Switching strategy for a switched reluctance machine

Abstract

Method (140) for controlling a motor (110), comprising the following steps: Determining a speed of the motor (110); and Activating a soft break routine on a first switch (132-1) and a second switch (132-2) of each phase when the motor speed is relatively low, the first switch (132-1) being driven by a first pulse width modulated (PWM) signal and the second switch (132-2) being driven by a second PWM signal, the first and second PWM signals being alternately configured such that at any point during the distributed soft break routine at least one of the first switch (132-1) and the second switch (132-2) is closed and the first switch (132-1) and the second switch (132-2) are never both open at the same time.

Description

Technical FieldThe present disclosure relates generally to electric motors, and more particularly to systems and methods for controlling the switching routine of a switched reluctance motor operating in a low speed motor mode.BackgroundBecause of the increasing interest in energy conservation, industrial work machines are increasingly being equipped with electric drive assemblies or systems for driving the work machine and operating the various tools and functions thereof. Electric drive developments have enabled electrically powered work machines to effectively achieve or exceed the performance of mechanically powered work machines while requiring significantly less fuel and energy. As electric drives are being used more and more frequently in industrial work machines and the like, the need for more efficient generators and techniques for controlling the same has also increased.An electric motor of an electrically-driven machine is typically used to convert mechanical power obtained from a primary power source, for example, a fuel engine, to electrical power for performing one or more operations of the work machine. Additionally, an electric motor may be used to convert electrical power stored in a common collection or storage device to mechanical power. Among the various types of electric motors available for use with an electric drive system, switched reluctance motors are of great interest because they are robust, cost effective, and overall more efficient. While current systems and methods for controlling switched reluctance motors provide adequate control, there is still room for improvement.Typical control methods for switched reluctance motors may include actuating two switches of each phase of the motor in one of two general modes of operation, for example, single pulse and current regulation modes of operation. Single pulse modes are used for higher speed tasks requiring greater power output, while current regulation modes are used for lower speed tasks requiring greater torque output of a driven machine. Moreover, in typical current regulation modes, tasks at a nominal speed may be performed by hard chopping (one current to the two switches of each phase), while tasks at a relatively lower speed may be performed by soft chopping (one current to the two switches of each phase.A hard chopping routine (hard chopping routine) generates a pulsed phase current by simultaneously opening and closing both switches of each phase at the required frequency, while a soft chopping routine (soft chopping routine) generates a soft pulsed phase current by holding a first switch closed while only a second switch is opened and closed at the required frequency. Soft break routines repeatedly represent significant thermal stress for only one of the two switches in a phase leg. The additional loading of such switches may result in premature failure of the associated power converter circuit. Such a power converter circuit failure, in turn, may cause premature failure of the electric propulsion system, thus preventing work machines and other related tools from meeting original performance requirements.Accordingly, there is a need to improve the overall efficiency and functionality of an electric propulsion system. Moreover, there is a need for an improvement in the switching strategy associated with operating a switched reluctance motor during a motor mode in high current, high load situations, for example, at low speeds to zero speed. Further, there is a need to more efficiently activate switches of a phase leg in soft interrupt routines so as to extend the life of the switches and the entire electric drive system.DE 100 42 199 A1 discloses a control device for interrupted excitation. At this time, in a range where the revolution number of the engine is low, a soft cut-off mode is performed in which one of two switching elements is cut-off and turned on while the other switching element is continuously turned off.DE 100 27 586 A1 likewise discloses a control device for interrupted excitation.SUMMARY OF THE DISCLOSUREAccording to one aspect of the present disclosure, a method of controlling an engine is provided. The method may include determining a speed of the engine and activating a soft interrupt routine at a first switch and a second switch of each phase when the engine speed is relatively low. The first switch may be driven by a first pulse width modulated (PWM) signal and the second switch may be driven by a second PWM signal. The first and second PWM signals may be alternately configured such that at any point during the distributed