Method and device for controlling the operation of a rotating electric machine with a permanent magnet

Abstract

A method for controlling a rotating electric machine (110) with a permanent magnet (17) comprising a stator (16S) and a rotor (16R), comprising: Determining a first reactive power term associated with the electrical machine (110) based on the voltage; Determining a second reactive power term associated with the electrical machine (110) based on the flux; Determining a first motor temperature in conjunction with the electric machine (110) on the basis of the first and second reactive power terms, wherein determining the first motor temperature associated with the electric machine (110) on the basis of the first and second reactive power terms includes determining a temperature of a rotor magnet of the electric machine (110); and Controlling the power output of the permanent magnet electric machine (110) based on the first motor temperature.

Description

INTRODUCTION

[0001] The present disclosure relates to methods and systems for real-time estimation of the temperature of a rotating permanent magnet electric machine and the operation control based thereon.

[0002] Machine temperature is a useful control variable in regulating the operation of an electrical machine. Accurate knowledge of the machine temperature is also required for accurate diagnosis. Existing methods for estimating machine temperature typically depend on complex thermal models. The robustness of a thermal model may depend on reliable information about an electrical machine's cooling system, such as coolant temperature and flow rate, and may be susceptible to signal errors in the various required input signals. Similarly, temperature estimation approaches that rely on knowledge of machine resistance can tend to be undermined by core, eddy current, and other loss effects, all of which are difficult to decouple from temperature effects.

[0003] DE 10 2014 213 103 A1 describes a method for determining a rotor temperature of an electrical machine, in particular an asynchronous machine, wherein the rotor temperature is determined at least as a function of reactive power and / or as a function of losses of the electrical machine. It is provided that a first rotor temperature is determined as a function of the reactive power and is verified or replaced by a second rotor temperature determined as a function of the losses, depending on an operating range of the electrical machine. DESCRIPTION

[0004] According to the invention, a method and a system for controlling and regulating the operation of a rotating electrical permanent magnet machine having a stator and a rotor are provided.

[0005] This includes determining a first reactive power term associated with the electric machine based on voltage and determining a second reactive power term associated with the electric machine based on flux. A first motor temperature associated with the electric machine is determined based on the first and second reactive power terms, and the power output of the permanent magnet electric machine is controlled based on the first motor temperature.

[0006] One aspect of the disclosure includes determining the first voltage-based reactive power term associated with the electric machine by determining a voltage, current, speed, and torque command associated with operation of the electric machine, and determining a flux term based on the torque command. A first stator voltage term is determined based on the speed, current, and flux term, and a second stator voltage term is determined based on the speed, current, and flux term. The first reactive power term is determined based on the first and second stator voltage terms. An advantage of determining the first reactive power term based on the first and second voltage terms is that the first reactive power term is not affected by temperature-related changes in stator resistance.

[0007] Another aspect of the disclosure includes determining the second reactive power term associated with the electric machine based on the flux by determining a current and a speed, determining a temperature of the electric machine, determining a first flux term based on the current and the temperature of the electric machine, determining a second flux term based on the current and the temperature of the electric machine, and determining the second reactive power term based on the speed and the first and second flux terms.

[0008] Another aspect of the disclosure includes that the temperature of the electric machine is an estimated temperature of a rotor magnet of the electric machine.

[0009] Another aspect of the disclosure includes determining the second reactive power term based on the speed and the first and second flux terms by determining the second reactive power term based on the speed and a difference between the first and second flux terms.

[0010] According to the invention, determining the first motor temperature associated with the electric machine based on the first and second reactive power terms comprises determining the temperature of a rotor magnet of the electric machine.

[0011] Another aspect of the disclosure includes determining a second motor temperature via a thermal model, comparing the first motor temperature to the second motor temperature, and controlling the power output of the permanent magnet electric machine based on the comparison of the first motor temperature to the second motor temperature.

[0012] Another aspect of the disclosure includes controlling the power output from the permanent magnet electric machine based on the comparison of the temperature of the first motor and the temperature of the second motor by decreasing the power output from the permanent magnet electric machine when a difference between the temperature of the first motor and the temperature of the second motor is greater than a threshold.

[0013] Another aspect of the disclosure includes controlling the power output of the permanent magnet electric machine based on comparing the temperature of the first motor and the temperature of the second motor by maximizing the power output of the permanent magnet electric machine when a difference between the temperature of the first motor and the temperature of the second motor is below a threshold.

