US3239749A - Transformer system - Google Patents

Description

March 8, 1966 R. L. ORIEZ 3,239,749

TRANSFORMER SYSTEM Original Filed Oct. 17, 1960 2 Sheets-Sheet l AC. Fame-A Jomece' [In/6176b)": aberdL. 27292,

March 8, 1966 QRIEZ TRANSFORMER SYSTEM 2 Sheets-Sheet 2 Original Filed Oct. 1'7, 1960 mnno Hummus Z. i M

Attorney United States Patent 3,239,749 TRANSFORMER SYSTEM Robert L. Oriez, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Continuation of application Ser. No. 63,104, Get. 17, 1960. This application July 6, 1964, Ser. No. 382,687 3 Claims. (Cl. 323-435) My invention relates to transformers, and more particularly to transformer systems having high current ratings and employing tap changing switches. This application is a continuation of my copending application Serial No. 63,104, filed October 17, 1960, for Transformer System, and now abandoned.

In supplying power to electrical equipment, such as induction and resistance furnaces, which may require currents ranging from about 200 to 1600 amperes, it is fre quently necessary to provide a variable voltage output. In many applications tap changing switches are employed in the transformer system to provide the variable voltage required by the electric furnace. The voltage is varied by selectively picking off the desired voltage from taps provided on the secondary winding of a main transformer. In the past, one or more tap changing switches having stationary contacts connected to the taps on the secondary winding and having movable contacts engageably associated with the stationary contacts were employed to provide the desired voltage variations at the output terminals connected to the movable contacts. The tap changing switches utilized in a particular transformer system must be able, of course, to withstand the high current, and the transformer system must provide a specified range of voltages in specified increments at the output terminals.

The tap changing switches of said prior art transformer systems were bulky and expensive to manufacture because of the high current at the tap changing switches. Also, in many applications, because of the many taps required in a secondary winding to provide the voltage increments and the low nominal secondary voltage, the available volts per turn has not in some cases been sufficient to provide economically the voltage increments required at the output terminals. It is evident, therefore, that there is a need for a transformer system which will allow a greater degree of flexibility in the choice of current rating for the tap changing switch so that switches having a lower current capacity can be employed in a given application as compared with prior art systems. Further, it is desirable that such a transformer system also permits the use of higher nominal secondary voltages in order to provide a greater degree of flexibility in the selection of taps.

Accordingly, a general object of the invention is to provide an improved transformer system that will achieve the desired results set forth above.

Another object of the invention is to provide an improved transformer system utilizing tap changing switches of lower rated current capacity as compared with transformer systems having the same rated power output and employing only tap changing switches in the secondary of a main transformer.

It is a more specific object of the invention to provide an improved transformer system wherein higher nominal secondary voltages can be employed as compared with transformer systems having the same rated power output and employing only tap changing switches in the secondary of the main transformer.

A further object of the invention is to provide an improved transformer system that can be more economically manufactured than transformer systems having the same rated power output and employing only tap changing switches in the secondary of the main transformer.

In general, according to the invention, I provide a transformer system comprising a main transformer having at least one primary winding for connection with an alternating current source and at least one secondary winding which has a plurality of taps. Taps on the secondary winding and one end of the secondary winding are connected to the stationary contacts of at least one tap changing switch which has a movable contact. The movable contact is connected to one end of an autotransformer. The other end of the autotransformer is connected to the other end of the secondary winding of the main transformer. Such an arrangement provides an increased nominal voltage in the secondary winding of the main transformer and a decreased current at the tap changing switches as compared with a transformer system having the same rated power output and having only tap changing switches in the secondary of the main transformer. The autotransformer has a common winding or secondary to which the output leads are connected.

In another aspect of the invention, I have provided a transformer system which comprises a main transformer having at least one primary winding for connection with an alternating current source and at least a secondary winding with a plurality of taps. The taps and the ends of the secondary winding are connected to the stationary contacts of a fine control tap changing switch and a coarse control tap changing switch, each having a movable contact. The movable contact of the fine control tap changing switch is connected to one end of an autotransformer and the movable contact of the coarse control switch is connected to the other end of the autotransformer. A pair of output leads connected to a common winding on the autotransformer are provided for connection to a load.

