SYSTEM AND METHOD FOR MANAGING THE TORQUE WRONGNESS DURING THE OPERATION OF AN ELECTRIC MOTOR

Abstract

System for controlling an electric motor (116), the system comprising: a switching bank (114) designed to individually control the current in each of a plurality of phases (142, 146, 150) of a stator (118) in the electric motor (116); and a control unit (166) designed to: Determining a desired torque to be generated by the electric motor (116); Calculating a current in a first phase (142) of the electric motor (116) that is required to deliver the desired torque; Determine whether the calculated current in the first phase (142) is greater than a threshold current for the first phase (142); if the calculated current is greater than the threshold current, limit the calculated current in the first phase (142) to the threshold current; and Increasing a current in a second phase (146) of the stator (118) adjacent to the first phase (142) in order to at least partially compensate for a deficit of torque on the first phase (142) resulting from the limitation of the calculated current in the first phase (142).

Description

Technical area

[0001] The present disclosure relates generally to electric drive systems, and more particularly to systems and methods for managing torque ripple during operation of an electric motor. background

[0002] With growing interest in energy conservation, more and more industrial machines are being equipped with electric drive assemblies or systems to power the machine and operate its various tools or functions. Ongoing developments in electric drive systems have made it possible for electrically powered machines to effectively match or even exceed the performance of primarily mechanically powered machines, while requiring significantly less overall fuel and energy. As electric drive systems become more commonplace for industrial machines and the like, the need for more efficient motors and generators, as well as for techniques to control them, has also increased.

[0003] An electric drive machine generator is typically used to convert mechanical power received from a primary power source, such as an internal combustion engine, into electrical power to perform one or more operations of the work machine. Additionally, an electric motor may be used to convert electrical power stored within a common bus or storage device into mechanical power. Among the various types of electric motors available for use with an electric drive system, switched reluctance machines have attracted the most interest because of their robustness, cost-effectiveness, and overall efficiency. Although currently existing systems and methods for controlling switched reluctance machines can provide adequate control, there is still room for improvement.

[0004] For example, control profiles with square-wave input current are easy to implement and protect the windings of an electric motor from high currents. However, such current control profiles can generate significant fluctuations in the torque produced by the electric motor over the course of a rotor cycle, known as torque ripple. Other current profiles can produce a flatter, high-torque output but may require winding currents that exceed the rated winding current. Manufacturing motors with overly high ratings to accommodate high transient currents is not cost-effective.

[0005] Control schemes for switched reluctance machines may involve operating two switches for each phase to pulse or split the electrical current, effectively providing a waveform, such as a pulse-width modulated (PWM) waveform. US Pat. No. 6,756,757 B2 (the '757 patent) teaches the use of a solver to calculate a desired torque profile and uses a lookup table to produce a flat torque output. However, the '757 patent does not consider maximum phase current limits or the effect of limiting the phase current on the generated torque.

[0006] The present disclosure solves one or more of the problems set forth above and / or other problems in the prior art. Summary of Revelation

[0007] According to one aspect of the disclosure, a method for controlling an electric motor may include determining a desired torque at the electric motor and calculating, in a controller, a calculated current at a first phase of the electric motor, wherein the calculated current is a current that results in delivery of the desired torque to the electric motor. The method may continue with comparing the calculated current to a predetermined threshold current in the controller, and if the calculated current is greater than the predetermined threshold current, reducing the calculated current to the predetermined threshold current in the controller. The method may also include adjusting a current in a second phase adjacent to the first phase of the electric motor to continue delivering the desired torque to the electric motor.

[0008] According to another aspect of the disclosure, a system for controlling an electric motor includes a switch bank configured to individually control the current in each of a plurality of phases of a stator in the electric motor, and a controller. The controller may be configured to determine a desired torque to be produced by the electric motor, calculate a current during a first phase of the electric motor required to deliver the desired torque, and determine whether the calculated current during the first phase is greater than a threshold current for the first phase.If the calculated current is greater than the threshold current, the controller may limit the calculated current to the threshold current during the first phase and increase a current in a second phase of the stator adjacent to the first phase to at least partially compensate for a torque deficit at the first phase resulting from limiting the calculated current during the first phase.