soft interrupt routine, at least one of the first switch and the second switch is closed and the first switch and the second switch are never both simultaneously opened.In an improvement, the first and second PWM signals may be different.In another improvement, the motor may be a low to medium voltage three-phase switched reluctance motor operable in one of a generator operating mode and a motor operating mode.In a related improvement, the distributed soft interrupt routine may be activated during the engine operating mode.In another improvement, relatively low engine speeds may correspond to engine speeds that are approximately between zero and a base speed. The base speed may be a maximum speed at which the engine is able to deliver a constant torque before torque delivery begins to decrease in proportion to engine speed.In an improvement related thereto, the method may further include activating a single pulse routine at the first switch and the second switch when the engine speed is relatively high, and activating a hard interrupt routine at the first switch and the second switch when the engine speed is a nominal speed. Relatively high engine speeds may be about greater than the base speed, and rated engine speeds may be about the base speed.According to another aspect of the present disclosure, a control system for an engine is provided. The control system may include a converter circuit connected for operation with a stator of the motor and a controller in communication with the motor and the converter circuit, respectively. The converter circuit may include at least a first switch and a second switch connected to each phase of the stator. The controller may be configured to determine an engine speed and activate the first switch and the second switch with a soft interrupt routine during an engine operating mode with relatively low engine speeds. The first switch and the second switch can be controlled by alternating signals, so that the first switch and the second switch are never both simultaneously opened.In an improvement, the controller may be configured to generate a first PWM signal to drive the first switch and a second PWM signal to drive the second switch during the soft break distributed routine, wherein the first PWM signal may be different from the second PWM signal.In an improvement related thereto, the controller may be configured to alternate the first and second PWM signals such that at any point during the distributed soft break routine, at least one of the first switch and the second switch is closed.In another improvement, the converter circuit may include insulated gate bipolar transistor switches.In another improvement, the soft interrupt routine may be pre-programmed in memory to which the controller has access.In another improvement, the controller may be adapted for use with a three-phase switched reluctance motor low to medium voltage electric drive operable in one of a generator mode and a motor mode.In another improvement, the controller may be further configured to activate the first switch and the second switch with a hard interrupt routine when the engine speed is a nominal speed and activate the first switch and the second switch with a single pulse routine when the engine speed is relatively high. Relatively low engine speeds may correspond to engine speeds that are approximately between zero and a base speed. Rated engine speeds can be approximately the base speed. Relatively high engine speeds may correspond to engine speeds that are approximately greater than the base speed. The base speed may be a maximum speed at which the engine is able to deliver a constant torque before torque delivery begins to decrease in proportion to engine speed.According to another aspect of the present disclosure, an electric drive system is provided. The electric drive system may include an electric motor having a rotor and a stator, a converter circuit in communication with the motor, and a controller in communication with the motor and the converter circuit, respectively. The rotor and the stator may each have a plurality of phases, and the converter circuit may include at least a first switch and a second switch connected to each phase of the stator. The controller may be configured to determine an engine speed and activate the first switch and the second switch with a soft interrupt routine during an engine operating mode with relatively low engine speeds. The first switch and the second switch can be controlled by alternating signals, so that the first switch and the second switch are never both simultaneously opened.In an improvement, the controller may be configured to generate a first PWM signal to drive the first switch and a second PWM signal to drive the second switch during the soft break distributed routine, wherein the first PWM signal may be different from the second PWM signal.In a related improvement, the controller may be configured to alternate the first and second PWM signals such that at any point during the distributed soft-break routine, at least one of the first switch and the second switch is closed.In another improvement, the soft interrupt routine may be pre-programmed in memory to which the controller has access.In another improvement, the motor may be a low to medium voltage three-phase switched reluctance motor operable in one of a generator mode