The subject matter which I regard as my invention is set forth in the appended claims. The invention, however, together with further objects and advantages thereof may be understood by referring to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a schematic circuit diagram illustrating one embodiment of the invention;

FIGURE 2 is a schematic circuit diagram illustrating another embodiment of the invention;

FIGURE 3 is a schematic circuit diagram illustrating another embodiment of the invention showing a three phase application of the circuit illustrated in FIGURE 1 wherein the power to the load on each phase is controlled independently of the other two phases;

FIGURE 4 is a schematic circuit diagram of a three phase transformer system of the invention having combined phase control for use with a delta or Y connected load without a grounded neutral; and

FIGURE 5 is a schematic circuit diagram of a three phase transformer system of the present invention with combined phase control intended for use with a load as a four-wire system having a neutral connection and being Y connected.

Referring now to FIGURE 1, I have shown therein a transformer system which includes a main transformer 11 having a primary winding P and a secondary winding S inductively coupled on a magnetic core 14. A pair of input leads 15, 16 are provided at the end of the primary winding P for connection to an alternating current power source as shown. The secondary winding S is provided with leads 17, 18 and with a plurality of taps having leads 19, 20, 21, 22, 26, which leads are connected to the stationary contacts of a pair of tap changing switches 24, 25. The tap changing switches 24, 25 are provided with movable arms or contacts 26, 27 which are selectively engageable with the stationary contacts. The movable contacts 26, 27 are connected in the primary circuit ,of an autotransformer 30 by leads 28, 2 9. The primary circuit of autotransformer 30 includes the leads 28, 29 and the autotransformer winding A disposed on a magnetic core 31. The autotransformer 30 includes a secondary circuit which is comprised of the common winding C and the output terminal leads 32, 33 provided for connection to a load as shown. The common winding C obviously is a portion of the autotransformer winding A.

It will be seen that the autotransformer 30 of the transformer system of the invention steps down the voltage impressed across the leads 28, 29. Thus, as the movable contacts or contact arms 26, 27 of the tap changing switches 24, are changed, the voltage across leads 28, 29 is varied. The number of diiferent voltage variations possible is equal to the product of the number of stationary contacts associated with the switch 24 multiplied by the number of stationary contacts associated with the switch 25. The number of taps per switch is limited by the available volts per turn of the secondary winding S.

Since the autotransformer is connected so that the voltage across the movable contacts 26, 27 is stepped down, the secondary winding S of the main transformer 11 can be designed for a higher nominal voltage across the secondary winding S. Thus, for a given value of voltage desired across the output terminals 32, 33, the transformer system of the present invention, the corresponding value of the voltage across the movable contacts 26, 27 is greater thereby making it possible to utilize a secondary winding S having more turns as compared to a transformer system of the same rated power output employing only tap changing switches in the secondary winding. Thus, the autotransformer arrangement of the invention provides the advantage of greater flexibility in the selection of an economical volts per turn for the secondary winding. Further, since the secondary current of an autotransformer, neglecting the resistance and reactance of the winding and the magnetizing current, is equal to the ratio of the number of turns in the primary circuit to the number of turns in the secondary circuit multiplied by the primary current, it will be seen that the current at the tap changing switches 24, 25 is less than the current at the output leads 32, 3-3. Thus, the transformer arrangement of the invention makes it possible to design the tap changing switches 24, 25 for lesser maximum current as compared with a transformer system of the same rated power output employing only tap changing switches. By being able to employ tap changing switches of lower current capacity, it is possible to achieve substantial savings in cost and a reduction in weight and space requirements for the switches. The added expense of the autotransformer 30 is more than oifset by the savings which result from the use of a tap changing switch of reduced current capacity.

The transformer system of my invention permits the designer to take advantage of the fact that the conditions of highest core loss and highest coil loss occur at opposite ends of the operating range of the output voltages. Further, with the autotransformer connection small variations in the nominal secondary voltage can be made by changes in the autotransformer and not in the main transformer. Thus, it is possible to utilize the same design of a main transformer for a greater number of applications and thereby realize a savings in design time.

