Reduction of delays in pulsed wound-field synchronous machines.

Abstract

An electric machine is provided. A power converter is provided between a power source and the wound field synchronous machine. The power converter is configured to provide pulsed operation by supplying a pulsed direct current to a field winding of the wound field synchronous machine.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority to U.S. Application No. 63 / 322,376, filed March 22, 2022, which is incorporated by reference herein for all purposes. [Background technology]

[0002] This application relates generally to pulse control of electric machines to selectively provide a desired output in a more energy efficient manner, and more particularly to pulsed electric wound field synchronous machines. DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0003] The term "machine" as used herein is intended to be broadly interpreted to mean both electric motors and generators. Electric motors and generators are very similar in construction. Both include a stator and a rotor with a number of poles. When a machine is operating as a motor, it converts electrical energy into mechanical energy. When operating as a generator, it converts mechanical energy into electrical energy. [Means for solving the problem]

[0004] Various methods, controllers, and electromechanical systems are described that facilitate pulse control of a multiple electric machine (e.g., electric motor and generator) drive system and improve the energy conversion efficiency of the electric machines when operating conditions permit. More specifically, an electric machine is provided. A power converter is coupled between a power source and a wound field synchronous machine. The power converter is configured to provide pulsed operation by supplying a pulsed direct current to a field winding of the wound field synchronous machine.

[0005] In another embodiment, a method is provided for operating a wound field synchronous machine having a field winding and an armature winding, the pulsed operation being achieved by supplying a pulsed direct current to the field winding.

[0006] These and other features of the present disclosure are described in more detail below in the detailed description of the present disclosure and in conjunction with the following figures. [Brief description of the drawings]

[0007] The invention and its advantages are best understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

[0008] [Figure 1] FIG. 1 is a typical torque / speed / efficiency graph illustrating the energy conversion efficiency of a typical electric motor under different operating conditions.

[0009] [Diagram 2] FIG. 2 is a graph illustrating a pulsed torque signal applied to an electric motor.

[0010] [Figure 3A] FIG. 3A is a torque vs. efficiency map for a motor operating at a fixed speed during the transition from zero to peak efficiency torque.

[0011] [Figure 3B] FIG. 3B is torque versus lost work for an example motor operating at a fixed speed during the transition from zero to peak efficiency torque.

[0012] [Figure 4] FIG. 4 is a diagram illustrating a pulse controlled electric machine, according to a non-limiting embodiment.

[0013] [Figure 5A] FIG. 5A illustrates a continuous three-phase AC waveform having a peak value of 50 amps for the armature windings.

[0014] [Figure 5B] FIG. 5B is a graph of a continuous DC signal at 5 amps on the field winding.

[0015] [Figure 5C] FIG. 5C is a pulsed waveform with a 50% duty cycle that provides the same power output as the continuous waveform of FIG. 5A. [Figure 5D] FIG. 5D is a pulsed DC signal with a 50% duty cycle that provides the same power output as the continuous DC signal of FIG. 5B.

[0016] [Figure 5E] FIG. 5E is a pulsed waveform with a 50% duty cycle that provides the same power output as the continuous waveform of FIG. 5A. [Figure 5F] FIG. 5F is a pulsed DC signal with a 50% duty cycle that provides the same power output as the continuous DC signal of FIG. 5B.

[0017] [Figure 6A] FIG. 6A is a signal diagram illustrating an advantage of a non-limiting embodiment. [Figure 6B] FIG. 6B is a signal diagram illustrating an advantage of a non-limiting embodiment. [Figure 6C] FIG. 6C is a signal diagram illustrating an advantage of a non-limiting embodiment.

[0018] [Figure 7A] FIG. 7A is a graph showing current rise time. [Figure 7B] FIG. 7B is a graph showing current rise time.

[0019] [Figure 8A] FIG. 8A is a schematic diagram illustrating a portion of a single-turn wound field synchronous motor according to a non-limiting embodiment. [Figure 8B] FIG. 8B is a schematic diagram illustrating a portion of a single-turn wound field synchronous motor according to a non-limiting embodiment.

[0020] [Figure 9] FIG. 9 is a schematic diagram of a rotary transformer for use in a non-limiting embodiment.

[0021] [Figure 10] FIG. 10 is a schematic diagram of a power supply for use in a non-limiting embodiment.

[0022] [Figure 11] FIG. 11 is a flow diagram illustrating steps of a pulse control operation for an electric machine in a vehicle according to a non-limiting embodiment.

[0023] In the drawings, like reference numbers may be used to designate like structural elements, and it should be understood that the depictions in the figures are schematic and not to scale. BEST MODE FOR CARRYING OUT THEINVENTION

[0024] Modern electric machines have relatively high energy conversion efficiencies. However, the energy conversion efficiency of most electric machines can vary significantly with operating load. In many applications, machines are required to operate under a wide variety of operating load conditions. As a result, machines are typically operated at or near their maximum level of efficiency at some times and at lower efficiency levels at other times.

