BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of a tap changer position sensor incorporating a load tap changer position sensor according to one embodiment of the invention;
FIG. 2 is an isometric view of the load tap changer position sensor shown in FIG. 1;
FIG. 3 is an exploded view of the load tap changer position sensor shown in FIGS. 1-2;
FIG. 4 is a plan view of a circuit board containing a series of radially spaced magnetic reed switches according to one aspect of the present invention and incorporated into the load tap changer position sensor shown in FIGS. 1-3; and
FIG. 5 is a schematic of an electrical circuit containing magnetic reed switches to generate an analog signal indicative of a position of a load tap changer according to another aspect of the invention.
DETAILED DESCRIPTION
This invention relates to a load tap changer position sensor, such as for use with an electrical transformer. As will be described more fully below, the load tap changer position sensor uses magnetic reed switches to indicate tap changer position thereby avoiding the drawbacks associated with resistor boards and other conventional changer position sensors.
Referring now to FIG. 1, a load tap changer assembly 10 includes a position indicator dial 12 with a series of equidistantly spaced radial markers 14 identifying possible tap positions. A needle 16 is caused to rotate in a known way relative to the series of radial markers 14 to provide a visual indication of tap position. The needle 16 is rotated by a shaft 18 that is coupled to a gear 20. Gear 20 has a serrated edge defining a series of gear teeth 22 that interface with grooves 24 formed along the edge of gear 26. Rotation of gear 20 causes gear 26 to also rotate. Gear 26 carries a shaft 28 that is interconnected to a load tap changer position sensor assembly 30. One skilled in the art will appreciate that the load tap changer assembly 10 is coupled to a tap position changer means in a known manner to change the tap position within a transformer. For example, gear 20 may interface with another gear (not shown) that is rotated when a change in tap position is made. This change in tap position causes gear 20 to rotate which then causes needle 16 to rotate, in this embodiment, along dial 12 to indicate the tap position. Similarly, as noted above, rotation of gear 20 causes gear 26 to rotate. Shaft 28 is carried by gear 26 and thus rotates therewith. This rotation of shaft 28, as will be described below, is detected by the load tap changer position sensor assembly 30.
Referring now to FIG. 2 and FIG. 3, the load tap changer position sensor assembly 30, in the illustrated embodiment, includes a cylindrical shaped housing 32 coupled to a disc-shaped cover plate 34. The cover plate 34 and the housing 32 may be interconnected to another in a known manner. A bore 35 is provided through the center of the cover plate 34. A drive mechanism 36 is coupled to the cover plate and includes a shaft 38 that extends centrally through the bore formed in the cover plate 34 and into the interior volume of the housing 32. A coupler 40 is mounted to an end of shaft 38 and includes a pair of interlocking prongs 42 that are designed to interlock with a corresponding coupler (not shown) on shaft 28 to interconnect the load tap changer position sensor assembly 30 to shaft 28.
As shown in FIG. 3, the load tap changer position sensor assembly 30 includes a circuit board 44 and an indicator wheel 46. The circuit board 44 is mounted to a bottom surface of cover plate 34 by a series of rivets 48. The circuit board 44 and indicator wheel 46 each have a bore 50, 52, respectively, that is aligned with bore 35 of the cover plate 34 so that shaft 38 extends through bores 35, 50, and 52 when the assembly 30 is assembled. Rivets 48 fixedly couple the circuit board 44 to the bottom surface of the cover plate 34. The indicator wheel 46 is coupled to the shaft 38 by a bushing 54 in a manner that allows the indicator wheel 46 to be rotated by shaft 38. Thus, as shaft 38 is rotated by shaft 28, the indicator wheel 46 will also rotate. The indicator wheel 46 carries a magnet 56 that, as will be described, is used to indicate the rotational position of the indicator wheel 46 and thus ultimately the position of the tap.
