Patent Application
20200162005
Electric powertrains, power plants, and other systems employ high-voltage electrical systems providing voltage levels well in excess of 12-volt auxiliary levels. When used as part of an electric drive system, for instance, a high-voltage bus may supply 60-300 volts or more to an electric traction motor. A direct current (DC)-side of such a high-voltage bus could be connected to a rectifier system, or to a rechargeable energy storage system (RESS) containing a battery pack having an application-specific number of high-energy battery cells along with associated thermal regulation hardware and other power electronics.
When using a polyphase electric machine as part of an electrical system, a power inverter module is interposed between the RESS and the electric machine. Pulse-width modulation, pulse-density modulation, or other common switching control techniques are used to establish respective on/off conducting states of the individual semiconductor switches of the power inverter module. In this manner, phase leads of the electric machine are supplied with an alternating current (AC) voltage when the electric machine operates in its capacity as a motor. The power inverter module is also operable for converting an AC output voltage from the electric machine, operating in this instance as a generator, into a DC voltage suitable for charging the battery cells of the RESS. Thus, more or less electrical current is typically pushed through the collective phase windings as needed depending on the motor torque that is requested from the electric machine.
High-voltage electric drive systems having polyphase electric machine and power inverter modules of the type noted generally above tend to have lower efficiencies when operated under partial-load conditions relative to operation under full-load conditions. “Partial-load conditions” may be thought of as the collective set of torque operating points of the electric machine that are substantially less than the electric machine's available torque capacity. Thus, “full-load conditions” may be experienced at the electrical equivalent of wide-open throttle, i.e., when substantially all of the electric machine's available torque capacity is required to meet an instantaneous requested torque operating point. For instance, an electrified vehicle may operate under full-load conditions when accelerating quickly from a standstill or while passing another vehicle on a highway.
An illustrative example application is that of an electric drive system of a vehicle, which operates under normal drive conditions, for example during commuting or stop-and-go urban driving. Under such conditions, the requested torque may be but a small fraction of the electric machine's total torque capability or rated torque. Much of the time, the requested torque may be as little as 20 percent or less of the rated torque. The majority of the electric machine's life is therefore spent operating in “lossy” partial-load regions. The disclosed strategy may therefore be used to increase efficiency under such partial-load operating conditions.
In particular, the disclosure pertains to a method for selectively deactivating some of the available electrical phases of a polyphase electric machine responsive to entering a predetermined partial-load region of the electric machine. Prevalent electrical losses under partial-load conditions are (I) copper and core losses within the respective windings and magnetic structure of the electric machine itself, and (II) switching and conduction losses occurring within the switching and circuitry components of the power inverter module. Therefore, ratios of such losses may be predetermined for the electric machine offline as a calibrated set of partial-load regions, each of which is associated with corresponding torque-speed operating points of the electric machine. Responsive to a real-time determination that the electric machine is operating in one of the pre-identified partial-load regions, the controller may disable up to all but two of the available electrical phases of the electric machine.
In an example embodiment, the electrical system includes a rechargeable energy storage system (RESS) connected to a high-voltage bus. The electrical system includes a traction power inverter module (TPIM), a polyphase electric machine, and a controller configured to selectively deactivate some of the available phases of the electric machine responsive to entering a predetermined partial-load region. In a two-level arrangement of the TPIM, the TPIM contains multiple switching sets, e.g., IGBTs, MOSFETs, or other semiconductor switches, with each switching set in an example two-tier inverter topology having an upper switch and a lower switch. As will be understood in the art of power inverter controls, the upper and lower switches of a given switching pair are connected to each other and to respective positive and negative bus rails of the high-voltage bus. Alternative multi-level TPIMs, such as neutral point clamped (NPC) inverters, cascaded h-bridge inverters, flying capacitor inverters, or other power converter configurations, have more than two switches per phase. Such inverter topologies are also usable within the scope of the present control strategy, and therefore the term “switching pair” is used interchangeably with the term “switching set” when referencing an example two-tier TPIM, with “switching set” possibly encompassing three or more switches.
For a two-level inverter in particular, an available phase multiple (m) of the electric machine equals the number of switching pairs, with an exemplary and non-limiting six-phase embodiment (m=6) used herein to illustrate the present control strategy. The controller in this embodiment is configured to determine when the electric machine enters or has entered a predetermined partial-load region of operation, and, responsive to entry into the predetermined partial-load region, to selectively deactivate a predetermined number (n) of the (m) electrical phases. Deactivation is accomplished via transmission of individual switching state signals to corresponding switches of the (n) deactivated switching pairs, with n≤m−2.
In some embodiments,
i.e., exactly half of the (m) available phases are deactivated, where m is an even number.
The controller may be programmed with a lookup table of electrical losses indexed by a corresponding speed and a torque point of the electric machine, and to determine when the electric machine enters the partial-load region of operation by comparing data from the lookup table to a calibrated threshold value. Optionally, the electrical losses may be a ratio of core losses to copper losses of the electric machine, or a ratio of switching losses to conductive losses of the TPIM.
