WO2010001415A1

WO2010001415A1 - A controller for an electric traction system

Info

Publication number: WO2010001415A1
Application number: PCT/IN2009/000374
Authority: WO
Inventors: Gangadurai Mohan; Krishnamoorthy Sugantha; Rengarajan Babu; Samraj Jabez Dhinagar
Original assignee: TVS Motor Co Ltd
Current assignee: TVS Motor Co Ltd
Priority date: 2008-07-02
Filing date: 2009-07-02
Publication date: 2010-01-07
Anticipated expiration: 2011-01-02
Also published as: MY175273A; WO2010001415A4; BRPI0913938B1; BRPI0913938A2

Abstract

The subject matter described herein relates to a system including a power-limiting controller (306) and a motor (308). The power-limiting controller (306) is configured to modulate unmodulated power (312) received from a power source (304). The power-limiting controller (306) modulates the unmodulated power (312) based on a throttle position signal (310) to provide modulated power (314). The modulated power (314) is drawn by the motor (308). Further, the power-limiting controller (306) adjusts the modulated power (314) based on a predefined range of speeds (403) of the motor (308), such that an output power of the motor (308) is maintained within a predefined power limit (412).

Description

A CONTROLLER FOR AN ELECTRIC TRACTION SYSTEM

TECHNICAL FIELD

The present subject matter described herein, in general, relates to an electric traction system for vehicles and, in particular, relates to a controller for an electric motor in the electric traction system.

BACKGROUND

Typically, in electric or hybrid vehicles, an electric traction system includes an electric motor, a power source, and an electronic controller. The electric motor, interchangeably referred to as motor hereinafter, is used to generate the required driving force or traction. The motor generally implemented in such traction systems is a brushless direct-current (BLDC) motor. The motor has an ability to render high starting torque, precise speed control, and linear torque-speed characteristics.

The motor draws varying amounts of current from the power source, for example, a battery, based on the operating conditions of the vehicle, for example, initial torque requirement, speed, etc. An uncontrolled drawing of current from the source could cause overheating and can damage not just the motor but also the associated circuitry. Generally, to prevent the damage associated with excess current drawn by the motor, a current limiting electronic controller is employed. The application of a current limit results in limiting the maximum torque that can be provided by the motor, reducing the effectiveness of the motor.

Further, the efficiency of the motor is directly proportional to its speed, and thus the efficiency of the motor decreases with a decrease in its speed. In essence, the efficiency of the motor would be low if it is operating at low speeds or a speed much lower than a rated speed. In general, the motors are designed to achieve an optimum efficiency when they perform at their rated speeds. The rated speed of any motor is the maximum allowable speed of the motor for a continuous reliable performance, and is preset during manufacture. Therefore, to achieve efficiency close to the optimum efficiency, the motor must be operated close to its rated speed.

Typically, to operate the motor at its rated speed and to meet the initial torque requirement, a large input power has to be supplied. In such a case, the peak output power P_max derived from the motor is also high. However, in some cases, it may be required to limit the peak power P_max derived from the motor to a value below a particular threshold value, for example due to prevailing vehicle standards or regulations. Consequently, in addition to limiting the current drawn by the motor, the electronic controller also limits the peak power Pmax- The peak output power P_max delivered by the motor is limited by the conventional electronic controller at the expense of the operating motor speed. Thus, when the current drawn by the motor and the peak output power P_max delivered by the motor are limited by the controller, there is a loss in the rated speed of the motor, and the initial torque requirement of the vehicle may also not be met. Also, it is seen that the peak output power Pm_3x is delivered by the motor over a very narrow range of motor speeds, thereby reducing the effectiveness of the motor.

SUMMARY

The subject matter described herein is directed towards an electric traction system with a power-limiting controller for an electric motor. In an embodiment, the electric traction system includes an electronic controller having a power-limiting configuration, interchangeably referred to as power-limiting controller hereinafter, and an electric motor. The power-limiting controller, is configured to modulate unmodulated power received from a power source. The unmodulated power is modulated based on a given actuation of a throttle device. Further, the modulated power is adjusted by the power-limiting controller such that an output power of the electric motor is maintained within a predefined power limit. Thus, the power-limiting controller facilitates the motor to meet the initial torque requirement of a vehicle at low motor speeds. Additionally, at high motor speeds, the motor is capable of operating at speeds close to a maximum allowable speed at the given actuation of the throttle device. Also, by using the power-limiting controller, the motor can operate over a wide range of speeds within the predefined power-limit. These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts and is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF DRAWINGS

The above and other features, aspects, and advantages of the subject matter will become better understood with regard to the following description, appended claims, and accompanying drawings where: Figure 1 shows a typical characteristic plot for a motor in a conventional electric traction system employing a conventional current limiting electronic controller for the motor.