In FIGURE 2 there is illustrated another modification of the transformer system of the invention wherein a main transformer 35 is adapted for connection to either a 220 or 440 volt power supply and wherein only one tap changing switch 36 is employed. As shown in FIGURE 2, the main transformer 35 includes a pair of primary windings P P and a secondary winding S inductively coupled on a magnetic core 37. The primary windings P P are provided with four input leads 38, 3-9, 40, 41. For connection with a 220 volt commercial power supply, lead 38 is connected to lead 40 and lead 39 is connected to lead 41. High voltage connections to the power supply are made with leads 33, 411. For connection across a 440 volt commercial alternating current supply, lead 3-9 is connected to lead 40, and the high voltage connections from the power supply are made to leads 38, 411.

The secondary winding S is provided with a plurality of taps which are connected to stationary contacts of the tap changing switch 36 having a movable contact 42. One end of the secondary winding of the main transformer and the movable contact 42 of the tap changing switch 36 are connected across the input of a step-down autotransformer 43 by means of the autotransformer leads 44, 45. The autotransformer 43 has a primary circuit which includes an autotransformer winding A and the V leads 44, 45 and a secondary circuit which includes a common winding C and output terminal leads 46, 47 for connection to a load circuit. In operation, as the movable contact or contact arm 42 is adjusted, portions of the secondary winding S between a selected lead and taps are connected in circuit with the autotransformer winding A Thus, the voltage applied across the autotransformer winding A is selectively varied. It will be seen that the transformer system shown in FIGURE 2 is similar in operation to the transformer system illustrated in FIG- URE 1 and affords the same advantages which were described in connection with the latter system.

In FIGURES 3, 4 and 5 I have illustrated the trans former system of my invention in three-phase applications. Referring now specifically to the schematic circuit diagram of FIGURE 3, there is shown a three-phase transformer system which provides output leads 49, 50, 511, 52, 53, 54 connected with three separate single phase loads (not shown), the load on each phase being controlled independently of the other two phases. Main transformer 55 has a magnetic core (not shown) on which primary windings P P and P and secondary windings S S and S are inductively coupled. Although, as shown in FIGURE 3, the primary windings P P P are delta connected, it will be appreciated that any of the transformer connections commonly used in power services which permit unbalanced loading of the secondary may be used. The secondaries S S S of the main transformer 55 are each separately connected across the autotransformer windings A A A If desired the secondaries S S S may be Y connected to a neutral as in a four-wire system.

Input leads 56, 57, 58 are provided for connecting the primary windings P10, P P to a three phase power source. The secondary windings S S S each have five leads brought out from taps and two leads connected to ends of each secondary S S S which are connected to the stationary contacts of tap changing switches T10! T40! T20, T50 T30 T60 The autotransformer windings A A A have three single phase cores, mounted either separately or with a common base but magnetically independent of each! other. Common windings C C 0, C are connected across output leads SI and 52, 53 and 54, 49 and 50, respectively. As in the single phase autotransformer of FIGURE 1, the leads 6t), 61, 62 brought out from one end of autotransformer windings A A A are connected with movable contacts of the tap changing switches T T T respectively. Leads 63, 64, 65 brought out from the other end of the autotransformer windings A A A are connected to the tap changing switches T T T 0 respectively in circuit with the output terminal leads 52, 53, 50.

It will be appreciated that the connections to the load are made from leads 49, 50, 51, 52, 53, 54 as if there were three separate single phase loads, and the power to each phase is controlled separately. If secondaries. S S S are connected with a neutral by joining; the movable contacts of tap changing switches T T T at a common junction, the power supplied to the load on each phase can also be controlled independently of.

the other two phases. Each of the separate phases are individually controlled by their respective pairs of tap changing switches T and T T and T 0, and T and T The tap changing switches T T T provide a coarse control while the tap changing switches T T and T provide a fine control of the voltage. Thus, in this transformer system the voltages of each individual phase is independently controlled of the other two phases. As in the single phase circuits of this invention, the autotransformer arrangement in accordance with the inven tion makes it possible to reduce the size and current carrying capacity of the tap changing switches and to achieve the advantage of more economical volts per turn in the secondary windings due to the increased nominal voltages in the secondary windings.