In another optional configuration, the controller may be configured to receive a mode selection signal indicative of a requested deactivation ramp-in rate. Responsive to receipt of the mode selection signal, the controller ramps-in deactivation of the (n) electrical phases at the requested deactivation ramp-in rate.
When
the controller may automatically reference a deactivation schedule to determine an order of deactivation of the (n) electrical phases which minimizes a deactivation-based torque ripple of the electric machine.
The polyphase electric machine includes a rotor, which in certain disclosed embodiments is coupled to a set of drive wheels of a motor vehicle, or to another driven load.
Also disclosed is a method for use with the above-noted electrical system. The method includes determining, via the controller, when the electric machine enters a predetermined partial-load region of operation. Responsive to entry of the electric machine into the predetermined partial-load region of operation, the method includes selectively deactivating a predetermined number (n) of the (m) electrical phases via transmission of switching state signals from the controller to corresponding ones of the switching sets, wherein n≤m−2.
The above summary is not intended to represent every possible embodiment or every aspect of the present disclosure. Rather, the foregoing summary is intended to exemplify some of the novel aspects and features disclosed herein. The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of representative embodiments and modes for carrying out the present disclosure when taken in connection with the accompanying drawings and the appended claims.
The present disclosure may be modified or have alternative forms, with representative embodiments shown by way of example in the drawings and described in detail below. Inventive aspects of the disclosure are not limited to the particular forms as disclosed herein. Rather, the present disclosure is intended to cover modifications, equivalents, combinations, and alternatives falling within the scope of the disclosure as defined by the appended claims.
Referring to the drawings, wherein like reference numbers refer to like components,
The vehicle 10 includes a controller 50 which, as shown schematically in
The controller 50 is programmed to execute a method 100 in response to a set of input signals (CCI). An example of method 100 is shown in
When the electric machine 18 is used as part of the example vehicle 10, for instance to generate and deliver motor torque (arrow TM) to an input member 23 of a transmission (T) 24 for propulsion of the vehicle 10, the voltage level on the high-voltage DC bus 20 and the AC voltage bus 22 may exceed 60-volts, and may be over 300-volts depending on the configuration of the vehicle 10. Thus, the term “high-voltage” as used herein is application-specific, but in general extends to voltage levels in excess of 12-volt auxiliary levels on the DC bus 120. Optionally, the vehicle 10 may include an internal combustion engine (E) 15 that is selectively coupled to the input member 23 of the transmission 24 via a clutch 17, e.g., a friction clutch or a hydrodynamic torque converter assembly. The engine 15 and/or the electric machine 18 may, depending on the operating mode, generate and deliver an input torque (arrow TI) to the transmission 24. The transmission 24 delivers output torque (arrow TO) to an output member 25.
When the vehicle 10 is a motor vehicle as depicted, a set of drive axles 26 may be coupled to a driven load in the form of a set of drive wheels 28, each of which is in rolling frictional contact with a road surface (not shown). In other vehicular embodiments, the driven load may be a wheel of a rail vehicle, or a propeller shaft of an aircraft or marine vessel. Likewise, non-vehicular embodiments such as power plants or to power pumps or hoists, e.g., in support of water removal or lode extraction in mining operations, and therefore such embodiments may similarly benefit from the present teachings. Thus, the vehicle 10 of
The TPIM 16 shown in
Regardless of the total number of available electrical phases of the electric machine 18, the present approach may provide another control degree of freedom in addition to, e.g., control of the phase angle and current or voltage amplitude. The method 100 may be advantageously applied to electric machines 18 having different winding technologies or rotor types. Particular benefits may be enjoyed in machine configurations lacking a rotor field or having a controllable rotor field, such as switch reluctance machines, wound-field synchronous machines, and synchronous reluctance machines. Likewise, the electric machine 18 will ideally have magnetically-isolated windings such that the described phase deactivation according to the method 100 results in unexcited core segments, as will be appreciated by those of ordinary skill in the art.
The TPIM 16 of
In its various configurations, the electric machine 18 has a plurality (m) of available electrical phases, with m=6 in
When executing the method 100, the controller 50 shown in
Deactivation of exactly half of the (m) available phases may be beneficial in terms of the resultant torque quality. That is, when an even number of electrical phases is presented, i.e., m=4, 6, 8, 10, etc., a reduction in perceived torque ripple or other noise, vibration, and harshness effect may be enjoyed when
However, other values of (n) may be used to provide efficiency gains under partial-load operating conditions, with m being even or odd without limitation. As few as one deactivated phase, i.e., n=1, may therefore fall within the scope of the present disclosure. The sequence of deactivation should take into consideration the spatial distribution of the stator windings 30 of the electric machine 18, with the quality of the resultant torque about the rotor 11 being a function of the timing of phase deactivation and identity/relative position of the (n) deactivated phases.
Electromagnetic power losses occurring in the electric machine 18 consist of core losses (Pfe) and copper losses (Pcu), i.e., P18=Pfe Pcu. Power losses in the TPIM 16 (P16) mainly consist of switching losses (Psw) and conduction losses (Pcond), i.e., P16=Psw Pcond. These four prevalent categories of power losses may be quantified off-line and recorded in memory (M) of the controller 50, and thereafter used as lookup tables or performance curves when detecting partial-load regions 42 or 142 in which to selectively deactivate some of the available phases.
