Electric motor control

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.

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.

DETAILED DESCRIPTION

Referring first toFIGS. 1 and 2, a permanent magnet (PM) electric machine10is depicted. For ease of illustration and description,FIG. 2shows a linear arrangement of the electric machine10ofFIG. 1. However, it is to be understood that the machine10is generally preferred to have the circular architecture ofFIG. 1, with an inside or outside rotor (FIG. 1shows 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 machine10may 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 machine10will now be referred to as motor10.

The motor10has a rotor12with permanent magnets14, interposed by spacers16, which rotor12is mounted for rotation relative to a stator20. A retention sleeve (not shown) is typically provided to hold the permanent magnets14and the spacers16. Stator20has at least one phase winding22and preferably at least one control, winding24(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 stator20has a 3-phase design with three essentially electromagnetically-independent phase windings22(the phases are denoted by the circled numerals 1, 2, 3, respectively inFIG. 2) and, correspondingly, three control windings24. The phase windings22and control windings24are separated in this embodiment by a winding air gap26and are disposed in radial slots28, divided into slot portions28aand28b, provided in the stator20between adjacent teeth30. For ease of description, the adjacent slots23a,28bare indicated inFIG. 2as A, B, C, D, etc. The phase windings22are electrically insulated from the control windings24. A back iron32, also referred to as the control flux bus32in this application, extends between and at the bottom of the slots28b. A rotor air gap34separates rotor12and stator20in a typical fashion. A core or “bridge” portion, also referred to as the “power flux bus”36portion of stator20extends between adjacent pairs of teeth30in slot28to form the two distinct slots23aand28b. The first slots28ahold the phase windings22only, and the second slots28bhold both the phase windings22and control windings24.

The materials for the PM motor10may 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 windings22in this embodiment consists of a conductor with turns per slot, which enters, for instance, the first slot portion28aof a selected slot28(e.g. at slot “A”), extends through the slot and exits the opposite end of the slot, and then radially crosses the power flux bus36to enter the second slot portion28bof the same slot28(e.g. at slot “A”), after which it extends back through the length of the selected slot, to exit the second slot portion28b, and hence exits the slot28on 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 set28a,28b, before proceeding to the next relevant slot set in the stator. The conductor of phase winding22then proceeds to the second slot28bof the next selected slot28(e.g. slot “D” inFIG. 2), where the phase winding22then enters and passes along the slot28, exits and radially crosses the power flux bus36, and then enters the adjacent first slot portion28aof the selected slot28, and then travels through the slot again to exit slot28aand the stator adjacent where the winding entered the slot28bof the selected slot28. This path is also repeated times to provide the turns of the phase winding in this slot set28a,28b, before proceeding to the next relevant slot set in the stator. The phase winding then proceeds to the next selected slot28(e.g. slot “G”), and so the pattern repeats for second phase winding22corresponding to phase2(not shown), begins in an appropriate selected slot (e.g. slot B ofFIG. 2) and follow an analogous path, but is preferably wound in an opposite winding direction relative to winding22of phase1. That is, the phase2winding22would enter the selected slot (slot B) via slot portion28b(since phase1winding22entered slot A via slot portion28a, above), and then follows a similar hut opposite path to the conductor of phase1, from slot to slot (e.g. slots B, E, etc.). Similarly, the phase3winding22is preferably oppositely-wound relative to phase2, and thus enters the selected slot (e.g. slot “C”) of the stator via slot portion23a, and follows the same general pattern as phase1, but opposite to the pattern of phase2, from slot to slot (e.g. slots C, F, etc.). Thus, as mentioned, the phases of the phase winding22are oppositely-wound relative to one

another, for reasons described further below.FIG. 3shows an isometric free-space view of a portion of a phase winding22wound, as just, described, but for the fact that only two turns are shown for reasons of drawing clarity.