In FIGURE 4 I have illustrated a schematic circuit diagram of a three phase transformer system in accordance with the invention having combined phase control and intended for use in connection with a three-phase, three-wire connected load. Primary windings P P P of main transformer 66 are inductively coupled on a magnetic core (not shown) with secondary windings S S S As in the circuit shown in FIGURE 3, the primary windings P P P are delta connected. Although the primary windings P P 1, P are preferably delta connected, it will be appreciated that any of the well known transformer primary connections can be used. The secondaries S S S of the main transformer 66 are delta connected. Although pairs of tap changing switches T and T T and T51, and T and T are shown connected with the secondary windings S S S it will be appreciated that a single tap changing switch can be used for each phase.

It is to be understood that the fine control tap changing switches T T and T must be gang operated. Also, the coarse control tap changing switches T T and T associated with secondary winding S S S must be gang operated from a single control. As in the other circuits of this invention, the secondary windings S S S are provided with five taps and leads brought out from the taps and the two end leads which are connected to stationary contacts of the tap changing switches T T21, T T T T It will be appreciated that additional taps or fewer taps may be provided as required by particular applications. Autotransformer windings A A A are inductively coupled on a magnetic core (not shown) with common windings C C C respectively. One end of autotransformer winding A A A is connected to the common junction N. Leads 70, 71, 72 connect the other ends of the autotransformer windings A A A with the movable contacts of the tap changing switches T and T T and T T and T respectively. It will be noted that no neutral connection either to ground or to the load is provided in the transformer system shown in FIGURE 4. Input leads 67, 68, 69 are provided for connecting the primary windings P P P across a three phase power source. Output leads 73, 74, 75 provide a three-wire connection for the load.

In addition to the hereinbefore described advantages of the invention, the transformer system of FIGURE 4 provides an additional advantage resulting from the use of the delta connected secondaries S S S of the main transformer 66. The delta connection makes it possible for the tap changing switches T T21, T T T T to carry the phase current which is approximately .577 times the line current. Thus, the use of a delta connected secondary in a main transformer permits either a further reduction in the required current capacity of the tap changing switches employed or makes it possible to decrease the size of the autotransformer used in the transformer system of the invention.

In FIGURE 5 I have illustrated the transformer system of the invention for three phase, four-wire applications. A main transformer 76 has a magnetic core (not shown) on which a primary comprising primary windings P P P are inductively coupled with a secondary comprising Y-connected secondaries S S S The primary windings P P P are delta connected and are provided with input leads 82, 83, 84. A neutral connection N to which the secondary windings S S S are connected through tap changing switches T T T is connected by lead 77 with neutral point N to which autotransformer windings A A A are connected. The autotransformer windings A A A and common windings C C C are inductively coupled on a common core (not shown). Tap changing switches T T T provide coarse control of the voltage output and have stationary contacts connected to two taps and an end lead of the secondary winding S S S Fine control of the voltage output at output leads 78, 79, 80, 81 is obtained by tap changing switches T T T each having stationary contacts connecting with three taps and an end lead one of the secondary windings S S S The movable taps of tap changing switches T T T are connected to a neutral connection N while the movable contacts of changing switches T T T are connected to the ends of the autotransformer windings A A and A respectively.

The operation of the transformer system shown in FIG- URE 5 is substantially similar in principle to the other three phase transformer systems of FIGURE 4. When input terminals 82, 83, 84 are connected to a three-phase power supply and the output terminals are connected to a Y-connected load (not shown), fine control of the phase voltage is obtained by the tap changing switches T T T which must be ganged to provide for combined phase control. Similarly, coarse control of the voltage is achieved by tap changing switches T T T which must also be ganged to provide for combined phase control. These voltage variations are accomplished with reduced current at the switches T T T T T52, T and a higher nominal voltage across the connected portions of the secondary windings S S S as compared with transformer systems of the prior art of the same rated power output employing only tap changing switches in the secondary winding.