[0025] Battery-powered electric vehicles are a good example of electric machines that operate over a wide range of efficiency levels. In a typical drive cycle, the electric vehicle accelerates, cruises, decelerates, brakes, corners, etc. Within certain rotor speed and / or torque ranges, the electric machine operates at or near its most efficient operating point, or "sweet spot." Outside these ranges, the electric machine operates less efficiently. As driving conditions change, the rotor speed and / or torque change, causing the machine to transition between higher and lower operating efficiency levels. If the electric machine could be made to operate in a higher efficiency operating region for a greater percentage of the drive cycle, the range of the vehicle for a given battery charge level would be increased. Since the limited range of battery-powered electric vehicles is a major commercial impediment to their use, extending the operating range of the vehicle would be highly advantageous. Thus, there is a need to operate electric machines, such as motors and generators, at higher levels of efficiency.

[0026] This application relates generally to pulse control of an electric wound field synchronous machine that is otherwise continuously operated. With pulse control, the machine is intelligently and intermittently pulsed on and off to (1) meet operational demands while (2) improving overall efficiency. More specifically, under selected operating conditions, the electric machine is intermittently pulsed at a more efficient energy conversion operating level to provide a desired average power output more efficiently than can be achieved by conventional continuous machine operation. Pulsed operation intentionally modulates the torque of the electric machine, but the modulation is managed to minimize noise and vibration levels for the intended application.

[0027] For simplicity, the pulse control of an electric winding field synchronous machine provided herein is described in the context of a three-phase electric winding field synchronous motor in a vehicle. However, this description should not be construed as limiting in any way. Rather, the pulse control described herein can be used in many types of electric winding field synchronous motor machines (i.e., meaning both electric motors and generators). Furthermore, such pulse control of an electric winding field synchronous machine can be used in any application, not limited to electric vehicles. In particular, pulse control can be used in systems that require lower acceleration and deceleration rates than vehicle applications, such as electric motors for heating, cooling, and ventilation systems.

[0028] Pulsed engine control is described in U.S. Patent Application No. 16 / 353,159, filed March 14, 2019, and U.S. Provisional Patent Application Nos. 62 / 644,912, filed March 19, 2018, 62 / 658,739, filed April 17, 2018, and 62 / 810,861, filed February 26, 2019. Each of the foregoing applications is incorporated herein by reference in its entirety.

[0029] (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. Wound field synchronous machines include wound field synchronous motors (WFSMs) and wound field synchronous generators. Wound-field synchronous machines, also known as Externally Excited Synchronous Machines (EESM) or Wound Rotor Synchronous Machines (WRSM), include: -Wound field synchronous rotor. The field coil (also called the field winding) is located on the rotor and the armature phase windings are on the stator -Flux switching. Both the field coil and armature winding are located on the stator.

[0030] In a wound field synchronous machine, the field coil is powered by a DC power source. In most wound field synchronous machines, the armature winding is powered by an AC power source. In most wound field synchronous machines, the field coil is on the rotor and the armature winding is on the stator. In such wound field synchronous machines, slip rings are sometimes used to provide electrical contact between the DC power source and the field coil on the rotor. In other embodiments, an air gap may be used to provide electrical contact. DC motors place the field coil on the stator and use a commutator connected to the rotor to convert DC power to AC power.

[0031] (Three-phase winding field synchronous machine) In a three-phase wound field synchronous machine, the stator contains a three-coil winding that is excited by a three-phase AC input, and the field winding is on a rotor that is powered by a DC input. When the three-phase AC input is passed through the three-phase armature windings, a rotating magnetic field (RMF) is generated. The rotational speed of the RMF is equal to the synchronous speed (N s The interaction of the field winding field with the armature winding field generates an electromagnetic force (EMF) that causes the rotor to rotate.

[0032] (Vehicle motor efficiency map) 1, an exemplary vehicle motor efficiency map 10 is illustrated under different load and speed conditions. Map 10 plots torque (N*m) on the vertical axis as a function of motor speed (RPM) on the horizontal axis. Maximum steady-state output power is shown by curve 12. This exemplary vehicle motor efficiency map is shown for purposes of illustrating the increased efficiency of a wound field synchronous motor (WFSM) that may be provided by one embodiment.

[0033] The area under the peak torque / speed curve 12 is mapped into regions, each labeled by its operating efficiency percentage. For the particular motor shown, the following characteristics can be seen: The most efficient or "sweet spot" region of the operating range is the operating region labeled 14, which is typically in the 4,500-6,000 RPM range and has a torque output in the range of approximately 40-70 N*m. In region 14, the energy conversion efficiency is around 96%, making this the "sweet spot" where the motor operates in its most efficient operating range. When the motor speed increases above about 6,000 RPM, efficiency tends to decrease regardless of output torque. As the output torque exceeds 70N*m or falls below 40N*m, the efficiency percentage tends to drop from its peak, sometimes quite significantly. For example, if a motor is running at about 2,000RPM and the output torque is 100N*m, the efficiency is about 86%. As the torque output falls below about 30N*m, the efficiency decreases and approaches zero at zero load, regardless of motor speed. For any particular motor speed, there exists a corresponding most efficient output torque, which is diagrammed by the maximum efficiency curve 16.

