Abstract:
The electric machine comprises a rotor and stator, the rotor and stator having generally equal axial lengths. The stator has at least one primary and at least one control winding disposed around the stator. The electric machine comprises a stator extension of a magnetic material. The stator extension is disposed coaxially with the stator and has only the at least one control winding wrapped therearound. The stator extension extends axially adjacent the stator beyond an axial terminus of the rotor and defines a magnetic circuit around the at least one control winding. The stator extension is configured such that in use, the magnetic circuit remains unsaturated thereby increasing inductance of at least one control winding in an electric alternator/motor.

Description:
TECHNICAL FIELD 
     The technical field relates to electric machines, and in particular to the control of such machines. 
     BACKGROUND 
     The output voltage and frequency of permanent magnet (PM) alternators are essentially determined by the rotor speed, which present challenges where the rotor speed cannot be independently controlled, such as in a gas turbine engine. Also, controllability in low- or no-load conditions may require using a regulation apparatus, such as a shunt regulation system, to draw current from the machine if the voltage increases above a given maximum allowable value. But these systems add weight and complexity. Instead, in one solution presented in U.S. Pat. No. 7,262,539, a set of control windings is provided around a portion of the stator. 
     The number of control winding turns generally sets the desired inductance of the control windings. However, the number of turns and the inductance can be the subject of conflicting requirements for designers. For example, a lower number of turns can be desirable to minimize voltage loss and reduce heating. However, increasing the number of turns to increase the inductance may be desirable to minimize current ripple or for other benefits. To date, the designers must sometimes make hard choices in optimizing inductance and machine performance, especially where the machine is used as both a generator and a motor. Room for improvement thus exists. 
     SUMMARY 
     In one aspect, the present concept provides an electric machine comprising: a rotor and stator, the rotor and stator having generally equal axial lengths, the stator having at least one primary and at least one control winding disposed around the stator; and a stator extension of a magnetic material, the stator extension disposed coaxially with the stator, the stator extension having only the at least one control winding wrapped therearound, the stator extension extending axially adjacent the stator beyond an axial terminus of the rotor, the stator extension defining a magnetic circuit around the at least one control winding, the stator extension configured such that in use the magnetic circuit remains unsaturated thereby increasing inductance of at least one control winding in an electric alternator/motor. 
     In another aspect, the concept provides an electric machine comprising: a rotor; a stator having a plurality of first slots and a plurality of second slots, the first slots being located on a periphery of the stator adjacent to the rotor, the second slots being located on an opposite side of the first slots relative to the rotor; at least a first winding disposed in a plurality of said first and second slots; at least a first magnetic circuit, defined in the stator and the rotor, encircling at least one first slot in which the first winding is disposed; at least a second magnetic circuit encircling at least one of the second slots in which the first winding is disposed, the second magnetic circuit sharing a common portion of the stator with the first magnetic circuit and including a magnetically saturable portion distinct from the common portion; a stator extension coaxial with the stator and extending axially beyond the rotor, the stator extension having slots in registry with corresponding ones of the second slots; at least a second winding disposed adjacent to the first winding in a plurality of said second slots and in their corresponding slots in the stator extension; a third magnetic circuit defined in the stator and including the magnetically saturable portion of the stator, the third magnetic circuit operatively associated with a current passing through the second winding; and a fourth magnetic circuit defined in the stator extension, the fourth magnetic circuit operatively associated with the current passing through the second winding for increasing inductance in the electric machine. 
     In another aspect, the concept provides an electric machine comprising: a rotor and a stator, the stator having a plurality of first slots and a plurality of second slots, the second slots being located on an opposite side of the first slots relative to the rotor; at least one power winding disposed in the first and second slots; at least one control winding disposed adjacent to the power winding in the second slots and around a magnetically saturable portion of the stator; and means for increasing inductance of the control winding regardless of saturation in the magnetically saturable portion. 