and a motor mode.In another improvement, the controller may be further configured to activate the first switch and the second switch with a hard interrupt routine when the engine speed is a nominal speed and activate the first switch and the second switch with a single pulse routine when the engine speed is relatively high. Relatively low engine speeds may correspond to engine speeds that are approximately between zero and a base speed. Rated engine speeds can be approximately the base speed. Relatively high engine speeds may correspond to engine speeds that are approximately greater than the base speed. The base speed may be a maximum speed at which the engine is able to deliver a constant torque before torque delivery begins to decrease in proportion to engine speed.In a related improvement, the controller may be configured to select one of a single pulse routine, a hard break routine, and a distributed soft break routine based at least in part on the detected engine speed and use one or more control maps pre-programmed in a memory to which the controller has access.Brief Description of the DrawingsFIG. 1 is a schematic view of an example machine with an electric drive system; FIG. 2 is a schematic view of an example electric drive control system constructed in accordance with the teachings of the present disclosure; FIG. 3 is a flowchart of an example method for controlling an engine; and FIG. 4 is a schematic view of an exemplary power converter circuit for a low to medium voltage switched reluctance motor; and FIG. 5 is a graphical representation of a typical soft break waveform versus an exemplary distributed soft break waveform.Detailed DescriptionReference will now be made in detail to specific embodiments or features, examples of which are shown in the accompanying drawings. In general, corresponding reference numerals are used in the drawings to refer to the same or corresponding parts.FIG. 1 schematically illustrates a mobile machine 100 that may use an electric drive means to effect movement. More specifically, machine 100 may include a power source 102 connected to an electric drive system 104 for causing movement via a traction device 106. Such a mobile machine 100 may be used as a work machine for performing a particular type of operation in connection with an industry such as mining, construction, agriculture, transport, or other suitable industry. For example, the machine 100 may be an earthmoving machine, a marine vehicle, an aircraft, a tractor, an off-road truck, a road-grade passenger vehicle, or another mobile machine. The power source 102 of the electric drive 104 may include, for example, a diesel engine, a gasoline engine, a gas engine, or any other type of internal combustion engine commonly used for power generation. The internal combustion engine 102 may be configured to mechanically transmit power to a generator or motor 110 of the electric drive 104 via a clutch or axially rotating driveshaft 112.FIG. 2 schematically illustrates an example electric drive system 104 that may be used to transfer power between the internal combustion engine 102 and one or more electrical loads 114. The motor 110 of the electric drive 104 in FIG. 2 may be a switched reluctance motor or the like configured to generate electrical power in response to a rotational input from the internal combustion engine 102 and pass the electrical power to one or more electrical loads 114 of the engine 100. The load 114 may include, for example, motors for causing movement of the machine 100, as well as motors for operating various mechanical tools of the machine 100. As known, the motor 110 may include a rotor 116 rotatably disposed within a fixed stator 118. The rotor 116 may be connected to an output of the engine 102 via the driveshaft 112 or, in other embodiments, via a direct crankshaft, transmission, hydraulic circuit, and the like. The stator 118 may be electrically connected to a common bus 120 of the electric drive 104 via a converter circuit 122.During a generator mode of operation, when the rotor 116 is rotated by the internal combustion engine 102 within the stator 118, an electrical current may be induced in the stator 118 and supplied to the converter circuit 122. The converter circuit 122, in turn, may convert the electrical signals to a suitable direct current (DC) voltage for distribution to the various electrical loads 114 of the machine 100. Additionally, the motor 110 may be capable of causing rotation of the rotor 116 in response to electrical signals supplied to the stator 118 from the common bus 120, for example during a motor operating mode. The common bus 120 may include a positive line 124 and a negative or grounded line 126, through which a common DC bus voltage may be passed to one or more loads 114 of the machine 100 connected thereto. For example, the converter circuit 122 may provide a DC signal that is transmitted through the common bus 120 to a rectifier circuit where the DC voltage may be converted to suitable alternating current (AC) signals for driving the one or more traction motors or the like to cause movement of the machine 100 across the traction device 106. The common bus 120 may further pass the common DC voltage to other loads 114 of the engine 100, such as a hybrid system, electrically powered pumps, electrically powered fans, and the like.With continued reference