Abstract:
A method and apparatus is disclosed for controlling a system comprising at least one electric motor. The system includes aspects which permit, among other things, electromagnetically disconnecting a failed permanent magnet motor from said system, weight savings in motor control electronics, controllability benefits and other benefits.

Description:
TECHNICAL FIELD 
     The invention relates generally to electric motors and, more particularly, to the control of electric motors. 
     BACKGROUND 
     Motors, such as permanent magnet motors, may be controlled in a variety of ways, but the control electronics can often be heavy and lack compactness. Also, the control of multiple electric motors connected to drive one load typically requires a mechanical disconnect system to disconnect a failed motor from the load, since the failed motor may begin to operate as a generator, potentially creating drag torque and internal heating of the motor. Accordingly, there is a need to provide improvements which address these and other limitations of prior art motor control systems. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides an electric motor system comprising a motor having a magnetic rotor and a magnetically conductive stator, the stator having at least two windings connected with one another in series, the rotor and stator together defining at least a first magnetic circuit encircling a first portion of a first one of the stator windings, the stator defining at least a second magnetic circuit therein, a second one of the stator windings wrapped around a portion of the stator remote from the first magnetic circuit, the second stator winding and said portion of the stator thereby providing an inductor assembly, the second magnetic circuit passing through said stator portion and encircling a second portion of the first stator winding and a portion of the second stator winding, the second magnetic circuit remote from the first magnetic circuit and remote from the rotor, the motor system having a buck regulation apparatus connected in series between a direct current (DC) electricity source and the second winding, wherein the inductor assembly provides a filter inductor function for the buck regulator. 
     In another aspect, the present invention provides a method for controlling an electric motor system, the system including at least one motor having a magnetic rotor and a magnetically conductive stator having at least one winding, the rotor and stator together defining at least a first magnetic circuit encircling a first portion of a first one of the stator windings, the stator defining at least a second magnetic circuit therein, the second magnetic circuit encircling a second portion of the first stator winding, the second magnetic circuit remote from the first magnetic circuit and remote from the rotor, the method comprising the steps of operating the motor to drive an output shaft thereof, the step of operating including the step of saturating at least a portion of the second magnetic circuit to maintain an impedance of said winding at a first, value during operation, detecting a fault in the motor requiring motor shutdown, and then shutting down the motor, including the step of de-saturating at least said portion of the second magnetic circuit to increase the impedance of the winding to a second value, the second value significantly higher than the first value such that current flow in the winding is effectively limited to a desired value. 
     Further details of these and other aspects will he apparent from the detailed description and Figures included below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying Figures in which: 
         FIG. 1  is a cross-section of a permanent magnet motor; 
         FIG. 2  is a partial schematic of the motor of  FIG. 1 ; 
         FIG. 3  is a isometric view of a portion of a phase winding of the motor of  FIG. 1 ; 
         FIG. 4  is a schematic of an arrangement of two motors according to  FIG. 1 ; 
         FIG. 5  is a schematic diagram of a control, scheme arrangement for the motors of  FIG. 1  and/or  4 ; 
         FIG. 6  is a cross-sectional view, similar to  FIG. 1 , of a another arrangement for a motor; and 
         FIG. 7  is a schematic diagram of a control scheme for the motor of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1 and 2 , a permanent magnet (PM) electric machine  10  is depicted. For ease of illustration and description,  FIG. 2  shows a linear arrangement of the electric machine  10  of  FIG. 1 . However, it is to be understood that the machine  10  is generally preferred to have the circular architecture of  FIG. 1 , with an inside or outside rotor ( FIG. 1  shows an inside rotor, which is preferred but not required). It will also be understood by the skilled reader that the Figures, as well as the accompanying description, are schematic in nature, and that routine details of machine design, have been omitted for clarity, as will be apparent to the skilled reader. The machine  10  may be configured as an alternator to generate electrical power, a motor to convert electrical power into mechanical torque, or both. The motor aspects of such a machine are primarily of interest in the following description, and hence machine  10  will now be referred to as motor  10 . 
