Method for improving the torque increase rate by stator current bias in an electric machine

Abstract

A method of controlling an electric machine having separately excitable rotors and stators includes pulsing the electric machine and controlling the electric machine to an off state. Pulsing the electric machine includes generating magnetic flux in the rotor in two separate paths by simultaneously exciting the rotor with a direct current and the stator with a stator bias current. Pulsing the electric machine may also include terminating the stator bias current when a desired magnetic flux is generated in the rotor. Pulsing the electric machine may include applying stator flux to the stator such that the electric machine provides a pulse torque.

Description

Technical Field

[0001] The present disclosure relates to the pulsed control of an electromechanical device, and more specifically, to a method for improving the torque rise rate of an externally excited synchronous machine by stator current bias.

Background Art

[0002] The wound field synchronous machine includes a wound field synchronous motor (WFSM) and a wound field synchronous generator (WFSG). The wound field synchronous machine is also called an externally excited synchronous machine and has a separately excited rotor and a separately excited stator. This rotor can be powered through slip rings, a rotating magnet, or a capacitively coupled rotary power transformer. A direct current can be supplied to the rotor to excite the rotor and generate the magnetic flux of the rotor.

[0003] A multiphase inverter can be used to generate a magnetic flux in the stator, that is, a stator magnetic flux. For example, the multiphase inverter can be a three-phase inverter that generates a stator magnetic flux in the stator. The magnetic flux of the rotor interacts with the stator magnetic flux in the air gap between the stator and the rotor, causing the rotation of the rotor and generating electric power in the form of an electromotive force (EMF).

[0004] The current EESM is designed such that the amount of current applied to the rotor can be less compared to the current applied to the stator. For example, a current in the range of 10 - 90 Amp is applied to the rotor, and a current in the range of 100 - 1000 Amp, and in some cases exceeding 1000 Amp, is applied to the stator. As a result, the rotor has a large number of turns, high resistance and high inductance, a high time constant, and is not suitable for turning on and off at frequencies in the range of 5 - 100 Hz.

[0005] When the EESM is supplying a certain level of power, the high resistance and high inductance do not pose a problem. This is because once the magnetic flux of the rotor magnetic flux is established, the current applied to the rotor is not turned on and off, but is controlled to a certain level based on the maximum efficiency operating point of the EESM. However, when the EESM is turned on and off in a pulsed manner, such as during dynamic motor drive (DMD) control, it is necessary to turn on and off the rotor current as fast and efficiently as possible.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present disclosure generally relates to a method of controlling an electromechanical machine to increase the rate of rise of magnetic flux in the rotor of an EESM, and thus increase the rate of rise of torque provided by the EESM. For example, the method disclosed herein can shorten the time for the electromechanical machine to transition from zero or near-zero torque to a desired pulsed torque when the electromechanical machine is turned on and off in a pulsed manner. The method detailed herein can be used to improve the rate of fall of magnetic flux in the rotor of an EESM. In some embodiments, the method detailed herein can be implemented on a conventional EESM configured to operate in a continuous control mode without considering pulsed control. In certain embodiments, the method detailed herein can be implemented on an EESM modified for operation in a pulsed mode.

[0007] Furthermore, within a non - conflicting range, any of the embodiments or aspects described herein can be used in combination with any or all of the other embodiments or aspects described herein.

Brief Description of the Drawings

[0008] Various aspects of the present disclosure will be described below with reference to the drawings incorporated herein and forming a part of this specification:

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Figure 1

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Figure 3A

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Figure 3B

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DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, the present disclosure will be described more fully with reference to its exemplary embodiments with reference to the drawings. In the drawings, the same reference numerals in each of the plurality of figures indicate the same or corresponding elements. These exemplary embodiments are described such that the present disclosure is thorough and complete and fully conveys the scope of the present disclosure to those skilled in the art. Features from one embodiment or aspect can be combined with features from any other embodiment or aspect in any suitable combination. For example, the individual or collective features of a method aspect or embodiment can be applied to an apparatus, product, or component aspect or embodiment, and vice versa. The present disclosure can be embodied in many different forms and should not be construed as limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will meet the legal requirements to which it is applied. As used in this specification and the appended claims, the singular forms "a", "an", "the", etc. include plural referents unless the context clearly dictates otherwise. Further, in this specification, quantitative measures, values, geometric relationships, etc. may be referred to, but unless otherwise specified, any one or more of these (even if not all) may be approximate values that take into account possible acceptable variations, such as absolute values or those due to manufacturing tolerances or engineering tolerances, etc.

[0023] As used herein, the term "machine" is intended to be broadly construed to mean both electric motors and generators. Both electric motors and generators include a stator and a rotor with multiple poles and are very similar in structure. When the machine operates as a motor, it converts electrical energy into mechanical energy, and when it operates as a generator, it converts mechanical energy into electrical energy.

[0024] Modern electric machines have relatively high energy conversion efficiency. However, the energy conversion efficiency of most electric machines varies greatly depending on the operating load. In many applications, machines are required to operate under a variety of operating load conditions. As a result, machines typically operate at, or near, the highest level of efficiency at some times and at a low efficiency level at other times.

[0025] Battery-powered electric vehicles are a good example of electric machines that operate at a wide range of efficiency levels. In a typical driving cycle, an electric vehicle accelerates, cruises, decelerates, brakes, corners, etc. Within a specific rotor speed and / or torque range, the electric machine operates at, or near, the most efficient operating point, i.e., the "sweet spot". Outside of these ranges, the operation of the electric machine becomes less efficient. As the driving conditions change, the machine transitions between high and low operating efficiency levels while the rotor speed and / or torque change. If the electric machine can operate in the high-efficiency operating region for a large portion of the driving cycle, the vehicle's range for a given battery charge level will increase. Since the limited range of battery-powered electric vehicles is a significant commercial obstacle to their use, it is highly advantageous to extend the operating range of the vehicle. Therefore, there is a need to operate electric machines such as motors and generators at a higher level of efficiency.