[0014] The above summary is not intended to represent every possible embodiment or aspect of the present disclosure. Rather, the above summary is intended to illustrate some of the novel aspects and features disclosed herein. The above features and advantages, as well as other features and advantages of the present disclosure, will be readily apparent from the following detailed description of the representative embodiments and modes for carrying out the present disclosure when taken in conjunction with the accompanying figures and the appended claims. BRIEF DESCRIPTION OF THE CHARACTERS

[0015] One or more embodiments will now be described by way of example with reference to the accompanying figures, in which: Fig.1 is a schematic diagram of a system utilizing a permanent magnet rotating electric machine in accordance with the present disclosure. Fig. 2A and Fig. 2B are representations of alternative surface-mounted and embedded rotor magnet configurations of the Fig. 1 schematically illustrated electrical machine, in accordance with the disclosure. Fig. 3 is a logical flow diagram showing a logical configuration of the Fig. 2 shown controller in accordance with the disclosure. Fig. 4 is a logical flow diagram illustrating part of the logical configuration described with reference to Fig. 3 is described in accordance with Revelation.

[0016] The accompanying figures are not necessarily to scale and may present a somewhat simplified representation of various preferred features of the present disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes. Details associated with such features will be determined in part by the particular intended application and use environment. DETAILED DESCRIPTION

[0017] The components of the disclosed embodiments described and illustrated herein can be arranged and configured in a variety of different configurations. The following detailed description, therefore, is not intended to limit the scope of the disclosure, as claimed, but is merely representative of possible embodiments thereof. Although numerous specific details are set forth in the following description in order to provide a thorough understanding of the embodiments disclosed herein, some embodiments may be practiced without some of these details. Furthermore, for clarity, certain technical material understood in the related art has not been described in detail so as not to unnecessarily obscure the disclosure. Furthermore, the disclosure as illustrated and described herein may be practiced without any element not expressly disclosed herein.As used herein, the term "system" may refer to mechanical and electrical hardware, software, firmware, electronic control components, processing logic, and / or processors, individually or in combination, including, but not limited to: application-specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or grouped) that executes one or more software or firmware programs, storage device(s) that electrically store(s) software or firmware instructions, a combinational logic circuit, and / or other components that provide the described functionality.

[0018] With reference to the figures, wherein like reference numerals refer to the same or similar components in the different figures, Fig.1, a system 100 is schematically illustrated, including a multi-phase electric machine (“electric machine”) 110 arranged to generate and transmit torque to an actuator 120 to effect work, and a controller 130 executing a control routine 200 to control and manage operation. Details regarding the control routine 200 will be described with reference to Fig. 3 and Fig. 4. In one embodiment, the system 100 is mounted on a vehicle (not shown). When mounted on a vehicle, the vehicle may include, but is not limited to, a mobile platform in the form of a commercial vehicle, an industrial vehicle, an agricultural vehicle, a passenger car, an aircraft, a watercraft, a train, an off-road vehicle, a personal mobility device, a robot, and the like to fulfill the purposes of this disclosure.

[0019] The system 100 includes the electric machine 110, an inverter 104, and a DC power source 102. The DC power source 102 is connected to the inverter 104 via a high-voltage DC bus 103, and the inverter 104 is connected to the electric machine 110 via a plurality of power lines 106. The inverter 104 includes a plurality of semiconductor switches (not shown) arranged and controllable to convert DC power to AC power and AC power to DC power using pulse width modulation or another control technique.The inverter 104 is arranged and controllable to convert DC power from the DC power source 102 into AC power to actuate the electric machine 110 to rotate and generate mechanical torque, which is transmitted to the actuator 120 via a rotatable member 112 when operating in a torque-generating mode. The electric machine 110 is controllable to generate AC electrical power from the mechanical torque generated at the actuator 120, which is converted by the inverter 104 into DC electrical power for storage in the DC power source 102 when operating in an electrical power-generating mode.In one embodiment, actuator 120 includes a hydraulic system including a pumping element 122 and an associated sump 124 that supplies pressurized hydraulic fluid to a hydraulic actuator 126. Hydraulic actuator 126 may be controllable to effect a vehicle function when deployed within the vehicle. DC power source 102 may be embodied in the form of a rechargeable electrochemical battery device, a fuel cell, an ultracapacitor, and / or other battery technology.