[0034] The illustrated map 10 was derived from an electric motor used in a 2010 Toyota Prius. The map 10 is for an interior permanent magnet synchronous motor. It should be understood that the map 10 is merely exemplary and should not be construed as limiting in any respect. Similar maps can be generated for any electric motor, for example, a three-phase induction motor, whether used in a vehicle or other application.

[0035] As can be seen from map 10, motors are generally most efficient when operating within the speed and torque range of sweet spot 14. If the operating conditions can be controlled so that the motor operates at or near sweet spot 14 a greater percentage of the time, the overall energy conversion efficiency of the motor can be significantly improved.

[0036] From a practical standpoint, however, many driving situations will force the motor to operate outside the sweet spot 14 speed and torque range. Electric vehicles typically lack a transmission, so the ratio of the electric motor's rotational speed to the rotational speed of the wheels is fixed. In this case, the motor speed can vary from zero when the vehicle is stationary to a relatively high RPM when cruising at highway speeds. The torque required can also vary significantly depending on factors such as whether the vehicle is accelerating or decelerating, going uphill or downhill, traveling on a flat road, braking, and so on.

[0037] As seen in Figure 1, at any particular motor speed there exists a corresponding most efficient output torque as diagrammed by maximum efficiency curve 16. From a conceptual standpoint, if the desired motor torque falls below the most efficient output torque for the current motor speed, the overall efficiency of the motor can be improved by pulsing the motor so that it operates at or near its sweet spot a percentage of the time and at a low or zero torque output level the remainder of the time. The average torque thus produced is controlled by controlling the duty cycle of the sweet spot operation.

[0038] Referring to FIG. 2, a graph 20 is shown plotting torque on the vertical axis versus time on the horizontal axis. In conventional operation, the motor produces 10N*m, represented by dashed line 22, continuously for as long as the desired torque remains at this value. In pulsed control operation, the motor is pulsed with a current pulse signal, represented by pulses 24, to produce a torque of 50N*m 20% of the time. The remaining 80% of the time the motor is off. The net output of the motor therefore meets the operational requirement of an average torque level of 10N*m. Since the motor operates more efficiently at providing 50N*m than it does at a continuous torque of 10N*m, the overall efficiency of the motor can be improved by pulsing the motor using a 20% duty cycle while still meeting the average torque requirement. Thus, pulsed operation provides greater energy efficiency than continuous operation.

[0039] In the above example, the duty cycle is not necessarily limited to 20%. The duty cycle can be changed to meet the desired motor output as long as the desired motor output does not exceed 50N*m. For example, if the desired motor output changes to 20N*m, the duty cycle of a motor operating at 50N*m can be increased to 40%, if the desired motor output changes to 40N*m, the duty cycle can be increased to 80%, if the desired motor output changes to 5N*m, the duty cycle can be decreased to 10%, and so on. In many cases, motor pulse control can be advantageously used whenever the desired motor torque is below the maximum efficiency curve 16 in FIG. 1.

[0040] On the other hand, if the desired motor torque is at or above the maximum efficiency curve 16, the motor can be provided by operating it in a conventional manner (continuous or non-pulsed). Pulsed operation provides an opportunity for efficiency improvements when the motor is called upon to provide an average torque less than the torque corresponding to its maximum operating efficiency point.

[0041] It should be noted that the torque values ​​and time scales shown in FIG. 2 are merely illustrative and are not intended to be limiting in any way. In actual motor pulsing embodiments, the pulse durations used may vary widely based on the design needs of a particular system. In general, however, it is expected that the period of each on / off cycle will be on the order of 10 milliseconds (ms) to 0.10 seconds (i.e., pulsing at frequencies ranging from 10 to 100 Hz). Furthermore, there are many different types of wound field synchronous motors, each with its own efficiency characteristics. Furthermore, for a given motor, different motor speeds will have different efficiency curves. The nature of the curves may vary depending on the particular wound field synchronous motor and the particular application. For example, the torque output need not be flat-topped as depicted in FIG. 2, and / or the torque need not be zero during the off period, but may be some non-zero value. However, regardless of the particular curve used, the wound field synchronous motor is preferably operating at or near its highest efficiency region for a given wound field synchronous motor speed for a certain percentage of the time.