[0026] This application generally relates to pulsed control of an externally excited synchronous machine (EESM) that is operable in a continuous or pulsed manner. With pulsed control, the machine is intelligently and intermittently turned on / off pulsed in order to (1) meet the operational requirements while (2) improving the overall efficiency compared to continuous control. More specifically, under selected operating conditions, the electric machine is pulsed intermittently at a more efficient energy conversion operating level to provide the desired average output more efficiently than can be achieved with continuous control. As a result of the pulsed control, the torque of the electric machine is intentionally modulated, but the modulation is managed such that the levels of noise and vibration are minimized for the intended application.

[0027] For the sake of brevity, the pulsed control of the EESM provided herein is described in the context of a three-phase electric winding field-oriented synchronous motor in a vehicle. However, this description should not be construed as limiting in any way. On the contrary, the pulsed control as described herein can be used for many types of electric winding field-oriented synchronous motor machines (i.e., meaning both electric motors and generators). Further, such pulsed control of an electric winding field-oriented synchronous machine can be used for any application, not just electric vehicles. In particular, the pulsed control can be used in systems that require lower acceleration and deceleration than vehicle applications, such as electric motors for heating, cooling, and ventilation systems. Pulsed engine control is described in U.S. Patent Publication No. 2019 / 0288629, which is hereby incorporated by reference in its entirety.

[0028] Wound-field synchronous machine A wound-field synchronous machine is a motor or generator that can convert electricity into mechanical motion or mechanical motion into electricity without using permanent magnets. There are a wound-field synchronous motor (WFSM) and a wound-field synchronous generator (WFSG) in the wound-field synchronous machine. The wound-field synchronous machine is also called an externally excited synchronous machine (EESM). The WFSM includes a wound-field synchronous rotor in which the field coil (also called the field winding) is arranged on the rotor and the armature phase winding is arranged on the stator. In the WFSM, the field coil receives power from a DC power source. In most WFSMs, the armature winding receives power from an AC power source. In the WFSM, slip rings may be used to provide an electrical connection between the DC power source and the field coil on the rotor. In some embodiments, an air gap may be used to provide an electrical connection to the field coil. A DC motor arranges the field coil on the stator and uses a commutator connected to the rotor to convert DC power into AC power.

[0029] Three-phase externally excited synchronous machine In a three-phase EESM, the stator can include a three-coil winding excited by a three-phase AC input and a field coil on the rotor that receives power from a DC input. When a three-phase AC input is passed through the three-phase armature winding, a rotating magnetic field (RMF) is generated. The rotational speed of the RMF is known as the synchronous speed (N s ) of the electromechanical device. Due to the interaction between the field coil of the rotor and the armature winding, an electromagnetic force (EMF) is generated, causing the rotor to rotate.

[0030] Vehicle motor efficiency map Referring to FIG. 1, an exemplary vehicle motor efficiency map 10 under different load and speed conditions is illustrated. The map 10 plots the torque (N*m) along the vertical axis as a function of the motor speed (RPM) along the horizontal axis. The maximum steady-state output power is given by curve 12. An example of a vehicle motor efficiency map is shown to illustrate the improvement in the efficiency of the EESM by pulse-width modulation control of the EESM.

[0031] The area below the peak torque / speed curve 12 is mapped into a plurality of areas, and each area is labeled with an operating efficiency percentage. For the particular motor shown, the following characteristics are evident: · The most efficient, or "sweet spot" area within the operating range is the operating area labeled 14, which is generally in the range of 4,500 to 6,000 RPM, and the torque output is in the range of approximately 40 to 70 N*m. In area 14, the energy conversion efficiency is on the order of 96%, which becomes the "sweet spot" where the motor operates in its most efficient operating range. · When the motor speed exceeds approximately 6,000 RPM, the efficiency tends to decrease regardless of the output torque. · When the output torque exceeds 70 N*m or is less than 40 N*m, the efficiency percentage tends to decrease from the peak and, depending on the situation, may decrease significantly. For example, when the motor operates at approximately 2,000 RPM and the output torque is 100 N*m, the efficiency is approximately 86%. When the torque output is less than approximately 30 N*m, the efficiency decreases regardless of the motor speed and approaches zero at zero load. · At any particular motor speed, there is a corresponding most efficient output torque, which is formulated by the maximum efficiency curve 16.

[0032] The illustrated map 10 is obtained from an electric motor used in a 2010 model Toyota Prius that utilizes an interior permanent magnet synchronous motor. It should be understood that this map 10 is merely illustrative and should not be construed as limiting in any way. Similar maps can be generated for any electric motor, such as a three-phase induction motor, regardless of whether it is used in a vehicle or for other applications.

[0033] As can be seen from Map 10, a motor is generally most efficient when operating within the speed and torque range of the sweet spot 14. If the operating conditions can be controlled such that the proportion of time the motor operates at or near the sweet spot 14 increases, the overall energy conversion efficiency of the motor can be significantly improved.

[0034] However, from a practical perspective, in many operating situations, the motor is defined to operate outside the speed and torque range of the sweet spot 14. Electric vehicles usually do not have a transmission, so the ratio of the electric motor rotation speed to the wheel rotation speed is fixed. In this case, the motor speed can vary from zero when the vehicle is stopped to a relatively high RPM when cruising at highway speeds. The torque requirements can also vary significantly based on factors such as whether the vehicle is accelerating or decelerating, going uphill or downhill, driving on a flat road surface, or applying the brakes.

