1. Field of the Invention
The present invention generally relates to motor control and more particularly relates to a partitioning of torque and flux currents (Iq, Id) supplied from a DC source when operating a permanent magnet (PM) motor, for example, in a constant power range.
2. Description of the Related Art
The PM synchronous motor possesses many appealing 7 characteristics for various applications, including pure-electric and hybrid-electric vehicles. The maximum input power of a vehicle is dictated by the size of the source (i.e. battery, fuel cell engine, supercapacitor, etc.) and is the product of the DC voltage and DC current. Quite often, the DC bus voltage varies with motor output power (i.e., Torque*Speed). As a result, rapid changes in vehicle load may cause large fluctuations in the DC bus voltage. A traction electric motor drive is often required to perform over a wide operating range. Typically the operating range of an electric machine such as a traction electric motor is divided into two regions: the constant torque region and the constant power region. It is important to maintain the ability to change between both modes of operation quickly and smoothly.
Existing PM motor control, particularly in vehicle applications, performs poorly when the DC bus voltage varies. Rapid fluctuation of the DC bus voltage, for example due to rapidly changing power demands, exacerbates this problem, and existing PM control systems are typically unable to adequately compensate. Fast switching between these two regions requires an improved current regulation methodology and apparatus.
Proper current partitioning between the flux and torque currents is important to produce the desired torque while avoiding commanding voltages beyond the instantaneous capability of the DC source. If the current is partitioned incorrectly, the motor will not be able to produce the desired torque. To address this issue, a closed-loop flux-weakening control system and method to partition the current optimally between the torque and flux-weakening currents to produce the desired torque without exceeding the capability of the DC source are disclosed. The control system and method shows superior stability of the dynamic performance during high-speed operation (i.e., constant power range) as compared to performance results previously published. Thus, the present invention provides a way to regulate and partition the current so as to improve the performance of a PM motor over an extended speed range.
In one aspect, a method and a system allows the setting and controlling of a minimum threshold of the maximum torque current allowed while operating in a flux-weakening region. This minimum threshold compensates for the variability of the DC source voltage and ensures a smooth torque response because the torque current regulates the flux current.
The method determines a maximum limit for the allowable torque current (IqMaxCalc), while also determining a minimum threshold (IqMaxBottomLine) for this maximum torque current limit to compensate for the DC source variability. If the maximum torque current limit (IqMaxCalc) is greater than the minimum threshold (IqMaxBottomLine), then the limit, IqMax, equals IqMaxCalc. If, however, the minimum threshold IqMaxBottomLine is greater than IqMaxCalc, the magnitude of the flux current is increased by the output of a first proportional-plus-integral (PI) controller. The increase in the magnitude of the flux current allows the maximum torque current limit IqMaxCalc to increase until it equals the minimum threshold IqMaxBottomLine.
If the demanded torque current (Iq*) is higher than the maximum torque current limit (IqMax), then the magnitude of the flux current is increased by adding together the peak-torque-per-amp (PTPA) component of the flux current and the output of a second proportional-plus-integral (PI) controller. This PI controller operates on the difference between the demanded torque current (Iq*) and the commanded torque current (Iq_req) and ensures that Iq* does not exceed IqMax. These features allow the motor to continue to produce torque smoothly and at the maximum capability of the motor, while transitioning into and out of the flux-weakening region at whatever rate is dictated by the DC source variation.
Further aspects and advantages of the present invention will be more readily apparent to those skilled in the art during the course of the following description, references being made to the accompanying drawings which illustrate preferred forms of the present invention.
In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements, as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.
In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with PM motors, controllers, microprocessors, and various electrical components have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.
Unless the context requires otherwise, throughout this specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.”
The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.
Glossary of Symbols
The following symbols, related to a PM motor, appear in the description that follows:
The most convenient manner of analyzing a sinusoidal permanent magnet alternating current (“PMAC”) machine uses the instantaneous current, voltage and flux linkage vectors in a synchronously rotating reference frame locked to the rotor. The d-axis has been aligned with the permanent magnet flux linkage vector so that the orthogonal q-axis is aligned with the resulting back-EMF vector. The equivalent circuit 1 for a PM motor can be developed for dq-axes as shown in FIG. 1.