[0009] According to yet another aspect of the disclosure, a method for managing torque and phase current in an electric motor using a controller may include limiting, by the controller, the current in a first phase of the electric motor to a predetermined threshold current in response to a load on the electric motor requiring a current in the first phase greater than the predetermined threshold current. If the current in the first phase is limited, the method includes increasing, by the controller, the current in a second phase of the electric motor by an amount sufficient to allow the electric motor to handle the load. Short description of the drawings Fig. 1 is a schematic view of an exemplary machine having an electric drive system; Fig. 2 is a sectional view of an exemplary electric motor; Fig. 3 is another sectional view of the exemplary electric motor of Fig. 2; Fig. 4 is a schematic diagram of an exemplary control electronics unit; Fig. 5 is a block diagram of an exemplary controller for the control electronics unit; Fig. 6 is a flowchart of an exemplary method for implementing a current profile strategy to minimize torque ripple and current; Fig. 7 is a partial sectional view of the exemplary electric motor of Fig. 2; Fig. 8 is a flowchart of an alternative exemplary method for implementing the current profile strategy to minimize torque ripple and current; Fig. 9A and Fig. 9B are input current and output torque curves in accordance with the present disclosure; and Fig. 10A and Fig. 10B are input current and output torque graphs in accordance with another embodiment of the present disclosure. Detailed description

[0010] Fig. 1 diagrammatically illustrates a drivetrain 102 of a machine 100 that may employ electric propulsion means to cause motion and / or perform work. In particular, the machine 100 may include a power source 104 that may be coupled to an electric drive system 106 to cause motion via a traction device 108. Such a machine 100 may be used as a work machine to perform a particular type of operation associated with an industry, such as mining, construction, agriculture, transportation, or any other industry known in the art. For example, the machine 100 may be an earthmoving machine, a marine vehicle, an aircraft, a tractor, an off-highway truck, a passenger-carrying road vehicle, or any other mobile machine.In other alternative implementations, the electric propulsion system 106 may be used in conjunction with stationary applications and may be implemented, for example, with wind turbines, hydroelectric dams, batteries, fuel cells, or any other suitable energy sources. The power source 104 of the electric propulsion system 106 may be, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other type of internal combustion engine commonly used to generate power.

[0011] In the exemplary illustrated embodiment, power source 104 drives a generator 110 or other power generating mechanism, with the generated energy being stored in a battery 112 or other energy storage device, such as a capacitor. Control electronics 114 may then drive the poles of a stator 118 of an electric motor 116. Rotor 120 may transmit power to an axle 124 via a drive shaft 122 to drive traction device 108. In other embodiments, drive shaft 122 may be coupled to a power take-off unit that drives hydraulic pumps instead of, or in addition to, powering traction device 108. In the illustrated exemplary embodiment, electric motor 116 is a switched reluctance (SR) motor.

[0012] Fig. Figure 2 is a side sectional view of one embodiment of electric motor 116. Electric motor 116 may include a rotor 120 and a stator 118, as discussed above. Rotor 120 may be coupled to drive shaft 122. Bearings, such as bearing 134, may be used to support rotor 120.

[0013] The stator 118 may be mounted by brackets 136. It will be appreciated that numerous variations exist to provide the mechanical mounting of the electric motor 116 and its various components. As explained in more detail below, the stator 118 includes numerous poles with windings 130, which may be coupled to the control electronics 114 via leads 132.

[0014] Fig. 3 is a sectional view of the electric motor 116 of Fig. 2. This view shows the rotor 120 and the stator 118. This view also shows a first stator pole 142 and its corresponding stator windings 144, a second stator pole 146 and its corresponding windings 148, and a third stator pole 150 and its corresponding stator windings 152. In this exemplary embodiment, the rotor 120 has four poles, and the stator 118 has six poles. Opposing stator poles can be excited simultaneously. Other configurations of poles in the stator 118 and the rotor 120 are also possible.

[0015] Fig. Figure 4 illustrates an embodiment of the control electronics 114 adapted for use with the electric motor 116 of Fig. 2 is suitable. Considering that opposite pairs of stator poles may be energized together, the control electronics may include a switch bank of drivers for each of the three sets of stator windings 144, 148, 152, depicted in the figures as inductors. Each of the drivers may be identical and may include a low-side drive transistor 156, a high-side drive transistor 158, and a pair of diodes 160 and 162. A capacitor 164 may be used to store electricity or filter power to reduce the DC ripple in electrical current supplied by the battery 112 or the generator 110, which also include a rectifier.

[0016] A controller 166 receives position information from position sensors (not shown) on the rotor 120 and / or the drive shaft 122. The controller 166 also includes output drivers for each of the paired transistors that drive the stator windings 144, 148, 152. The transistors 156 and 158 may be insulated gate bipolar transistors (IGBTs), known for their high current capacity and fast switching speed. In operation, when both transistors 156 and 158 are turned on, current flows through each stator winding 144 and its paired poles, establishing a magnetic field. When the transistors 156 and 158 are turned off at an appropriate time during the rotation of the rotor 120, the coincident magnetic field generates electrical current, which is transferred back to the capacitor 164 via the diodes 160 and 162.