Referring now to FIGS. 4-5, the circuit board 44 has a series of radially spaced magnetic reed switches 58 that are arranged in a circular fashion. The design of the circuit board 44 is such that the resultant analog output does not require a microprocessor or decoder to convert the reed switch signals into a useful scale and value. The reed switches 58 are configured so as to operate in the presence of a magnetic field. In a manner as is known, each reed switch 58 has a pair of contacts 60, 62 of ferrous metal reeds inside a hermetically sealed glass envelope 64. In the presence of a magnetic field, the contacts 60, 62 in the glass envelope 64 are pulled together, thus completing an electrical path in an electrical circuit 66, schematically shown in FIG. 5. When the magnetic field is removed, the contacts 60, 62 are pulled apart and the electrical path is interrupted. The aforementioned configuration of reed switches 58 is but one example contemplated by the present invention, and is not intended to limit the present invention in any way. Other reed switch configurations as known in the art are also intended to be within the scope of the present invention.
As noted above, the indicator wheel 46 carries a magnet 56. More particularly, the indicator wheel 46 includes an opening within which a small magnet 56 is positioned. The magnet 56 acts to create a magnetic field in the presence of a reed switch 58 of the circuit board 44 over which the magnet 56 is positioned. Thus, as the indicator wheel 46 is turned in response to the advancement of the gear 26, the magnet 56 housed within the indicator wheel 46 is moved over a corresponding one of the reed switches 58 of the circuit board 44. The resultant magnetic field created by the magnet 56 causes the contacts 60, 62 of the reed switch 58 directly below the magnet 56 to come together, thus completing the electrical path within the reed switch 58. The completion of the electrical path with the reed switch 58 causes an analog signal to be output to indicate the precise position of the load tap changer.
As shown in FIG. 5, the electrical circuit 66 includes reed switches 58 as will as a number of resistors 68. When the magnet 56 is above a given reed switch 58, the contacts 60, 62 of the reed switch close thereby forming an electrical path through the closed contacts. Current will then flow from the voltage source, Vcc, through the resistors 68 between the closed reed switch 58 and the voltage source and then through the closed reed switch 58 to ground. In this regard, the number of resistors between the voltage source and the closed reed switch 58 will depend upon the position of the closed reed switch 58. Device Q171 can be configured as either a constant voltage source or constant current source. If Q1 is configured as a voltage source, then as the number of resistors in the closed electrical path increases, the current passing through current sensing resistor 70 decreases. Conversely, as the number of resistors in the closed electrical path decreases, the current passing through the current sensing resistor 70 increases. In this regard, the current sensing resistor 70 and the resistors 68 in the closed electrical path effectively form a voltage divider such that the voltage provided at Vout provides a voltage indicative of the position of the closed reed switch. If Q171 is configured as a constant current source, then Since the reed switch is only closed by the magnetic field created by the magnet 56, the voltage provided at Vout indicates the position of the magnet 56 which is rotated by shaft 38. As described above, shaft 38 rotates in unison with the load tap changer and therefore the voltage at Vout provides at indication of the position of the load tap changer. One skilled in the art will appreciate that the Vout terminal is connected to a processor or other suitable device (not shown) that compares the voltage at the Vout terminal to a database or similar data structure to determine the position of the load tap changer based on the voltage at the Vout terminal.
Additionally, it is understood that the electrical circuit 66 may include additional components not shown or specifically described herein to enhance performance of the electrical circuit such as filters, voltage regulators, and the like. It is also recognized that the present invention may be embodied in electrical circuits different from that shown in FIG. 5.
Accordingly, in one embodiment the present invention is directed to a method of determining the position of a load tap changer. The method includes detecting closing of a magnetically activated switch and then determining the position of that switch. More particularly, a magnetic indicator is rotated by shaft connected either directly or through a series of gears to the load tap changer. As that magnetic indicator is rotated, it creates a magnetic field of sufficient strength to close the contacts of a reed switch when in the magnetic field. Through a series of radially spaced reed switches, the position of the magnetic indicator and thus the load tap changer can be determined from the position of a closed reed switch.
Many changes and will modifications could be made to the invention without departing from the spirit thereof. The scope of these changes will become apparent from the appended claims.
Patent Application
20080073189