Figure 2 shows another typical characteristic plot for the motor in the conventional electric traction system employing a conventional current and peak power limiting electronic controller. Figure 3 illustrates a block diagram of an exemplary electric traction system with a power-limiting controller, according to an embodiment.

Figure 4 shows an exemplary characteristic plot for a motor controlled via the power- limiting controller, in accordance with an embodiment of the present subject matter

Figure 5 shows an exemplary flowchart illustrating working of the power-limiting controller.

DETAILED DESCRIPTION

The described subject matter relates to an electric traction system for electric and hybrid vehicles. The electric traction system employs a power-limiting controller to operate an electric motor, for example a BLDC motor, interchangeably referred to as motor hereinafter. The power-limiting controller limits the maximum output power, delivered by the motor, over a predetermined range of motor speeds. The maximum output power of the motor is limited below a predefined power limit. Such a power-limiting controller facilitates operation of the motor even at speeds close to a maximum allowable speed, for example, a rated speed. . The present subject matter can be understood in light of the conventional electric traction system and conventional controller as described with reference to figure 1 and figure 2.

Figure 1 shows a plot 100 which illustrates typical characteristics of a motor with respect to the motor speed at a given actuation of a throttle device. In the figure, dotted curves 104, 108, 110 illustrate the current, torque and output power characteristics of the motor respectively, without application of any controller. Solid curves 102, 105 and 106 illustrate current, torque and power characteristics of the motor respectively, on application of a conventional current limiting controller.

As is clear from the solid curve 102, a maximum current drawn by the motor is limited to a current limit 103 by the conventional current limiting controller. In such a case, the motor draws a constant current equal to the current limit 103 till the motor reaches a motor speed at which the solid curve 102 intersects the dotted curve 104. Beyond this motor speed, the motor current reduces along the dotted curve 104.

The solid curve 105 illustrates that the initial torque generated at very low speeds is also reduced due to application of a current limit. The solid curve 106 shows that the power characteristic of the motor also shifts due to application of a current limit.

Figure 2 shows a plot 200 which illustrates characteristics of a motor controlled with a conventional current and peak power limiting controller. The plot 200 illustrates the characteristics of a motor at a given actuation of a throttle device. In the figure, the dotted curves illustrate the characteristics of the motor without application of any controller, the hashed lines illustrate the characteristics of the motor on application of a conventional current limiting controller, and the solid lines illustrate the characteristics of the motor on application of a conventional current and peak power limiting controller.

In the figure, curve 202 depicts torque characteristics of the motor,- curve 204 depicts the corresponding current characteristics, and curve 206 defines the corresponding power characteristics of the motor. The curves 202, 204, 206 depict the various characteristics of the motor with respect to the motor speed.

Horizontal line 212 illustrates the limit of output power P_maXL to which the peak output power P_max of the motor is limited using the conventional current and peak power limiting controller. The loss in torque 208 of the motor and the loss in the maximum allowable speed 210 of the motor due to the application of the current and peak power limit are also depicted in the plot 200. Thus, as shown in the figure, when the peak output power P_maχ delivered by the motor is limited to P_maXL, then the maximum operating motor speed that can be achieved and the maximum torque that can be obtained are much less than that which can be delivered by the motor without applying the peak power limit. Figure 3 illustrates a block diagram of an exemplary electric traction system 300 with a power-limiting controller for an electric or a hybrid vehicle. The electric traction system 300 includes a throttle device 302, a power source 304, a power-limiting controller 306, and a motor 308. The motor 308 can be a BLDC motor. It will be appreciated by a person skilled in the art that other motors, which are known in the art, may also be used.

In operation, depending upon the actuation of the throttle device 302 by a user, a throttle position signal 310, interchangeably referred to as TP signal 310 hereinafter, may be generated by, for example, a throttle position sensor. The throttle device may be, for example, a twist-grip throttle, an accelerator pedal, etc. The power-limiting controller 306 receives the TP signal 310 and, based on the TP signal 310, modulates unmodulated power 312 received from the power source 304. In one implementation, the power-limiting controller 306 regulates the unmodulated power 312 using a modulation technique, for example, pulse width modulation (PWM).