[0042] (Improved efficiency due to faster current rise and fall) Most modern motor converters are typically designed for continuous operation rather than pulsed operation. Such motors generally transition from a de-energized to an energized state relatively infrequently. As a result, little design effort is devoted to managing such transitions. If design effort is devoted to managing transitions, it is typically directed toward achieving smooth transitions rather than fast transitions. As a result, for most motors, transitions from energized to energized states are often speed limited (i.e., not relatively fast).

[0043] It has been discovered that in electric motor systems that periodically transition from a non-energized motor state to a peak efficiency state, such as pulsed operation, further efficiency improvements can be realized if the transition occurs as quickly as possible. For example, a fast transition from zero torque to peak efficiency torque improves the overall average motor efficiency because the motor spends less time in a transition where its efficiency is less than peak. This relationship is depicted in Figures 3A and 3B.

[0044] 3A, a torque vs. efficiency map is shown for an exemplary electric motor operating at a fixed speed (e.g., 6000 rpm). In this exemplary map, the range of torque output from 0.0 Nm to 250 Nm is plotted along the horizontal axis, and the efficiency of the motor from 0.0 percent to 100 percent is plotted along the vertical axis. Curve 26 shows the transition of the motor from zero to peak efficiency torque. During this transition (in this example, from zero torque to torque T), torque is being produced at an efficiency less than the peak efficiency torque at T shown at 28.

[0045] 3B, a map showing torque versus work loss for an example motor operating at a fixed speed during the transition from zero to peak efficiency torque is shown. In this map, work loss (W) is plotted along the vertical axis and the torque output of the motor is plotted along the horizontal axis. As shown by curve 29, the work loss of the motor increases as the torque output increases during the transition from zero to peak efficiency torque. Thus, 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.

[0046] By substituting time for torque along the horizontal axis and integrating the area under the curve 29, the energy consumed by the electric motor for a given transition time can be calculated. For example, in the exemplary motor, a transition time of 0.5 seconds used 7234.5 Joules of energy, whereas a transition time of 0.05 seconds used only 723.4 Joules of energy. This comparison shows that the faster the transition time from zero to peak efficiency torque, the less energy is consumed in losses. Note that this example assumes that no acceleration of the load occurs, so no energy is added to the load inertia. Just as a shorter ramp time increases efficiency, a shorter ramp time increases efficiency.

[0047] The motor's zero to peak efficiency torque transition, peak efficiency torque, and work loss are all different in different motors, therefore the maps of Figures 3A and 3B should be considered merely illustrative and should not be construed as limiting in any way.

[0048] (Power Converter) Power inverters are well-known devices used with electric motors to convert a DC source of power, such as from a battery or capacitor, into three-phase AC input power that is applied to the motor's stator windings, in response which generate a rotating magnetic field.

[0049] Referring to FIG. 4, a diagram of a power controller 30 for pulsed operation of an electric machine is shown. The power controller 30 includes a power converter 32, a DC power source 34, and an electric machine 36. In some embodiments, the electric machine 36 is a wound field synchronous machine. In this non-limiting embodiment, the power converter 32 includes a pulse controller 38. The power converter 32 can be operated as a power inverter or a power rectifier depending on the direction of energy flow in the system. When the electric machine is operated as a motor, the power converter 32 is responsible for generating three-phase AC power from the DC power source 34 to drive the electric machine 36. Three-phase input power, designated as phase A 37a, phase B 37b, and phase C 37c, is applied to the stator windings of the electric machine 36 to generate the RMF, as described above. The lines representing the various phases 37a, 37b, 37c have arrows on either end to indicate that current flows both from the power converter 32 to the electric machine 36 when the machine is being used as a motor, and from the electric machine 36 to the power converter 32 when the machine is being used as a generator. When the electric machine is operating as a generator, the power converter 32 operates as a power rectifier and converts the AC power coming from the electric machine 36 to DC power that is stored in the DC source. The field current line 37d carries the DC field current, which is typically unidirectional for both the motor and generator operating modes.

[0050] The pulse controller 38 is responsible for selectively pulsing the three-phase input power. In conventional (i.e., continuous) operation, the input power to the three-phase field coils is continuous, i.e., not pulsed, whereas in pulsed operation, the input power to the three-phase field coils is pulsed. Pulsed operation can be implemented, in non-limiting embodiments, using any of the approaches described herein, including, but not limited to, the approaches described below.

[0051] 5A-5F, plots are shown to illustrate the difference between continuous and pulsed, three-phase field current input power supplied to electric machine 36. In each plot, phase and field current are plotted on the vertical axis and time is plotted along the horizontal axis.

[0052] FIG. 5A shows conventional sinusoidal three-phase input currents 42a, 42b, 42c applied to the armature windings of the electric machine 36. Phase B, shown by curve 42b, lags phase A, shown by 42a, by 120 degrees. Phase C, shown by curve 42c, lags phase B, by 120 degrees. The period of the sinusoids is τ. The three-phase input currents 42a, 42b, 42c are continuous (not pulsed) and have a specified maximum amplitude of about 50 amperes. FIG. 5B shows conventional DC field current 42d applied to the field coils. This field current is continuous (not pulsed) and has an amplitude of 5 amperes. It should be understood that 50 amperes (for the phase currents applied to the armature windings) and 5 amperes (for the field currents applied to the field coils) are only representative maximum currents and that the maximum currents may have any value.

