Patent Publication Number: US-2007115085-A1

Title: Direct current link inductor for power source filtration

Description:
FIELD OF THE INVENTION  
      The invention relates to electrical power sources for supplying and filtering direct current (DC) power, and more particularly to such power sources that have inductive filter elements.  
     BACKGROUND OF THE INVENTION  
      Electrical power sources that supply and filter DC to a load, such as sources that convert alternating current (AC) current to DC current for a load, generally comprise a rectifier circuit for converting the AC current to pulsating DC current and a filter circuit for converting the pulsating DC to steady-state DC. The rectifier circuit connects to the filter circuit by way of a DC link that generally comprises an inductor that serves as part of the filter circuit to form a choke-input filter circuit. Of course, the total current, including ripple current through the filter circuit and steady-state current through a load applied to the output of the filter circuit passes through the inductor. The total energy stored in the inductor is ½ LI 2 , wherein L is the inductance of the inductor and I is the total current passing through the inductor. The inductor has to be large enough to store this total energy.  
      Other applications that use such a DC link inductor as part of a power source include electrical controls for loads, such as motor speed controls for brushless DC motors, that tend to generate unwanted harmonics. The DC link inductor filters out the unwanted harmonics in such applications.  
      When a high level of DC passes through an inductor, the magnetic core for the inductor generally must have an air gap to avoid magnetic saturation of the inductor. The air gap has the effect of increasing the length of the magnetic path through the magnetic core. The resulting increase in magnetic path length causes the magnetic field “H” in the inductor to decrease. The reduced H field puts the magnetic operating point of the inductor in a linear region of the inductor&#39;s hysteresis loop where the permeability of its magnetic core is relatively large. Even though the core permeability is large, the air gap causes the effective permeability to be less than the magnetic core permeability. Since the inductance of the inductor is proportional to the effective permeability and inversely proportional to the magnetic length of the inductor, the introduction of an air gap into the magnetic core of the inductor reduces its inductance.  
      Since the air gap is required to prevent magnetic saturation of the inductor, to achieve the same inductance as before the introduction of the air gap, the inductor must have an increased number of winding turns or an increased magnetic core area. It is generally preferable to increase the magnetic core area since the addition of turns also increases the inductor&#39;s H field, which may require an increase in the air gap to prevent saturation due to the increased H field. In short, high-level DC passing through an inductor requires that the inductor be larger, heavier and more costly than if there were no DC passing through it.  
      An alternative to using an air gap to prevent magnetic saturation of the inductor involves placing a permanent magnet within the magnetic core to serve as a secondary magnetic field source that opposes the magnetic field generated by current that passes through the inductor&#39;s winding. Although this alternative approach is simple and requires no extra components, it has several disadvantages.  
      First, there is no convenient way to control the magnetic field generated by the permanent magnet. Thus, the opposing magnetic field that the permanent magnet generates cannot change in response to varying inductor current. In fact, the magnetic field of the permanent magnet may dominate when the level of inductor current is low. Another disadvantage is that materials that have sufficient magnetic retentivity to be suitable for use as the permanent magnet have low permeability and therefore introduce an equivalent air gap when placed within the magnetic core of the inductor.  
     SUMMARY OF THE INVENTION  
      The present invention inserts an electromagnetic H field into the inductor that opposes the H field generated by the DC that passes through its primary winding. The net H field is thus reduced and the magnetic operating point is then within a linear region of the inductor&#39;s hysteresis loop without the introduction of a large air gap. In one possible embodiment, the inductor has an auxiliary winding and a current passes through the auxiliary winding that creates an opposing H field. A feedback circuit may control the amount of current passing through the auxiliary winding to adjust the opposing H field to keep the magnetic operating point of the inductor in a linear region of the inductor&#39;s hysteresis loop regardless of DC current that passes through its primary winding.  
      Generally, the invention comprises an inductor with a primary winding on a magnetic core that produces a primary magnetic field H 1  with a current I 1 , comprising: an electromagnetic field source that generates a secondary magnetic field H 2  in the core that opposes the primary magnetic field H 1  to produce a low net magnetic field H NET  in the core to prevent magnetic saturation of the core. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a typical inductor with an air gap in its magnetic core according to the prior art.  
       FIG. 2  is a perspective view of an inductor that has a permanent magnet inserted in a gap of its magnetic core according to the prior art.  
       FIG. 3  is a perspective view of an inductor according to a possible embodiment of the invention that has an auxiliary winding wound around its magnetic core.  
       FIG. 4  is a simple schematic of one possible embodiment of a feedback circuit to control current through the auxiliary winding of the embodiment of the invention shown in  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a perspective view of a typical inductor  2  according to the prior art. The inductor  2  has a primary winding  4  of N turns on a magnetic core  6 . The primary winding  4  carries a current I along a direction indicated by arrow  8 . The current I generates a magnetic field H along a magnetic path indicated by arrows  10 . The magnetic core  6  has an air gap  12  that is placed across the magnetic path  10 . For the typical inductor  2 , the magnetic field H may be represented by:  
       H   =     KNI     I   e             
 wherein K is a constant and I e  is the effective length of the magnetic path  10 . Of course, the air gap  12  increases the effective length of the magnetic path  10 , and thereby it reduces the possibility of magnetic saturation by reducing H. 
 
