Abstract:
An AC-DC power converter has a phase-shifting autotransformer based rectifiers and DC capacitors. Soft start of the AC-DC power converter is achieved by designing the autotransformer to operate at a low peak flux density at a point of AC voltage step application (initial turn on). The addition of a controlled impedance segregates capacitor charging from the initial magnetizing process of the autotransformer.

Description:
BACKGROUND 
     The present disclosure relates to AC-DC converters incorporating a phase-shifting autotransformer for AC input power factor correction. 
     Electric aircraft often includes three-phase power generators, which are used to generate the power needed to operate on-board electronic systems during flight. The three phase power from the generators is converted to DC power using an AC-DC converter. One type of AC-DC converter used in aircraft systems is a phase-shifting autotransformer with integrated rectifiers. 
     Phase-shifting autotransformer-based AC-DC converter systems require a large initial input of energy (referred to as an inrush current) on startup when a zero voltage to rated AC voltage step is applied in order to magnetize the phase-shifting autotransformer and charge a DC capacitor. Due to the initial inrush requirement, as much as 10 times the rated working current of the AC-DC converter can be drawn from the AC power connections. 
     SUMMARY 
     Disclosed is an AC-DC converter that includes a phase-shifting autotransformer module having an AC power input and a DC power output, a capacitor connected across the DC power output, and a controlled impedance component interrupting the DC power output, such that the autotransformer magnetization current is segregated from the capacitor charging current. 
     Also disclosed is a method for operating a phase-shifting autotransformer based AC-DC converter. The method includes segregating phase-shifting autotransformer initial magnetizing and DC capacitor charging to control inrush current drawn from an AC source, by way of a controlled impedance component. The controlled impedance component is in an off mode when the AC step voltage is applied, the controlled impedance component allows the autotransformer to establish an initial magnetization without charging the capacitor. After autotransformer initial magnetization, the controlled impedance component is in a high impedance mode thereby establishing capacitor slow charging. The controlled impedance component is in an on mode after the capacitor is charged up, thereby establishing a steady state operation mode. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an electric aircraft power system. 
         FIG. 2  illustrates a phase-shifting autotransformer based AC-DC converter. 
         FIG. 3  illustrates an exemplary autotransformer. 
         FIG. 4  illustrates an example autotransformer magnetization curve. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an electric aircraft  10  that includes a three phase power generator  20 . The generated three phase power is distributed throughout the aircraft  10  via a power distribution system  30 . A phase-shifting autotransformer based AC-DC power converter  40  converts AC power from the power distribution system  30  into DC power for use with the DC components  50 . 
       FIG. 2  illustrates an example phase-shifting autotransformer based AC-DC power converter  40 . In the example phase-shifting autotransformer based AC-DC power converter  40 , three phase power  110  is input into a phase-shifting autotransformer based rectifier  120 . DC power is output from the phase-shifting autotransformer based rectifier  120  on a pair of DC outputs  132 ,  134 . One of the DC outputs  132  is interrupted by a controlled impedance component  130 , such as a semi-conductor switch/transistor, that can be operated in an off mode (open circuit), a high impedance mode, or an on mode. 
     The high impedance mode limits a capacitor charging current provided to the capacitor  140 . A controller  136  controls the mode of the controlled impedance component  130 . By way of example, the controlled impedance component  130  can be a semi-conductor switch controlled by the controller  136 . 
     When the phase-shifting autotransformer based AC-DC power converter  40  is initially powered up, the controlled impedance component  130  is in the off mode, thereby preventing any power from passing to the capacitor  140  or the DC load connection  150 . While the controlled impedance component  130  is in the off mode, a zero AC voltage to rated AC voltage step is applied to the phase-shifting autotransformer based rectifier  120 , a startup current is drawn from the three phase power  110  and magnetizes the autotransformer portion of the phase-shifting autotransformer based rectifier  120 , thereby establishing transformer flux in the core  320 , illustrated in  FIG. 3 , of the autotransformer portion of the phase-shifting autotransformer based rectifier  120 . 
       FIG. 3  illustrates a phase shifting autotransformer  300  usable in the power converter  40 , having a core  320  about which a set of phase windings  310  are wound. In order to function, a transformer flux is established in the core  320  via the use of magnetization current provided to the windings  310  according to known principles. The initial flux generated when the autotransformer  300  is turned on is referred to as a startup flux. 
     The startup flux density within the autotransformer portion of the phase-shifting autotransformer based rectifier  120  peaks at a high value before declining to a steady state flux density after the autotransformer core is fully magnetized. Startup current from three phase power  110  peaks at high value before settling to steady state. Such current is referred to as inrush current. Once the autotransformer portion of the phase-shifting autotransformer based rectifier  120  is fully magnetized, the controller  136  switches the controlled impedance component  130  into the high impedance mode, thereby slowly charging the capacitor  140 . 
     When the capacitor  140  is charged, the controller  136  switches the controlled impedance component  130  into the on mode, and rectified power is allowed to pass through the DC load connection  150  into an attached load. An inrush current exceeding the rated current of the AC-DC converter is referred to as a hard start, and causes instability and stress within the aircraft electrical system. In contrast, an inrush current that is less than a full rated AC input current is referred to as soft start. Additional power converters  40  in the power system simultaneously undergoing a hard start compound the stresses resulting from hard start inrush currents. 
     In order to allow the above described “soft start” performance, the magnetization of the autotransformer portion of the phase-shifting autotransformer based rectifier  120  is designed to have a peak startup flux that falls within either a linear region or a shallow saturation region of the magnetization curve. 
       FIG. 4  illustrates a magnetization curve  200  of an example autotransformer. The magnetization curve  200  includes a linear region  240  and a saturation region  250 . The saturation region  250  is broken into two sub regions, a shallow saturation region  252  (between B 1  and B 2 ) and a deep saturation region  254  (above B 2 ). In one example autotransformer design, the autotransformer core flux density falls entirely within the linear region  240 . As a result, the peak flux density during startup is less than B 1 , where B 1  is the transition point between the linear region  240  and the saturation region  250 . 
     In the above example, the autotransformer typically draws a steady state magnetization current (I mag ) of &lt;10% of a full rated AC input current in order to maintain autotransformer magnetization during steady state operations. The magnetization current is drawn from the three phase power  110 . During the initial startup of the autotransformer system  40 , the inrush current is 2×I mag  or &lt;20% of the full rated AC input current and lasts for three times the autotransformer magnetization inductance time constant (τ). The initial current of 2×I mag  results in an autotransformer flux density that is near, but under, B 1 . Thus, the peak startup flux density falls within the linear region  240 . The magnetization inductance time constant is τ=L/R, where L is the autotransformer inductance and R is the autotransformer magnetizing winding resistance. After 3τ, the autotransformer flux density reduces to ½ B 1 , where it stays steady during high impedance mode and on mode operations. An autotransformer designed according to the above principles can be large and, thus, unsuitable for certain applications. 
     Alternately, the autotransformer can be designed such that the peak startup flux falls in a shallow saturation region  252  between B 2  and B 1 . In such a design, the inrush current can vary from 2×I mag  to the full rated AC input current, and is a function of the peak flux density during the startup. An increase in initial flux density increases the current draw. The shallow saturation region  252  prevents the inrush current from reaching levels that exceed the rated current of the autotransformer, thereby avoiding a hard start. Designing the autotransformer such that the peak startup flux density is in the shallow saturation region  252  provides for a soft start performance and reduces the physical size of the autotransformer. 
     Although an example has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.