Patent Application
20230208263
The present invention relates to an Integrated Starter Generator system.
Conventional ISG systems comprise a three-phase brushless direct-current (BLDC) machine coupled to an engine crankshaft. The BLDC machine satisfies two purposes—to start the engine by rotating engine crankshaft to sufficiently high speeds to enable self-sustaining combustion, and to act as an alternator once the engine starts.
An electronic control unit (ECU) is typically coupled to the battery and to the BLDC machine. One of the primary purposes of the ECU is to apply optimal commutations to the BLDC machine to generate positive torque during engine starting, and to generate negative torque once the engine has started. The ECU typically comprises a three-phase H-Bridge circuitry to achieve the above. The ECU typically uses either 6-step commutation, sinusoidal commutation or space vector PWM to actuate the three-phase H-Bridge to produce required torque. Furthermore, typical ISG systems also comprise position sensors such as hall-effect sensors, to detect the relative position of rotor with respect to stator, and provide the sensed position to the ECU to perform optimal commutation, the position sensors and ECU typically connected using a wiring harness. Position sensors and the related wiring harness are prone to failure because of high temperature, vibration, and proximity to mechanical objects rotating at high speeds, therefore presenting a need for sensor-less operation of the BLDC machine. However, BLDC machines used in conventional ISG systems are not necessarily favourable to be used in sensor-less ISG systems.
While acting as a motor to start the engine, the BLDC machine is required to generate sufficiently high torque to make speed of engine crankshaft reach a threshold, wherein self-sustaining combustion of engine can sustain with high probability. The said threshold is typically higher when ambient temperature is substantially lower than room temperature, because of increase in viscosity of oil lubricating the engine. Furthermore, the said threshold is typically higher when the engine has not been started for a substantial amount of time. BLDC machines with high back-emf voltage per unit RPM, pose a challenge to generate high amount of torque as speed of engine crankshaft increases. As speed of engine crankshaft increases, the difference between battery voltage and back-emf voltage reduces, hence limiting the amount of current that can be provided to the machine. This limit in current correspondingly reduces the torque producing ability of machine, hence adversely affecting engine starting function.
While acting as an alternator, the speed of the BLDC machine reaches substantially high rotations per minute, resulting in large value of back-emf voltage being induced in the stator windings of the BLDC machine, the value of back-emf induced being substantially larger than the nominal voltage of battery. Because of the large value of back-emf induced compared to the nominal voltage of battery, the battery terminals appear as a short-circuit as perceived by the BLDC machine. Therefore, the amount of current flowing through the machine lines are approximately equal to the short-circuit current of the BLDC machine. The short circuit current of the BLDC machine is defined as the amount of current flowing through the machine lines if the motor line terminals are shorted, and the BLDC machine is rotated at high speeds. The short-circuit current typically increases with speed and eventually saturates. While acting as an alternator, the BLDC machine line current also flows through the power switches of the ECU. The line currents result in heat generation in stator winding of the BLDC machine because of resistance of stator winding, as well as result in heat generation inside the ECU because of the resistance of power switches. Therefore, it is preferable to reduce the amount of short circuit current of the machine to improve system efficiency.
BLDC machines used in conventional ISG systems comprises a stator and a rotor. The number of stator teeth and magnets are typically 3n and 2n respectively where n is a positive natural number. The stator is disposed with 3 sets of windings resulting in three phases, each phase being typically 120 electrical degrees apart from the other. Furthermore, for traditional ISG systems, BLDC machines are typically chosen such that they have line-to-line back-emf voltage amplitude to be greater than 75% of the nominal battery voltage, when the machine is rotated at 1000 RPM. It is perceived that BLDC machines with high back-emf voltage per unit RPM are well suited for ISG application. BLDC machine as described above has low value of stator winding inductance, hence resulting in large value of short circuit current. Furthermore, the high back-emf value of the BLDC machine adversely affects the engine starting function of the BLDC machine as discussed previously.
Thus, there is a need in the art for an Integrated Starter Generator system which addresses at least the aforementioned problems.
In one aspect of the invention, the present invention is directed at an Integrated Starter Generator system having a battery and a three-phase brushless DC electric machine. The electric machine has a stator with 6n stator teeth, ‘n’ being a natural number, and each stator tooth has a coil corresponding to one of the three phases. Further, the electric machine has a rotor with 6n±2 rotor poles facing the stator, and magnets on the rotor poles are disposed with an alternating sequence of magnet polarity facing the stator.
In an embodiment of the invention, average width of each stator teeth is smaller than 1.2 times the diameter of the stator divided by number of stator teeth.
In a further embodiment of the invention, back-emf constant of the electric machine is substantially between 25% of a nominal battery voltage and 75% of the nominal battery voltage.
In a further embodiment of the invention, the battery has the nominal battery voltage between 10V and 14V.
In another embodiment of the invention, the stator has 18n stator teeth, and the rotor has 18n±2 rotor poles facing the stator.