Further, the power-limiting controller 306 limits the output power of the motor 308 to a value below the predefined power limit. The output power of the motor 308 may be limited, for example, on the basis of prevalent standards in the industry. In an implementation, the predefined power limit remains constant for different percentage actuations of the throttle device 302. In another implementation, the predefined power limit varies with the percentage of actuation of the throttle device 302.

In one mode of operation of the power-limiting controller 306, a motor characteristic, for example motor current, is varied by the power-limiting controller 306 such that the power delivered by the motor 308 is not adjusted until a predefined power limit is reached. In said mode, the initial torque requirement of the motor 308 is achieved by allowing the motor 308 to draw a high current initially. Additionally, below the lower limit of the predetermined range of motor speeds 403, the power limiting controller 306 may apply a current limit so as to avoid damage of electrical and electronic components due to excessively high currents. In one implementation, the output power derived from the motor 308 is limited or controlled by using a control logic in the power-limiting controller 306. The control logic may be implemented by using hardware, software, or a combination of both.

In an embodiment, to limit the output power of the motor, the current input to the motor 308 is continuously varied by the power-limiting controller 306 as a function of the motor speed. For example, the current may be varied according to predetermined values of motor current as a function of the motor speed, which will be explained in detail later. For this, the power-limiting controller 306 receives an indication of the motor speed in the form of a speed signal 316. For example, a speed sensor may be used to measure the motor speed and generate the speed signal 316. It will be understood that the measurement of the motor speed can also be implemented through other techniques known in the art. The power-limiting controller 306 then determines whether the motor speed is in the predetermined range of motor speeds at the given actuation of the throttle device 302. If the measured motor speed lies within the predetermined range, then the current drawn by the motor 308 is adjusted to a predetermined value of current corresponding to the measured motor speed such that the output power obtained from the motor 308 is limited to a value below the predefined power limit. If the measured motor speed lies outside the predetermined range of motor speeds, then the power-limiting controller 306 may not adjust the current drawn by the motor 308.

In an implementation, a range of predetermined values of current is provided for the predetermined range of motor speeds. For example, the range of predetermined values of current, can be stored in a look-up table in the power-limiting controller 306. The look-up table can be referred to by the power-limiting controller 306 to vary the current drawn by the motor 308.

As a result, the motor 308 provides the required initial torque at low motor speeds and, at higher speeds, the motor 308 operates close to the maximum allowable speed corresponding to the given actuation of the throttle device 302. Thus, at a given actuation of the throttle device 302, the power-limiting controller 306 facilitates operation of the motor 308 over a wide range of motor speeds and in addition meets the initial torque requirement of the vehicle, while the maximum output power obtained from the motor is limited.

Figure 4 shows an exemplary plot 400 of motor characteristics against different motor speeds.. In the plot 400, motor speed is plotted on the x-axis and the motor characteristics, namely, torque, current, and output power, are plotted on the y-axis. The dotted curves in the plot 400 illustrate the characteristics of the motor on application of a conventional current limiting electronic controller as discussed in figure 1, while the solid curves illustrate the power characteristics for the motor 308 controlled by the power limiting controller 306, according to an embodiment of the present subject matter. The plot 400 illustrates the motor characteristics at a given actuation of the throttle device 302. The solid curve 402 represents output power of the motor 308 due to application of the power-limiting controller 306. As is clear from the figure, at low speeds, the variation of the output power from the motor 308 on application of the power-limiting controller 306 is similar to that on application of the conventional current limiting controller. This is true until the speed of the motor 308 is less than a lower limit of the predetermined range of motor speeds 403. The power limiting controller 306 can apply a current limit at motor speeds below the lower limit of the predetermined range of motor speeds 403, so as to avoid damage of electrical and electronic components due to excessively high currents.

In an implementation, the lower limit of the predetermined range of motor speeds 403 corresponds to a predefined power limit. In the plot 400, the horizontal line 412 illustrates the predefined power limit to which the output power obtained from the motor 308 is limited by the power-limiting controller 306. In an implementation, the predefined power limit is a constant for any given percentage of actuation of the throttle device 302. In another implementation, the predefined power limit varies with the percentage of actuation of the throttle device 302. Also, in an implementation, the predetermined range of motor speeds 403 may vary with the percentage of actuation of the throttle device 302.