     The motor  10  has a rotor  12  with permanent magnets  14 , interposed by spacers  16 , which rotor  12  is mounted for rotation relative to a stator  20 . A retention sleeve (not shown) is typically provided to hold the permanent magnets  14  and the spacers  16 . Stator  20  has at least one phase winding  22  and preferably at least one control, winding  24  (both windings are represented schematically in the Figures as a solid rectangles in cross-section, but the skilled reader will appreciate each may comprise multiple turns of a conductor, as described below). In the illustrated embodiment, the stator  20  has a 3-phase design with three essentially electromagnetically-independent phase windings  22  (the phases are denoted by the circled numerals 1, 2, 3, respectively in  FIG. 2 ) and, correspondingly, three control windings  24 . The phase windings  22  and control windings  24  are separated in this embodiment by a winding air gap  26  and are disposed in radial slots  28 , divided into slot portions  28   a  and  28   b , provided in the stator  20  between adjacent teeth  30 . For ease of description, the adjacent slots  23   a ,  28   b  are indicated in  FIG. 2  as A, B, C, D, etc. The phase windings  22  are electrically insulated from the control windings  24 . A back iron  32 , also referred to as the control flux bus  32  in this application, extends between and at the bottom of the slots  28   b . A rotor air gap  34  separates rotor  12  and stator  20  in a typical fashion. A core or “bridge” portion, also referred to as the “power flux bus”  36  portion of stator  20  extends between adjacent pairs of teeth  30  in slot  28  to form the two distinct slots  23   a  and  28   b . The first slots  28   a  hold the phase windings  22  only, and the second slots  28   b  hold both the phase windings  22  and control windings  24 . 
     The materials for the PM motor  10  may be any one deemed suitable by the designer. Materials preferred by the inventor are samarium cobalt permanent magnets, copper phase and control windings, a suitable electromagnetic material(s) for the stator teeth and power and control flux buses, such, as electrical silicon steels commonly used in the construction of electromagnetic machines. The stator teeth, power and control flux buses may be integral or non-integral with one another, as desired. Each of the phase windings  22  in this embodiment consists of a conductor with turns per slot, which enters, for instance, the first slot portion  28   a  of a selected slot  28  (e.g. at slot “A”), extends through the slot and exits the opposite end of the slot, and then radially crosses the power flux bus  36  to enter the second slot portion  28   b  of the same slot  28  (e.g. at slot “A”), after which it extends back through the length of the selected slot, to exit the second slot portion  28   b , and hence exits the slot  28  on the same axial side of the stator as it entered. This path is repeated times to provide the 4 turns of the phase winding in that slot set  28   a ,  28   b , before proceeding to the next relevant slot set in the stator. The conductor of phase winding  22  then proceeds to the second slot  28   b  of the next selected slot  28  (e.g. slot “D” in  FIG. 2 ), where the phase winding  22  then enters and passes along the slot  28 , exits and radially crosses the power flux bus  36 , and then enters the adjacent first slot portion  28   a  of the selected slot  28 , and then travels through the slot again to exit slot  28   a  and the stator adjacent where the winding entered the slot  28   b  of the selected slot  28 . This path is also repeated times to provide the turns of the phase winding in this slot set  28   a ,  28   b , before proceeding to the next relevant slot set in the stator. The phase winding then proceeds to the next selected slot  28  (e.g. slot “G”), and so the pattern repeats for second phase winding  22  corresponding to phase  2  (not shown), begins in an appropriate selected slot (e.g. slot B of  FIG. 2 ) and follow an analogous path, but is preferably wound in an opposite winding direction relative to winding  22  of phase  1 . That is, the phase  2  winding  22  would enter the selected slot (slot B) via slot portion  28   b  (since phase  1  winding  22  entered slot A via slot portion  28   a , above), and then follows a similar hut opposite path to the conductor of phase  1 , from slot to slot (e.g. slots B, E, etc.). Similarly, the phase  3  winding  22  is preferably oppositely-wound relative to phase  2 , and thus enters the selected slot (e.g. slot “C”) of the stator via slot portion  23   a , and follows the same general pattern as phase  1 , but opposite to the pattern of phase  2 , from slot to slot (e.g. slots C, F, etc.). Thus, as mentioned, the phases of the phase winding  22  are oppositely-wound relative to one 
     another, for reasons described further below.  FIG. 3  shows an isometric free-space view of a portion of a phase winding  22  wound, as just, described, but for the fact that only two turns are shown for reasons of drawing clarity. 