[0020] The sensors are arranged to monitor the parameters of the system 100. The monitored parameters include, for example, the voltage and current between the electric machine 110 and the inverter 104, and the speed of the electric machine 110. Other monitored parameters may be, for example, the hydraulic pressure transmitted to the hydraulic actuator 126, the voltage on the high-voltage DC bus 103, etc. In one embodiment, the sensors may include a voltage sensor 138 arranged to monitor the high-voltage DC bus 103, one or more current sensors 135 arranged to monitor the current flow between the inverter 104 and the electric machine 110, a speed sensor 136 arranged toIt may include a hydraulic pressure sensor 137 arranged to monitor the speed of the rotatable member 112 of the electric machine 110, and a hydraulic pressure sensor 137 arranged to monitor the pressure in the hydraulic actuator 126. Other monitored parameters may include motor commands, including a motor torque command, which may be from an operator torque request or a system request associated with the actuator 120.

[0021] The controller 130 may be embodied as one or more digital computing devices and may include one or more processors 134 and sufficient amounts of tangible, non-volatile memory 132, including read-only memory in the form of optical, magnetic, flash memory, etc. The control routine 200 may be stored as an executable instruction set in the memory 132 and executed by one of the processors 134 of the controller 130. The controller 130 is in communication with the inverter 104 and the aforementioned sensors to control their operation in response to the execution of the control routine 200 to operate the electric machine 110.

[0022] The term "controller" and related terms such as microcontroller, control module, assembly, controller, control unit, processor, and similar terms refer to one or various combinations of application-specific integrated circuit(s) (ASIC), field-programmable gate array (FPGA), electronic circuit(s), central processing unit(s), e.g., microprocessor(s), and associated non-volatile storage component(s) in the form of memory and storage devices (read-only, programmable read-only, random access, hard disk, etc.).The non-transitory memory component is capable of storing machine-readable instructions in the form of one or more software or firmware programs or routines, combinational logic circuits, input / output circuits and devices, signal conditioning and buffer circuits, and other components accessible by one or more processors to provide a described functionality. Input / output circuit(s) and devices include analog-to-digital converters and related devices that monitor inputs from sensors, where these inputs are monitored at a preset sampling frequency or in response to a trigger event. Software, firmware, programs, instructions, control routines, code, algorithms, and similar terms mean control-executable instruction sets, including calibrations and lookup tables. Each controller executes control routine(s) to provide the desired functions.The routines can be executed at regular intervals, for example, every 100 microseconds during operation. Alternatively, routines can be executed in response to the occurrence of a triggering event. Communication between controllers, actuators, and / or sensors can take place via a direct wired point-to-point connection, a networked communications bus connection, a wireless connection, or any other suitable communications link. Communication involves the exchange of data signals in any suitable form, including, for example, electrical signals over a conductive medium, electromagnetic signals over air, optical signals over fiber optics, and the like. The data signals can be discrete, analog, or digitized analog signals representing inputs from sensors, actuator commands, and communication between controllers.

[0023] The term "signal" refers to a physically perceptible indicator that conveys information and may be any suitable waveform (e.g., electrical, optical, magnetic, mechanical, or electromagnetic), such as direct current, alternating current, sine wave, triangular wave, square wave, oscillation, and the like, capable of passing through a medium.

[0024] The term "model" refers to processor-based or processor-executable code and associated calibration that simulates the physical existence of a device or physical process. As used herein, the terms "dynamic" and "dynamic" describe steps or processes that execute in real time and are characterized by monitoring or otherwise determining states of parameters and regularly or periodically updating the states of the parameters during the execution of a routine or between iterations of the routine's execution.

[0025] The terms "calibration," "calibrated," and related terms refer to a result or process that compares an actual or standard measurement associated with a device or system with a perceived or observed measurement or a commanded position for the device or system. Calibration, as described here, may be reduced to a storable parametric table, a plurality of executable equations, or another suitable form that can be used as part of a measurement or control routine.

[0026] A parameter is defined as a measurable quantity that represents a physical property of a device or other element, detectable using one or more sensors and / or a physical model. A parameter can have a discrete value, e.g., either "1" or "0," or it can be continuously variable.

