Abstract:
A brushless variable transformer. Variable autotransformers, use brushes, and as such, have moving parts requiring maintenance and periodic cleaning of the brushes. A variable transformer without brushes is advantageous in that it eliminates the cleaning and maintenance of brushes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/831,068 filed Jun. 4, 2013, the contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This description relates generally to electronic transformers and more specifically to brushless variable transformers. 
     BACKGROUND 
     An electronic transformer is an AC electronic component that will change, or transform an AC input voltage to a different output voltage level. An important characteristic of typical transformers is that circuitry connected to the primary is electrically isolated from circuitry connected to the secondary winding. An output voltage higher than the input voltage will generate a lower output current, and a lower output voltage will generate a higher output current. After accounting for losses in the transformer the power into the transformer is substantially equal to the output power produced. A transformer may have a first, primary winding upon a core, with a second winding, or secondary, also disposed upon the same core. The primary core to which an input voltage is applied, through electromagnetic coupling induces a voltage across the secondary. Accordingly the output voltage of a transformer may be changed by adding or removing secondary turns 
     Alternatively, discrete voltages may be selected by attaching wires (taps) at various taps. The taps if connected to a rotary switch provide discrete, but variable output voltages. 
     In an alternative construction a more continuous output voltage may be produced by allowing a conductor (typically a carbon brush), to slide over exposed turns of a secondary winding. Typically, a knob is provided, and turning it in one direction increases the voltage output, and the opposite direction decreases the output voltage. 
     Transformers of this sort may be desirable in applications which require a variable voltage, such as light dimmers, welders, motor controls, audio applications, testing equipment at low and high end operating conditions, and the like. However, using a conventional transformer with a bulky core and two windings in such applications would not be practical. If electrical isolation is not needed a device called an autotransformer may be substituted for a transformer. It advantageously utilizes a single winding in which taps or brushes may be applied as previously described in a transformer. 
       FIG. 1 . shows a schematic of an autotransformer  100 , which has a single winding  102  over a core material  104  with two primary terminals  106  and  108  at the extreme ends of that single, or primary winding. It also has one or more terminals or taps  110  at intermediate tap points along the single winding  102  that forms the secondary winding or circuit. Thus the primary and secondary coils have part or all of their turns in common. 
     The primary voltage  112  is applied across two of the primary terminals, and the secondary voltage  114  taken from the tap terminals. The autotransformer almost always has one terminal  108 , in common with the primary voltage. The primary and secondary circuits, therefore, have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer, part of the current flows directly from the input to the output, and only part of the current is transferred by induction. 
     Autotransformers may also include many taps and include additional automatic switchgear to allow them to act as automatic voltage regulators to maintain a steady voltage over a wide range of load conditions. If a sliding tap is used that contacts more than one turn at a time, the turns are shorted. However if a resistance is inserted sliding tap the shorting problem may be eliminated. An autotransformer that is designed to produce continuous voltage variation, without shorting adjacent turns is known as a variable autotransformer, such as the VARIAC® variable autotransformer from Instrument Service and Equipment, Inc., Cleveland, Ohio 
       FIG. 2  shows an electrical schematic of a variable autotransformer. In a variable autotransformer, part of the winding coils  202  may be exposed and the secondary connection is made with a sliding brush  204 . The brush is typically a carbon brush. The primary connection is  206 . The addition of the brush, which may be controlled with an external knob (not shown) allows a continuously variable turns ratio to be obtained, which is established by the location in the winding the brush makes contact. This allows for very smooth control of voltage. The output voltage  208  is not limited to the discrete voltages represented by actual number of turns. The input voltage  210  can be smoothly varied between turns as the brush has a relatively high resistance (compared with a metal contact) and the actual output voltage is a function of the relative area of brush in contact with adjacent windings. The primary connection  206  can be connected to only a part of the winding allowing the output voltage to be varied smoothly from zero to above the input voltage. This allows a variable autotransformer to be used for testing electrical equipment at the limits of its specified voltage range. 
     Brushes make physical and electrical contact in conducting electricity between moving parts and tend to wear from use. Typical applications of brushes include electric motors, alternators, electric generators, and variable autotransformers. Accordingly it would be desirable to eliminate the use of brushes in a variable transformer design. 
     Those having skill in the art would understand the desirability of having a variable transformer that uses circuitry to vary and regulate output voltage without brushes. The variable transformer described herein allows the use of a variable transformer not requiring cleaning and maintenance of moving parts, nor mechanical brushes. 
     SUMMARY 
     The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later. 
     The present example provides a brushless variable transformer using electronic switches and a unique circuitry to provide a variable voltage output. A variable transformer without brushes is advantageous in that it eliminates the cleaning and maintenance of brushes. 
     Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein: 
         FIG. 1  shows an electrical schematic of an autotransformer. 
         FIG. 2  shows an electrical schematic of a continuously variable autotransformer having brushes. 
         FIG. 3  shows an electrical schematic of a continuously variable autotransformer utilizing switches rather than brushes. 
         FIG. 4  shows an electrical schematic of the brushless variable transformer with switches set in a first position wherein the switches are closed to create an increase in line voltage. 
         FIG. 5  shows an electrical schematic the brushless variable transformer with switches set in a second position wherein the switches are closed to create a decrease in line voltage. 
         FIG. 6  shows an electrical schematic of the brushless variable transformer with switches set in a third position wherein the output voltage equals the input voltage. 
         FIG. 7  shows an electrical schematic of multiples of present invention used in series. 
         FIG. 8  shows a table with exemplary total voltage output variation for various switch configurations. 
     
    
    
     Like reference numerals are used to designate like parts in the accompanying drawings. 
     DETAILED DESCRIPTION 
     The detailed description provided below in connection with the appended drawings is intended as a description of the present examples of a brushless variable transformer and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples. 
     The examples below describe a brushless variable transformer. As those skilled in the art will appreciate, the present examples are suitable for application in a variety of different types of voltage control and regulation systems. 
       FIG. 1  shows an electrical schematic of an autotransformer. 
       FIG. 2  shows an electrical schematic of a variable autotransformer. 
       FIG. 3  shows an electrical schematic of a brushless variable transformer  300  constructed as described herein. A conventional transformer  302  has a primary winding  304  and a secondary winding  306  sharing a common core  308 . Voltage is induced in the secondary winding  306  solely by inductive coupling to the primary winding  304 . The transformer  302  is characterized by the ratio of the number of turns of the primary winding  304  around the common core  308  to the number of turns of the secondary winding  306  around the common core. 
     Power switches  310 ,  312 ,  314 , and  316  are conventionally constructed switches, and may be of any suitable construction. These switches may be relays, contactors, or solid state power devices such as insulated gate bipolar transistors (IGBT) and silicon-controlled rectifiers (SCR), which are also known as thyristors. The switches are isolated from the line current, and operate at much lower voltage than line voltage. Alternating current (AC) at line voltage is provided at an input  318 , and modified alternating current at variable voltage is at an output  322 . The line voltage may be low, in the range of 200 to 400 VAC, or may be in a medium voltage range of 4600 to 13,600 VAC. The circuit is provided with a neutral connection  320 . 
     Switches  310 ,  312 ,  314 , and  316  are not operated at line voltage, and may be controlled using microcontrollers and/or a programmable logic controllers (PLC)  324  using proportional-integral-derivative control (PID) and or a microcontroller, or the like. The construction and wiring of such controllers is well known and is not shown in  FIG. 3  for simplification of the diagram. The methods for implementing and controlling a brushless variable transformer as described herein are unique to the examples described below. Power switches  310 ,  312 ,  314 , and  316  can be configured to allow or prevent current from passing through them, and subsequently alter the direction of current applied to the secondary winding of transformer  302 , thereby making the output voltage buck or boost due to changes in the inductive voltage transfer from the secondary winding  306 . The various switch configurations and subsequent variation in the output voltage are described in  FIGS. 4, 5, and 6 . 
       FIG. 4  shows an electrical schematic of a brushless variable transformer wherein the switches are opened or closed to create an increase in line voltage. The controller  324  directs the switches  310  and  316  to allow current to flow through them, and switches  314  and  316  not allow current to pass. Input alternating current  318  passes through the primary winding of transformer  302 . Arrow  502  shows the direction of current flow. Simultaneously, the condition of switches  312  and  316  allow current from input  318  to pass through the secondary winding  306  of transformer  302  to neutral  320 . Arrow  504  shows the direction of current flow in the secondary winding. The condition of switch  316  connects the circuit to the neutral  320 . 
     Inductive coupling of the primary and secondary windings in this example provides for an increase in the voltage at the circuit output  406 . The magnitude of the output depends on the ratio of the number of wire turns in the primary winding  304  to the number of wire turns in the secondary winding  306  in transformer  302 . If, for example, when the secondary winding of transformer  402  is wound to produce 1% of the output, the output voltage  406  will equal the input voltage of the input current  318  plus 1%. 
       FIG. 5  shows an electrical schematic of the present invention wherein the switches are configured to create a decrease in line voltage. Here, the controller  324  activates switches  312  and  314  to allow current to flow through them, and switches  310  and  316  not to allow current to pass. Input alternating current  318  passes through the primary winding of transformer  302 . Arrow  502  shows the direction of current flow. Simultaneously, the condition of switches  312  and  314  allow current from input  318  to pass through the secondary winding  306  of transformer  302  to neutral  320 . Arrow  504  shows the direction of current flow in the secondary winding. The condition of switch  314  connects the circuit to the neutral  320 . 
