The disclosure relates generally to modification of stator current for enhanced flux weakening in an electric machine assembly.
An electric machine, such as an interior permanent magnet machine, includes a rotor having a plurality of permanent magnets of alternating polarity. The rotor is rotatable within a stator which generally includes multiple stator windings and magnetic poles of alternating polarity. Reducing the magnetic flux inside the electric machine at higher speeds improves power characteristics of the electric machine.
An electric machine assembly includes an electric machine having a stator configured to have a stator current. A controller is operatively connected to the electric machine and is configured to receive a torque command (T). The controller has a processor and tangible, non-transitory memory on which is recorded instructions for executing a method of modifying the stator current for enhanced flux weakening. Execution of the instructions by the processor causes the controller to obtain a base stator current [IdLU, IqLU] from a look-up table based at least partially on the torque command (T). The controller is programmed to obtain a characteristic angle (θi, i=1, 2, 3) based at least partially on the torque command (T) and the base stator current [IdLU, IqLU].
The controller is programmed to obtain a stator current modifier [ΔId, ΔIq] based at least partially on the characteristic angle (θi, i=1, 2, 3) and a flux weakening factor (ΔIS). The stator current modifier [ΔId, ΔIq] may be defined as: ΔId=(ΔIS*cosine (θi)) and ΔIq=(ΔIS*sine (θi)). The controller is operative to control at least one operating paramater of the electric machine based at least partially on the stator current modifier [ΔId, ΔIq]. Enhancing the flux weakening achieves improved high speed current control and improved motor torque linearity. The method enhances magnetic flux weakening beyond the weakening associated with voltage constraints in the electric machine.
The characteristic angle (θi, i=1, 2, 3) may be at least one of a first characteristic angle (θ1), a second characteristic angle (θ2) and a third characteristic angle (θ3). If the torque command (T) is greater than a high torque threshold (TH), the controller is programmed to obtain a first characteristic angle (θ1) defined as: θ1=(β±90), such that β=a tan 2[IqLU, IdLU]. If magnitude of the torque command is less than a low torque threshold (TL), the controller is programmed to obtain a second characteristic angle (θ2) defined as: θ2=a tan 2[IqLU, (IdLU−(λm/Ld))], such that λm is a magnetic flux and Ld is a d-axis inductance. If the torque command is between the low torque threshold (TL) and the high torque threshold (TH), inclusive, the controller is programmed to obtain a third characteristic angle (θ3) based at least partially on the first characteristic angle (θ1), the second characteristic angle (θ2) and a ratio R, such that R=(|T|−TL)/(TH−TL) and θ3=[R*θ1+(1−R)*θ2].
The magnetic flux (λm) may be obtained from a look-up table based at least partially on data from a rotor temperature sensor operatively connected to the controller. The controller may be further programmed to obtain a modified stator current [IdN, IqN], based at least partially on the stator current modifier [ΔId, ΔIq] and the base stator current [IdLU, IqLU] such that IdN=(IdLU+ΔId) and IqN=(IqLU+ΔIq). The flux weakening factor (ΔIS) may be obtained based at least partially on the torque command (T) and a DC link voltage. Obtaining the flux weakening factor (ΔIS) may include generating respective d-axis and q-axis command voltages based on the torque command (T) and a DC link voltage. A voltage magnitude is generated based on the respective d-axis and q-axis command voltages. The flux weakening factor (ΔIS) may be generated based on a comparison of the voltage magnitude and a predefined reference voltage.
A battery pack may be operatively connected to the controller and configured to provide the DC link voltage. A pulse-width-modulator (PWM) inverter may be operatively connected to the controller and the battery pack.
The above features and advantages and other features and advantages of the present disclosure are readily apparent from the following detailed description of the best modes for carrying out the disclosure when taken in connection with the accompanying drawings.
Referring to the drawings, wherein like reference numbers refer to like components,
Referring to
The stator 14 includes a stator core 24 which may be cylindrically shaped with a hollow interior. The stator core 24 may include a plurality of inwardly-protruding stator teeth 26A-F, separated by gaps or slots 28. In the embodiment shown in
The stator 14 is configured to have electric current, referred to herein as stator current, flowing in the stator windings 30 and causing a rotating magnetic field in the stator 14. Referring to
Referring to
The controller 40 of
The method 100 enhances magnetic flux weakening beyond the flux weakening associated with voltage and current constraints in the electric machine 12. The method 100 improves functioning of the assembly 10 by enabling transition in and out of a six-step pulse-width-modulation (PWM) operation mode based on operating conditions of the electric machine 12 or in steady state during the six-step pulse-width-modulation (PWM) operation. As understood by those skilled in the art, a six-step pulse-width-modulation (PWM) operation is a mode of operation wherein the voltage vector is applied at six intervals (for a 3 phase inverter) during one fundamental cycle (i.e. electrical speed). The six-step pulse-width-modulation (PWM) operation is a desired mode of operation for increasing the efficiency of the assembly 10 (electric machine 12 plus the pulse-width-modulation (PWM) inverter 56) at low to light load or torque conditions and for increasing voltage utilization to increase peak torque of the electric machine 12. The six-step pulse-width-modulation (PWM) operation is employed in a high speed region from zero torque to the peak torque capability of the electric machine 12. During these modes of operation, the method 100 also ensures torque accuracy along a constant torque curve.