[0053] 5C and 5D show an example of pulsed three-phase current waveforms 44a, 44b, 44c shown in FIG. 5C, along with a pulsed DC field current 44d shown in FIG. 5D, with a duty cycle of 50% and a peak amplitude of about 100 amps for the three-phase waveforms 44a, 44b, 44c, and a peak amplitude of about 10 amps for the field current 44d. As in FIG. 5A, the period of the basic sine wave is τ, but now the sine wave is modulated on and off. The currents provided in FIG. 5C and 5D provide the same average torque as the continuously applied three-phase input current of FIG. 5A and 5B (assuming that the torque is proportional to the current, which is often the case). In FIG. 5C and 5D, the current pulses 44a-d are interleaved with "off" periods of equal length. The length of each on and off period is 2τ. In this example, the duty cycle is 50%. The frequency of the pulse modulation may vary based on the type of electric machine used, noise and vibration considerations, the current operating rotor speed, and other factors.

[0054] This example of Figures 5C and 5D shows an application in which the "on" motor drive pulses are evenly spaced while the motor is operating at a steady-state desired power level. While such an approach works well in many situations, it is not a requirement. The duty cycle does not need to be 50% and can be adjusted to suit the desired average output torque. In Figures 5C and 5D, the phase of the on / off pulses is synchronous with the applied power, but in some embodiments, the phase of the on / off pulses need not be synchronous with the phase of the applied power. Thus, the relative magnitude and / or timing of the motor drive pulses can be varied as long as they average out to provide the desired average torque.

[0055] This example shows pulsing of both the armature winding AC current and the DC field coil current designed to operate the wound field synchronous machine at an efficient torque level while reducing the amount of power required to provide the desired torque level.

[0056] 5E and 5F show another example of pulsed three-phase current waveforms 46a, 46b, 46c shown in FIG. 5E, along with a pulsed DC field current 46d shown in FIG. 5F, with a duty cycle of 50% and a peak amplitude of about 100 amps for the three-phase waveforms 46a, 46b, 46c, and a peak amplitude of about 10 amps for the field current 46d. As in FIG. 5A, the basic sine wave has a period τ, but now the sine wave is modulated on and off. The currents provided in FIG. 5E and 5F provide the same average torque as the continuously applied three-phase input current of FIG. 5A and 5B (assuming that the torque is proportional to the current, which is often the case). In FIG. 5E and 5F, the current pulses 46a-d are interleaved with "off" periods of equal length. The length of each on and off period is τ / 2. In this example, the duty cycle is 50%. The frequency of the pulse modulation may vary based on the type of electric machine used, noise and vibration considerations, the current operating rotor speed, and other factors.

[0057] (Power conversion circuit) The inductance inherent in the motor transiently delays / slows down the voltage / power steps between on and off motor states. In continuous (non-pulsed) operation, such transient effects tend to have a relatively small impact on overall motor operation. However, when rapid pulsing is used as contemplated herein, the net effect of the transient effects can be greater, and so there is an incentive to reduce the transition times of the leading and trailing edges of the pulses. This is particularly important for the field currents, which exhibit significantly higher time constants than the armature phase currents.

[0058] As previously mentioned, the goal of motor pulse control is to operate the electric machine 36 at substantially the most efficient level for the current machine speed during the "on" periods and to cut off power (deliver zero or negligible power) during the "off" periods. For example, the power delivered during the off periods may be less than 10%, less than 5%, less than 1%, less than 0.5%, less than 0.1% of the power delivered during the "on" periods. The operating point during operation during the "on" periods may have an efficiency within 5%, 2%, or 1% of the motor's maximum operating efficiency point 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. Thus, the power transition between the "on" and "off" states of the machine power ideally has a leading edge that transitions straight up vertically and a trailing edge that transitions straight down vertically. Such a "perfect" pulse 60 is diagrammed in FIG. 6A, which shows the ideal motor drive current versus time for pulse control with a 50% duty cycle. In this diagram, the current pulses represent the field winding currents. Although the current pulses are shown as flat-topped, this is not necessarily the case.

[0059] In the real world, such perfect pulse generation is difficult due to many practical limitations. For example, the inductive aspects of both the electric machine 36 and the power converter 32 circuits slow down the current rise and fall times. The actual response of a particular machine varies with the electrical characteristics of the electric machine 36, the speed at which the electric machine rotates, and the available bus voltage. In general, the actual rise and fall of the pulse occurs more gradually; that is, the transition occurs over time. The nature of the rise and fall in the real world is diagrammed in FIG. 6B. As can be seen, 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.