to FIG. 2, the electric drive 104 may further include a control system 128 for controlling the motor 110. The control system 128 may generally include a controller 130 in communication with at least the converter circuit 122 associated with the electric drive 104. The converter circuit 122 may include a series of transistors or gated switches 132, for example insulated gate bipolar transistors, and diodes 134 for selectively activating one or more phase windings of the motor 110. For example, a switched three-phase reluctance motor 110 may be driven using a converter circuit 122 having six switches 132 and six diodes 134 for selectively enabling or disabling each of the three phase legs of the motor 110. Each of the switches 132 may be activated or deactivated via gate signals that may be provided by the controller 130. In certain modifications, the control system 128 may be further provided with encoders, speed sensors 136, or the like, adapted to generate a speed sensor signal corresponding to the rotational position and / or frequency of the rotor 116 relative to the stator 118 and pass the speed sensor signal to an input of the controller 130. The speed sensors 136 may include a Hall effect sensor, a variable reluctance sensor, an anisotropic magneto-resistance sensor, or the like. Power for the control system 128 and the converter circuit 122 may be supplied from an external or secondary power source, such as a battery (not shown), a residual voltage stored in a capacitor 138 of the common bus 120, or any other current limited DC power supply.The controller 130 of FIG. 2 may be implemented using at least one processor, microprocessor, microcontroller, electronic control module (ECM), electronic control unit (ECU), or other suitable means for providing electronic control to the electric drive system 104. More specifically, the controller 130 may be configured to operate the motor 110 of the electric drive 104 according to a predetermined algorithm or set of instructions configured to optimize performance of the engine 100 based on observed characteristics of the motor 110, the internal combustion engine 102, the electric drive 104, and the like. For example, the controller 130 may be programmed in advance to determine the operating mode for a switched reluctance motor 110 that is most suitable for an observed motor speed, observed load characteristics, and / or observed phase current requirements. Moreover, based on the observed parameters, the controller 130 may set the phase current supplied to each phase leg of the motor 110 so that the motor 110 operates in one of a single pulse mode, a current regulation mode, a hard break mode, a soft break mode, and the like. Such algorithms may additionally or alternatively include predefined control maps or tables that may suggest to the controller 130 a predefined control scheme that best corresponds to a given situation and optimizes performance. Algorithms or command sets and conditions may be pre-programmed or included in a memory of the controller 130 in a known manner.Referring to FIG. 3, a flowchart of an example algorithm or method 140 is shown with which the controller 130 may be configured to drive a motor 110 in a motor or drive mode of operation. As shown, the controller 130 may be configured to determine a current engine speed in an initial step 140- 1. More specifically, the controller 130 may monitor the observed speed and compare it to one or more predetermined speed thresholds or a range of thresholds to determine whether the engine speed corresponds to a relatively high speed, a nominal or medium speed, a relatively low speed, or the like. Such engine speed thresholds may be differentiated based on a base speed. Although the base speed for a particular engine 110 and / or application may vary with the applied load, the base speed may generally be defined as the maximum speed at which the engine 110 is able to deliver a constant torque before torque delivery begins to decrease in proportion to engine speed. Relatively low engine speeds may be defined to be in a range between approximately zero and the base speed, while relatively high engine speeds may be defined to be engine speeds that exceed approximately the base speed. Rated engine speeds can correspond approximately to the base speed.If the observed speed is relatively high, the controller 130 may be configured to activate a single pulse mode of operation of the motor 110 in step 140- 2. During the single pulse mode of step 140- 2, the controller 130 may transmit gate signals configured to continuously activate and close both switches 132 of the converter circuit 122 associated with each phase branch of the motor 110 such that the motor 110 is operated in a substantially constant power output range. Alternatively, if the observed speed is the nominal speed or relatively low, the controller 130 may be configured to activate a current regulation mode to operate the motor 110 in step 140- 3.During the electric current regulation mode, the controller 130 may be configured to further distinguish whether the observed engine speed corresponds to a nominal speed or a relatively low engine speed. As with the previous conditions, the controller 130 may distinguish between the nominal speed and relatively low engine speeds based on one or more predetermined speed thresholds or a range of