     Meanwhile, a control winding (s)  24  is wrapped around the control flux bus  32 , in a manner as will now be described. In this embodiment, control winding  24  preferably forms loops wrapped preferably in a positive turns ratio relative to the phase winding. In this case, a control-to-phase turns ratio of 3:2 is preferred, such that the control winding is wrapped 6 times around the control flux bus  32  (relative to the phase winding&#39;s 4 turns), for reasons described below. The control winding  24  and control flux bus  32  thus provide an integral saturable inductor in stator  20 , as will be discussed below. The direction of winding between adjacent second slots  28   b  is preferably the same from slot to slot, and thus alternatingly opposite relative to the phase winding  22  of a same phase wound as described above, so that a substantially net-zero voltage is induced in each control winding  24 , as will also be described further below. Preferably, all loops around the control flux bus  32  are in the same direction. Note that the control winding  24  does not necessarily need to be segregated into phases along with the phase windings, but rather may simply proceed adjacently from slot to slot (e.g. slots A, B, C, D, etc.). Although it is preferred to alternate winding direction of the phase windings, and not alternate direction of the control windings, the phase and control windings are preferably wound in relative opposite directions and in equal slot numbers to ensure a substantially net-zero voltage is induced in each control winding  24  as a result of current flow in the phase windings  22 , so that the function described below is achieved. If the control winding is segregated into phase correspondence with phase windings  22 , for example to reduce its inductance by a series parallel arrangement, preferably there are equal numbers of slots of a given phase in which the phase winding and control winding are wound in opposite directions, to yield the desired induced net-zero voltage. 
     In use, in a motor mode, a 3-phase power source drives phase windings  22 , which result in current flow in phase windings  22  and a primary magnetic flux along magnetic flux path or magnetic circuit  60 . Interaction of permanent magnets  14  and primary magnetic flux causes rotor  12  to move relative to stator  20 . When the current flow in phase windings  22  is appropriately controlled, the motor  10  rotates with a speed and torque. A current or voltage controller appropriately controls the current flow to the phase windings  22  such that an appropriate speed and torque is obtained. The current in the control windings in normal operation of the motor is substantially the same as the current flow in the phase windings, because they are connected in series, except that in this embodiment current is preferably DC in the control windings, and AC in the phase windings. The implications for motor control will be discussed further below. 
     Primary magnetic circuit  60  includes rotor  12 , magnets  14 , rotor air gap  34 , power flux bus  36  and the portion of stator teeth  30  between rotor  12  and power flux bus  36 . Primary magnetic circuit  60  encircles a portion of phase winding  22  and is generated in motor  10  by the combined effect, of the rotor magnets and an electrical current in phase windings  22 . Secondary magnetic circuit  62  includes power flux bus  36 , control bus  32  and the portion of stator teeth  30  between control bus  32  and power flux bus  36 . In this embodiment, secondary magnetic circuit encircles the portions of the phase winding  22  and control winding  24  in slot  28   b . Power flux bus  36  divides slot  28  into two slot portions or openings  28   a  and  28   b , with one opening  28   a  for the phase winding only, and another opening  28   b  for the phase and control windings. The primary magnetic circuit encircles an opening  28   a  while the secondary magnetic circuit encircles an opening  28   b . Opening  28   a  is preferably radially closer to the rotor than opening  28   b . Power flux bus  36  is common to both the primary and secondary AC magnetic circuit paths in this embodiment. AC current in the phase windings  22  causes a secondary magnetic flux to circulate secondary magnetic circuit  62  when the control bus  64  is not in a saturated state. The primary and secondary magnetic circuits are non-overlapping (i.e. non-intersecting), and remote or isolated from one another. The second magnetic circuit is remote from, and does not include, the rotor and is preferably defined wholly within the stator assembly. 