     Inductive coupling of the primary and secondary windings in this example provides for a decrease in the voltage at the circuit output  506 . The magnitude of the output depends on the ratio of the number of wire turns in the primary winding  304  to the number of wire turns in the secondary winding  306  in transformer  302 . If, for example, when the secondary winding of transformer  302  is wound to produce 1% of the output, the output voltage  506  will equal the input voltage of the input current  318  minus 1%. 
       FIG. 6  shows an electrical schematic of the present invention wherein the output voltage equals the input voltage. Here, switches  314  and  316  are activated by the controller  324  to allow current to flow through them, and switches  310  and  312  do not allow current to pass. Input alternating current  318  passes through the primary winding of transformer  302 . Arrow  602  shows the direction of current flow. Simultaneously, the condition of switches  310  and  412  do not allow current from input  318  to pass through the secondary winding  306  of transformer  302 . As such, there is no current to provide inductive coupling to the current passing through the primary winding  602  and its voltage remains unchanged from the voltage of the input current  318 . 
       FIG. 7  shows an electrical schematic of multiples of present invention used in series. For simplification of the diagram, the controller for the switches is not shown. Although an exemplary pair of circuits  300  and  700  providing brushless variable transformers are shown, it is obvious to those skilled in the art that a plurality of such circuits can be connected in series to provide a wide range of possible voltage outputs. Each brushless variable transformer circuit can be provided with a different ratio of the number of wire turns in the primary windings to the number of turns in the secondary windings providing a wide range of possible outputs. 
     When multiple circuits shown above are coupled, or cascaded in series, the amount of buck (decrease in voltage) or boost (increase in voltage) can be controlled to get desired voltage at the output. In  FIG. 7 , two circuits  300  and  700  are coupled in series with different primary winding to secondary winding turns ratio transformers  302  and  702 . The output current  322  from circuit  300  is the input current  750  to circuit  700 . 
     If, for example, transformer  302  provides an exemplary 1% variation in the output current  322  voltage, there are three possible conditions transformer  302  can effect on the output current. These are +1%, −1%, and 0%. The +1% condition occurs when the switched in the circuit  300  are as shown in  FIG. 4 , −1% occurs when the switches in circuit  300  are as shown in  FIG. 5 , and 0% when in the switches are as shown in  FIG. 6 . 
     Similarly for circuit  700 , if the ratio of the primary winding  706  turns to the secondary winding  708  are such that the transformer  704  provides an exemplary 3% variation, the three conditions circuit  700  can effect on the input current is +3%, −3%, and 0%. By linking the circuit  300  and circuit  700  in series such that the output current  322  is also the input current  750  to circuit  700 , the voltage variation range is +/−4%. 
     By simultaneously activating with a controller, the switches  310 ,  312 ,  314 ,  316  and  710 ,  712 ,  714 ,  716  on the brushless variable transformer circuits  300  and  700  can be positioned to allow or not allow current to pass. An example of the possible voltage variations possible for this example is shown in  FIG. 8 . 
       FIG. 8  shows a table with exemplary total voltage output variation for various switch configurations for the exemplary variable transformers shown in  FIG. 7 . With two circuits with transformer  302  providing +/−1% of variation and transformer  704  providing a variation of +/−3%, it is possible to vary the output voltage of the cascade from −4% to +4%. Column  802  of  FIG. 8  shows the possible effects on the input current voltage provided by circuit  300  in  FIG. 7 . Column  804  shows the possible effects on its input current voltage provided by circuit  700  in  FIG. 7 . Column  806  shows the total variation in voltage provided by the two circuit operating in series as shown in  FIG. 7 . 
     For the positive values in each of column  802  and  804 , the switches are configured as shown in  FIG. 4 ; for negative values, the switches are configured as shown in  FIG. 5 , and zero values occur when the switches are as shown in  FIG. 6 . By varying the switch positions systematically using the controller, total output variation in column  806  can be varied from +4 to −4%. 
     The examples provided above are but exemplary, and not limiting. The basic circuit may be varied in construction as long as a buck and boost may be applied to the output, causing a controlled variation without use of brushes. Alternatively the cascaded configurations and their ratios of primary winding turns to secondary winding turns may be adjusted to produce a variety of outputs. 
     For example, it is possible to couple more stages and get output variation of −31% to +31%, or −46% to +46%. If additional precision is required, additional stages of ½% or ¼% could be added. Similar stages may be constructed for use in three phase input/output needs 
     Those skilled in the art will realize that the process sequences described above may be equivalently performed in any order to achieve a desired result. Also, sub-processes may typically be omitted as desired without taking away from the overall functionality of the processes described above.