[0035] As shown in FIG. 1, at any given motor speed, there is a corresponding most efficient output torque, schematically represented by the maximum efficiency curve 16. From a conceptual perspective, when the desired motor torque is below the most efficient output torque for the current motor speed, the overall efficiency of the motor can be improved by operating the motor in pulses such that the proportion of time the motor operates at or near its sweet spot and the remaining time at a low torque or zero torque output level. The average torque generated in this way is controlled by controlling the duty cycle of the sweet spot operation.

[0036] Referring to FIG. 2, a graph 20 is shown with torque on the vertical axis and time on the horizontal axis. In the conventional operation, the motor continuously generates 10 N*m as indicated by the dashed line 22 as long as the desired torque is maintained at this value. In the pulsed control operation, as represented by the pulse 24, the motor is pulsed with a current pulse signal and supplies a torque of 50 N*m for 20% of the time. The motor is off for the remaining 80% of the time. Thus, the net output of the motor meets the operational requirement of an average torque level of 10 N*m. Since the motor operates more efficiently when supplying 50 N*m than when supplying 10 N*m continuously, by pulsing the motor using a 20% duty cycle while meeting the average torque requirement, the overall efficiency of the motor can be improved. Thus, the pulsed operation provides higher energy efficiency than the continuous operation.

[0037] In the above example, the duty cycle is not necessarily limited to 20%. As long as the desired motor output does not exceed 50 N*m, the desired motor output can be met by changing the duty cycle. For example, if the desired motor output is changed to 20 N*m, the duty cycle of the motor operating at 50 N*m can be increased to 40%. If the desired motor output is changed to 40 N*m, the duty cycle can be increased to 80%. If the desired motor output is changed to 5 N*m, the duty cycle can be decreased to 10%, and so on. Generally, pulsed motor control can potentially be advantageously used whenever the desired motor torque is below the maximum efficiency curve 16 in FIG. 1.

[0038] On the other hand, when the desired motor torque is above the maximum efficiency curve 16, the motor can supply the desired torque by operating conventionally (continuously i.e., non-pulsed). The pulsed operation provides an opportunity for efficiency gain when the motor is required to supply an average torque below the torque corresponding to its maximum operating efficiency point.

[0039] It should be noted that the torque values and time scales provided in FIG. 2 are merely illustrative and are not intended to be limiting in any way. In actual motor parsing embodiments, the pulse duration used can vary widely based on the design needs of a particular system. However, generally, the scale of the period of each on / off cycle is expected to be on the order of 10 milliseconds (ms) to 0.10 seconds (i.e., parsing at a frequency in the range of 10 Hz to 100 Hz). Further, there are a wide variety of EESMs, and each EESM has its own efficiency characteristics. Additionally, if the motor speed is different, the efficiency curve of the motor is also different. The nature of the curve can vary depending on a particular wound-rotor synchronous motor or a particular application. For example, the torque output does not have to be flat-topped as depicted in FIG. 2, and / or the torque does not have to go to zero during the off period and may be a non-zero value. However, regardless of the particular curve used, it is preferred that, for a certain percentage of the time the EESM is operating, it is in or near the maximum efficiency region of a given EESM.

[0040] Efficiency improvement by improving the rise and fall speed of rotor current Most current motor converters are typically designed for continuous operation rather than pulsed operation. Such motors generally transition from a non-energized state to an energized state relatively infrequently. Therefore, little design effort has been made to manage the rate of increase of the rotor current during such transitions. Even if design effort is expended to manage the transition, it is typically directed at achieving a smooth transition as opposed to a high-speed transition. As a result, in most motors, the transition from a non-energized state to an energized state is often speed-limited.

[0041] In an electric motor that regularly transitions from a non-energized motor state to a peak efficiency state, such as in a pulsed operation, it has been discovered that further efficiency improvements can be achieved if the transition occurs as quickly as possible (e.g., if the rate of increase of the rotor current is improved). For example, in a high-speed transition from zero torque to peak efficiency torque, the overall average motor efficiency improves because the time the motor spends in a transition where the efficiency is lower than the peak is reduced. This relationship is shown in FIGS. 3A and 3B.

[0042] Referring to FIG. 3A, a torque versus efficiency map of an exemplary electric motor operating at a fixed speed (e.g., 6000 rpm) is illustrated. In this map example, a torque output range from 0.0 Nm to 250 Nm is plotted along the horizontal axis, and the motor efficiency from 0.0 percent to 100 percent is plotted along the vertical axis. Curve 26 shows the motor transition from zero to peak efficiency torque. During this transition, as depicted in the shaded area 27, the peak efficiency torque has a rather low efficiency at peak efficiency torque 28.

[0043] Referring to FIG. 3B, a map showing torque versus lost work of an exemplary motor operating at a fixed speed during the transition from zero to peak efficiency torque is provided. In this map, the work loss (W) is plotted along the vertical axis, and the motor torque output is plotted along the horizontal axis. As shown by curve 29, the motor work loss increases as the torque output increases during the transition from zero to peak efficiency torque. Therefore, the faster the transition time from zero to peak efficiency torque, the less work is done and the less energy is consumed by the electric motor.

[0044] By substituting time for torque on the horizontal axis and integrating the area under curve 29, the energy consumed by the electric motor for a certain transition time can be calculated. For example, in one example of the motor, 7234.5 joules of energy were used for a transition time of 0.5 seconds, while only 723.4 joules of energy were used for a transition time of 0.05 seconds. This comparison shows that the faster the transition time from zero to peak efficiency torque, the less energy is consumed by losses. Note that in this example, it is assumed that no load acceleration occurred, so no energy was added to the load inertia. Efficiency increases by shortening the rise time, and efficiency also increases by shortening the fall time.