As an illustrative example of a specific transformer system in accordance with the invention, I have con structed a main transformer as shown in FIGURE 1 having a primary winding of 116 turns of aluminum wire having a cross-sectional of .08968 square inch and a secondary winding having 36 turns of aluminum wire, 20 turns having a cross-sectional area of .4216 square inch and 16 turns having a cross-sectional area of .2845 square inch. The 20 turns provided a nominal voltage of 75.8 volts at switches 24, 25 which was reduced by autotransformer 30 to 65.5 volts while the 16 turns provided percent of the nominal voltage. In order to obtain the widest range of selectable voltages with the smallest number of fine and coarse increments (an increment being a voltage or a number of turns), each coarse increment should be equal to the fine increment multiplied by the number of fine increments plus one. Expressed in another way, each coarse increment should be equal to the fine increment multiplied by the number of fine taps. Referring again to FIGURE 1, the fine taps were located so as to provide two turns between leads 17 and 19, between leads 19 and 20 and between leads 20 and 21. With the relation given above, the coarse taps were located at increments of two turns multiplied by three increments plus one, or eight turns between leads .18 and 23 and between leads 23 and 22. Thus, between leads 21 and 22, 14 turns were left. With the input leads 15, 16 connected to 220 volts alternating current course, the transformer system was designed to provide a nominal voltage of 65.5 volts at the output leads 32, 33 when the movable contacts 26, 27 of the tap changing switches 24, 25 were positioned so as to engage the stationary contacts connected to leads 17, 22, respectively. By selectively shifting the position of tap changing switches 25,

26, it was possible to vary the output voltage from 70 to 180 percent of the specified nominal voltage of 65.5, the voltages being available in ten percent increments. The twelve (four fine taps times three coarse taps) voltages obtainable at the output leads 32, 33 of the autotransformer for various switch positions identified by the reference numerals of the leads are summarized in Table I below:

For the particular application for which the transformer system of the illustrative example was adapted, it was found that the maximum current at the output terminals 32, 33 was 417 amperes for the transformer system which was designed for a rated power output of 27% k.v.a. Without the autotransformer connection of the present invention for such an application, it will be appreciated that tap changing switches would be required to handle a maximum of 417 amps. Since commercial manufactured tap changing switches, suitable for such applications, are currently available in ratings of 200, 400 and 800 amperes, the transformer system using only switches in the secondary would require the switches having an 800 ampere rating. With the autotransformer connection of the invention it will be seen that maximum current at the tap changing switches 24, corresponding to 417 amperes at the output terminals 32, 33 was 360 amperes. In the transformer system of the invention it was therefore possible to use tap changing switches with a rating of 400 amperes. Also, it was possible to proportionately increase the nominal voltage across the secondary windings S of the main transformer 11 to make available a more practicable volts per turns. In view of the limited number of turns (a total of 36 turns) of the secondary winding, it will be appreciated that the available volts per turn poses a problem in the design of such transformer systems. By reducing the maximum current at the switches from 417 to 360 amperes it was possible to effect a substantial savings in cost, to reduce the space requirements of the switches and to decrease the overall weight of the unit. The additional cost of the autotransformer represented less than 50 percent of the saving resulting from the lower cost of the switches having a rated capacity of 400 amperes. Also, it will be seen that the autotransformer connected in accordance with the invention permits greater flexibility in design to the extent that the voltage of the secondary winding S of the main transformer 11 can be picked to provide exact ratios on the taps at an economical volts per turn and that small variations in the nominal secondary voltage required for different applications may be readily effected by changes in design of the autotransformer 30 and thereby make it possible to use the same main transformer design for different applications.

While I have described and illustrated my invention with reference to several particular embodiments of the invention, it will be readily apparent to those skilled in the art that many modifications may be made. It is to be understood, therefore, that I intend by the appended claims to cover all such modifications that fall Within the true spirit and scope of the invention.

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

1. A transformer system comprising: an alternating current source, a main step-up transformer having at least one primary winding connected across the alternating current source and at least one isolated secondary winding having a plurality of taps spaced at relatively fine and relatively coarse increments therealong, a first tap changing switch, a second tap changing switch, each of said tap changing switches having a plurality of stationary contacts and having a movable contact arm for selectively engaging its stationary contacts, means connecting said finely spaced taps to said stationary contacts of said first switch and said coarsely spaced taps to said stationary contacts of said second switch, and a step-down auxiliary autotransformer having an autotransformer Winding connected across said movable contact arms and a common winding, said common winding having a pair of output leads for connection with a load and the current supplied at said output leads during operation being greater than the current at said tap changing switches.

