阅读说明：本技术 基于转子电阻的电机热保护 (Motor thermal protection based on rotor resistance ) 是由 R.S.科尔比 于 2021-03-22 设计创作，主要内容包括：电机过载保护装置配有热监测器,可根据转子电阻估计值确定预期的转子温度升高值。热监测器通过使用根据转子电阻估计值计算出的转子温度升高值和电机热状态估计值之间的相关性来确定预期的转子温度升高值。相关性可以通过将线拟合到多个点并确定该线的斜率来导出,其中每个点由转子温度升高估计值和相应的电机热状态估计值组成的有序对来定义。然后,在给定转子电阻估计值和相应的电机热状态估计值的情况下,热监测器可以使用该转子温度斜率来确定转子温度升高值。在一些实施例中,如果转子温度升高值超过预期的转子温度升高值大于预定义阈值,热监测器发出警报和/或采取校正动作。(The motor overload protection device is equipped with a thermal monitor that determines an expected rotor temperature rise based on an estimated rotor resistance value. The thermal monitor determines an expected rotor temperature increase value by using a correlation between a rotor temperature increase value calculated from the rotor resistance estimate value and a motor thermal state estimate value. The correlation may be derived by fitting a line to a plurality of points and determining the slope of the line, wherein each point is defined by an ordered pair of an estimate of the rotor temperature rise and a corresponding estimate of the thermal state of the motor. The thermal monitor may then use the rotor temperature slope to determine a rotor temperature rise value given the rotor resistance estimate and the corresponding motor thermal state estimate. In some embodiments, the thermal monitor issues an alarm and/or takes corrective action if the rotor temperature increase exceeds the expected rotor temperature increase by more than a predefined threshold.)

1. A method of protecting an induction motor from a thermal overload condition, comprising:

calculating a rotor temperature rise estimation value according to a rotor resistance estimation value of the induction motor;

calculating a motor thermal state estimated value of the induction motor;

deriving a correlation between the thermal state estimate and the rotor temperature rise estimate;

calculating a new temperature rise estimate based on the new rotor resistance estimate for the induction motor;

calculating a new motor thermal state estimation value of the induction motor;

calculating an expected rotor temperature rise value for the induction motor using the correlation and the new motor thermal state;

checking whether the new estimate of the temperature increase exceeds the expected rotor temperature increase by more than a predefined rotor temperature increase threshold for the induction machine; and

a corrective action is performed in response to the new estimated temperature increase value exceeding the expected rotor temperature increase value by more than a predefined rotor temperature increase threshold.

2. The method of claim 1, further comprising obtaining an estimate of rotor resistance based on the real part of the complex motor input admittance and a slip value of the induction motor.

3. The method of claim 2, further comprising determining a plurality of motor input admittances from the motor voltage and current in the synchronous reference frame, wherein the motor voltage is oriented on the Q-axis of the synchronous reference frame.

4. The method of claim 1, wherein deriving the correlation comprises fitting a line to a plurality of points and determining a slope of the line, each of the plurality of points being defined by an ordered pair consisting of an estimate of rotor temperature rise and a corresponding estimate of a thermal state of the motor.

5. The method of claim 4, wherein calculating the expected rotor temperature rise value for the induction motor comprises applying the slope to a new motor thermal estimate.

6. The method of claim 1, wherein performing a corrective action comprises one or more of: the power supply of the induction motor is cut off, and an alarm is given.

7. The method of claim 1, wherein calculating the expected rotor temperature rise value for the induction motor is performed in one of a motor overload protection device or an edge device.

8. An apparatus for protecting an induction motor from a thermal overload condition, comprising:

a processor;

a storage unit communicatively coupled to the processor, the storage unit storing thereon computer-readable instructions for causing the processor to:

calculating a temperature rise estimation value according to a rotor resistance estimation value of the induction motor;

calculating a motor thermal state estimated value of the induction motor;

calculating an expected rotor temperature rise value of the induction motor;

checking whether the estimated temperature rise value exceeds an expected rotor temperature rise value by more than a predefined rotor temperature rise threshold value of the induction machine; and

a corrective action is performed in response to the temperature increase estimate exceeding the expected rotor temperature increase by more than a predefined rotor temperature increase threshold.