[0017] Fig. 5 is an exemplary embodiment of a controller 166 that includes a processor 170 and a memory 172 coupled together via a data bus 174. The controller 166 may also include a control input 176 from a cab that relays requests from an operator for a particular action, such as a request for forward movement. The controller 166 may also include sensor inputs 178, such as from position sensors on the rotor 120 or drive shaft 122, that receive signals indicative of the position of the rotor for use in calculating the winding current setpoints. The controller 166 may also include a control output 180 used to energize or de-energize individual pole pairs in the stator 118.

[0018] Processor 170 may be any of a variety of known computer processor architectures, including, but not limited to, single-chip processors or conventional computer architectures. Memory 172 may be any combination of volatile and non-volatile memory, including rotating media, flash memory, conventional RAM, ROM, or other non-volatile programmable memory, but not carrier waves or other propagation media. Controller 166 may also include a communications port (not shown) that enables communication with external devices, such as another engine computer or chassis electronics (not shown).

[0019] The memory 172 may include modules containing computer-executable instructions for various operating and control functions, and may include an operator system 182 and utility programs 184 used, for example, to send and receive signals and perform basic operating tasks. The memory 172 may also include a torque module 186 that processes operator input to calculate a requested torque output for the electric motor 116. A phase current module may accept data from the torque module and use rotor position data and torque characteristics for the electric motor 116 to calculate a required phase current to achieve the requested torque output. Another module 190 may include limits and constraints used by the phase current module to make decisions about maximum or other threshold current values ​​and to modify torque characteristics. Industrial applicability

[0020] Fig. 6 is a flowchart of a method 200 for minimizing torque ripple and phase current. Fig. Figure 7 illustrates an SR motor with two poles or phases energized at a time. Stator pole 142 generates magnetic flux 143, which causes rotor pole 151 to be attracted toward stator pole 142. Similarly, current in stator pole 146 causes rotor pole 153 to rotate counterclockwise toward stator pole 146. Because rotor pole 153 is almost completely aligned with stator pole 146, its magnetic field imparts little torque to the rotor.

[0021] Fig. 9A and 9B disclose an approach to providing a low torque ripple output while simultaneously preventing phase overcurrent conditions. Some benefits of avoiding phase overcurrent conditions in accordance with various implementations of this disclosure may include, but are not limited to, a reduction in the amount of heat generated by the SR motor and the ability to use components with lower current ratings than would otherwise be required to withstand phase overcurrents. The input phase currents 300, 304, and 308 of Fig. 9A are processed according to procedure 200 of Fig. 6 to shape the phase torque outputs 302, 306, 310 to provide a substantially smoothed total torque output 312. In one embodiment, the shape of an individual input phase current, e.g., input phase current 300, may conform to a particular current profile suitable for use with a particular operating mode, such as high torque at low rpm. In other operating modes, other input phase current profiles may be used, such as a constant current profile at higher rpm, for example, operation above 250 rpm.

[0022] Continue to Fig. 6, an exemplary method 200 for implementing a current profiling strategy to minimize torque ripple and current is shown. A controller may determine a desired torque at the SR machine (step 202). This determination may be based, for example, on a load on the electric motor 116. Relatively high torque conditions and correspondingly high current conditions may occur at low speeds and, therefore, relatively low speeds (revolutions per minute) for the electric motor 116. In various exemplary implementations, the method 200 may only be invoked at speeds below approximately 200 rpm.

[0023] Controller 166 may determine the desired torque at the SR machine by receiving an input signal via control input 176 associated with an operator request requiring power from electric motor 116. The input signal may correspond to a change in a throttle lever or may be responsive to a command associated with a hydraulic work attachment, such as raising a bucket on a loader. Controller 166 may determine a desired torque required to meet the demand associated with the operator request. For example, an operator request to increase the speed of machine 100 from 0 to 5 mph may result in a desired torque equal to a maximum rated output of electric motor 116.

[0024] A given electric motor, such as electric motor 116, can produce an amount of torque that is a function of the rotor angle and phase current. The current level required to produce a particular torque at a particular rotor angle can be calculated or obtained from a lookup table. At step 204, controller 166 can evaluate a condition of electric motor 116, e.g., the position of rotor 120, and calculate a required current at a particular phase required to deliver the desired torque.