As shown in the solid curve 402, once the output power from the motor 308 reaches the predefined power limit of the motor 308, the output power is limited to the predefined power limit over the predetermined range of motor speeds 403, as shown by the line 404. In an implementation, the predetermined range of motor speeds 403 is determined on the basis of the predefined power limit. The predetermined range of motor speeds 403 correspond to the range of motor speeds at which conventionally the output power would be greater than the predefined power limit if the power limiting controller is not applied.

Once the motor speed increases to a value beyond the predetermined range of motor speeds 403, the output power from the motor 308 decreases with an increase in the motor speed and follows the solid curve 406. Thus, the solid curve 406 illustrates the output power derived from the motor 308 corresponding to the motor speeds more than an upper limit of the predetermined range of motor speeds 403.

Further, the curves 408 and 410 illustrate to the current and torque characteristics respectively. It can be seen that the areas in the plot 400 where the output power from the motor 308 is less than the predefined limit of power output 412 (and correspondingly, outside the predetermined range of motor speeds 403), the characteristics of the motor 308 with power-limiting controller 306 follow the characteristics of the motor with conventional current limiting controller.

Thus, the solid curve 408 illustrates that an initial high current can be provided to meet the initial torque requirement of the vehicle, until the motor speed is less than a lower limit of the predetermined range of motor speeds 403. The power limiting is achieved by varying the current drawn by the motor 308 as a function of motor speeds. For this, the current is varied over the predetermined range of motor speeds 403 following the solid curve 408. In the predetermined range of motor speeds 403, the current drawn by the motor 308 is decreased with an increase in the motor speed. For motor speeds greater than the upper limit of the predetermined range of motor speeds 403, the current drawn by the motor 308 is as per the conventional current limiting characteristic.

The solid curve 410 illustrates a torque curve for the power-limiting controller 306. As can be seen, an initial high torque can be generated at low motor speeds below the predetermined range of motor speeds 403. Within the predetermined range of speeds 403, application of the power-limit results in a lower torque curve as compared to the conventional torque curve. Further, based at motor speeds greater than the upper limit of the predetermined range of motor speeds 403, the torque of the motor 308 varies as per the conventional torque curve. As can be noted from the plot 400, by using the power-limiting controller 306, it is possible to achieve high initial torques at low motor speeds. It is also possible to achieve maximum speeds close to the maximum allowable speed of the motor 308 at a given actuation of the throttle device 302, while operating the motor 308 within the predefined power limit 412. In effect, at motor speeds within the predetermined range of motor speeds 403, an increase in speed is obtained at the cost of the torque generated, thereby maintaining the power output within the predefined limit 412.

Figure 5 shows an exemplary flowchart 500 illustrating the working of the electric traction system 300 employing the power-limiting controller 306 in an electric or a hybrid vehicle. The method in the flowchart 500 has been described with reference to figure 1 to figure 4. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the methods, or alternate methods. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein.

At block 502, a TP signal 310 generated, for example, by a throttle position sensor, based on the actuation of a throttle device 302 of a vehicle is monitored. In said embodiment, the TP signal 310 is received by the power-limiting controller 306. Also, unmodulated power, provided by a power source 304, for example, a battery, is received by the power-limiting controller 306.

At block 504, modulation of the unmodulated power 312 supplied from the power source 304 to the power-limiting controller 306 is achieved. In one embodiment, this unmodulated power 312 is modulated based on the TP signal 310 received by power-limiting controller 306. In one implementation, the unmodulated power 312 is modulated by using a technique such as pulse width modulation.

At block 506, motor speed is received. In an implementation, the modulated power 314 is supplied to motor 308 for its operation and a corresponding speed of the motor 308 is measured. For example, a speed sensor may be used to measure the speed of the motor 308.