9. The apparatus of claim 8, wherein the computer readable instructions further cause the processor to obtain the rotor resistance estimate based on a real part of a complex motor input admittance and a slip value of an induction motor.

10. The apparatus of claim 9, wherein the computer readable instructions further cause the processor to determine the complex motor input admittance from a motor voltage and a current in a synchronous reference frame, wherein the motor voltage is oriented on a Q-axis of the synchronous reference frame.

11. The apparatus of claim 8, wherein the computer readable instructions further cause the processor to derive a correlation between the estimate of the thermal state of the induction machine and the estimate of the increase in rotor temperature.

12. The apparatus of claim 11, wherein the computer readable instructions further cause the processor to calculate an expected rotor temperature increase value by applying the correlation to a motor thermal estimate.

13. The apparatus of claim 8, wherein the processor performs the corrective action by performing one or more of: the power supply of the induction motor is cut off, and an alarm is given.

14. The apparatus of claim 8, wherein the apparatus is one of: motor overload protection devices or edge devices.

15. A non-transitory computer-readable medium containing program logic that, when executed by operations of one or more computer processors, causes the one or more computer processors to:

calculating a rotor temperature rise estimation value according to a rotor resistance estimation value of the induction motor;

calculating a motor thermal state estimated value of the induction motor;

deriving a correlation between the thermal state estimate and the rotor temperature rise estimate;

calculating a new motor thermal state estimation value of the induction motor;

calculating an expected rotor temperature rise value for the induction motor using the correlation and the new motor thermal state;

checking whether the new estimate of the temperature increase exceeds the expected rotor temperature increase by more than a predefined rotor temperature increase threshold for the induction machine; and

a corrective action is performed in response to the new estimated temperature increase value exceeding the expected rotor temperature increase value by more than a predefined rotor temperature increase threshold.

16. The computer readable medium of claim 15, wherein the program logic further causes the one or more processors to obtain the rotor resistance estimate based on a real part of a complex motor input admittance and a slip value of an induction motor.

17. The computer readable medium of claim 16, wherein the program logic further causes the one or more processors to determine the complex motor input admittance from a motor voltage and a current in a synchronous reference frame, wherein the motor voltage is oriented on a Q-axis of the synchronous reference frame.

18. The computer readable medium of claim 15, wherein the correlation is derived by fitting a line to a plurality of points, each of the plurality of points defined by an ordered pair consisting of an estimate of rotor temperature rise and a corresponding estimate of a thermal state of the motor, and determining a slope of the line.

19. The computer readable medium of claim 18, wherein the one or more processors calculate an expected rotor temperature rise value for the induction motor by applying the slope to a new motor thermal estimate.

20. The computer-readable medium of claim 15, wherein the one or more processors perform the corrective action by performing one or more of: the power supply of the induction motor is cut off, and an alarm is given.

Technical Field

The present disclosure relates to induction motors, and more particularly to methods and systems for monitoring and protecting such motors from thermal overload based on rotor resistance estimates.

Background

Induction motors are widely used in industrial fields because of their advantages of low cost, high efficiency, high reliability, etc. A typical induction motor includes a stationary member or stator having a plurality of windings thereon, and a rotating member or rotor rotatably disposed within the stator. Application of a sinusoidal or alternating voltage to the stator windings induces a rotating magnetic field, causing the rotor to rotate. Induction motors are typically operated at single and three phase voltages, although two phase induction motors are also available.

Most induction motors employ overload protection devices to protect the motor from over-current, thermal overload, and the like. The overload protection device detects excessive current or heat in the motor and interrupts the motor power supply to prevent damage from occurring. These devices may include circuit breakers, overload relays, and other types of circuit interrupting devices, which typically have a resistive element connected in series with the power supply line of the motor. The overload protection device cuts off the power supply to the motor when the resistance element becomes overheated.

While many advances have been made in the field of thermal overload protection for induction motors, it is readily appreciated that continued improvements are still needed.