[0045] In different motors, the transition of the motor from zero to peak efficiency torque, peak efficiency torque, and work losses are different. Therefore, the maps in FIGS. 3A and 3B should be regarded as merely illustrative and should not be construed in a limiting sense in any way.

[0046] Power converter A power inverter is a known device used with an electric motor to convert a DC power source, such as that generated by a battery or capacitor, into a polyphase AC input power applied to the stator windings of the motor, for example, three-phase AC input power. In response, the stator windings generate the RMF as described above.

[0047] Referring to FIG. 4, a diagram of a power controller 30 for pulsed operation of an electromechanical device is shown. The power controller 30 includes a power converter 32, a DC power supply 34, and an electromechanical device 36. In this non-limiting embodiment, the power converter 32 includes a pulse controller 38. The power converter 32 can operate as a power inverter or a power rectifier depending on the direction of the energy flow through the system. When the electromechanical device operates as a motor, the power converter 32 is responsible for generating three-phase AC power from the DC power supply 34 to drive the electromechanical device 36. The three-phase input power denoted as phase A 37a, phase B 37b, and phase C 37c is applied to the windings of the stator of the electromechanical device 36 to generate the RMF as described above. Arrows are drawn at both ends of the lines indicating phase A 37a, phase B 37b, and phase C 37c, showing that current can flow from the power converter 32 to the electromechanical device 36 when the machine is used as a motor and from the electromechanical device 36 to the power converter 32 when the machine is used as a generator. When the electromechanical device 36 operates as a generator, the power converter 32 operates as a power rectifier, and the AC power coming from the electromechanical device 36 is converted into DC power stored in the DC power supply. The line 37d indicating the field current typically carries a DC field current in a single direction for both the motor and generator operating modes.

[0048] The pulse controller 38 is responsible for selectively pulsing the three-phase input power. In conventional (i.e., continuous) operation, the three-phase and field coil input powers are continuous, i.e., not pulsed. On the other hand, in pulsed operation, the three-phase and field coil input powers are pulsed. The pulsed operation can be implemented using any of the approaches described herein in non-limiting embodiments, for example, but not limited to, the approaches described below.

[0049] With reference to FIGS. 5A - 5F, plots are provided to illustrate the differences between continuous and pulsed three - phase and field current input powers applied to the electromechanical device 36. In each plot, the phase currents and the field current are plotted on the vertical axis, and time is plotted along the horizontal axis.

[0050] FIG. 5A shows the conventional sinusoidal three - phase input currents 42a, 42b, 42c supplied to the armature windings of the electromechanical device 36. The B - phase shown by curve 42b lags the A - phase shown by 42a by 120 degrees. The C - phase shown by curve 42c lags the B - phase by 120 degrees. The period of the sine wave is τ. The three - phase input currents 42a, 42b, 42c are continuous (non - pulsed), and the specified maximum amplitude is approximately 50 Amp. FIG. 5B shows the conventional DC field current 42d supplied to the field coil. This field current is continuous (non - pulsed) and has an amplitude of 5 Amp. It should be understood that 50 Amp (for the phase current supplied to the armature winding) and 5 Amp (for the field current supplied to the field coil) are only representative maximum currents, and the maximum current can have any value.

[0051] FIGS. 5C and 5D show an example of the pulsed three - phase current waveforms 44a, 44b, 44c shown in FIG. 5C and the pulsed DC field current 44d with a 50% duty cycle shown in FIG. 5D. For the three - phase waveforms 44a, 44b, 44c, the peak amplitude is approximately 100 Amp, and for the field current 44d, the peak amplitude is approximately 10 Amp. Similar to FIG. 5A, the period of the basic sine wave is τ, but now the sine wave is on / off modulated. The supply currents in FIGS. 5C and 5D supply the same average torque as the continuously applied three - phase input currents in FIGS. 5A and 5B (assuming torque is proportional to current - which is often the case). In FIGS. 5C and 5D, the current pulses 44a - d are interleaved with "off" periods of the same length. The length of each on - off period is 2τ. In this example, the duty cycle is 50%. The frequency of the pulse modulation can vary based on the type of electromechanical device used, considerations of noise and vibration, the current operating rotor speed, and other factors.

[0052] The examples of FIGS. 5C and 5D illustrate an application where "on" motor drive pulses are equally spaced while the motor is operating at a desired output level in a steady state. Such an approach works well in many situations but is not essential. The duty cycle need not be 50% and can be adjusted to match the desired average output torque. In FIGS. 5C and 5D, the phase of the on / off pulses is synchronized with the applied power, but in some embodiments, the phase of the on / off pulses need not be synchronized with the phase of the applied power. Thus, the relative magnitude and / or timing of the motor drive pulses can vary as long as they are averaged to provide the desired average torque.

[0053] This example shows how both the armature winding AC current and the DC field coil current are pulsed. This pulsing is designed to operate the EESM at an efficient torque level while reducing the amount of electrical power required to provide the desired torque level.

[0054] FIGS. 5E and 5F show another example of the pulsed three-phase current waveforms 46a, 46b, 46c shown in FIG. 5E and the pulsed DC field current 46d having a 50% duty cycle shown in FIG. 5F, where for the three-phase waveforms 46a, 46b, 46c, the peak amplitude is about 100 Amp and for the field current 46d, the peak amplitude is about 10 Amp. Similar to FIG. 5A, the period of the basic sine wave is τ, but now the sine wave is on / off modulated. The supply currents in FIGS. 5E and 5F supply the same average torque as the continuously applied three-phase input currents in FIGS. 5A and 5B (assuming torque is proportional to current - which is often the case). In FIGS. 5E and 5F, the current pulses 46a - d are interleaved with "off" periods of the same length. The length of each on / off period is τ / 2. In this example, the duty cycle is 50%. The frequency of the pulse modulation can vary based on the type of electromechanical device used, considerations of noise and vibration, the current operating rotor speed, and other factors.