thresholds. If the observed speed corresponds to a rated engine speed, the controller 130 may activate a hard interrupt shift routine at step 140- 4. During the hard interrupt routine, the controller 130 may transmit gate signals configured to generate current pulses through the phase legs of the motor 110 by simultaneously switching, opening, and closing both switches 132 for each phase of the motor 110 at a required frequency. Such a hard-disconnect routine may drive the motor 110 to generate an output range with a substantially constant torque. Alternatively, if the observed speed corresponds to a relatively low speed, the controller 130 may be configured to activate a soft-disconnect shift routine at step 140- 5. For example, during step 140- 5, the controller 130 may transmit gate signals configured to alternately (alternately) switch one of the two switches 132 of a phase leg of a switched reluctance motor 110 at a required frequency while maintaining the remaining switch in the closed state.By the example power converter circuit 122 of FIG. 4 and the corresponding waveforms shown in FIG. 5, the distributed soft interrupt routine of step 140- 5 may be a modification of a conventional soft interrupt routine that is commonly used. In conventional soft interrupt routines, only one of the upper or first switches 132-1 of a phase leg is pulsed or opened and closed at a required frequency, while one of the lower or second switches 132-2 is continuously maintained in the closed position. Moreover, in conventional soft interrupt routines, only the first switch 132- 1 is switched or driven by a pulse width modulated (PWM) signal. Soft switching routines may typically be used to drive a switched reluctance motor 110 at zero speed or at relatively low speeds for high current applications requiring a substantially higher torque output. Such high current applications represent a significant load and thermal load on only one of the two switches 132- 1, 132- 2 that is used continuously for switching, often resulting in premature failure.However, the soft break distributed routine shown in FIG. 5 may distribute the switching pattern of the high phase currents to both of the first and second switches 132- 1, 132- 2, such that the thermal load is distributed more evenly, and reduce the overall load on the power converter circuit 122. As shown, each of the first and second switches 132- 1, 132- 2 may be driven by two different and generally alternating PWM signals. For example, the controller 130 may be configured to transmit a first PWM gate signal for driving the first switch 132- 1 and a second PWM gate signal for driving the second switch 132- 2. More specifically, the first and second PWM signals may generally be configured to alternate such that at any point during the soft break distributed routine, at least one of the first and second switches 132- 1, 132- 2 is closed and the first and second switches 132- 1, 132- 2 are never both simultaneously opened during the soft break routine. Moreover, the PWM signals may be configured such that each pulse segment of the conventional soft-break routine, e.g., the first waveform of FIG. 5 generated by switching only the first switch 132- 1, effectively matches the distributed soft-break routine, e.g., the second waveform of FIG. 5 generated by switching both the first and second switches 132- 1, 132- 2. Accordingly, the controller 130 may be further configured to generate PWM signals that maintain the effects of conventional soft interrupt routines while improving the overall life of the converter circuit 122 and the electric drive system 104.Industrial applicabilityIn general, the disclosure described above finds use in various industrial applications, such as agriculture, construction, and mining, in which smoother and more efficient control of engines conventionally used in connection with work vehicles and / or machines such as tractors, backhoe loaders, compressors, cellar bunchers, forest machines, industrial loaders, skid steer loaders, wheel loaders, and the like is provided. More specifically, the disclosed control systems and methods may be applied to electric drive systems and machines with switched reluctance motors or other comparable motors that are commonly used. The systems and methods disclosed herein provide an efficient electric motor drive strategy that aims to maintain the state of an associated electric drive system and extend machine performance. In addition, a distributed soft switching strategy for driving switched reluctance motors during a motor or drive mode of operation at zero or relatively low motor speeds is provided. By distributing soft break more evenly over more than one switch per phase of the motor in high current, high load applications, component failure caused by thermal stress is reduced. Moreover, by alternating the switching of all switches of the power converter circuit, efficiency is improved while maintaining the effective functionality of conventional soft-break strategies.From the foregoing, it will be apparent that although only certain embodiments have been described for purposes of illustration, alternatives and modifications may be apparent to those skilled in the art from this specification. These and other alternatives are intended to be equivalents and fall within the scope of this disclosure and the appended claims.