[0025] At step 206, the requested current may be compared to a maximum rated or other threshold current for the phase. If at step 206 the requested current is less than the threshold current (step 206: No), the actual current for the phase may be set to the requested current (step 208). The control electronics 114 may actuate the drive circuits discussed above to deliver the actual current in the phase.

[0026] If, at step 206, the requested current is greater than the threshold current for the phase (step 206: Yes), the current in the phase can be set to the threshold current (step 210). That is, instead of delivering the requested current, the current actually delivered to the phase can be set to the threshold current.

[0027] At step 212, the amount of torque lost by reducing the current in a phase may be calculated. An actual torque generated by the phase at the threshold current for the current rotor position may be calculated, as well as a torque loss or torque deficit compared to the desired torque. In an exemplary embodiment, the desired torque may be the rated torque of the electric motor 116.

[0028] At step 214, a current may be calculated that must be applied to an adjacent stator phase on the SR machine to compensate for the torque loss. However, depending on the rotor position, the required current for the adjacent phase may be above the maximum rated or another predetermined threshold current.

[0029] In step 216, a determination is made as to whether the required current in the adjacent phase is greater than the threshold current. If the required current is less than the threshold current for the phase (step 216: No), the current in the adjacent phase may be set to the requested current (step 218). In some embodiments, the threshold current may be greater than a maximum rated current for the circuit, depending on other conditions, such as an expected duration or temperature.

[0030] If the required current is greater than the threshold current for the phase (step 216: Yes), the required current can be reduced to the threshold current or another setpoint, and the control electronics 114 can supply only the predetermined current to the adjacent phase. The method 200 can be performed continuously, so that a primary phase current and an adjacent phase current can be calculated dozens or hundreds of times per revolution.

[0031] As in Fig. As can be seen in Figure 9B, a situation may arise where overlapping phase currents, e.g., phase currents 300 and 304, produce corresponding phase torques that do not necessarily produce an overall torque output 312 that is perfectly smooth. If the expected level of torque ripple is within a desired range such that the ripple meets the requirements of a particular application, this ripple level may be acceptable. Reducing the ripple to a nearly completely smooth torque output is possible using any or all of the following factors: i) more accurate characterizations of current and torque, ii) more accurate rotor position sensing, and iii) faster cycle times, allowing a greater number of phase current adjustments per revolution.The use of more accurate phase current sensors and / or instantaneous torque measurements can also be used to reduce torque ripple.

[0032] Fig. 8 is a flowchart of an alternative exemplary method 240 for implementing a current profiling strategy to minimize torque ripple and current. At step 242, controller 166 may determine a desired torque at the SR machine. Controller 166 may receive an input signal via control input 176 associated with an operator request requiring power from electric motor 116. The input signal may correspond to a change in a throttle lever or may be responsive to a command associated with a hydraulic work attachment, such as raising a bucket on a loader. Controller 166 may determine a desired torque required to meet the demand associated with the operator request.

[0033] As explained above, a given electric motor, such as electric motor 116, can produce an amount of torque that is a function of rotor angle and phase current. The current level required to produce a particular torque at a rotor angle can be calculated or obtained from a lookup table. At step 244, controller 166 can evaluate a state of electric motor 116, e.g., the position of rotor 120, and calculate a requested current at a first phase of electric motor 116 required to deliver the desired torque.

[0034] At step 246, the requested current may be compared to a maximum rated or other threshold current for the first phase. If the requested current is less than the threshold current (step 246: No), the actual current for the first phase may be set to the requested current (step 248). As explained above, the control electronics 114 may actuate the drive circuits discussed above to deliver the actual current in the first phase.

[0035] If, at step 246, the requested current is greater than the threshold current for the first phase (step 246: Yes), the actual current for the first phase may be set to the threshold current or another predetermined current (step 250). That is, instead of delivering the requested current, the actual current delivered to the first phase is set to the threshold current or another predetermined current.

[0036] At step 252, instead of calculating a reduction in first-phase torque, as explained above for method 200, the current in a second, adjacent phase may be set to a threshold for the second phase. This may reduce the processing burden for controller 166 while still allowing an acceptable increase in torque to at least partially compensate for the reduction in torque on the first phase. In some embodiments, the threshold current may be greater than a maximum current rating for the circuit, depending on other conditions, such as expected duration or temperature.