[0055] Power converter circuit Due to the inherent inductance of the motor, the voltage / power steps during the on / off motor state can be transiently delayed / slowed down. In continuous (non-pulsed) operation, such transient effects tend to have a relatively small impact on the overall motor operation. However, when rapid pulsing is used as intended herein, the transient effects can have a greater net impact, thus motivating the shortening of the leading and trailing edge pulse transition times. This is particularly important for the field current that requires a significantly longer time to accumulate flux in the rotor than the stator to accumulate stator flux when current is applied to the armature winding.

[0056] As described above, the goal of pulsed motor control is to operate the electromechanical device 36 at substantially its most efficient level with respect to the current mechanical speed during the "on" period and to cut off the power (supply zero or negligible power) during the "off" period. For example, the power supplied during the "off" period may be less than 10%, less than 5%, less than 1%, less than 0.5%, or less than 0.1% of the power supplied during the "on" period. The operating point when operating during the "on" period may have an efficiency within 5%, 2%, or 1% of the maximum operating efficiency point of the motor at the current motor speed. The transition through the low-efficiency operating region between the "off" and "on" periods should be as fast as possible to maximize efficiency. Therefore, the power transition between the "on" and "off" states of the mechanical power ideally has a leading edge that transitions vertically straight up and a trailing edge that transitions vertically straight down. Such a "perfect" pulse 60 is schematically shown in FIG. 6A, which shows the ideal motor drive current versus time for a 50% duty cycle pulsed control. In this figure, the current pulse represents the field winding current. The current pulse is shown as having a flat top, but this is not necessarily the case.

[0057] In the real world, there are numerous practical constraints that make it difficult to generate such perfect pulses. For example, the inductive aspects of both the electromechanical 36 and the power converter 32 circuits slow down the rise and fall times of the current. The actual response of a particular machine varies depending on the electrical characteristics of the electromechanical 36, the rotational speed of the electromechanical machine, and the available bus voltage. Generally, the actual rise and fall of the pulse occur more gradually. That is, the transition takes time. The nature of the rise and fall in the real world is shown schematically in FIG. 6B. As can be seen here, there is a ramp-up period (rise time) 62 required for the current to actually rise from zero to the desired "on" power level, and a ramp-down period (fall time) 64 required for the current to actually fall from the "on" power level to zero.

[0058] During the power ramp-up and ramp-down periods, the wound-field synchronous machine 36 continues to consume or generate power. However, during these transition periods, the efficiency of the operation of the wound-field synchronous machine decreases. Generally, the efficiency of the wound-field synchronous machine decreases as the operating current decreases from the maximum efficiency condition (curve 16 in FIG. 1) towards zero. The energy conversion efficiency deteriorates significantly as the current level approaches zero. Thus, the pulse distortion represented by the current ramp-up period and the ramp-down period impairs the efficiency gain due to the pulsed operation. Generally, the smaller the ratio of the rise / fall time to the pulse length, the smaller the transient switching effect on the energy conversion efficiency of the machine during pulsing.

[0059] It should be understood that the transient effects shown in FIG. 6B are of an exemplary nature and do not necessarily reflect the actual rise / fall times associated with the operation of any particular wound field synchronous machine. The relative scale of the ratio of the rise time to the pulse length can vary significantly based on the characteristics of the wound field synchronous machine being used (which primarily define the rise and fall times), the frequency of the pulsing (which is primarily defined by the control scheme being used), and the pulse width (which is defined by the control scheme and the mechanical load). The voltage and mechanical rotational speed available to power the wound field synchronous electrical machine also impact the rise and fall times of the pulses. If the pulsing is slow compared to the response of the wound field synchronous machine, the rise / fall times will be a small percentage of the pulse width, and the transient switching effect may have a minimal impact on the performance of the machine. Conversely, if the pulsing is very fast and / or the response of the wound field synchronous machine is low, the rise / fall times will be a significant percentage of the pulse width and may even exceed the pulse width in some situations. Without careful management, the transient efficiency losses associated with switching can significantly reduce or even eliminate the theoretical gains achievable with the pulsed operation. Therefore, it is important to consider the transient switching effects associated with the pulsed operation when determining the pulsing frequency and control scheme suitable for a particular application.

[0060] As described above, in the continuous control of an electrical machine, there is no need to improve torque accumulation in the electrical machine. On the other hand, in pulsed control such as DMD, it is necessary to improve torque accumulation in the electrical machine.

[0061] One way to improve the torque accumulation rate in an electric machine is to maintain the rotor magnetic flux by continuously applying current to the rotor and modulating or pulsing the stator current. However, applying continuous current to the rotor reduces the efficiency of the electric machine, and in some cases, pulse control may be less efficient than continuous control. Therefore, it is necessary to improve the accumulation of the excitation magnetic flux when the rotor current is pulsed on / off.

[0062] By analyzing how the excitation magnetic flux of the EESM generates torque in the EESM, a method for increasing the torque accumulation rate in the EESM is shown. The torque of the EESM can be expressed by the following torque equation:

Equation

[0063] As detailed below, by using simultaneous excitation of both the rotor and the stator, the magnetic flux can be generated quickly and the torque accumulation rate of the electric machine can be improved. For example, using a first path that applies current i re to the rotor to generate magnetic flux, and applying a bias current i d to the statorBy utilizing a second path that generates magnetic flux by applying [something], the rise of magnetic flux in the electric machine can be improved, and the rate of increase in torque in the electric machine can be improved. By improving the rate of increase in torque in the electric machine, the transition time between the on state and the off state of the electric machine can be shortened.

