US2780768A - Load tap changing circuit - Google Patents

Description

1957 w. c. SEALEY 2,780,768

/ LOAD TAP CHANGING CIRCUIT Filed Nov." '7, 1955 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 62 ,3 6w mm United States Patent LOAD TAP CHANGING CIRCUIT William C. Sealey, Wauwatosa, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

Application November 7, 1955, Serial No. 545,98

5 Claims. (Cl. 323--43.5)

This invention relates in general to load tap changing transformers and more particularly to load tap changing circuits which employ a preventive autotransformer or a tapped reactor.

Load tap changing transformers are used extensively in electrical systems where it is desired to maintain the load voltage between predetermined limits, referred to as the band width. Most load tap changing transformers include a winding which is provided with a plurality of electrically spaced taps and a tap changing switch having a plurality of spaced stationary contacts each of which is connected to a different tap on the transformer winding. The switch further includes a pair of movable contacts which cooperate with the stationary contacts to define a plurality of operating positions of the transformer. The movable contacts are spaced in accordance with the spacing of the stationary contacts so that both movable contacts may be positioned on one stationary contact or be positioned on adjacent stationary contacts in a bridging relationship thereto. An impedance, usually a reactor or a preventive autotransformer, is connected between the movable contacts to limit the flow of circulating current when the movable contacts are in the bridging position which occurs at alternate operating positions of the load tap changing transformer.

The reactor is connected to an output terminal of the transformer either directly or through a series transformer depending on the particular current and voltage rating of the tap changing transformer.

The tap changing switch is usually motor operated so that the movable contacts of the switch may be moved to change the output voltage of the transformer as required. Most often the output voltage of the tap changing trans former is maintained between predetermined limits automatically by action of a motor control circuit responsive to the load voltage although manual operation may be used if desired.

In the past the connection from the load terminal of the tap changing transformer to the reactor was always made to the midtap of the reactor so that an equal number of turns of the reactor were disposed between each movable contact and the midtap.

Such an arrangement is satisfactory where equal output voltage steps of the transformer are not necessary and not required. However, where it is desired to have the output voltage of the transformer varied in equal voltage steps by the tap changing switch such an arrangement is unsuccessful because the exciting current of the reactor is different for a bridging position than for a nonbridging position. When the movable contacts are in a bridging position the reactor draws exciting current which causes a voltage drop in the main transformer. However, when the movable contacts are in the nonbridging position, little or no exciting current is drawn by the reactor and hence causes only a negligible voltage drop in the main transformer. It is this difference in voltage drop on adjacent operating positions of the transformer which cause adjacent voltage steps to be unequal. The size of successive voltage steps will be alternately greater than half the voltage between physical taps and less than half this value with the pattern repeated over the operating range of the tap changing transformer.

It has been found according to the present invention that the voltage steps of the load tap changing transformer employing a reactor or preventive autotransformer may be made equal by tapping the reactor or pereventive-auto oif center sufiiciently to compensate for the voltage drop caused by its exciting current.

It is therefore an object of the present invention to provide a load tap changing transformer having equal output voltage steps.

A further object of the present invention is to provide a load tap changing transformer employing a reactor or preventive-auto in which the output voltage steps of the transformer are equal.

A still further object of the present invention is to provide a step type voltage regulating system which is capable of maintaining the load voltage of the system within a relatively narrow band without excessive hunting.

Objects and advantages. other than those mentioned above will be apparent from the following description when read in connection with the drawing in which:

Fig. l is a diagrammatic view of a step type voltage regulating system having a load tap changing circuit embodying the present invention;

Fig. 2 is a schematic view of the system shown in Fig. 1;

Fig. 3 is a chart illustrating the equal voltage steps obtained by the system shown in Figs. 1 and 2; and

Fig. 4 is a chart similar to Fig. 3 illustrating the unequal voltage steps which would be obtained with the use of a midtapped reactor.

Referring to the drawing the step type voltage regulating system shown in Fig. 1 comprises a supply circuit 11, a load circuit 12, and a tap changing under load transformer 13 for maintaining the voltage of load circuit 12 between predetermined limits independently of changes in the voltage supply circuit 11.

Load tap changing transformer 13 may be of any suitable type known in the art which has a tap changing switch connected to the output terminal of the transformer through a reactor or preventive-auto for facilitating changing taps under load. The load tap changing transformer shown diagrammatically in the preferred embodiment is of the general type disclosed by L. H. Hill in U. S. 2,177,109, October 24, 1939, Voltage Regulator, which is assigned to the assignee of the present invention. Load tap changing transformer 13 comprises an exciting winding 14 provided with a plurality of taps 15 between any electrically adjacent pair of which the potential difference is the same, a tap changing switch 16, a reactor 17 and a series transformer 18. Tap changing switch 16 comprises a plurality of spaced stationary contacts 19 which may be arranged in a circle as shown with each contact 19 connected to a different tap 15 of winding 14. Switch 16 further includes a pair of movable contacts 20 which are spaced apart a distance corresponding to the spacing of the stationary contacts 19 so that both movable con tacts 20 may be disposed on one stationary contact or in a bridgin relationship with a pair of adjacent stationary contacts 19. Means are provided for moving contacts 20 in steps from one stationary contact to the next in a predetermined manner and as shown this means comprises a reversible motor 23. If desired the switch 16 may be provided with a detent means for changing the relatively slow uniform rotation of the motor to a rapid snap action of the movable contacts but since such detent means comprices no part of the present invention is has not been illustrated in the drawing.