[0027] With reference to Fig. 2A and Fig. 2B, the electric machine 110 can be Fig. 1 as a synchronous device 16A of the surface permanent magnet type (“SPM”) ( Fig. 2A) or as a synchronous device 16B of the internal permanent magnet (“IPM”) type ( Fig.2B). In the SPM-type synchronous device 16A, the permanent magnets 17 are mounted on the surface of a rotor 16R. In the IPM-type synchronous device 16B, the permanent magnets 17 may be embedded in laminations of the rotor 16R. The SPM-type synchronous device 16A and the IPM-type synchronous device 16B each include a stator 16S concentric with and surrounding the rotor 16R. The stator 16S includes a plurality of inwardly directed, radially projecting stator teeth with gaps therebetween. Stator windings in the form of lengths of conductive wire are inserted into the gaps between adjacent stator teeth and connected to the inverter 104 to generate a rotating magnetic field. In the depicted configurations, the stator 16S surrounds the rotor 16R. However, in other embodiments, the rotor 16R may surround the stator 16S. Individual stator windings 19 are also shown.If the individual stator windings 19 are connected via the operation of the . Fig. 1, a rotating stator magnetic field is created, whereby the rotating stator field interacts with the magnetic field of the permanent magnets 17. The rotating magnetic field interacts with the permanent magnets 17 of the rotor 16R to force rotation of the rotor 16R about its axis, thereby inducing a torque on the rotor shaft. The mutual field interaction ultimately imparts rotation to the rotor 16R about its central axis 11, which rotation generates the motor output torque. Fig. 2A and Fig. 2B show two possible configurations of the electric machine 110 and the permanent magnets 17. However, the present teachings are not limited to a particular configuration of the electric machine 110, which is why the Fig. only illustrate two common positions of the permanent magnets 17 in the rotor 16R.

[0028] Fig. Figure 3 schematically shows an embodiment of the control routine 200 that can be implemented as executable code to control operation on a rotating permanent magnet electric machine, e.g., an embodiment of the electric machine 110 described with reference to Fig. 1. The control routine 200 is represented as a collection of blocks in a logical flowchart representing a sequence of operations that may be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer instructions that, when executed by one or more processors, perform the recited operations. Table 1 is provided as a guide, illustrating the numerically labeled blocks and the corresponding functions corresponding to the control routine 200 as follows. Table 1 BLOCK BLOCK CONTENT 202 Initiate routine 204 Monitor parameters including motor speed and torque command 206 Is engine speed > threshold speed? AND Is torque command > threshold torque? 210 Determine the first reactive power parameter based on the voltage 220 Determine the second reactive power parameter based on the flow 230 Estimate magnet temperature 232 Updating the flux based on the estimated magnet temperature 234 Estimation of the rotor temperature from the thermal model 250 Comparison of magnet temperature and rotor temperature 252 Operating the electric machine in response to a torque request when the difference between the magnet temperature and the rotor temperature is less than the threshold 254 Reducing the torque output of an electrical machine when the difference between the magnet temperature and the Rotor temperature is greater than the threshold 256 Iteration beenden

[0029] The execution of the control routine 200 can proceed as follows. The steps of the control routine 200 can be executed in any suitable order and are not limited to the steps described with reference to Fig. 3. As used herein, the term "1" means an affirmative answer or "YES" and the term "0" means a negative answer or "NO".

[0030] The control routine 200 may be executed periodically, with each iteration beginning (202) by determining parameters such as the speed and torque command for the electric machine 110 (204). The speed and torque command serve as input criteria for executing the remaining elements of the control routine 200 by avoiding their execution under conditions that include low-speed and / or low-power operation. If the speed is greater than a minimum threshold speed and the torque command is greater than a minimum threshold torque, operation of the control routine 200 continues (206)(1). Otherwise (206)(0), monitoring of the speed and torque command continues (206)(0) without further action.

[0031] If the execution continues, a first reactive power term Q voltdetermined on the basis of the motor voltage (210) in accordance with the following system of equations. vdse=Rsidse−ωrλqse vqse=Rsiqse+ωrλdse QVolt=vqseidse−vdseiqse where: R s represents the stator resistance; ω r stands for the rotation speed; λ represents a corresponding flow concept; i stands for a current term; s indicates the stator; d, q are linked to machine parameters described in a synchronous reference system indicating direct and quadrature axes, respectively; and e indicates that the respective value is an estimate.

[0032] It is estimated that the first reactive power term Q volt can be determined without changing the stator resistance R s must be determined using the relationship described in Equation 3. The stator resistance R svaries with temperature and speed and would otherwise introduce variability that would have to be taken into account. By using the first reactive power term Q volt determined in this way, robustness is introduced into the result.