[0064] Referring to FIG. 7, according to an embodiment of the present disclosure, a model of rotor current versus time for different stator bias currents is illustrated. The rotor current represents the torque generated by the electric machine. As shown in the model, the stator bias current is applied at 1 ms. In the illustrated model, the maximum torque of the electric machine is provided at a rotor current of 20 Amp, and the normal pulsed torque is provided in the range of rotor currents of 7 - 10 Amp.

[0065] As shown in the model, when only the rotor is excited as a single path, for example, when the stator bias current is zero (i d = 0), the electric machine takes 8.5 ms to supply a rotor current of 10 Amp and 10 ms to supply 20 Amp. The improvement in the rate of rise by supplying a DC current to the stator to excite the second path of the stator is shown. For example, when a stator bias current of 100 Amp of DC current is supplied to the stator, the time for the electric machine to supply 10 Amp is shortened to 5.5 ms, and the time to supply 20 Amp is shortened to 6.75 ms. Furthermore, by applying a stator bias current of 700 Amp of DC current, it is shown that the time for the electric machine to supply 10 Amp is shortened to 0.5 ms, and the time to supply 20 Amp is shortened to 0.75 ms.

[0066] It has been found that the stator bias current can be applied to the EESM simultaneously with the rotor current without modifying the existing EESM. For example, the power controller 30 is configured to simultaneously excite both the first path of the rotor and the second path of the stator during the transition of the electric machine from the off state to the on state, so that a stator bias current is applied to the stator, thereby improving the rise of the magnetic flux in the rotor. In an embodiment, the power controller 30 supplies the available full voltage to the stator and the rotor, and adjusts the phase angle of the stator voltage applied by saturating the stator current controller, so that each path is simultaneously excited, improving the maximum rise rate of the magnetic flux and accumulating the motive flux of the electric machine. The power controller 30 can achieve the required pulse torque in the shortest time by removing this voltage saturation and controlling the magnitude and phase of the stator bias current and the magnitude of the rotor current. The power controller 30 may control the voltage saturation so that the magnetic flux is accumulated in an efficient path for applying pulse control, such as DMD. In some embodiments, by using the symmetry of the control of the stator current parallel to the rotor current and shortening the time required to extract the magnetic field, the pulse of the electric machine can be quickly and efficiently turned off.

[0067] When the magnetic flux is accumulated in the rotor so that the electric machine generates a desired torque, such as a pulsed torque, the power controller 30 terminates the stator bias current, and the rotor current is maintained until the end of the pulse. In some embodiments, when the electric machine transitions from the on state to the off state or from the off state to the on state, the power controller 30 can smooth the transition between states to reduce the abruptness of the transition. This transition can improve the noise, vibration, or harshness characteristics of the electric machine.

[0068] The improvement in the rise of the rotor current described above has been verified in an electrical machine that realizes magnetic flux with a rotor current of 8 Amp. When the rotor current was applied without flowing the stator bias current, the electrical machine took 15.4 ms to achieve a rotor current of 8 Amp. On the other hand, when the rotor current was applied with a stator bias current of 300 Amp, the electrical machine achieved a rotor current of 8 Amp in 1 ms.

[0069] Next, referring to FIG. 8, a method for improving the torque rise rate in an electrical machine is provided in accordance with an embodiment of the present disclosure, and this is generally referred to as method 800. Method 800 will be described with reference to the exemplary electrical machine 36 and the power controller 30 of FIG. 4. This exemplary electrical machine 36 may be an EESM. As will be described in detail below, method 800 is used to increase the torque rise rate of each pulse of the electrical machine during the pulsed control of the electrical machine, for example, an EESM. However, method 800 may also be used for the transition of the electrical machine from an initial off state to a continuous on torque of the electrical machine.

[0070] As detailed above, if it is determined that the electrical machine 36 is more efficient when operating in pulsed control rather than continuous control, the power controller 30 may initialize the pulsed control of the electrical machine 36 (step 810). The decision to enter pulsed control can be made within the power controller 30 or within another controller that is in signal communication with the power controller 30. In pulsed control, the power controller 30 may determine the pulsed torque and duty cycle of the electrical machine 36, or the pulsed torque and duty cycle may be provided by another controller that is in signal communication with the power controller 30.

[0071] When the power controller 30 enters the pulsed control of the electric machine 36, the power controller 30 may transition from continuous control to pulsed control (step 815). In order to transition the electric machine 36 from continuous control to pulsed control, the power controller 30 terminates the supply of rotor current in the form of direct current to the rotor and stator magnetic flux in the form of polyphase AC current to the stator, and causes the electric machine 36 to enter the off state (step 820).

[0072] The electric machine is in the off state, and the power controller 30 pulses the electric machine 36 (step 830). To pulse the electric machine 36, the power controller 30 applies a direct current to the rotor of the electric machine 36 via the field line or path 37d to excite the rotor (step 832). While exciting the rotor, the power controller 30 also excites the stator with a stator bias current as a direct current via the phase lines 37a, 37b, and 37c (step 834). The power controller 30 may apply the stator bias current by saturating each of the phase lines 37a, 37b, and 37C. As detailed above, by exciting the rotor and the stator simultaneously, the rise of the magnetic flux of the electric machine 36 increases. When the electric machine 36 reaches the magnetic flux for pulse torque, the power controller 30 terminates the stator bias current (step 836) and applies the stator magnetic flux to the stator in the form of a polyphase alternating current (step 838). The power controller 30 can effect the transition or termination from the stator bias current to the stator magnetic flux by controlling the magnitude and phase of the current applied to each of the phase lines 37a, 37b, and 37c such that the electric machine 36 provides the desired torque. When the pulse is complete, the power controller 30 controls the electric machine 36 back to the off state (step 820) and repeats the pulsing of the electric machine until the electric machine 36 is returned to continuous control or is controlled to an extended OFF state (step 830). During the pulsing of the electric machine 36, each pulse (step 830) can have a different magnitude and / or duty cycle such that the electric machine 36 provides the desired torque. In some embodiments, controlling the electric machine 36 to the off state from the continuous control mode or after a certain pulse may include shortening the time for the excitation magnetic flux of the rotor to reach zero using the stator bias current. For example, the power controller 30 can shorten the time for the excitation magnetic flux of the rotor to become zero by modifying the magnitude and / or phase of the stator magnetic flux to the stator bias current.By shortening the time until the excitation magnetic flux of the rotor becomes zero, the torque ramp-down in the electric machine 36 can be improved.