In an embodiment, n=1, with the stator having 18 stator teeth (132′), teeth numbered 1, 3, 10 and 12 are wound with a coil (A-A′) corresponding to the first phase in the clockwise sense, teeth numbered 2 and 11 are wound with the coil (A-A′) corresponding to the first phase in the anti-clockwise sense. Teeth numbered 4, 6, 13 and 15 are wound with the coil corresponding to the second phase in the clockwise sense, and teeth numbered 5 and 14 are wound with the coil corresponding to the second phase in the anti-clockwise sense. Finally, teeth numbered 7, 9, 16 and 18 are wound with the coil corresponding to the third phase in the clockwise sense, and teeth numbered 8 and 17 are wound with the coil corresponding to the third phase in the anti-clockwise sense.
In another embodiment of the invention, n=1 with the stator having 18 stator teeth and the rotor having 16 rotor poles facing the stator. In an alternative embodiment, n=1 with the stator having 18 stator teeth and the rotor having 20 rotor poles facing the stator.
In a further embodiment of the invention, the stator has 12n stator teeth, and the rotor has 12n±2 rotor poles facing the stator.
In an embodiment, n=1, with stator having 12 stator teeth (132′), teeth numbered 1 and 8 are wound with the coil corresponding to a first phase in a clockwise sense, teeth numbered 2 and 7 are wound with the coil corresponding to the first phase in an anti-clockwise sense, teeth numbered 4 and 9 are wound with the coil corresponding to a second phase in the clockwise sense, teeth numbered 3 and 10 are wound with the coil corresponding to the second phase in the anti-clockwise sense, teeth numbered 5 and 12 are wound with the coil corresponding to a third phase in the clockwise sense, and teeth numbered 6 and 11 are wound with the coil corresponding to the third phase in the anti-clockwise sense.
In a further embodiment of the invention, n=1 with the stator having 12 stator teeth and the rotor having 10 rotor poles facing the stator. In an alternative embodiment, n=1 with the stator having 12 stator teeth and the rotor having 14 rotor poles facing the stator.
Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
The present invention relates to an Integrated Starter Generator system. More particularly, the present invention relates to an Integrated Starter Generator system with an improved starting performance, improved generating performance, and being favourable for sensor-less operation.
In the embodiment referenced in
In the embodiment specifically depicted in
In an alternative embodiment, n is equal to 1 with the stator 132 having 18 (eighteen) stator teeth 132′ and the rotor 134 having 20 (twenty) rotor poles 134′ facing the stator 132.
In the embodiment referenced in
In the embodiment specifically depicted in
In an alternative embodiment, n is equal to 1 with the stator 132 having 12 (twelve) stator teeth 132′ and the rotor 134 having 10 (ten) rotor poles 134′ facing the stator 132.
It is understood that typical sensor-less position estimation methods rely on inductance variation of stator phase windings as a function of rotor position with respect to the stator. The inductance variation is significant if magnetic saturation of stator teeth varies significantly with rotor position. The magnetic saturation of stator teeth as a function of rotor position can be increased by reducing the thickness of stator teeth, resulting in increase in the magnetic flux density in the stator teeth.
Reference is made to
As illustrated in
It is known that starting operation of IC engines requires the crankshaft of IC engine to be rotated at a reasonable speed. The probability of self-sustaining combustion, resulting in engine starting increases as the cranking speed of IC engine increases.
It is further known that short-circuit current of the electric machine is a good indicator of efficiency of an ISG system when the electric machine is acting as a generator. The efficiency of the system is higher if short-circuit current is lower.
The flux linkage in stator winding is provided by the maximum value of f(a)=∫02πN(θ)B(θ+α)dθ, where N(θ) is the winding function, and B(θ) is the fundamental component of magnetic field caused by the rotor poles in the air gap. To achieve substantially equal flux linkage in the electric machine 130 of the present invention, and therefore achieve a substantially equal back-emf compared to a conventional electric machine, the number of turns in the electric machine 130 should be increased compared to the conventional electric machine. Since inductance of machine varies quadratically with number of turns, the inductance of the electric machine will be significantly increased and higher than the conventional electric machine for similar back-emf constant.
Advantageously, the present invention provides an ISG system with a three-phase brushless DC electric machine with increased magnetic flux density, resulting in increased stator winding inductance variation as a function of rotor position and rendering the electric machine favorable for sensor-less operation.
Further, the present invention provides the electric machine with low back-emf constant where the difference between battery voltage and induced back-emf is higher, resulting in higher torque producing capability of the ISG system in a starting operation. The electric machine of the ISG system of the present invention also achieves higher cranking speed.
Furthermore, the electric machine as per the present invention has a reduced short-circuit current due to increased stator winding inductance, thereby having an increased efficiency when the ISG system is acting as a generator. The reduced short circuit current also results in reduction of heat generation in the power switches of the Electronic Control Unit.
While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202021022575 | May 2020 | IN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IN2021/050524 | 5/28/2021 | WO |