[0033] A second reactive power term Q flux is determined based on the motor flux (220), either in parallel or in series with the execution of step 210. The second reactive power term Q flux , which is a temperature-dependent flow parameter and is determined according to the following system of equations. Qflux=ωr(λds_Teiqse−λqs_Teidse) =ωr(λds@90Ceiqse−λqs@90Ceidse+ΔλfTiqse) where: λds_Te=f(idse,iqse,Tmagnet_est) λqs_Te=g(idse,iqse,TMagnet_est) where f(idse,iqse,TMagnet_Nest) and g(idse,iqse,TMagnet_Nest) represent functions used to determine the respective flux value with respect to estimated DC and quadrature axis stator currents and an estimated magnet temperature; and where: T magnet_est represents the estimated magnet temperature, ω r stands for the rotation speed of the rotor, λds@90Ce represents an estimated d-axis flux for the electric machine with the stator operating at 90° C, λqs@90Ce represents an estimated q-axis flux for the electric machine with the stator operating at 90° C, i stands for a current term, s indicates the stator, d, q are linked to synchronous machine parameters and indicate direct and quadrature axes, respectively, and e indicates that the respective value is an estimate.

[0034] The estimated magnet temperature T magnet_est438 can be calculated depending on the difference between the first reactive power term Qvolt and the second reactive power term Q flux (232). Details on determining the estimated magnet temperature T magnet_est 438 are made with reference to Fig. 4 described.

[0035] The magnetic flux linkage term is calculated based on the estimated magnet temperature T magnet_est (234) updated, where the update of the magnetic flux linkage term is based on the relationship between the flux density in a rotating electric permanent magnet machine and the temperature, where the flux density decreases with an increase in the temperature of the permanent magnet.

[0036] An estimated rotor temperature can be determined using a thermal model (240). Examples of thermal models for estimating rotor temperature include thermal models for predicting the temperature of mass clumps.

[0037] The estimated magnet temperature T magnet_est and the estimated rotor temperature are compared (250), and if a difference between them is less than a temperature threshold (250)(1), the operation of the electric machine 110 is controlled without restriction in response to a torque command up to a maximum output torque achievable by the electric machine 110 (252).

[0038] However, if there is a difference between the estimated magnet temperature T magnet_est and the estimated rotor temperature is greater than a temperature threshold (250)(0), the operation of the electric machine 110 is reduced (254). In one embodiment, the power reduction comprises limiting the magnitude of the torque output by the electric machine 110, wherein the limitation of the magnitude of the power reduction is directly related to the difference between the estimated magnet temperature T magnet_estand the estimated rotor temperature. The iteration of control routine 200 then ends (256).

[0039] Fig. 4 schematically shows a logic diagram 400 containing additional details on aspects of the control routine 200 described with reference to Fig. 3, including determining the estimated magnet temperature T magnet_est 438. The logic diagram is in the form of a closed control loop that provides temperature-dependent flux compensation. Step 410 contains details for determining the first reactive power parameter Q volt 412 based on DC and quadrature voltage and current inputs 402 according to Eq. 3. Step 420 provides details for determining the second reactive power parameter Q flux 422 based on DC and quadrature current inputs and the speed 402 and in accordance with Eq. 4 (see above).

[0040] A difference operator 425 calculates a difference between the first reactive power term Q volt and the second reactive power parameter Q flux , the result of which is used in the magnet temperature estimator 430 to determine the magnet temperature T magnet_est 438 to be estimated.

[0041] The magnet temperature estimator 430 includes a scalar multiplier 432 that calculates the estimated magnet temperature T magnet_es t 438 based on the difference between the first reactive power term Q volt and the second reactive power parameter Q flux The scalar multiplier 432 may be in the form of a proportional term, a proportional-integral (PI) term, or a proportional-integral-derivative (PID) term.

[0042] The magnet temperature estimator 430 includes a magnetic flux linkage table 434 that determines a flux correction term Δλ f_T 436 with respect to the estimated magnet temperature Tmagnet_est 438. The flux correction term Δλ f_T 436 is provided as a feedback term to step 420 to determine the second reactive power parameter Q flux 422 based on DC and quadrature current inputs and the speed 402.