[0073] The method 800 detailed above can be used without modification for an electric machine 36, such as an EESM, constructed or configured for operation in continuous control mode. In some embodiments, the method 800 detailed above may be combined with a modified electric machine. For example, in combination with the method detailed above, the number of turns of the rotor may be modified to increase the torque rise rate of the EESM. For additional information regarding changing the number of turns of the rotor to increase the torque rise rate of the EESM, reference can be made to U.S. Provisional Patent Application No. 63 / 322,376, filed on March 22, 2022, entitled "DELAY REDUCTION FOR PULSED WOUND FIELD SYNCHRONOUS MACHINES", the entire content of which is incorporated herein by reference.

[0074] FIG. 9 is a block diagram of an exemplary controller 900 that can perform one or more of the operations described herein, according to some embodiments. For example, the controller 900 can be used as the power controller 30 or the pulse controller 32 detailed above. The controller 900 can communicate signals by being integrated into or connected to other computing devices or controllers via a LAN, intranet, extranet, and / or the Internet. In some embodiments, only a single controller is shown, but the term "controller" can be interpreted to include any set of controllers that individually or jointly execute a set (or sets) of instructions for performing the methods described herein.

[0075] Exemplary controller 900 includes a processing device (e.g., a general-purpose processor, a PLD, etc.) 902, a main memory 904 (e.g., a synchronous dynamic random access memory (DRAM), a read-only memory (ROM)), and a static memory 906 (e.g., a flash memory, a data storage device 918), and these can communicate with each other via a bus 930.

[0076] The processing device 902 may be provided by one or more general-purpose processing devices such as a microprocessor, a central processing unit, etc. In an exemplary embodiment, the processing device 902 may be composed of a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or a processor implementing a combination of instruction sets. The processing device 902 may be composed of one or more dedicated processing devices such as an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), a DSP (Digital Signal Processor), a network processor, etc. The processing device 902 may be configured to execute the operations and steps discussed herein in accordance with one or more aspects of the present disclosure.

[0077] Computing device 900 may include a network interface device 908 that can communicate with communication network 920. Computing device 900 may include a video display unit (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device (e.g., a keyboard), a cursor control device (e.g., a mouse), and an acoustic signal generating device (e.g., a speaker). In one embodiment, the video display unit, the alphanumeric input device, and the cursor control device may be combined into a single component or device (e.g., an LCD touch screen).

[0078] Data storage device 918 may include a computer-readable storage medium 928 in which one or more sets of instructions 925 may be stored, the instructions 925 including instructions for one or more components (e.g., electromechanics 36) to perform the operations described herein. The instructions 925 may be present, in whole or at least in part, within main memory 904 and / or within processing device 902 during execution by computing device 900, and main memory 904 and processing device 902 constitute a computer-readable medium. The instructions 925 may be transmitted or received via communication interface 920 via interface device 908.

[0079] Although computer-readable storage medium 928 is illustratively shown as a single medium, the term "computer-readable storage medium" should be interpreted to include a single medium or multiple media (e.g., a centralized or distributed database and / or associated caches and servers) that store one or more sets of instructions. The term "computer-readable storage medium" can be interpreted to include any medium that is capable of storing, encoding, or carrying a set of instructions for machine execution and that causes a machine to perform the methods described herein. Thus, the term "computer-readable storage medium" shall include, but not be limited to, solid state memories, optical media, and magnetic media.

[0080] The embodiments described in this specification may relate to an apparatus for performing the operations described in this specification. This apparatus may be specially constructed for the required purposes, or it may be composed of a general-purpose computing device selectively programmed by a computer program stored in a computing device. Such a computer program may be stored in a non-transitory computer-readable storage medium.

[0081] The methods and exemplary embodiments described herein are not inherently related to a particular computer or other device. A variety of general-purpose systems may be used in accordance with the teachings described herein, and it may be convenient to construct more specialized devices to perform the required method steps. The structure required for such various systems is as described in the above description.

[0082] The above description is intended to be illustrative and not restrictive. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the present disclosure is not limited to the described embodiments. The scope of the present disclosure should be determined with reference to the following claims, along with the full scope of equivalents to which the claims are entitled.

[0083] As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises", "comprising", "includes", and / or "including", when used herein, may specify the presence of the stated feature, integer, step, operation, element, and / or component, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Accordingly, the terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting.

[0084] In some embodiments, the indicated functions / acts may occur out of the order indicated in the figures. For example, two figures shown in succession may in fact be executed substantially simultaneously or, depending on the related functions / acts, may sometimes be executed in the reverse order.

[0085] Although method operations are described in a particular order, other operations may be performed between the described operations, the described operations may be adjusted to occur at slightly different times, or the described operations may be distributed in a system that can generate processing operations at various intervals related to the processing, which should be understood.