Meanwhile, a control winding (s)24is wrapped around the control flux bus32, in a manner as will now be described. In this embodiment, control winding24preferably 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 bus32(relative to the phase winding's 4 turns), for reasons described below. The control winding24and control flux bus32thus provide an integral saturable inductor in stator20, as will be discussed below. The direction of winding between adjacent second slots28bis preferably the same from slot to slot, and thus alternatingly opposite relative to the phase winding22of a same phase wound as described above, so that a substantially net-zero voltage is induced in each control winding24, as will also be described further below. Preferably, all loops around the control flux bus32are in the same direction. Note that the control winding24does 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 winding24as a result of current flow in the phase windings22, so that the function described below is achieved. If the control winding is segregated into phase correspondence with phase windings22, 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 windings22, which result in current flow in phase windings22and a primary magnetic flux along magnetic flux path or magnetic circuit60. Interaction of permanent magnets14and primary magnetic flux causes rotor12to move relative to stator20. When the current flow in phase windings22is appropriately controlled, the motor10rotates with a speed and torque. A current or voltage controller appropriately controls the current flow to the phase windings22such 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 circuit60includes rotor12, magnets14, rotor air gap34, power flux bus36and the portion of stator teeth30between rotor12and power flux bus36. Primary magnetic circuit60encircles a portion of phase winding22and is generated in motor10by the combined effect, of the rotor magnets and an electrical current in phase windings22. Secondary magnetic circuit62includes power flux bus36, control bus32and the portion of stator teeth30between control bus32and power flux bus36. In this embodiment, secondary magnetic circuit encircles the portions of the phase winding22and control winding24in slot28b. Power flux bus36divides slot28into two slot portions or openings28aand28b, with one opening28afor the phase winding only, and another opening28bfor the phase and control windings. The primary magnetic circuit encircles an opening28awhile the secondary magnetic circuit encircles an opening28b. Opening28ais preferably radially closer to the rotor than opening28b. Power flux bus36is common to both the primary and secondary AC magnetic circuit paths in this embodiment. AC current in the phase windings22causes a secondary magnetic flux to circulate secondary magnetic circuit62when the control bus64is 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 circuit64preferably circulates around control bus32, as partially indicated inFIG. 2(i.e. only a portion of the tertiary circuit is shown, as in this embodiment the tertiary circuit circulates around the entire stator20). The control flux bus32is 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 bus32, as will, be discussed further below. At least a portion of control flux, bus32is saturable by the flux density of the tertiary magnetic circuit.

Magnetic flux preferably circulates the tertiary magnetic circuit64in the same direction around the control flux bus32. As mentioned above, although the control winding24is provided in the second slots28bcorresponding to a particular phase of the three-phase machine described, the phase windings22are wound in the opposite direction in each first slot28awhich is due to the opposite polar arrangement of the magnets14associated with each adjacent first slot28aof the phase. To ensure that a uniform direction for the tertiary magnetic circuit64is provided, as mentioned, the control windings24are preferably wound in the same direction in ail second slots28b.

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 motor10are found in applicant'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 windings22, and control windings24. Since in one particular arrangement depicted inFIG. 5the 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 bus32is magnetically saturated by the action of current flowing through the control winding24, the inductance (thus impedance) of the phase windings22is 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 inFIG. 5the 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 winding24, 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's co-pending application Ser. No. 10/996,311.

FIG. 4shows a redundancy arrangement in which two motors10are co-mounted on the same output shaft66, and driven by suitable motor drives68, each in communication with a system controller69, and operated as described above. If one motor10should fail in a short circuit, open circuit or ground (whether in the motor itself or the drive electronics or lead wires), the drive(s)68preferably adjusts control of the remaining motor10(or motors10, 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 controller69provides the appropriate control to motor drives68. 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 detection84may 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. 5shows a simplified example control scheme for a motor drive68for driving a motor10. It should be noted that the motor10schematically depicted inFIG. 5depicts 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 motor10is driven by a motor drive68, preferably comprising a 3-phase H-bridge commutation circuit70driving the phase windings22of the motor10. 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 diode73reduces high amplitude voltage spikes occurring at the input of the inverter section of the H-bridge commutation circuit70due to the inductive effect of the control winding24of motor10. Current flow to the motor, and thus the motor's torque and speed, is adjusted using a suitable pulse width modulated supply system or “buck regulator” circuit72connected to control winding24of the motor10. 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 regulator72uses the control winding(s)24as its inductor, thus eliminating the need for an additional inductor, and consequently reducing the weight of the buck regulator72. 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 winding24is connected to the inverter section of the H-bridge commutation circuits70, such that a DC-current in the control winding24becomes AC current to the phase windings22of the motor10.

A feedback82of the drive current level is provided to a buck regulator controller74using a current sensor76. 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 regulator72varies the current flow to the phase windings22of the motor10, and thus controls the torque and speed of the motor10, based on an input torque/speed request78received from system controller69. Current is provided from a DC source80to the phase windings22, via the control winding24, as already described.

Referring again toFIG. 4, preferably both motors10and their associated controllers68are arranged as described with reference toFIG. 5, to provide a dual-redundant motor system. To enhance redundancy protection, preferably separate DC sources80are provided for each motor system. The operation of such a dual redundant system according toFIGS. 1-5will now be described.