Depicted loss regions I, II, III, and IV are indicative of decreasing power losses in terms of a predefined loss ratio, i.e.,
in
As will be appreciated, the vast majority of torque-speed operating points of the electric machine 18 will occur at substantially less than the rated torque of the electric machine 18, such as 20 percent or less of the rated torque. As a result, partial-load operating conditions may account for over 95 percent of the electromagnetic losses in the electric machine 18 and inverter losses in the TPIM 16, with power losses in the electric machine 18 generally being at least twice the amount of inverter losses in the TPIM 16. Core losses are several times higher than copper losses over a majority of operating points, as seen in
As an example of power loss reduction that is made possible by the present disclosure, consider an example m-phase permanent magnetic motor as the electric machine 18 in case (1), and deactivation of (n) phases in case (2). Assuming T≈kI:
P
1
=P
fe,1
+P
cu,1
+P
sw,1
+P
cond,1
P
2
=P
fe,2
+P
cu,2
+P
sw,2
+P
cond,2
Simplified inverter conduction losses and motor copper losses increase by a factor of
Simplified inverter switching losses at low currents (I) remain unchanged. Furthermore, assuming that motor iron losses (Pfe) also decrease by a factor of k when n phases are deactivated:
The losses encompassed by term kPfe,1+Psw,1 are higher at partial loads, while the losses represented by the sum (Pcu,1+Pcond,1) are higher at full loads. Thus, to some extent the deactivation of (n) phases comes with a tradeoff in the form of increased copper losses. However, as the greatest amount of loss under a partial load occurs in the core, i.e., Pfe, the reduction of such core losses under partial-load conditions is enjoyed by reducing the number of active phases in such a region.
The loss disparity may be illustrated as follows. Assuming Pfe,1 10Pcu,1, and that Psw,1=10Pcond,1 at partial load, the controller 50 may disable n=3 phases in an example six-phase embodiment of the electric machine 18 where m=6. In such an embodiment:
P
1
=P
fe,1
+P
cu,1
+P
sw,1
+P
cond,1=11Pcu,1+11Pcond,1
P
2
=P
fe,2
+P
cu,2
+P
sw,2
+P
cond,2
=kP
fe,1+2Pcu,1+Psw
→P2=(10k+2)Pcu,1+12Pcond,1
For k=0.5, for instance, a 36% power loss reduction in the electric machine 18 is possible relative to a 9% increase in losses for the TPIM 16. Assuming that overall motor losses are 2× or 200% of inverter losses, this will result in a 21% system power loss reduction. Avoidance of such losses may be enabled by execution of the method 100.
An example embodiment of the method 100 is shown in
At step S104, the controller 50 determines a corresponding torque operating region of the electric machine 18. As part of step S104, the controller 50 may use torque and speed point values from step S102 to determine whether the electric machine 18 is working within a permissible range of its calibrated maximum rated torque for that particular speed and operating temperature. The method 100 then proceeds to step S106.
Step S106 includes comparing the torque or load on the electric machine 18 from step S104 to a calibrated threshold indicative of partial-load conditions. As noted above, torque and speed points may be associated with a loss ratio of electromagnetic losses, such as iron/core-to-copper losses as depicted in
Step S108 of method 100 as shown in
Step S110, in contrast, deactivates (n) of the available (m) phases and then proceeds to step S112. Once deactivated, the switches 35 for the deactivated (n) phases are no longer available in the switching control circuit, akin to a sustained binary 0/off signal to the switches 35. PWM or other switching control signals used to vary the output voltage of the TPIM 16 in
At step S112, the controller 50 controls the output torque or speed of the electric machine 18 using N phases, with N=m if step S112 is arrived at from step S108 and N=(m−n) if step S112 is instead arrived at from step S110.
Optionally, the method 100 may include step S114 to enable use of a trigger signal in the form of a mode selection signal (M/S). The mode selection signal (M/S) may be transmitted by the external device 13 of
Responsive to receipt of the mode selection signal, the controller 50 may ramp in the deactivation of the (n) electrical phases at the requested deactivation ramp-in rate. Such an approach may allow an operator of the vehicle 10 to customize torque feel when deactivating the (n) phases, for instance as an economy (energy-efficient), sport (faster torque response), or normal operating mode, with normal possibly balancing torque responsiveness with energy efficiency, e.g., using a cost function. Or, the controller 50 may automatically reference a phase deactivation schedule to determine an order of deactivation of the (n) phases, particularly when
so as to minimizes deactivation-based torque ripple along a driveline of the vehicle 10 caused by such phase deactivation.
Therefore, the method 100 as described above provides a strategy for reducing losses in multi-phase electric machines such as the example electric machine 18 of
While some of the best modes and other embodiments have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims. Those skilled in the art will recognize that modifications may be made to the disclosed embodiments without departing from the scope of the present disclosure. Moreover, the present concepts expressly include combinations and sub-combinations of the described elements and features. The detailed description and the drawings are supportive and descriptive of the present teachings, with the scope of the present teachings defined solely by the claims.