[0060] During the power ramp-up and ramp-down periods, the wound-field machine 36 continues to consume or generate power. However, during these transition periods, the wound-field machine operates less efficiently. In general, the efficiency of a wound-field machine decreases as the operating current decreases from the maximum efficiency condition (Fig. 1 curve 16) toward zero, with the energy conversion efficiency deteriorating noticeably as the current level approaches zero. Thus, the pulse distortion represented by the current ramp-up and ramp-down periods negates the efficiency gains from pulsed operation. In general, the smaller the ratio of rise / fall time to pulse length, the less impact transient switching effects have on the energy conversion efficiency of the machine during pulsing.

[0061] It is understood that the transient effects shown in FIG. 6B are exemplary in nature and do not necessarily reflect the actual rise / fall times associated with the operation of a particular wound-field machine. The relative scale of the rise time to pulse length ratios can vary widely based on the characteristics of the wound-field machine used (which primarily dictates the rise and fall times), the frequency of pulsing (which is primarily dictated by the control scheme used), and the pulse width (which is dictated by the control scheme and the machine load). The voltage available to power the wound-field machine and the rotational speed of the machine also affect the pulse rise and fall times. If the pulsing is slow compared to the response of the wound-field machine, the rise / fall times will be a small fraction of the pulse width and the transient switching effects may have minimal impact on the machine's performance. Conversely, if the pulsing is very rapid and / or the response of the wound-field machine is poor, the rise / fall times will be a significant fraction of the pulse width and may even exceed the pulse width in some circumstances. If not carefully managed, the transient efficiency losses associated with switching can significantly reduce or even eliminate the theoretical gains that can be achieved with pulsed operation. It is therefore important to consider the transient switching effects associated with pulsed operation when determining the appropriate pulsing frequency and control scheme for a particular application.

[0062] In a wound field synchronous machine, the rise and fall times of the field current follow the equation:

number

number

number

number

[0063] From this formula, the ampere-turns of the magnetic winding, AT f and / or voltage U f and / or increase the cross-sectional area S of the winding wire f By increasing

number

[0064] In some embodiments, the number of turns is reduced by providing at least two wires in parallel to form each turn. In some embodiments, the parallel wires may be in the form of a ribbon cable. In some embodiments, there are at least five wires in parallel. In some embodiments, there are fewer than three turns. In some embodiments, there is only one turn and at least ten wires in parallel. In some embodiments, a rotary transformer is used to provide power across the air gap. In some embodiments, power is transferred via slip rings or other types of wireless transmission. In some embodiments, the diameter of the wire may be increased to increase the current.

[0065] (Embodiments for reduction of windings and increase of wire cross section) In some embodiments, the reduction in rise and fall times is obtained by reducing the number of turns in each winding. FIG. 7A is a graph plotting ampere turns versus time for 140 turns (704), 70 turns (708), and 50 turns (712). FIG. 7B shows a portion of the curve for 140 turns (704), 70 turns (708), and 50 turns (712) from 0 to 30 ms. For the 140 turn machine, it takes about 15 ms to reach 400 ampere turns. For the 50 turn machine, it takes about 5 ms to reach 400 ampere turns. It should be noted that by reducing the turns from 140 to 70 turns, the time is reduced by 50%. As a result, the rise and fall times are also reduced by about 50%. In this example, it is assumed that the current is regulated by adjusting the applied voltage.

[0066] In some embodiments, the winding has 70 turns or less. In other embodiments, the winding has less than 50 turns. In some embodiments, the winding has only one turn. FIG. 8A is a schematic cross-section of a motor having a single turn. The motor has an armature winding 802 on the stator and a field coil or winding 804 on the rotor. FIG. 8B is a perspective view of a portion of the single turn. Showing the rear connection 812 that forms the single turn, and the front connection 816 that connects the single turn to an adjacent single turn in an adjacent winding. In some embodiments, the single turn field winding 804 is a foil formed by a thin sheet of conductive material.

[0067] In other embodiments, the rise and fall times are reduced by increasing the cross section of the wire in the winding. In one embodiment, a thicker wire may be used to increase the cross section of the wire. However, in high frequency pulsing, the skin effect caused by high frequency pulsing reduces the current in the thicker wire. Due to the skin effect, the high frequency pulse only travels along the skin of the conductor and not through the core of the conductor. Therefore, in other embodiments, multiple thin wires are used in parallel instead of a thick wire. In some embodiments, the multiple thin wires are in the form of Litz wire. In various embodiments, the parallel wires are twisted or have random windings to reduce the proximity effect. Increasing the cross section of the wire can result in high current levels that brushes and slip rings cannot tolerate. In various embodiments, a brushless design may be employed. Such a brushless design may use inductive or capacitive coupling to transfer energy across the air gap. In some embodiments, the number of turns in the winding may be reduced and the cross section of the wire may be increased.