Referring again toFIGS. 4 and 5, in a normal operation mode of the motors10, the drive68to each motor10is adjusted so that the motors contribute in desired proportions to the torque delivered to shaft66, and the shaft rotates at a desired speed, as requested by system controller69. Both motors10are preferably driven concurrently to provide torque and, when a higher efficiency operation or higher power operation is desired, the respective drives68can be adjusted accordingly to adjust the contribution proportion of each motor10. The control winding24of each motor10functions as the filter inductor for its respective buck regulation circuit, as described above. Also as described above, the control winding24of 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 motor10. Should one motor10fail, such as in a short circuit, open circuit or ground, the drive68to the other motor10can be adjusted using its buck regulator72to increase the AC input to the phase windings22of the operational motor10to compensate for the loss in torque caused by loss of the other motor10. As the skilled reader will appreciate, the failed PM motor10can tend to add drag and heat to the system, however with the present arrangement the failed motor10, 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 motor10to 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 controller69requests zero current, then the relevant buck regulator stops providing current accordingly. This control command is preferably based on the system, controller69detecting 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 motor10, 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. 6illustrates a 3-phase, “dual channel” m motor10* according to the general “multi-channel” principles described in applicant'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. Stator20of dual channel FM machine10′ 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 windings22and24will be described in terms of phase winding sets22A and22B and control winding sets24A and24B, 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,

Motor10′ 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 windings22A and22B and respective control windings24A and24B (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 motor10′ is otherwise generally as described above with respect to the single channel embodiment of motor10.

The dual channel FM motor10′ provides a single rotor rotating relative to two effectively independent stators, or stator sections. Thus, rotor12rotates relative to a stator sector20A (i.e. the portion of stator20with phase windings22A) and also relative to a stator sector20B (i.e. the portion of stator20with phase windings22B). When operated as a motor, the two “motors” (i.e., in effect, motors10′A and10′B) are driven independently, as described generally above with respect to motor10, but are synchronized such that they co-operate, as if only one “motor” is present. In normal motoring mode, the two “motors” (10′A and10′B) of motor10′ are operated as described above with respect to motors10inFIG. 4. Likewise, if one channel of the machine10′ should fail in a short circuit, open circuit or ground (whether in the motor10′ 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 toFIG. 6, the stator of the multi-channel motor10′ preferably includes means for impeding cross-talk between the tertiary magnetic circuits of channels A and B, such as is described in applicant'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 slit21acts to substantially contain, the tertiary magnetic within the channel. As such, the tertiary magnetic preferably travels along the entire length of the control flux bus32to the channel boundary, where the presence of the cross-talk reduction slit21redirects the flux up to power flux bus36, where it then travels back alexia entire length of the power flux bus36(this flux is not present, and therefore not depicted, in the single channel embodiment ofFIG. 2), until the path joins up again with the beginning of the tertiary path, in the vicinity of another cross-talk reduction slit21.

Referring toFIG. 7, a control system for dual-channel motor10′ is shown.FIG. 7is similar toFIG. 4, but for the configuration of motor10′ inFIG. 7relative to two motors10ofFIG. 4. Motor drives68A and68B are preferably each as described above with respect toFIG. 5, and this two independent motor drives are provided, one for each channel of motor10′. In use, a similar operation is obtained when the control scheme ofFIG. 5is applied to the dual channel motor10′ ofFIG. 7. Accordingly, in normal operation, channels A and. B may be operated separately, or conjunctively, and motor drives68A and68B are controlled accordingly by controller69. When, a failure is detected on one motor channel, the current flow in its respective control windings24is 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 ofFIGS. 6 and 7offers obvious size and weight savings over the two motors system as shown inFIGS. 4 and 5. The two motor design ofFIG. 4 and 5, however, has its own advantages over the dual-channel arrangement ofFIGS. 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 inFIG. 7, the dual motor arrangement ofFIG. 4, or as the case may be, the dual, channel motor ofFIG. 7, is controlled using a modified motor drive68′ in which buck regulator72has a dedicated filter inductor83independent from the control windings24. Separate DC current sources80and81respectively drive the phase and control windings independently from one another. Phase windings may be driven as described above with respect toFIGS. 5 and 7, so that torque is split as desired among the motors or channels in normal operation, during which time the DC source81provides 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 source81to 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 ofFIG. 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 ofFIGS. 1 and 4are 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 windings24of 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.