     In another aspect, the concept provides a method of controlling an electric machine having at least one power winding and at least one control winding, the machine also having first, second and third magnetic circuits and a control device, the first magnetic circuit including a rotor and encircling at least a first portion of the power winding, the second magnetic circuit encircling at least a second portion of the power winding remote from the first magnetic circuit and at least a portion of the control winding, the first and second magnetic circuits coupled when current flows in the power winding, the second magnetic circuit including a portion which is selectively magnetically saturable using the third magnetic circuit, the method comprising: maintaining a minimum inductance in the control winding when the magnetically saturable portion is at a fully saturated level; and controlling at least one among an output voltage and an output current of the electric machine using the control device regardless of the saturation level of the magnetically saturable portion. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding and to show more clearly how the present concepts may be carried into effect, reference will now be made by way of example to the accompanying figures, in which: 
         FIG. 1  is an isometric view of an example of an inductance augmenter that is used with an electric machine stator; 
         FIG. 2  is an enlarged view of some of the slots in the inductance augmenter of  FIG. 1 ; 
         FIG. 3  is an isometric view showing the stator control windings extending around the inductance augmenter and the stator of  FIG. 1 ; 
         FIG. 4  is a somewhat schematic radial cross-sectional view of a portion of the inductance augmenter; and 
         FIG. 5  is a view similar to  FIG. 4 , showing another example of an inductance augmenter. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a stator  20  generally in accordance with U.S. Pat. No. 7,262,539, the contents of which patent are hereby fully incorporated by reference into this application. Briefly, stator  20  comprises teeth  26 , extending from a back iron or control flux bus  32 , through a primary flux bus  30 . Windings  22  and  24  (see  FIG. 3 ) are provided on the stator  20  (and the stator extension  40 , as will be described further below), about an inside rotor (not indicated). Other details are available in U.S. Pat. No. 7,262,539, and need not be repeated here. 
     When the electric machine  10  is used as a motor, and is driven using for instance the techniques disclosed in co-pending U.S. patent application Ser. No. 11/964,123, inductance in the control circuit is used as part of a buck regulation circuit of the drive architecture. It can also be used as part of a buck regulator, using for instance the technique described in U.S. patent publication No. 20080019062 published Jan. 24, 2008. Because of the use of the inductance of the control windings  24  as a filter component of the buck regulator, it is desirable to always maintain the inductance above a minimum value. 
     Referring again to  FIG. 1 , an example of an inductance augmenter arrangement is provided in the form of a coaxial stator extension  40  juxtaposed to the side of the stator  20  and is therewith. It has a shape and cross section complementary to that of the stator  20  for receiving a control winding  24 , as will be described further below. The stator extension  40  of the illustrated example has two distinct segments  40   a ,  40   b , to thereby provide a “dual channel” configuration of the type described in U.S. Pat. No. 7,262,539. Segments  40   a ,  40   b  are separated by air spaces  41  at both ends. Although a two-part inductance augmenter  40  is shown, other designs are also possible. Each segment  40   a ,  40   b  corresponds to a distinct machine side. It is also possible that only one side of the electric machine  10  be provided with a stator extension segment. Still, it is possible to provide less that all control windings  24  on the stator extension  40 . 
     As shown in  FIG. 2 , the stator extension  40  has at least one row of slots  42  and has outer teeth  44  that are complementary to those of the stator  20 . In the illustrated example, two concentric and spaced-apart annular members  45 ,  46  are provided. These annular members  45 ,  46  are segmented, however, since the stator extension  40  has two segments  40   a ,  40   b . The inner annular member  46  of each segment of the stator extension  40  is also referred hereafter to as the solid flux bus. The slots  42  are separated from each other by a plurality of radially-extending walls  47 . The outer-teeth  44  are in registry with the walls  47 . Each slot  42  in the illustrated example also incorporates a discontinuity, in this case an individual air gap  48 , across the entire width of the outer annular member  45 , for preventing the flux density from reaching a saturation level in the stator extension  40  even though the control flux bus  32  in the stator  20  is fully saturated, as will be described further below. Stator extension  40  may also be provided without air gaps  48  and use, for instance, a material having a lower magnetic permeability than that of the magnetically saturable portion of the control flux bus  32  in the stator  20 . 
       FIG. 3  shows the electric machine  10  as assembled with the stator extension  40 . As is apparent from  FIG. 3 , the rotor and stator generally have the same axial length, and thus the stator extension  40  causes the control windings  24  thereon to extend well beyond the end of the rotor. The increase provided by stator extension  40 , discussed further below, in the non-saturable inductance is directly proportional to the length of the stator extension  40 . The skilled reader will appreciate that inductance is directly proportional to the magnetic cross section, (i.e. magnetic circuit cross-sectional area) which is proportional to stator extension  40  length. The stator extension  40  is provided on one or both sides of the stator  20  and the control windings  24  are wound thereon. The stator extension  40  is thus formed as a part of the electric machine  10 . The machine  10  and stator extension  40  may share the same cooling system and use the same coils as control windings  24 . This may reduce the overall weight, improves efficiency, reduces the size of the electronic system and reduces the part count. 