     A tertiary magnetic circuit  64  preferably circulates around control bus  32 , as partially indicated in  FIG. 2  (i.e. only a portion of the tertiary circuit is shown, as in this embodiment the tertiary circuit circulates around the entire stator  20 ). The control flux bus  32  is preferably common to both the secondary and tertiary magnetic circuit paths and thus the secondary and tertiary magnetic circuits are snare a common portion, namely the control bus  32 , as will, be discussed further below. At least a portion of control flux, bus  32  is saturable by the flux density of the tertiary magnetic circuit. 
     Magnetic flux preferably circulates the tertiary magnetic circuit  64  in the same direction around the control flux bus  32 . As mentioned above, although the control winding  24  is provided in the second slots  28   b  corresponding to a particular phase of the three-phase machine described, the phase windings  22  are wound in the opposite direction in each first slot  28   a  which is due to the opposite polar arrangement of the magnets  14  associated with each adjacent first slot  28   a  of the phase. To ensure that a uniform direction for the tertiary magnetic circuit  64  is provided, as mentioned, the control windings  24  are preferably wound in the same direction in ail second slots  28   b.    
     When the control flux bus is magnetically saturated, the inductance (thus impedance) of the phase windings is very low, as if there where no secondary AC magnetic circuit. However, if zero current is applied to the control winding (i.e. the control winding is open circuited, or otherwise switched off), the impedance of the phase windings increases significantly, thus limiting the current that can flow in the phase windings, which may be used to remediate, for example, a faulted condition, such as an internally shorted phase winding or short circuits in the drive electronics. This impedance control has beneficial implications for PM motor control, discussed further below. 
     It is to be understood that the above description applies only to phase “1” of the described embodiment, and that similar interactions, etc. occur in respect of the other phases. Further details and aspects of the design and operation of motor  10  are found in applicant&#39;s co-pending application Ser. No. 10/996,411, filed Nov. 26, 2004, the contents of which is incorporated herein by reference. 
     Thus, in use, in a motoring mode, a power source drives phase windings  22 , and control windings  24 . Since in one particular arrangement depicted in  FIG. 5  the two are effectively connected in series, the control winding current is equivalent, (i.e. in magnitude) to the phase winding current. As a result of the 3:2 turns ratio between these two windings, the slightly higher number of turns in the control winding helps ensure that the control bus is always in a fairly saturated condition during normal motor operation, so as to enable efficient functioning of the motor at any drive current. As discussed above, although the AC flux in the phase windings tends to cancel out the DC flux in the control winding in the control bus sections where the flux directions are in opposition, the 3:2 turns ratio bias in the control coil, prevents the fluxes from actually cancelling. Thus, when the control flux bus  32  is magnetically saturated by the action of current flowing through the control winding  24 , the inductance (thus impedance) of the phase windings  22  is very low, as if there where no secondary AC magnetic circuit, and hence the control windings and secondary magnetic circuit would be essentially “invisible” to the motor during normal motor operation. 