[0068] (Voltage increase) In other embodiments, the rise / fall time can be decreased by increasing the voltage applied to the field winding. FIG. 9 is a schematic diagram of a rotating transformer for supplying voltage to the field winding. By increasing the number of turns on the field winding side 904 of the transformer, the voltage is increased (boosted), which results in a decrease in rise time. In various embodiments, to decrease the rise time, the voltage is boosted from the voltage source to the voltage applied to the rotor winding 908. Many conventional wound field synchronous motors step down the voltage from the voltage applied from the voltage source to the voltage applied to the rotor winding. In some embodiments, the transformer may be an air core transformer. Air core transformers are more efficient at high frequencies. In this example, the rise time is decreased, but the fall time remains the same. More complex designs may be used to decrease both the rise and fall times.

[0069] In another embodiment, to reduce both the rise and fall times, the voltage is increased by providing two voltage sources. Figure 10 is a schematic diagram of a power supply consisting of two voltage sources. In this embodiment, the power supply comprises a high voltage source 1008 and a low voltage source 1012. In one way that this power supply may be used, the high voltage from the high voltage source 1008 is applied when a fast rise time is required to reach the desired current. This can be done by turning on Q2 and Q1 simultaneously. When the desired current is reached after a fast rise time, the high voltage is stopped and the voltage from the low voltage source 1012 is applied to maintain the desired current or when a fast rise time is not required. This can be done by turning off Q1 and turning on Q3. To achieve a fast current fall time, the rotor winding is open circuited. This can be done by turning off Q3 and Q2. Diodes D1 and D2 catch the flyback voltage that returns the field energy to the high voltage link. Both D1 and D2 are turned off as the feedback current naturally decays to zero. Due to the low time constant of the stator, the stator current returns to zero quickly, so the negative torque transient caused by the reversal of the current in the field winding should be short-lived. Furthermore, the captured field energy is returned to the motor upon application of the next pulse current. Electrical contacts 1020 are used to provide electrical contact between the power source and the field winding 1016. In some embodiments, the electrical contacts 1020 are slip rings or a type of contactless transmission, such as a rotary transformer or capacitive coupling to bridge the air gap. If logic signals are required to reconfigure the direct current applied to the rotor winding, they may be transmitted across the air gap by either optical or transformer-based coupling. Efficiency is further increased by shortening both the rise and fall times of the field winding. In some embodiments, the low voltage is on the order of 12 volts and the high voltage is on the order of 300 volts. Currently, electric vehicles have systems that provide such voltages. In various embodiments, the use of higher voltages may be combined with either a reduced number of windings and / or a larger wiring cross-section to provide faster rise and fall times.

[0070] An example of improved rise and fall times is shown diagrammatically in Figure 6C. As can be seen, the ramp-up rise time 66 of the pulse leading edge is faster / shorter as compared to the corresponding ramp-up time 62 shown in Figure 6B. Similarly, the ramp-down time 68 of the pulse trailing edge is faster / shorter as compared to the corresponding ramp-down time 64 shown in Figure 6B. It should thus be appreciated that electric machines designed with pulse control in mind, or modified to improve the machine's transient response to power pulses, can benefit even more from pulsed operation than existing machines.

[0071] A variety of fast rise times and fast fall times can be provided. Some embodiments have a fast rise time and a fast fall time. In some embodiments, the fast rise time is the same length as the fast fall time. In some embodiments, the fast rise time is symmetrical to the fast fall time. In some embodiments, the fast rise time and / or fall time of the field winding is the same length and / or symmetrical to the fast rise time and / or fall time of the armature winding.

[0072] (Operation flow chart) Figure 11 is a flow diagram 70 showing the steps of pulse control operation of an electric motor having the characteristics as depicted in Figure 1. In a first step 72, the current motor power and current motor speed are known.

[0073] In decision step 74, a decision is made whether to operate the motor in continuous mode or pulse mode based on the current motor power and the current motor speed. That is, a determination is made whether the desired motor torque is above or below the most efficient output torque for the current motor speed (i.e., the maximum efficiency curve 16 of the motor map illustrated in FIG. 1). If so, the motor is operated in continuous mode. If not, it is advantageous for the motor to be operated in pulse mode. In step 76, if the current motor torque is above the most efficient output torque for the current motor speed, the motor is operated in continuous mode 76.

[0074] In step 78, the power output or magnitude of the "on" pulse is determined that provides substantially maximum efficiency operation at the current motor speed. In step 80, the desired pulse duty cycle for operation in pulsed mode is determined such that the average output power or torque matches the desired output.

[0075] At step 82, the motor is operated in a pulsed mode with the determined pulse duty cycle and pulsed power output. Use of the power controller 30 in various embodiments reduces the pulse rise and fall times, further improving motor efficiency.