     In use, the basic operation inside the stator  20  in the electric machine  10  incorporating stator extension  40  is substantially similar to that shown and described in U.S. Pat. No. 7,262,539, with the exception that in use, a minimum inductance in the control winding  24  is maintained even when the magnetically saturable portion in the stator  20  is at a fully saturated level, as will now be described. Within the stator  20 , first, second and third magnetic circuits are set up, as described in U.S. Pat. No. 7,262,539. The stator extension  40 , however, generates a “fourth” magnetic circuit, as described with reference to  FIG. 4  below, so as to increase the inductance of the control winding  24 , particularly when the control winding  24  is operated at a high saturation current level required to minimize the power winding inductance or inductive reactance. The stator extension  40  is arranged such that it remains un-saturated at currents normally capable of fully saturating the control flux bus  32 . In this way, a minimum inductance in the control winding circuit can be maintained even when the control flux bus  32  is fully saturated, and this additional inductor remains as a part of the machine assembly and control winding  24  structure within the machine  10 , as opposed to being a separately provided inductor. The stator extension  40  increases the inductance of the control windings  24  compared to the inductance of the control windings of a machine without it, for a given current through the control windings  24  provided from the source. Thus, the inductance augmenter arrangement increases the inductance regardless of the saturation level of the magnetically saturable portion. 
       FIG. 4  shows an example of the magnetic circuit  50  inside a portion of the stator extension  40 . As can be seen, the magnetic path between both sides of the control windings  24 —one side corresponding to the upper (inner) slot row in the figure and the other side corresponding to the bottom (outer) slot row—adds to the magnetic flux circulating in the third magnetic circuit in the stator  20 . The magnetic path crosses the air gaps  48  in  FIG. 4 . It should be noted that the width of the air gaps  48  are larger for the purpose of illustration, and in practice are only as large as need be to provide the desired effect. The portion of the stator extension  40  between the two sides of the control windings  24  is in registry with the control flux bus  32  of the stator  20 . 
     The stator extension  40  may be provided on one or both sides of the stator  20 , and the control windings  24  are wound thereon. The new magnetic path(s) formed around the slots  42  results in a significant increase in the inductance of the control windings  24 , particularly when the control flux bus  32  is saturated. The stator extension  40  increases the inductance of the end turn section of the control winding  24  without saturating the stator  20  at the highest control current required for normal operation of the machine  10 . 
     When the machine  10  is used as a motor, and is driven using the techniques disclosed in applicant&#39;s co-pending application Ser. No. 11/420,602, filed May 26, 2006 (the entire contents of which are hereby fully incorporated by reference into this application), inductance in the control coil circuit is used as a part of a buck regulation circuit of the drive architecture, and in pending application Ser. No. 11/934,170, filed Nov. 2, 2007, (the entire contents of which are hereby fully incorporated by reference into this application), is used as a part of a buck regulator. Generally, when the machine is used in the motoring mode, the control winding current may be arranged such that the control flux bus  32  is saturated in both portions of the phase winding slots most of the time, in order to achieve the best performance (i.e. lowest possible reactance provides for the highest possible power factor). In the architectures referenced earlier in this paragraph, the control current and the phase current are effectively in series with each other, since the buck regulation circuit which uses the control coil inductance as a filter component, supplies the current delivered to the phase coils, and as such the control current and the phase current are always the same value except that the control current is always DC and the phase current is AC. Use of the control coil inductance as a filter component of the buck regulator is just one example where it becomes desirable to maintain the inductance of the control coil circuit above some minimum value, particularly when the control winding itself is at a saturated condition. 
       FIG. 5  illustrates a portion of another example of a stator extension  40 . The solid flux bus  46  is provided on the outside (corresponding to the bottom of the figure). However, the fourth magnetic circuit  50  is in the same direction as in  FIG. 4 . The fourth magnetic circuit  50  also crosses the air gaps  48 . The teeth  44  are on the inside. 
     As can appreciated, the new magnetic path  50  formed around the slots  42  results in a significant increase in the inductance of the control winding  24  of the electric machine  10  and a minimum inductance remains when the control flux bus  32  is saturated. This can be very useful in a wide variety of applications, including when the electric machine  10  is used in a gas turbine engine. 
     Still other modifications which fall within the scope of the present description will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. In this application, it is to be understood that the term ‘alternator’ is sometimes used generically to mean a device used for generating electricity, and is not always intended therefore to be limited to a device for generating an output alternating current. The electric machine needs not necessarily to be an alternator/motor, and can be exclusively an alternator or exclusively a motor. The rotor may include other arrangements that do not include permanent magnets. The rotor can also be designed as an outer rotor instead of an inside rotor as in the illustrated example. The stator extension does not need to be provided in two or more distinct sections. The discontinuities in the stator extension can include the presence of a non-magnetic material instead of an air gap. Other modifications can be devised as well.