     In the arrangement depicted in  FIG. 5  the number of turns on the control winding slots will typically be chosen to be more than the number of turns in the phase winding slots, such to ensure saturation of the control bus (however preferably not much into saturation, since some inductance in the control winding is a useful, inductor for the buck regulator filter function as described below) by having a preferably just marginally more ampere turns on the control winding than on the phase windings in the secondary magnetic circuit. The DC flux in the control bus typically dominates relative to the opposing AC flux density in the secondary magnetic circuit, holding the control bus in saturation down to quite low relative values of drive current provided, via the control winding to the phase windings, even under the effects of the counter fluxes from the phase windings (i.e. the portion of the phase winding carrying AC in the negative portion of the cycle tends to reduce saturation of the control flux bus, unless the control ampere turns are high enough to maintain saturation). 
     In use in a fault or shut-down mode, when the drive current to the motor is at or close to zero, i.e. such as when, the motor is shut down in response to a fault condition, the control bus de-saturates (as a result of no control current being supplied) and, as a result, the interaction between the primary and secondary magnetic circuits and the inductor-like effect of the control winding  24 , impedes any significant generated currents from flowing in the phase windings due to continued rotation of the shut-down motor and any short circuit failure in the main phase circuits. Further discussion is found in applicant&#39;s co-pending application Ser. No. 10/996,311. 
       FIG. 4  shows a redundancy arrangement in which two motors  10  are co-mounted on the same output shaft  66 , and driven by suitable motor drives  68 , each in communication with a system controller  69 , and operated as described above. If one motor  10  should fail in a short circuit, open circuit or ground (whether in the motor itself or the drive electronics or lead wires), the drive(s)  68  preferably adjusts control of the remaining motor  10  (or motors  10 , if there are more than two provided in total, and two or more are to remain operational in the event of the shutdown of one) to compensate for the resulting loss in torque, and the failed motor is no longer driven. The controller  69  provides the appropriate control to motor drives  68 . As described above, the failed motor is also in effect disconnected, by bringing current flow in its windings to zero, resulting in the impedance of the main phase windings of the failed motor increasing to a high value, as previously described, such that the drag torque due to a short circuit type failure is minimised. Motor failure detection  84  may be achieved using any suitable approach, such as incorrect speed or torque as a function of current, voltage, high temperature, machine impedance, etc. Failure detection preferably results in a signal provided to an appropriate controller for interrupting the current supply to the motor system (i.e. bringing current flow to zero, as mentioned above). 
       FIG. 5  shows a simplified example control scheme for a motor drive  68  for driving a motor  10 . It should be noted that the motor  10  schematically depicted in  FIG. 5  depicts only a single control winding for the 3 phases of its associated phase winding set, the control winding proceeding slot-to-slot in the stator irrespective of the phase arrangements of the phase windings. As discussed generally above, this is just one of many control winding arrangements possible, and the skilled reader will be able to apply the present teachings to such arrangements in light of the teachings herein. 
     The motor  10  is driven by a motor drive  68 , preferably comprising a 3-phase H-bridge commutation circuit  70  driving the phase windings  22  of the motor  10 . The commutation scheme is preferably a six step 120-degree overlapping scheme in a “drake before break” sequence. This sequence in conjunction with a feedback diode  73  reduces high amplitude voltage spikes occurring at the input of the inverter section of the H-bridge commutation circuit  70  due to the inductive effect of the control winding  24  of motor  10 . Current flow to the motor, and thus the motor&#39;s torque and speed, is adjusted using a suitable pulse width modulated supply system or “buck regulator” circuit  72  connected to control winding  24  of the motor  10 . The buck regulator may be any suitable circuit. The skilled reader will appreciate that buck regulators typically require a filter inductor as an energy storage device for stepping down the voltage level. In this configuration, the buck regulator  72  uses the control winding(s)  24  as its inductor, thus eliminating the need for an additional inductor, and consequently reducing the weight of the buck regulator  72 . This filter inductor replacement role of the control winding may dictate design features of the control winding, as the designer will consider the buck regulator requirements as well as the motor requirements in providing a suitable control winding configuration. The output of the control winding  24  is connected to the inverter section of the H-bridge commutation circuits  70 , such that a DC-current in the control winding  24  becomes AC current to the phase windings  22  of the motor  10 . 