Claims

A method (140) of controlling a motor (110), comprising: determining a speed of the motor (110); and activating a soft interrupt routine at a first switch (132-1) and a second switch (132-2) of each phase when the motor speed is relatively low, wherein the first switch (132-1) is driven by a first pulse width modulated (PWM) signal and the second switch (132-2) is driven by a second PWM signal, wherein the first and second PWM signals are alternately configured such that at any point during the soft interrupt distributed routine at least one of the first switch (132-1) and the second switch (132-2) is closed and the first switch (132-1) and the second switch (132-2) are never both simultaneously open.The method (140) of claim 1, wherein the first and second PWM signals are different.The method (140) of claim 1, wherein the motor (110) is a low to medium voltage three-phase switched reluctance motor operable in one of a generator mode of operation and a motor mode of operation, the distributed soft interrupt routine being activated during the motor mode of operation.The method (140) of claim 1, wherein relatively low engine speeds correspond to engine speeds that are approximately between zero and a base speed, the base speed being a maximum speed at which the engine (110) is capable of delivering a constant torque before torque delivery begins to decrease in proportion to the engine speed.The method (140) of claim 1, further comprising the steps of activating a single pulse routine on the first switch (132-1) and the second switch (132-2) when the engine speed is relatively high, and activating a hard interrupt routine on the first switch (132-1) and the second switch (132-2) when the engine speed is a nominal speed, wherein relatively high engine speeds are about greater than a base speed and nominal engine speeds are about the base speed, wherein the base speed is a maximum speed at which the engine (110) is capable of delivering a constant torque before torque delivery begins to decrease in proportion to the engine speed.A control system (128) for a motor (110), comprising: a converter circuit (122) operatively connected to a stator (118) of the motor (110), the converter circuit (122) including at least a first switch (132-1) and a second switch (132-2) connected to each phase of the stator (118); and a controller (130) in communication with the motor (110) and the converter circuit (122), respectively, wherein the controller (130) is configured to determine a motor speed and activate the first switch (132-1) and the second switch (132-2) with a soft interrupt routine during a motor operating mode with relatively low motor speeds, wherein the first switch (132-1) and the second switch (132-2) are controlled by alternating signals such that the first switch (132-1) and the second switch (132-2) are never both simultaneously open.The control system (128) of claim 6, wherein the controller (130) is configured to generate a first pulse width modulated (PWM) signal for driving the first switch (132-1) and a second PWM signal for driving the second switch (132-2) during the soft break distributed routine, wherein the first PWM signal is different than the second PWM signal.The control system (128) of claim 6, wherein the controller (130) is configured to alternate a first pulse width modulated (PWM) signal for driving the first switch (132-1) and a second pulse width modulated (PWM) signal for driving the second switch (132-2) such that at any point during the distributed soft break routine, at least one of the first switch (132-1) and the second switch (132-2) is closed.The control system (128) of claim 6, wherein the controller (130) is adapted for use with an electric drive (104) comprising a low to medium voltage three-phase switched reluctance motor operable in one of a generator mode and a motor mode.The control system (128) of claim 6, wherein the controller (130) is further configured to activate the first switch (132-1) and the second switch (132-2) with a hard interrupt routine when the engine speed is a nominal speed, and activate the first switch (132-1) and the second switch (132-2) with a single pulse routine when the engine speed is relatively high, wherein relatively low engine speeds correspond to engine speeds that are approximately between zero and a base speed, nominal engine speeds correspond to approximately the base speed, relatively high engine speeds correspond to engine speeds that are approximately greater than the base speed, wherein the base speed is a maximum speed at which the engine (110) is capable of delivering a constant torque before torque delivery begins to decrease in proportion to the engine speed.

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