       FIG. 2  is a perspective view of another inductor  14  according to the prior art. The inductor  14  has a primary winding  4  of N turns on a magnetic core  6 . The primary winding  4  carries a current I 1  along a direction indicated by arrow  8 . The current I 1  generates a primary magnetic field H 1  along a magnetic path that extends around the magnetic core  6  in a direction indicated by arrow  16 . A permanent magnet  18  serves as a secondary magnetic field generator for generating a magnetic field H 2  that extends around the magnetic core  6  in a direction that opposes the primary magnetic field H 1  as indicated by arrow  20 . The permanent magnet  18  may conveniently mount in place of the air gap  12  as shown in  FIG. 1  to generate a secondary magnetic field H 2  within the magnetic core  6  in a direction that opposes the primary magnetic field H 1 .  
      The magnetic field H 1  in this case may be represented by:  
         H   1     =       KNI   1       I   e           
 
 In this case, the effective length of the magnetic path I e  may be less than with the inductor  2  shown in  FIG. 1  because the actual air gap may be less than the air gap  12 . Thus, the inductor  14  may have a higher value of primary magnetic field H 1  for the number of turns N for the primary winding  4  and same size of magnetic core  6 . 
 
      Since the secondary magnetic field H 2  in the magnetic core  6  that the permanent magnet  18  generates opposes the primary magnetic field H 1 , the net magnetic field H NET  may be represented by:
 
H NET =H 1 −H 2 
 
 Thus, the secondary magnetic field H 2  generated by the permanent magnet  18  may cancel out part of the primary magnetic field H 1  to prevent magnetic saturation of the magnetic core  6  for the inductor  14 . Although the inductor  14  is simple and requires no extra components, it has several disadvantages. 
 
      First, there is no convenient way to control the secondary magnetic field H 2  that is generated by the permanent magnet  18 , particularly when the permanent magnet  18  intersects the magnetic core  6  as shown in  FIG. 2 . Thus, the secondary magnetic field H 2  cannot change in response to varying current I 1  so that the net magnetic field H NET  is always minimised. In fact, the secondary magnetic field H 2  may dominate when the level of current I 1  is low. Another disadvantage is that materials that have sufficient magnetic retentivity to be suitable for use as the permanent magnet  18  have low permeability and therefore introduce an equivalent air gap when intersecting the magnetic core  6  as shown in  FIG. 2 .  
       FIG. 3  is a perspective view of an inductor  22  according to a possible embodiment of the invention that obviates the disadvantages of the inductor  14  shown in  FIG. 2 . The inductor  22  has a primary winding  4  of N 1  turns on a magnetic core  6 . The primary winding  4  carries a current I 1  along a direction indicated by arrow  8 . The current I 1  generates a primary magnetic field H 1  along a magnetic path that extends around the magnetic core  6  in a direction indicated by arrow  16 .  
      The magnetic field H 1  in this case may be represented by:  
         H   1     =         KN   1     ⁢     I   1         I   e           
 
 The effective length of the magnetic path I e  may be less than with the inductor  2  shown in  FIG. 1  because there may be no air gap  12 . Thus, the inductor  22  may have a higher value of primary magnetic field H 1  for the number of turns N for the winding  4  and same size of magnetic core  6 . In addition, the magnetic path I e  may also be less than with the inductor  14  when the permanent magnet  18  intersects the magnetic core  6  as shown in  FIG. 2 . 
 