     A feedback  82  of the drive current level is provided to a buck regulator controller  74  using a current sensor  76 . The buck regulator and controller are of any suitable type, which includes suitable types well-known to the skilled reader, and thus need not be discussed further here. 
     In use, the buck regulator  72  varies the current flow to the phase windings  22  of the motor  10 , and thus controls the torque and speed of the motor  10 , based on an input torque/speed request  78  received from system controller  69 . Current is provided from a DC source  80  to the phase windings  22 , via the control winding  24 , as already described. 
     Referring again to  FIG. 4 , preferably both motors  10  and their associated controllers  68  are arranged as described with reference to  FIG. 5 , to provide a dual-redundant motor system. To enhance redundancy protection, preferably separate DC sources  80  are provided for each motor system. The operation of such a dual redundant system according to  FIGS. 1-5  will now be described. 
     Referring again to  FIGS. 4 and 5 , in a normal operation mode of the motors  10 , the drive  68  to each motor  10  is adjusted so that the motors contribute in desired proportions to the torque delivered to shaft  66 , and the shaft rotates at a desired speed, as requested by system controller  69 . Both motors  10  are preferably driven concurrently to provide torque and, when a higher efficiency operation or higher power operation is desired, the respective drives  68  can be adjusted accordingly to adjust the contribution proportion of each motor  10 . The control winding  24  of each motor  10  functions as the filter inductor for its respective buck regulation circuit, as described above. Also as described above, the control winding  24  of each motor preferably also keeps its respective control bus saturated (by virtue of the relative turns ratio between phase and control winding) to keep the control winding otherwise virtually “invisible” to the motor  10 . Should one motor  10  fail, such as in a short circuit, open circuit or ground, the drive  68  to the other motor  10  can be adjusted using its buck regulator  72  to increase the AC input to the phase windings  22  of the operational motor  10  to compensate for the loss in torque caused by loss of the other motor  10 . As the skilled reader will appreciate, the failed PM motor  10  can tend to add drag and heat to the system, however with the present arrangement the failed motor  10 , can be “turned off” by no longer energising the windings (i.e. and thus the current in the control winding is reduced to zero), which thus adjusts the failed motor  10  to a high impedance condition for the phase windings, as already described, thereby minimising drag and heat generation. The currents to the respective control windings and inverters is controlled by external control signals provided to the buck regulator circuits. If the system controller  69  requests zero current, then the relevant buck regulator stops providing current accordingly. This control command is preferably based on the system, controller  69  detecting a fault or other command to set the current to sera. The resulting adjustment of the impedance characteristics of the phase windings of the affected motor  10 , from low impedance during proper motor function to a high impedance in the failed condition, results in much improved operation and controllability, particularly in PM motors where rotor excitation cannot be independently controlled. 
       FIG. 6  illustrates a 3-phase, “dual channel” m motor  10 * according to the general “multi-channel” principles described in applicant&#39;s U.S. Pat. No. 6,965,133, but modified in accordance with the above teachings, as will now be discussed further. The same reference numerals are used to denote the analogous elements described with reference to the embodiments above, and thus all elements will not be redundantly described here. Stator  20  of dual channel FM machine  10 ′ is conceptually divided into an “A” half and a “B” half, thus providing a distinct stator sector for each channel, each channel provided with its own independent windings sets. Thus windings  22  and  24  will be described in terms of phase winding sets  22 A and  22 B and control winding sets  24 A and  24 B, as discussed further below. Other features associated with channels a and B are also described as “A” or “B”, specifically, to indicate their respective channels, 
     Motor  10 ′ has a multi-channel architecture (in this case, dual channel), in that a plurality of circumferentially distributed distinct and fully independent (i.e. electromagnetically separate) “sets” of phase and control windings are provided in each stator sector corresponding to the multiple channels. In this case, two such sets of 3-phase phase and control windings are provided, namely a 3-phase set of phase windings  22 A and  22 B and respective control windings  24 A and  24 B (which happen to be single phase in this embodiment). This multi-channel architecture provides a plurality of functional “motor elements” within the same machine structure, which may either be operated in conjunction, or independently, as desired. The construction of motor  10 ′ is otherwise generally as described above with respect to the single channel embodiment of motor  10 . 