[0076] The above steps 72-82 are performed continuously while the motor is operating. At any particular motor speed, there is a corresponding most efficient output torque, which is diagrammed by the maximum efficiency curve 16 in FIG. 1. As the instantaneous motor output demand and / or current motor speed change, a decision is made to operate the motor in either a continuous mode or a pulsed mode, as appropriate. From a conceptual standpoint, if the desired motor torque falls below the most efficient output torque for the current motor speed, pulsing the motor can improve the overall efficiency of the motor. As a result, in an electric motor powered vehicle, the overall vehicle efficiency is improved, which extends the vehicle's range between battery charges.

[0077] Although the present disclosure has been described in terms of several preferred embodiments, there exist changes, modifications, permutations, and various substitute equivalents, which fall within the scope of the present disclosure. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present disclosure. It is therefore intended that the following appended claims be interpreted as including all such changes, modifications, permutations, and various substitute equivalents, as fall within the true spirit and scope of the present disclosure.

Claims

1. Power supply, a wound field synchronous machine having an armature winding and a field winding; a power converter disposed between the power source and the wound field synchronous machine, the power converter configured to provide a pulsed operation by supplying a pulsed direct current to the field winding; An electric machine comprising:

2. the power converter is configured to provide a desired output, and the pulsing operation generates a current pulse signal in the field winding that causes the wound field synchronous machine to alternate between at least a first torque level and a second torque level to provide an average torque level; 2. The electric machine of claim 1, wherein the current pulse signal is selected to provide a higher energy conversion efficiency during the pulsed operation of the electric machine than when the electric machine is operated at a third torque level that would be required to drive the electric machine continuously to provide the same average torque level.

3. 3. The electric machine of claim 2, wherein the current pulse signal has a frequency of at least 10 Hz and at least one of a rise time and a fall time for the current pulse signal is less than or equal to 5 ms.

4. The electric machine of claim 3 , wherein both the rise time and fall time are less than or equal to 5 ms.

5. The electric machine of claim 1 , wherein the field winding of the wound field synchronous machine comprises a ribbon cable.

6. The electric machine of claim 1 , wherein the number of turns in the field winding of the wound field synchronous machine is less than 50 turns.

7. The electric machine of claim 1 , wherein the number of turns of the field winding of the wound field synchronous machine is one turn.

8. The electric machine of claim 1 , wherein the field winding comprises a foil.

9. The electric machine of claim 1 , wherein the power converter includes a rotary transformer capable of stepping up a voltage.

10. The electric machine of claim 1 , wherein the wound field synchronous machine is a wound field synchronous motor.

11. 2. The electric machine of claim 1, wherein the power converter has a first voltage source and a second voltage source, the first voltage source providing a higher voltage than the second voltage source, the power converter providing a current rise time by providing a voltage from the first voltage source, not providing a voltage from the first voltage source after the current rise time, maintaining a current by providing a voltage from the second voltage source, providing a current fall time by providing a voltage from the first voltage source, and not providing a voltage from the first voltage source after the current fall time is provided.

12. 2. The electric machine of claim 1, wherein the power converter is further configured to determine whether, for a target torque, providing a current pulse signal is a less efficient torque than providing a continuous current, and to provide a continuous current when it is determined that providing the current pulse signal is a less efficient torque than providing the continuous current.

13. A method of operating a wound field synchronous machine having a field winding and an armature winding, the method comprising providing pulsed operation by supplying a pulsed direct current to the field winding.

14. the pulsing operation generates a current pulse signal in the field winding that causes the wound field synchronous machine to alternate between at least a first torque level and a second torque level to provide an average torque level; 14. The method of claim 13, wherein the current pulse signal is selected to provide a higher energy conversion efficiency during the pulsed operation of the wound field machine than when the wound field machine is operated at a third torque level that would be required to drive the wound field machine continuously to provide the same average torque level as the average torque level.

15. 15. The method of claim 14, wherein the current pulse signal has a frequency of at least 10 Hz and at least one of a rise time and a fall time for the current pulse signal is less than or equal to 5 ms.

16. 16. The method of claim 15, wherein both the rise time and fall time are less than or equal to 5 ms.

17. The method of claim 13 , wherein the wound field synchronous machine is a wound field synchronous motor.

18. 14. The method of claim 13, wherein a power converter is connected to the wound field synchronous machine, the power converter having a first voltage source and a second voltage source, the first voltage source providing a higher voltage than the second voltage source, providing a current rise time by providing a voltage from the first voltage source; removing the supply of voltage from the first voltage source after the current rise time; maintaining a current by providing a voltage from the second voltage source; providing a current fall time by providing a voltage from the first voltage source; removing the supply of voltage from the first voltage source after the current fall time; The method further comprising:

19. a power converter coupled to the wound field synchronous machine, the method comprising: determining by the power converter whether a target torque is such that providing a current pulse signal is less efficient than providing a continuous current; providing a continuous current when it is determined that providing the current pulse signal is less torque efficient than providing the continuous current. The method of claim 13 further comprising:

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