[0086] Various units, circuits, or other components may be described or claimed as "configured to" or "configurable to" perform a task(s). In such contexts, the phrases "configured to" or "configurable to" are used to mean structure by indicating that the unit / circuit / component includes a structure (e.g., a circuit) that performs the task(s) during operation. Thus, it can be said that the unit / circuit / component is configured to perform the task or is configurable to perform the task even when the specified unit / circuit / component is not currently operating (e.g., is not turned on). Units / circuits / components used with the language "configured to" or "configurable to" include hardware - e.g., a circuit, a memory storing program instructions executable to perform an operation, etc. - and may include a general-purpose structure (e.g., a general-purpose circuit) that is operated by software and / or firmware (e.g., an FPGA or a general-purpose processor executing software) and operates in a manner capable of performing the described task(s). "Configurable to" may include adapting a manufacturing process (e.g., a semiconductor manufacturing facility) to fabricate a device (such as an integrated circuit) adapted to implement or perform one or more tasks. Stating that a unit / circuit / component is "configured to" perform one or more tasks or is "configurable to" perform one or more tasks is expressly intended not to contravene paragraph 6 of Title 35 of the United States Code."Configurable to ~" is expressly intended not to apply to blank media, unprogrammed processors or unprogrammed general-purpose computers, or unprogrammed programmable logic devices, programmable gate arrays, or other unprogrammed devices (except when accompanied by a programmed medium that gives the unprogrammed device the ability to be configured to perform the function(s) disclosed by the unprogrammed device).

[0087] The foregoing description has been presented for purposes of illustration and description of specific embodiments. However, the above exemplary discussion is not intended to be exhaustive or to limit the embodiments to the exact form disclosed. Many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the embodiments and their practical application, thereby enabling others skilled in the art to best utilize the embodiments and various modifications as are suited to the particular use contemplated. Accordingly, the embodiments are considered to be illustrative and not restrictive, and the embodiments are not limited to the details given herein, but may be changed within the scope of the appended claims and the equivalents thereof.

[0088] Although some embodiments of the present disclosure have been shown in the drawings, the present disclosure is not intended to be limited thereto, and the present disclosure is intended to be as broad as the technology permits and is intended to be read in the same manner herein. Any combination of the above embodiments is also envisioned and is within the scope of the appended claims. Accordingly, the above description should not be construed as limiting, but should be construed merely as illustrative of specific embodiments. Those skilled in the art will envision other modifications within the scope of the appended claims herein.

Claims

1. A method of controlling an electric machine having a separately excitable rotor and a stator, comprising: energizing the electric machine by: generating magnetic flux in the rotor in two separate paths by simultaneously exciting the rotor with a direct current and the stator with a stator bias current; applying a stator magnetic flux to the stator such that the electric machine provides torque; controlling the electric machine to an off state. A method as described above.

2. The method of claim 1, wherein energizing the electric machine includes terminating the stator bias current when a desired magnetic flux has been generated in the rotor.

3. The method of claim 2, wherein the desired magnetic flux is generated in the rotor when the rotor current reaches 8 Amp.

4. The method of claim 3, wherein exciting the rotor with the stator bias current includes the stator bias current being at least 300 Amp.

5. The method of claim 4, wherein the rotor current reaches 8 Amp in 1 ms.

6. The method of claim 2, wherein applying the stator magnetic flux occurs in response to a desired magnetic flux being generated in the rotor.

7. The method of claim 1, wherein energizing the electric machine generates a pulse torque.

8. The method of claim 1, wherein simultaneously exciting the rotor with a direct current and the stator with the stator bias current is configured to improve the rate of increase of magnetic flux in the rotor compared to exciting the rotor only.

9. The method of claim 1, wherein exciting the stator with the stator bias current includes exciting the stator with a direct current.

10. The method of claim 1, wherein terminating the stator bias current includes controlling the magnitude or phase of the current provided as the stator bias current for applying the stator magnetic flux.

11. The method of claim 1, wherein energizing the electric machine includes the electric machine being an externally excited synchronous machine (EESM) configured to operate in a continuous control mode.

12. The method of claim 11, wherein energizing the electric machine includes pulsing the EESM that has not been modified for operation in a pulsed control mode.

13. Ending the direct current to the rotor and the stator magnetic flux reduces the time for the magnetic flux of the rotor to reach zero by transitioning from the stator magnetic flux to a stator bias current before ending the direct current to the rotor, according to the method of claim 1.

14. Simultaneously exciting the rotor with a direct current and the stator with the stator bias current generates a desired magnetic flux in 1 ms, according to the method of claim 1.

15. A controller for controlling an electric machine having a separately excitable rotor and stator, a memory, a processing device operably coupled to the memory, generating magnetic flux in the rotor in two separate paths by simultaneously exciting the rotor with a direct current and the stator with a stator bias current, applying a stator magnetic flux to the stator so that the electric machine provides torque, and a processing device for controlling the electric machine to an off state by ending the direct current to the rotor and the stator magnetic flux. A controller comprising:

16. Applying the stator magnetic flux to the stator causes the electric machine to provide a torque pulse, according to the controller of claim 15.

17. The processing device further ends the stator bias current when a desired magnetic flux is generated in the rotor, according to the controller of claim 15.

18. A non-transitory computer-readable medium storing instructions that, when executed by the processing device, cause the processing device to control an electric machine having a separately excitable rotor and stator as follows: Generating magnetic flux in the rotor in two separate paths by simultaneously exciting the rotor with a direct current and the stator with a stator bias current, Applying a stator magnetic flux to the stator so that the electric machine provides torque, and Controlling the electric machine to an off state by ending the direct current to the rotor and the stator magnetic flux.

19. Applying the stator magnetic flux to the stator causes the electric machine to provide a torque pulse, according to the non-transitory computer-readable medium of claim 18.

20. The processing device further ends the stator bias current when a desired magnetic flux is generated in the rotor, according to the non-transitory computer-readable medium of claim 18.

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