In the preferred embodiment of the system shown the motor is provided with a suitable control circuit 24 which preferably includes an electroresponsive device 25, such as a contact making voltmeter, which is responsive to the voltage of the load circuit 12.

Since movable contacts are disposed in a bridging relationship with respect to adjacent stationary contacts on alternate operating positions of the load tap changing transformer 13, a tapped reactor 17 is provided in the tap changing circuit to limit the flow of circulating current when the switch is in the bridging position. The tap 2'7 of the reactor is connected to one end of the primary winding 28 of the series transformer 18 which supplies a buck or boost voltage to the load circuit 12 depending on the position of reversing switch 29.

The tap 27 of reactor 17 is disposed oti center a predetermined amount to compensate to a predetermined extent for the voltage drop in exciting winding 14 resulting from the flow of exciting current in reactor 17 when switch 16 is in a bridging position. Tap 27 is disposed off center towards the end of the reactor 17 which is connected to the contact having the higher potential when the switch 16 is in a bridging position.

The operation of the system shown in Fig. 1 together with the improved results obtained by providing an off center tap 27 on reactor 17 may be more readily seen by referring to Figs. 2, 3 and 4. Assume, in Fig. 2 that the voltage between taps is 2.5 percent of the total winding voltage and that the voltage drop due to exciting current in the reactor 17 is .2 percent. To correct this the reactor 17 would be tapped .2 percent off center so that it would have 1.05 percent winding on one side and 1.45 percent on the other side. This results in all the voltage steps being equal as shown in Fig. 3 which is a chart illustrating the output voltage of the transformer for successive operating positions thereof.

if a center tap would have been provided on the reactor 1'7 the voltage steps would be unequal as shown in Fig. 4 which is a chart similar to that shown in Fig. 3. It should be apparent that an advantage is obtained in the above disclosed system if the reactor is tapped off center to any appreciable extent, the optimum condition being reached when the tap is sufficiently off center to compensate for the voltage drop in the transformer caused by the exciting current of the reactor when the switch 16 is in a bridging position.

The advantages of equal steps in a step type voltage regulating system is that the voltage jumps a smaller amount when changing from one tap position to the next which allows closer adjustments of the voltage to a desired value. Another advantage of equal voltage steps is that the band width for which the voltage control relay is set can be made smaller when the steps are equal than when they are unequal because in order to avoid hunting the band width must be wider than the maximum step.

While only one embodiment of the present invention has been illustrated and described it will be apparent to those skilled in the art that modifications may be made therein without departing from the scope of the invention or from the spirit of the appended claims.

it is claimed and desired to secure by Letters Patent:

1. A load tap changing transformer comprising a winding having a plurality of electrically spaced taps, a reactor, switch means operable in alternate positions to connect the ends of said reactor to adjacent ones of said spaced taps, means connecting an output terminal of said transformer to a tap on said reactor, said reactor tap being disposed electrically off center sufliciently to compensate for the voltage drop in said transformer resulting from the exciting current of said reactor when said switch is in said alternate positions.

2. A tap changing under load transformer comprising a load terminal, a winding having a plurality of spaced taps between any electrically adjacent pair of which there is the same potential difference, a reactor provided with a tap, switch means for selectively connecting the ends of said reactor to the same or different ones of said taps to vary the output voltage of said transformer in steps, means connecting said reactor tap to said load terminal, said reactor tap being disposed electrically off center to compensate to a predetermined extent for the voltage drop in said transformer resulting from the exciting current in said reactor causing said output voltage steps to be substantially equal.

3. A. load tap changing transformer comprising a wind ing having a plurality of spaced taps between any electrically adjacent pair of which there is the same potential difference, a tap changing switch including a plurality of spaced stationary contacts connected to different ones of said taps, and a pair of movable contacts cooperating with said stationary contacts to vary the output voltage of said transformer in steps, a reactor connected between said movable contacts to limit the circulating current in said winding when said movable contacts are connected to different ones of said stationary contacts, and means connecting an output terminal of said transformer to said reactor including a tap on said reactor disposed electrically off center sutficiently to compensate for the voltage drop in said transformer resulting from the exciting current of said reactor when said movable contacts are connected to said different ones of said stationary contacts.

4. A step type voltage regulating system comprising a supply circuit, a load circuit, and load tap changing transformer means interconnecting said circuits for maintaining the voltage of said load circuit between predetermined limits independently of fluctuations of the voltage of said supply circuit, said transformer means including a step type voltage regulator responsive to the voltage of said load circuit for changing the output voltage of said transformer means in steps, a reactor for facilitating changing said output voltage without interrupting the flow of current in said load circuit, means connecting said load circuit to said reactor including a tap on said reactor disposed electrically otf center sufficiently to compensate for the voltage drop in said regulator resulting from the exciting current of said reactor drawn on alternate steps of said regulator.

5. A load tap changing transformer comprising a winding provided with a plurality of spaced taps between any electrically adjacent pair of which there is the same potential difference, a dial type tap changing switch including a plurality of circumferentially spaced stationary contacts and a pair of corresponding spaced movable contacts selectively cooperating with said stationary contacts to define alternate bridging and nonbridging positions of said switch, means connecting each of said stationary contacts to a different one of said taps, means connecting said movable contacts to an output terminal of said transformer including a reactor connected across said movable contacts, and means for actuating said switch to vary the output voltage of said transformer in steps, said reactor having a tap disposed between the midpoint and one terminal of said reactor to compensate for the voltage drop in said trans former caused by the exciting current of said reactor when said switch is in said bridging positions.

No references cited.

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