At block 508, it is determined whether the measured motor speed is within a predetermined range of motor speeds 403 at the given actuation of the throttle device 302 corresponding to the TP signal 310. The predetermined range of motor speeds 403 correspond to the speeds between which the maximum power output obtained from the motor 308 is to be maintained within the predefined power limit 412. In one implementation, the power-limiting controller 306 determines whether the motor speed is within the predetermined range of motor speeds 403 or not. When the motor speed is within the predetermined range of motor speeds 403, then instructions at block 512 are executed, else instructions at block 510 are executed. Block 510 is invoked when the measured motor speed does not lie within the predetermined range of motor speeds 403 at the given actuation of the throttle device 302. At block 510, it is determined by the power-limiting controller 306 whether the current drawn is greater than a current limit. If the current drawn is greater than the current limit, then at block 514 the current limit is applied to the modulated power signal by methods known in the art and the current limited modulated power signal can then be sent to the motor 508. Else, if the current drawn is less than the current limit, at block 516 the modulated power signal is sent to the motor 308.

At block 512, when the motor speed lies within the predetermined range of motor speeds 403, the modulated power 314 is adjusted by adjusting the current drawn. As discussed earlier, the power-limiting controller 306 adjusts the modulated power 314 based on the measured speed of motor 308 and the predetermined range of motor speeds 403. When the motor speed lies within the predetermined range of motor speeds 403, the power-limiting controller 306 reduces the current drawn by the motor 308, and therefore, the modulated power 314 drawn by the motor 308 is adjusted. With this regulation of motor current, the motor speed increases and approaches the maximum allowable speed for given actuation of the throttle device 302.

In one implementation, to adjust the modulated power 314, the current drawn by the motor 308 is varied by the power-limiting controller 306 such that the maximum power output obtained from the motor 308 is maintained within the predefined power limit 412, over the predetermined range of speeds 403. The current drawn by the motor 308 is varied according to a range of predetermined values of current associated with the predetermined range of motor speeds 403. In said implementation, the range of predetermined values of current is provided in a look-up table in the power limiting controller 306. These predetermined values of current can be obtained by experimentation by measuring current values required at various motor speeds for delivering constant limited power.

Thus, the power-limiting controller 306 for an electric traction system 300 ensures safety of the motor 308 from high current without compromising on the efficiency of the motor 308 by implementing both power and current limit. At a given actuation of the throttle device 302, the power-limiting controller 306 enables the motor 308 to operate even at speeds close to the maximum allowable speed, for example, rated speed. The power-limiting controller 306 also assists the motor 308 to generate a high initial torque even when the power output from the motor 308 is limited. Also, with the power-limiting controller 306, the motor 308 delivers a constant output power limited to the predefined power limit 412, over the predetermined range of motor speeds 403. Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

Claims

I/We claim:

1. A system comprising: a power-limiting controller (306) configured to modulate unmodulated power (312) received from a power source (304) based on a throttle position signal (310) to provide modulated power (314); and a motor (308) to receive said modulated power (314); characterized in that, said power-limiting controller (306) adjusts said modulated power (314) based on a predetermined range of speeds (403) of said motor (308) such that an output power of the motor (308) is maintained within a predefined power limit (412).

2. The system as claimed in claim 1, wherein said power-limiting controller (306) further determines whether a motor speed is within said predetermined range of motor speeds (403).

3. The system as claimed in claim 2, wherein said power-limiting controller (306) further adjusts said modulated power (314) based on said motor speed by varying a current drawn by said motor (308) such that said motor speed is within said predetermined range of motor speeds (403).

4. The system as claimed in claim 1, wherein said motor (308) is a brushless direct current motor.

5. The system as claimed in claim 1, wherein said power-limiting controller (306) receives said unmodulated power (312) from a battery.

6. A method comprising: receiving unmodulated power (312); modulating said unmodulated power (312) based on a throttle position signal (310) to provide a modulated power (314) to drive a motor (308); determining whether a motor speed is within a predetermined range of motor speeds (403); and adjusting said modulated power (314) based on said motor speed such that an output power of the motor (308) is maintained within a predefined power limit (412) over said predetermined range of motor speeds (403).

7. The method as claimed in claim 6, wherein said adjusting of said modulated power (314) is achieved by varying a current drawn by said motor (308) according to a range of predetermined values of current.

8. The method as claimed in claim 6, wherein said adjusting of said modulated power (314) is achieved by pulse width modulation.

9. The method as claimed in claim 6, wherein said modulation of said unmodulated power (312) from said power source (304) is achieved by pulse width modulation.

10. The method as claimed in claim 7, wherein said range of predetermined values of current corresponds to said predetermined range of motor speeds (403).