[0043] By using the closed-loop arrangement in the manner described with reference to Fig. 4 to determine the estimated magnet temperature T magnet_est , with two reactive power calculations and the compensator, the control routine 200 is robust against converter losses and variations in the motor series resistance, which is a function of rotor speed and temperature. As such, the Fig.1, the system 100 described above provides an electrically based temperature estimation method and associated system that provides a robust value without information about the thermal capacities of the various elements of the electric machine and without the need to compensate for the speed-based variation of the motor series resistance for temperature estimation. When deployed in the vehicle, the system 100 is capable of monitoring and detecting faults in an electric machine cooling system to cause reduced propulsion by (a) estimating the magnet temperature based on two independent reactive power calculations using voltage equations in the synchronous reference frame and reactive power calculation using flux tables, (b) designing the magnet estimator, and (c) comparing the estimated magnet temperature and the rotor temperature from the thermal model.

[0044] The present disclosure is susceptible to modifications and alternative forms, and representative embodiments are shown by way of example in the figures and described in detail below. The inventive aspects of this disclosure are not limited to the disclosed embodiments. Rather, this disclosure is intended to cover modifications, equivalents, combinations, and alternatives falling within the scope of the disclosure as defined by the appended claims.

[0045] While some of the preferred embodiments 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 changes 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 figures are supportive and descriptive of the present teachings, with the scope of the present teachings being defined solely by the claims.

Claims

[1] A method for controlling a permanent magnet (17) rotating electrical machine (110) including a stator (16S) and a rotor (16R), comprising: determining a first reactive power term associated with the electric machine (110) based on the voltage; determining a second reactive power term associated with the electric machine (110) based on the flux; Determining a first motor temperature in conjunction with the electric machine (110) based on the first and second reactive power terms, wherein determining the first motor temperature associated with the electric machine (110) based on the first and second reactive power terms comprises determining a temperature of a rotor magnet of the electric machine (110); and Controlling the power output of the permanent magnet electric machine (110) based on the first motor temperature. [2] The method of claim 1, wherein determining the first reactive power term associated with the electric machine (110) based on the voltage comprises: Determining a voltage, a current, a speed and a torque command associated with the operation of the electric machine (110); Determining a flux term based on the torque command; Determining a first stator voltage term based on the speed, current and flux terms; Determining a second stator voltage term based on the speed, current and flux terms; and Determining the first reactive power term based on the first and second stator voltage terms. [3] The method of claim 1, wherein determining the second reactive power term associated with the electric machine (110) based on the flux comprises: Determining a current and a speed; Determining a temperature of the electrical machine (110); Determining a first flux term based on the current and temperature of the electric machine (110); Determining a second flux term based on the current and the temperature of the electric machine (110); and Determine the second reactive power term based on the speed and the first and second flux terms. [4] The method of claim 3, wherein determining the second reactive power term based on the speed and the first and second flux terms comprises determining the second reactive power term based on the speed and a difference between the first and second flux terms. [5] The method of claim 3, wherein the temperature of the electric machine (110) comprises an estimated rotor magnet temperature. [6] The method of claim 1, further comprising: Determining a second engine temperature using a thermal model; Comparing the first engine temperature and the second engine temperature; and Controlling the power output of the permanent magnet electric machine (110) (17) based on the comparison of the first motor temperature and the second motor temperature. [7] The method of claim 6, wherein controlling the power output from the permanent magnet electric machine (110) based on the comparison of the first motor temperature and the second motor temperature comprises reducing the power output from the permanent magnet electric machine (110) when a difference between the first motor temperature and the second motor temperature is greater than a threshold. [8] The method of claim 6, wherein controlling the power output from the permanent magnet electric machine (110) based on the comparison of the first motor temperature and the second motor temperature comprises maximizing the power output from the permanent magnet electric machine (110) when a difference between the first motor temperature and the second motor temperature is less than a threshold. [9] A system comprising: a rotating electric machine (110) with a permanent magnet (17) having a stator (16S) and a rotor (16R); an inverter (104); a controller (130) operatively coupled to the inverter (104) and communicating with current, voltage and speed sensors (135, 138, 136) arranged to monitor the rotating electric machine (110), the controller (130) including an instruction set, the instruction set being executable to: determine a first reactive power term associated with the electric machine (110) based on the voltage; determine a second reactive power term associated with the electric machine (110) based on the flux; determine a first motor temperature associated with the electric machine (110) based on the first and second reactive power terms, wherein determining the first motor temperature associated with the electric machine (110) based on the first and second reactive power terms comprises determining a temperature of a rotor magnet of the electric machine (110); and to control the power output of the electric machine (110) with permanent magnet (17) via the inverter based on the first motor temperature.

Images