Referring now to
In block 120 of
In block 130 of
In sub-block 131, the controller 40 is programmed to determine if the torque command (T) is greater than a high torque threshold (TH). If the torque command (T>TH) is greater than the high torque threshold (TH), the method 100 proceeds to sub-block 133. In sub-block 133 (T>TH), the controller 40 is programmed to obtain a first characteristic angle (θ1) defined as:
θ1=(β±90), such that β=a tan 2(IqLU,IdLU).
When Id<0,Iq>0,θ1=(β+90) and β=a tan 2(IqLU,IdLU).
When Id<0,Iq<0,θ1=(β−90) and β=a tan 2(IqLU,IdLU).
As understood by those skilled in the art, a tan 2 is a math function for the four-quadrant arc tangent inverse whose values are bound from −π to π. A tan 2 is the inverse tangent function with two factors such as (A, B), for example. A tan 2 (A, B) may be defined as the angle in radians between the positive Id axis and the point given by the coordinates (A, B) on it. The angle is positive for counter-clockwise angles (upper half-plane, Iq>0), and negative for clockwise angles (lower half-plane, Iq<0).
In sub-block 135, the controller 40 is programmed to determine if the torque command (T) is less than a low torque threshold (TL). If the torque command (T) is less than the low torque threshold (TL), the method 100 proceeds to sub-block 137. In sub-block 137 (T<TL), the controller is programmed to obtain a second characteristic angle (θ2) defined as:
θ2=a tan 2[IqLU,(IdLU−(λm/Ld))].
Here λm is a magnetic flux and Ld is a d-axis inductance of the stator windings 30. The IdLU term may be a negative value. The magnetic flux (λm) may be estimated via any modeling or estimation method known to those skilled in the art or may be set to a pre-determined constant value. For example, the magnetic flux (λm) may be obtained from a pre-determined look up table as a function of rotor temperature. The rotor temperature may be estimated via a machine thermal estimator model running in the controller with inputs such as voltage, motor speed, stator currents, coolant temperature, coolant flow rates and others. The magnetic flux (λm) may be obtained via a flux observer or any other method known to those skilled in the art.
The inductance (Ld) may be obtained as a function of the number of turns (N) in the stator winding, the relative permeability of the winding core material (μ), the area of the winding/coil in square meters and the average length of the winding/coil in meters (l), such that: Ld=(N2*μ*A/l). The inductance (Ld) of the stator winding 30 may be obtained by any method known to those skilled in the art.
If the torque command (T) is neither greater than the high torque threshold (TH) nor less than a low torque threshold (TL), i.e., the torque command is between the low torque threshold (TL) and the high torque threshold (TH), inclusive, the method 100 proceeds to sub-block 139. In sub-block 139 (TL<T<TH), the controller 40 is programmed to obtain a third characteristic angle (θ3) based at least partially on the first characteristic angle (θ1), the second characteristic angle (θ2) and a ratio R, such that:
R=(|T|−TL)/(TH−TL); and
θ3=[R*θ1+(1−R)*θ2].
Referring now to block 140 of
In sub-block 141, the controller 40 is programmed to generate respective d-axis and q-axis command voltages (V*d, V*q) based on the torque command (T) and a DC link voltage (Vdc). The DC link voltage (Vdc) may be provided by the battery pack 54. The controller 40 may rely on a look-up table or data repository generated in a dynamo or test cell conditions or nay other method known to those skilled in the art. In sub-block 143, the controller 40 is programmed to generate a voltage magnitude (Vm) based on the d-axis and q-axis command voltages (V*d, V*q). In sub-block 145, the controller 40 is programmed to generate the flux weakening factor (ΔIS) on a comparison of the voltage magnitude and a predefined reference voltage (Vref). The reference voltage (Vref) may be selected based on the application.
In block 150 of
ΔId=(ΔIS*cosine(θi)); and
ΔIq=(ΔIS*sine(θi)).
The controller 40 is operative to control at least one operating paramater of the electric machine 12 based at least partially on the stator current modifier [ΔId, ΔIq], to achieve improved high speed current control and improved motor torque linearity.
In block 160 of
The controller 40 of
Look-up tables, databases, data repositories or other data stores described herein may include various kinds of mechanisms for storing, accessing, and retrieving various kinds of data, including a hierarchical database, a set of files in a file system, an application database in a proprietary format, a relational database management system (RDBMS), etc. Each such data store may be included within a computing device employing a computer operating system such as one of those mentioned above, and may be accessed via a network in any one or more of a variety of manners. A file system may be accessible from a computer operating system, and may include files stored in various formats. An RDBMS may employ the Structured Query Language (SQL) in addition to a language for creating, storing, editing, and executing stored procedures, such as the PL/SQL language mentioned above.
The detailed description and the drawings or figures are supportive and descriptive of the disclosure, but the scope of the disclosure is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed disclosure have been described in detail, various alternative designs and embodiments exist for practicing the disclosure defined in the appended claims. Furthermore, the embodiments shown in the drawings or the characteristics of various embodiments mentioned in the present description are not necessarily to be understood as embodiments independent of each other. Rather, it is possible that each of the characteristics described in one of the examples of an embodiment can be combined with one or a plurality of other desired characteristics from other embodiments, resulting in other embodiments not described in words or by reference to the drawings. Accordingly, such other embodiments fall within the framework of the scope of the appended claims.
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Entry |
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Bolognani, Calligaro, Petrella; “Adaptive Flux-Weakening Controller for Interior Permanent Magnet Synchronous Motor Drives”; IEEE vol. 2, No. 2, Jun. 2014, 2168-6777. |