[0037] Fig. Figures 10A and 10B disclose an alternative approach to provide a low torque ripple output while preventing phase overcurrent conditions. Fig. 9A and Fig. 9B disclose clipping a phase current, e.g., phase current 300, at a threshold level. Fig. Figure 10A illustrates phase currents 320, 322, and 324. If a particular phase current profile, e.g., phase current 320, were to exceed a threshold level, the entire curve can be shifted downward so that its peak is below the threshold level. Adjacent phase current profiles can be increased so that their peak values ​​are also below the threshold level, with further profiling below the threshold level to at least partially compensate for the reduction in first-phase torque while providing a torque output 326 with reduced ripple, as shown in Fig. 10B.

[0038] In general, the above disclosure finds application in various industrial applications, such as agriculture, construction, and mining, to provide smoother and more efficient control of electric motors typically used in conjunction with work vehicles and / or machines, such as tractors, backhoe loaders, compactors, fellers, industrial loaders, counter-rotating track loaders, wheel loaders, and many other machines, and the like. In particular, the disclosed control systems and methods may be applied to electric drive systems and machines that include switched reluctance motors or other comparable motors known in the art.

[0039] The systems and methods disclosed herein provide a strategy for delivering rated torque output from an electric motor with minimized ripple while protecting the stator windings from unacceptable current levels.

Claims

[1] A system for controlling an electric motor (116), the system comprising: a switch bank (114) configured to individually control the current in each of a plurality of phases (142, 146, 150) of a stator (118) in the electric motor (116); and a control device (166) which is designed to: Determining a desired torque to be generated by the electric motor (116); Calculating a current in a first phase (142) of the electric motor (116) required to deliver the desired torque; Determining whether the calculated current in the first phase (142) is greater than a threshold current for the first phase (142); if the calculated current is greater than the threshold current, limiting the calculated current in the first phase (142) to the threshold current; and Increasing a current in a second phase (146) of the stator (118) adjacent to the first phase (142) to at least partially compensate for a torque deficit at the first phase (142) resulting from limiting the calculated current in the first phase (142). [2] The system of claim 1, wherein the controller (166) is further configured to: calculate the torque deficit in the first phase (142) due to the limitation of the calculated current in the first phase (142); calculate a set current requirement in the second phase (146) to compensate for the torque deficit in the first phase (142); and to limit the set current in the second phase (146) to the threshold current if the set current is greater than the threshold current. [3] The system of claim 2, wherein the controller (166) is further configured to operate both the first phase (142) and the second phase (146) simultaneously at the threshold current for at least a portion of a revolution of a rotor (120) of the electric motor (116). [4] The system of claim 1, wherein the controller (166) is further configured to: to determine whether a speed of the electric motor (116) is below about 250 revolutions per minute before increasing a current in the second phase (146) of the stator (118) adjacent to the first phase (142) to at least partially compensate for the deficit in torque at the first phase (142). [5] The system of claim 1, wherein the controller (166) is further configured to control the current in at least one of the first phase (142) or the second phase (146) of the stator (118) of the electric motor (116) to a different current profile when a speed of the electric motor (116) is higher than about 250 revolutions per minute than when the speed of the electric motor (116) is lower than 250 revolutions per minute. [6] A method (200) for managing torque and phase current in an electric motor (116) using a controller (166), the method comprising: Limiting, by the controller (166), the current in a first phase (142) of the electric motor (116) to a predetermined threshold current in response to a load on the electric motor (116) requiring a current in the first phase (142) that is greater than the predetermined threshold current; and Increasing, by the controller (166), the current in a second phase (146) of the electric motor to an extent sufficient to allow the electric motor (116) to handle the load. [7] The method of claim 6, further comprising: Determining, at the controller (166), a required current in the first phase (142) to correspond to the load; and Calculating, at the controller (166), the torque delivered by the electric motor (116) when the first phase (142) is energized with the predetermined threshold current. [8] The method of claim 7, further comprising: Calculating, at the controller (166), a second current in the second phase (146) to provide a supplemental torque corresponding to a difference between the load and a torque provided by the electric motor (116) when the first phase (142) is energized at the predetermined threshold current; and Calculating, by the controller (166), the second current in the second phase (146) at the predetermined threshold current when the second current exceeds the predetermined threshold current. [9] The method of claim 6, wherein limiting, via the controller (166), the current in the first phase (142) of the electric motor (116) to the predetermined threshold current comprises selecting a phase current profile that limits the current to the predetermined threshold current during at least a portion of a revolution of the electric motor (116). [10] The method of claim 9, further comprising selecting a different phase current profile when a speed of the electric motor (116) is greater than about 250 revolutions per minute.

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