equal voltage increments from the output; a second voltage transforming device having a winding formed into at least two similar, separate coils wound on a closed magnetic core in the same direction; switch means for electrically connecting at least one coil to an increment of the output of the first voltage transforming device; a contactor movable along the winding and in engagement with the turns thereof; and means, forming an output of the unit, connected to the movablecontactor and the first voltage transforming device.

3. A variable voltage transforming unit comprising a first voltage transforming device having an output and an input connectible to a source of alternating current; means for providing substantially equal voltage increments from the output; a second voltage transforming device having a winding formed into two similarly wound, independent coils wound on a closed magnetic core; a contactor movable along the winding in electrical engagement with the turns thereof; switch means for connecting and disconnecting the coils to increments of the output of the first voltage transforming device and for disconnecting one coil from its increment of the output while the contactor is in engagement with the intermediate portion of the other connected coil; and means, forming an output of the unit, connected to the movable contactor and the first voltage transforming device.

4. A variable voltage transforming unit comprising a first voltage transforming device having an output and an input connectible to a source of alternating current; means for providing substantially equal voltage increments: from the output; a second voltage transforming device having a winding formed into two similar, independent coils wound on a closed magnetic core; a. contactor movable along the winding in electrical, engagement with the turns thereof; switch means for connecting and disconnecting the coils to increments of the output of the first voltage transforming device and for maintaining one coil disconnected from the output of the first voltage transforming device when the movable contactor is in an intermediate portion of the other coil; and means, forming an output of the unit, connected to the movable contactor and the first voltage transforming device.

5. A, variable voltage transforming unit comprising a first voltage transforming device having an output and an input connectible to a source of alternating current; means for providing substantially equal voltage increments from the output; a second voltage transforming device having a winding formed into at least two similar, independent coils, said coils being wound in the same direction on a closed magnetic core; a contactor movable, along the winding in electrical engagement With the turns thereof; switch means for causing connection of both coils to different increments of the output so that the magnetic flux caused by each coil in the core is additive at at least one position of the contactor; and means, forming an output of the unit, connected to the movable contactor and the first voltage transformingdevice.

6. A variable voltage transforming unit comprising a first voltage transforming device having an output and an input connectible to a source of alternating current; taps for providing substantially equal voltage increments from the output; a second voltage transforming devicehaving a winding formed into at least two similar, independent coils, said coils being wound in the same direction' on a closed magnetic core and having at least one adjacent end portion; a contactor movable along the winding and in electrical engagement with the turns thereof; switch means for connecting each coil to adjacent increments with the adjacent end portion of each coil being connected to the same tap; and means,. forming.

an outputv of the unit, connected to the contact and: the first voltage transforming device.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent Na; 2 37m476 March 21 1961 Arthur E. Snowdon It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 42 and 45,, for "=k.v. a-" each occurrence read KV-a column 4, line 29 for "a read a column 8 line 20, list of references cited, under UNITED STATES PATENTS, for "2572,1154" r d m. 2 5725 in Signed and sealed this 29th day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer I Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent N-d-w 2,97m476 March 21,, 1961 Arthur E. Snowdon It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 42 and 45,; for "k.V,'a each occurrence, read KV-a column 4, line 29 for "a read a column 8, line 20 list of references cited, under UNITED STATES PATENTS for "25724454" read 2572 545 Signed and sealed this 29th day of August, 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

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