     The dual channel FM motor  10 ′ provides a single rotor rotating relative to two effectively independent stators, or stator sections. Thus, rotor  12  rotates relative to a stator sector  20 A (i.e. the portion of stator  20  with phase windings  22 A) and also relative to a stator sector  20 B (i.e. the portion of stator  20  with phase windings  22 B). When operated as a motor, the two “motors” (i.e., in effect, motors  10 ′A and  10 ′B) are driven independently, as described generally above with respect to motor  10 , but are synchronized such that they co-operate, as if only one “motor” is present. In normal motoring mode, the two “motors” ( 10 ′A and  10 ′B) of motor  10 ′ are operated as described above with respect to motors  10  in  FIG. 4 . Likewise, if one channel of the machine  10 ′ should fail in a short circuit, open circuit or ground (whether in the motor  10 ′ itself, or in the drive electronics or lead wires), the drive to the remaining channel, is adjusted to compensate for the loss in torque, and the failed channel is no longer driven. The drive of the failed channel is effectively disconnected by bringing current flow in the windings to zero, resulting in the impedance of the main phase windings of the channel increasing to a high value, as previously described, such that the drag torque due to a short circuit type failure in the channel, is minimized. This multi-channel configuration offers two fully redundant systems (i.e. channel A and channel B) with a minimum, of hardware, thereby minimizing weight and space and increasing reliability- Channel failure detection may be achieved using any suitable approach, such as incorrect speed or torque as a function of current, voltage, high temperature, machine impedance, etc. 
     Referring again to  FIG. 6 , the stator of the multi-channel motor  10 ′ preferably includes means for impeding cross-talk between the tertiary magnetic circuits of channels A and B, such as is described in applicant&#39;s co-pending application Ser. No. 11/419,238 filed May 19, 2006. As described in that application, the presence of a cross-talk reduction feature, such a stator slit  21  acts to substantially contain, the tertiary magnetic within the channel. As such, the tertiary magnetic preferably travels along the entire length of the control flux bus  32  to the channel boundary, where the presence of the cross-talk reduction slit  21  redirects the flux up to power flux bus  36 , where it then travels back alexia entire length of the power flux bus  36  (this flux is not present, and therefore not depicted, in the single channel embodiment of  FIG. 2 ), until the path joins up again with the beginning of the tertiary path, in the vicinity of another cross-talk reduction slit  21 . 
     Referring to  FIG. 7 , a control system for dual-channel motor  10 ′ is shown.  FIG. 7  is similar to  FIG. 4 , but for the configuration of motor  10 ′ in  FIG. 7  relative to two motors  10  of  FIG. 4 . Motor drives  68 A and  68 B are preferably each as described above with respect to  FIG. 5 , and this two independent motor drives are provided, one for each channel of motor  10 ′. In use, a similar operation is obtained when the control scheme of  FIG. 5  is applied to the dual channel motor  10 ′ of  FIG. 7 . Accordingly, in normal operation, channels A and. B may be operated separately, or conjunctively, and motor drives  68 A and  68 B are controlled accordingly by controller  69 . When, a failure is detected on one motor channel, the current flow in its respective control windings  24  is set to zero in order to increase impedance of the phase windings and thereby minimise a drag torque and other undesirable effects otherwise brought on by the failed channel. 
     The dual-channel design of  FIGS. 6 and 7  offers obvious size and weight savings over the two motors system as shown in  FIGS. 4 and 5 . The two motor design of  FIG. 4 and 5 , however, has its own advantages over the dual-channel arrangement of  FIGS. 6 and 7 , such as simplicity of individual components. 