      The inductor  22  has a secondary auxiliary winding  24  that carries a current I 2  along a direction indicated by arrow  20 . The current I 2  in the secondary winding  24  lets it serve as an electromagnetic field source for generating a secondary magnetic field H 2  along the magnetic path that extends around the magnetic core  6  and opposes the primary magnetic field H 1  in a direction indicated by arrow  20 .  
      The secondary magnetic field H 2  in this case may be represented by:  
         H   2     =         KN   2     ⁢     I   2         I   e           
 
 Since the secondary magnetic field H 2  in the magnetic core  6  that the secondary winding  24  generates opposes the primary magnetic field H 1 , the net magnetic field H NET  may be represented by:
 
H NET =H 1 −H 2 
 
      The intensity of the secondary magnetic field H 2  may track the intensity of the primary magnetic field H 1  by appropriately adjusting the level of current I 2  to produce a net magnetic field H NET  of 0 regardless of the level of current I 1 . In this way, the magnetic core  6  of the inductor  22  cannot saturate regardless of the level of current I 1  yet the secondary magnetic field H 2  cannot dominate when the level of current I 1  is low. Furthermore, there is no permanent magnet  18  with its low permeability to adversely affect the effective length I e  of the magnetic path in the magnetic core  6 . Optionally, a small air gap  12 ′, such as shown in dotted line, may be introduced for control purposes, but if so used it may be much smaller than the air gap  12  used for the inductor  2  shown in  FIG. 1 .  
      One possible way to make the level of current I 2  track the level of current I 1  so that the net magnetic field H NET  remains at or near 0 is with a feedback circuit.  FIG. 4  is a simple schematic of one possible embodiment of a feedback circuit  28  to control current through the secondary winding  24  of the inductor  22 . Current I 1  from a rectifier circuit (not shown) passes through the primary winding  4  of the inductor  22  to a resistance  30  and capacitance  32  by way of an inductor input line  34  and an inductor output line  36 . The inductor  22 , resistance  30  and capacitance  32  serve as a choke-input power supply filter  38 .  
      An electrical potential sensor  40  measures AC back electromotive force (EMF) generated as a result of the pulsating DC current that flows through the primary winding  4  of the inductor  22 . The sensor  40  preferably is connected such that it measures the maximum AC back EMF across the primary winding  4 , such as across the primary winding  4  as shown. An output of the sensor  38  connects to one input of an amplifier  42  by way of a sensor output line  44 . A reference electrical potential, such as an electrical potential reference source  46 , connects to another input of the amplifier  42  by way of a reference source output line  48 . The amplifier  42  has an output connected to the secondary winding  24  by way of an amplifier output line  50 .  
      The reference source  46  has a level that lets the amplifier 42  generate a current I 2  level in the secondary winding  24  that minimises the net magnetic field H NET  in the magnetic core  6  of the inductor  22  for a given current I 1  level to produce a maximum AC back EMF across the primary winding  4 . As the current I 1  level increases or decreases in level, the back EMF across the primary winding  4  also changes, changing the output of the sensor  38  and thereby changing the current I 2  level that the amplifier  42  generates to keep the net magnetic field H NET  at a minimum.  
      Described above is an inductor with a primary winding on a magnetic core that produces a primary magnetic field H 1  with a current I 1  and an electromagnetic field source that generates a secondary magnetic field H 2  in the core that opposes the primary magnetic field H 1  to produce a low net magnetic field H NET  in the core to prevent magnetic saturation of the core. The described embodiment is only an illustrative implementation of the invention wherein changes and substitutions of the various parts and arrangements thereof are within the scope of the invention as set forth in the attached claims.