The d-q axis model of a PM motor with the reference axes rotating at synchronous speed (ωr) is:
νds=rsid+pλds−ωrλqs; (1)
νqs=rsiq+pλqs+ωrλds; (2)
where:
λds=Ldid+λPM=(Lld+Lmd)id+λPM; (3)
λqs=Lqiq=(Llq+Lmq)iq. (4)
Rotor mechanical speed is determined according to:
pωr=P(Te−TL−Fwωr)/J. (5)
Motor torque is determined according to:
Te=(3P/2)(λPMiq+idiq(Ld−Lq)). (6)
and power is found as:
Powerinput=(3/2)(νdsid+νqsiq); (7)
Poweroutput=Teωrm; (8)
where:
ωrm=ωr/P. (9)
Control System Model
Since the constant power range is very important for traction drive systems, an improved flux-weakening control method is implemented, while a peak torque per ampere (“PTPA”) method is employed for the constant torque region. With reference to
As shown in equation (6), the torque of the PM primarily depends on the id and iq currents. From the equivalent circuit, the phase current can be expressed as:
is=√{square root over (id2+iq2)}; (10)
thus, equation (6) can alternatively be expressed as:
Te=(3P/2)(λPMiq+iq√{square root over (is2−iq2)}(Ld−Lq)). (11)
For a given phase current, the peak torque per ampere can be found by varying the distribution of id and iq.
Unfortunately, as the rotor speed increases, the range of attainable motor currents is limited by the ability of the source voltage (i.e., DC bus voltage) to accommodate the growing motor terminal voltage due to the increasing Back-EMF voltage. At any speed, the attainable stator current component in the dq-plane lies within a voltage limit ellipse defined by:
Recall that the PM motor torque is defined as:
Te=(3P/2)(λPMiq+idiq(Ld−Lq)). (6)
If we assume that the reference torque commands the current defined by point “A” in
The new and unique flux-weakening algorithm modifies the current commands at high speed to force the id and iq operating point to the left, along the ellipse boundary 10, to the intersection of the constant torque curve T*=T0 at point “C” in FIG. 4. At point “C”, the PTPA characteristic is no longer applied.
To reach point “C”, the torque current command (iq*) is limited to remain within the ellipse boundary 10 while the magnitude of the flux current command (id*) is increased to move the operating point to the left along the ellipse boundary 10. At any given speed, bus voltage and flux current (id*), the maximum value of torque current (iqMax) is given by:
While operating at any given torque, as the motor speed increases the output power increases, the DC bus voltage is reduced according to the terminal characteristics of the DC source. At some point, the maximum torque current iqMax will decrease to zero according to equation (14). To maintain a minimum torque capability of the motor, a minimum threshold (IqMaxBottomLine) of the maximum torque current limit iqMax must be established to prevent the maximum torque current limit iqMax from becoming zero. When the maximum torque current limit iqMax exceeds IqMaxBottomLine, no limitation is applied to the maximum torque current limit iqMax, and the output of a first PI controller 62
Command dq Voltage Calculation
During steady state operation, the id and iq currents are not changing, thus the feed-forward voltages, VdCalc and VqCalc, are derived from the inverse voltage equations for a PM motor. These equations, previously presented in equations (1) and (2), can be simplified to:
νdCalc=rsid−ωrλq=rsid−ωrLqiq; (15)
νqCalc=rsiq+ωrλd=rsid+ωr(Ldid+λPM). (16)
To improve the system's transient performance, as well as to account for the IGBT voltage drops and dead-times, third and fourth PI controllers 30 and 40 (
Flux Current Calculation
Torque Current Calculation
As known, the PM torque is given as (6):
Te=(3P/2)(λPMiq+idiq(Ld−Lq)). (6)
For a given flux current, torque current can be calculated by:
Since the PM motor parameters may vary, a PI controller 70 is employed in the torque current command subsystem 7 to improve the torque control accuracy, as shown in
Although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art.
The teachings provided herein of the invention can be applied to other motor control systems, not necessarily the PM motor control system generally described above. For example, the PM motor control system may be embodied in software, hardware, and/or firmware. Additionally, or alternatively, many of the methods and processes described above may include optional acts or steps, and additional act or steps may be added as will be recognized by those skilled in the relevant arts. Further, the acts or steps of many of the methods and processes described above, may be executed in a different order, as will be recognized by those skilled in the relevant arts. The motor control system can have a different organization than the illustrated embodiment, combining some functions and/or eliminating some functions.
These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all motor control systems and methods that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.
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