     The skilled reader will appreciate that a failure is not required to turn a channel or motor “off” as described above, but rather the approach may be used in any suitable situation where it is desired to shut a channel “off”, including as part of a normal operation scheme. 
     In another control scheme, depicted in  FIG. 7 , the dual motor arrangement of  FIG. 4 , or as the case may be, the dual, channel motor of  FIG. 7 , is controlled using a modified motor drive  68 ′ in which buck regulator  72  has a dedicated filter inductor  83  independent from the control windings  24 . Separate DC current sources  80  and  81  respectively drive the phase and control windings independently from one another. Phase windings may be driven as described above with respect to  FIGS. 5 and 7 , so that torque is split as desired among the motors or channels in normal operation, during which time the DC source  81  provides control current at a sufficient level to keep the control flux bus fully saturated at all times, for reasons already described. In the event of a channel failure, phase winding current in the other motor/channel is adjusted to compensate for the loss of torque due to the failed channel, while the current from source  81  to the control winding(s) for the failed channel is brought to zero to minimize the drag torque due to the failed channel, 
     In this embodiment, the control winding has different design constraints than the above embodiments, and thus the control winding may have a higher number of turns relative to the phase windings, to minimise the amount of control current required to saturate and maintain saturation in under the influence of desaturating fluxes from the main phases. 
     In the arrangement of  FIG. 7 , where the control current is supplied from a source separate from the phase windings, and is independently variable relative to the phase windings, if the phase winding current in the motor/channel exceeds a specific value, such as a desired maximum limit, the inductance of the phase winding will abruptly increase, tending to limit the current in the phase winding to that specific value or limit. This can be used to simplify the drive system, of very low impedance (i.e. high speed) PM motors. For example, the motor can be designed using this feature to intrinsically limit inrush current on start-up by appropriately designing this feature into the motor, such that other typical inrush limiting techniques, such as duty cycle control, may be omitted or operated at lower frequencies. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the number of phases in the motors could be varied and could be to any number. The motors may be single or multi-phase, single or multi-channel. The windings may have single or multiple turns per slot, the number of turns of a winding not; necessarily has to be a whole number. The number of phase windings does not necessarily have to equal the number of control windings, and one or more windings may perhaps be present in a slot. The windings may be any conductor(s) (i.e. single conductor, more than one wire, insulated, laminated, hits etc) or may be superconductors. In multiphase alternators, there may be delta or Y-connected windings in accordance with suitable techniques. There need not be an air gap between the phase and control windings, as long as the windings are electrically isolated from one another. The rotor can be any electromagnetic configuration suitable (i.e. permanent magnet rotor not necessary), and may be provided in an outside or inside configuration, or any other suitable configuration. Other winding configurations are possible, and those described above need not be used at all, or throughout the apparatus. Also, the magnetic circuits described can be arranged in the stator (and/or rotor) in any suitable manner. The magentic circuits need not be provided in the same stator, but rather the primary and secondary magnetic circuits may be provided in separate stator elements. Any suitable stator configuration may be used, and the stator of  FIGS. 1 and 4  are exemplary only. The stator need not be slotted as shown, nor slotted at all. The arrangement of the primary, secondary and tertiary magnetic circuits, and the arrangement of phase winding saturation apparatus(s) in the motors may be any suitable arrangement. Likewise, the stator and rotor may also have any suitable configuration. Although DC is preferred in the control windings  24  of the motor or channel, any suitable saturating arrangement may be used. For example, a suitable saturation apparatus may be provided using permanent magnetic means to selectively saturate a portion of the secondary magnetic circuit, rather than using the electromagnetic means of the control winding. Any suitable motor drive arrangement may be employed. The present technique may also be employed with stand-alone motors if desired, and redundant systems are not required, but merely one apparatus arrangement: which may benefit from the application of the above principles. Still other modifications which fall, within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fail within the appended claims.