2. A transformer system comprising: an alternating current source, a load for energization from said alternating current source, a main step-up transformer having a primary winding connected across the alternating current source and an isolated secondary winding having a first and a second set 01 taps, said first set of taps being spaced at relatively large increments and said second set of taps being spaced at relatively small increments, a first tap changing switch including a plurality of stationary contacts connected in electrical circuit with said first set of taps to provide a coarse selection of voltage, a second tap changing switch having a plurality of stationary contacts connected in electrical circuit with said second set of taps to provide a fine selection of voltage, each of said tap changing switches having a movable arm for selectively engaging the stationary contacts of the tap changing switch, a step-down auxiliary autotransformer having an autotransformer winding connected across the movable arms of said tap changing switches, and output means coupling said load to a common Winding portion of said autotransformer winding to supply load current that is greater than the current at the tap changing switches.

3. A transformer system comprising a main step-up transformer having at least one primary winding for connection across an alternating current source and at least one isolated secondary winding having a plurality of first taps coupled to one end of said secondary winding and at relatively fine increments therefrom and a plurality of second taps coupled to the other end of said secondary winding and at relatively coarse increments therefrom, each of said coarse increments being equal to one of said fine increments multiplied by the number of said first taps, a first tap changing switch having a plurality of first stationary contacts and a movable contact arm for selectively engaging said first stationary contacts, a second tap changing switch having a plurality of second stationary contacts and a movable contact arm for selectively engaging said second stationary contacts, means respectively coupling said first taps to said first stationary contacts, means respectively coupling said second taps to said second stationary contacts, a step-down auxiliary autotransformer having a primary circuit and a secondary circuit, said primary circuit including an autotransformer winding and a lead extending from each end thereof, one of said leads of said autotransformer Winding being connected to said movable contact arm of said first tap changing switch to provide a fine control for the voltage at the output of said autotransformer and the other of the leads of said autotransformer winding being connected to said movable contact arm of said second tap changing switch to provide a coarse control for said voltage, said secondary circuit including a common winding of the autotransformer, and a pair of output terminals connected to said common winding for connection with a load circuit, the current supplied at said output terminals during operation being greater than the current at said tap changing switches.

References Cited by the Examiner UNITED STATES PATENTS 726,391 4/1903 Armstrong et al. 32343.5 1,422,653 7/1922 Berry 323-43.5

Uphoif 32343.5

Ramondo 32343.5 XR

Weyl 32343.5

Biermanns 323-43.5

Morrison 32347 XR Prescott 323-435 Annis 323-435 10 LLOYD MCCOLLUM, Primary Examiner.

W. E. RAY, Assistant Examiner.

Claims (1)

1. A TRANSFORMER SYSTEM COMPRISING: AN ALTERNATING CURRENT SOURCE, A MAIN STEP-UP TRANSFORMER HAVING AT LEAST ONE PRIMARY WINDING CONNECTED ACROSS THE ALTERNATING CURRENT SOURCE AND AT LEAST ONE ISOLATED SECONDARY WINDING HAVING A PLURALITY OF TAPS SPACED AT RELATIVELY FINE AND RELATIVELY COARSE INCREMENTS THEREALONG, A FIRST TAP CHANGING SWITCH, A SECOND TAP CHANGING SWITCH, EACH OF SAID TAP CHANGING SWITCHES HAVING A PLURALITY OF STATIONARY CONTACTS AND HAVING A MOVABLE CONTACT ARM FOR SELECTIVELY ENGAGING ITS STATIONARY CONTACTS, MEANS CONNECTING SAID FINELY SPACED TAPS TO SAID STATIONARY CONTACTS OF SAID FIRST SWITCH AND SAID COARSELY SPACED TAPS TO SAID STATIONARY CONTACTS OF SAID SECOND SWITCH, AND A STEP-DOWN AUXILIARY AUTOTRANSFORMER HAVING AN AUTOTRANSFORMER WINDING CONNECTED ACROSS SAID MOVABLE CONTACT ARMS AND A COMMON WINDING, SAID COMMON WINDING HAVING A PAIR OF OUTPUT LEADS FOR CONNECTION WITH A LOAD AND THE CURRENT SUPPLIED AT SAID OUTPUT LEADS DURING OPERATION BEING GREATER THAN THE CURRENT AT SAID TAP CHANGING SWITCHES.

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