Patent Application
20140070739
This disclosure relates to control of electric motors.
Permanent magnet synchronous motors (PMSMs) are utilized in various applications because they have generally favorable efficiency characteristics relative to other types of motors. Typically, PMSMs have three separate electrical windings within the stator which are each powered by alternating current (AC) voltages Va, Vb, and Vc. In operation, the winding currents Ia, Ib, and Ic each oscillate at a frequency proportional to the rotor speed and are separated by 120 degrees in phase from one another. These winding currents induce a rotating magnetic field which may be out of phase with the rotor. The resulting shaft torque depends upon both the magnitude of the magnetic field and the phase angle relative to the rotor. The magnetic properties of the permanent magnets are impacted by temperature which impacts the resulting torque. Accurate torque delivery requires compensation for the effects of temperature.
For convenience, the winding voltages and currents may be represented by vectors with respect to a rotating reference frame that rotates with the rotor. The mapping between rotor position and the rotating reference frame depends upon the number of poles in the motor. The voltage vector has a direct component Vd and a quadrature component Vq. Similarly, the current has a direct component Id and a quadrature component Iq. Vd, Vq, Id, and Iq do not oscillate based on rotor position.
In certain applications, such as electric vehicles and hybrid electric vehicles, electrical power is available from a non-oscillating direct current (DC) voltage source such as a battery. Therefore, inverters are utilized to convert the non-oscillating voltage Vdc into three oscillating voltages. Inverters contain a discrete number of switching devices and are therefore capable of supplying only a discrete number of voltage levels at each of the three motor terminals. For a 2-level inverter, at any moment in time, the switching devices are set to electrically connect each of the three AC motor terminals to either the positive or the negative DC terminal. Thus, eight switching states are available. Two of these switching states, in which all three AC terminals are connected to the same DC terminal, are called zero states. In the remaining six states, one AC terminal is connected to one of the DC terminals and the other two AC terminals are connected to the opposite DC terminal. The inverter is capable of switching rapidly among these eight states.
In one embodiment, a method of compensating for temperature is disclosed. In response to a torque request, the method adjusts the torque request based on temperature. The direct and quadrature components of the winding current are calculated based on this adjusted torque request. The calculated direct component is further adjusted based on the temperature. Finally, commands are issued to an inverter to effectuate the target winding current. In some embodiments, the calculated direct component is not adjusted when the torque request is less than a threshold. The threshold may be based on rotor speed and bus voltage.
In another embodiment, a vehicle includes a bus, an electric machine such as a permanent magnet synchronous motor, an inverter, and a controller. The inverter is configured to supply the electric machine a winding current having a direct component (Id) and a quadrature component (Iq). The controller is configured to issue pulse width modulation commands to the inverter to adjust the winding current such that for a given speed, torque, and voltage, the direct component increases as the temperature increases. The direct component may be less than zero at the given speed, torque, and voltage.
In another embodiment, a controller includes an input interface, an output interface, and control logic. The input interface receives signals indicating a voltage of an electrical bus and a temperature and a rotor speed of an electric machine. The output interface is configured to send pulse width modulation commands to an inverter causing the inverter to supply the electric machine with a winding current such that the electric machine produces torque. The control logic is programmed to adjust the pulse width modulation commands such that for a given rotor speed, bus voltage, and torque, the direct component increases as temperature increases. The direct component may be less than zero at the given speed, torque, and voltage.
Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
A schematic diagram of an exemplary hybrid electric powertrain is illustrated in
Controller 140 is illustrated schematically in more detail in
The output of generator controller 214 and motor controller 216 are switch states for switches within inverters 130 and 132 respectively. These controllers receive input signals indicating the angular position of the corresponding rotor, labeled ΘR, the voltage of DC bus 134, labeled Vdc, the current in each winding, labeled Ia, Ib, and Ic, and a temperature of the electric machine. Variable Voltage Controller 218 issues commands to DC to DC converter 138 to effectuate the target bus voltage. Controllers 210, 212, 214, 216, and 218 may be implemented as a single micro-controller or as multiple communicating controllers.
Two basic control methods are known for switching among inverter states to regulate torque output of a PMSM. In the six-step method, the inverter cycles through the six non-zero states once per cycle of the rotor, producing an oscillating voltage and current in each winding. A rotor cycle is defined relative to motor poles and does not necessarily correspond to a complete revolution. The amplitude of the AC voltage is dictated by the DC voltage. The torque is dictated by the DC voltage, the rotor speed, and the phase difference between these quasi-sinusoidal AC voltage signals and the rotor position. A controller issues commands to the inverter indicating when to switch to the next state in the sequence. In six-step mode, the components of the winding current, Id, and Iq, are located on a curve such as curve 122 or 124 in
Vdc and the rotor speed ω. The position along the curve is determined by the voltage angle, Θv. The controller adjusts Θv by adjusting the timing at which it switches the inverter to the next non-zero inverter states. The target voltage angle Θ*v is a non-linear function of the torque request, rotor speed, and bus voltage.
In the PWM method, the inverter switches very rapidly among two of the non-zero states and one of the zero states. A controller specifies what fraction of the time should be spent in each of these three states by specifying pulse width modulation (PWM) duty cycles. The controller updates these duty cycles at regular intervals such that the frequency of updates is significantly higher than the frequency of the rotor rotation. In PWM mode, the components of the winding current, Id, and Iq, are located in a region bounded by a curve such as curve 122 or 124 in
The target components of the winding current, I*d, and I*q, are non-linear functions of the torque request, rotor speed, and bus voltage. At low torque requests, low speeds, and high bus voltages, I*d and I*q are selected along the line 118 in
If a controller compensates for increasing temperature without compensating for the shift in the achievable region, then I*d will be decreased to a more negative value. However, when the method of
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.