Disclosure of Invention

Embodiments of the present disclosure relate to systems and methods for monitoring and protecting induction motors from thermal overload conditions. The method and system provide a motor overload protection device equipped with a thermal monitor that can determine an expected rotor temperature increase based on an estimated rotor resistance value. The thermal monitor determines an expected rotor temperature increase value by using a relationship or correlation between a rotor temperature increase estimate based on the rotor resistance estimate and a motor thermal state estimate. The relationship or correlation may be derived by fitting a line to a plurality of points, each point being defined by an ordered pair of an estimate of the rotor temperature rise and a corresponding estimate of the thermal state of the motor, and determining the slope of the line. This slope (which may also be referred to as a rotor temperature slope) may then be used by the thermal monitor to determine a rotor temperature rise value given the rotor resistance estimate and the corresponding motor thermal state estimate. In some embodiments, the thermal monitor issues an alarm and/or takes corrective action if the rotor temperature increase exceeds the expected rotor temperature increase by more than a predefined threshold.

In general, in one aspect, embodiments of the present disclosure relate to a method of protecting an induction machine from a thermal overload condition. The method includes, among other things, calculating an estimate of a rotor temperature rise based on an estimate of a rotor resistance of the induction machine and calculating an estimate of a machine thermal state of the induction machine. The method also includes deriving a correlation between the thermal state estimate and the rotor temperature rise estimate, and calculating a new temperature rise estimate based on a new rotor resistance estimate for the induction machine. The method also includes calculating a new motor thermal state estimate for the induction motor and calculating an expected rotor temperature rise value for the induction motor using the correlation and the new motor thermal state. The method then includes checking whether the new estimate of temperature increase exceeds the expected rotor temperature increase by more than a predefined rotor temperature increase threshold for the induction machine, and performing a corrective action in response to the new estimate of temperature increase exceeding the expected rotor temperature increase by more than the predefined rotor temperature increase threshold.

According to one or more of the foregoing embodiments, the rotor resistance estimate is obtained from real and slip values of a complex motor input admittance of the induction motor, and the complex motor input admittance is determined from a motor voltage and a current in a synchronous reference frame, wherein the motor voltage is oriented on a Q-axis of the synchronous reference frame. In accordance with one or more of the preceding embodiments, deriving the correlation includes fitting a line to a plurality of points, each point defined by an ordered pair of an estimate of rotor temperature rise and a corresponding estimate of a thermal state of the motor, and determining a slope of the line. In accordance with one or more of the foregoing embodiments, calculating the expected rotor temperature increase value for the induction motor includes applying a slope to the new motor thermal estimate, performing the corrective action includes one or more of powering down the induction motor and issuing an alarm, and performing the calculation of the expected rotor temperature increase value for the induction motor in one of the motor overload protection device or the edge device.

In general, in another aspect, embodiments of the present disclosure relate to an apparatus for protecting an induction machine from a thermal overload condition. The apparatus includes, among other things, a processor and a memory unit communicatively coupled to the processor. The memory unit has stored thereon computer readable instructions for causing the processor to calculate an estimate of the temperature increase, and an estimate of the motor thermal state of the induction motor, particularly based on an estimate of the rotor resistance of the induction motor. The computer readable instructions further cause the processor to calculate an expected rotor temperature increase value for the induction machine and check whether the temperature increase estimate exceeds the expected rotor temperature increase value by more than a predefined rotor temperature increase threshold for the induction machine. The computer readable instructions also cause the processor to perform a corrective action in response to the estimated temperature increase exceeding the expected rotor temperature increase by more than a predefined rotor temperature increase threshold.

In accordance with any one or more of the preceding embodiments, the computer readable instructions further cause the processor to obtain an estimate of rotor resistance from real and slip values of a complex motor input admittance of the induction motor, and to determine the complex motor input admittance from a motor voltage and a current in a synchronous reference frame, wherein the motor voltage is oriented on a Q-axis of the synchronous reference frame. The computer readable instructions further cause the processor to derive a correlation between the estimate of the thermal state of the induction motor and the estimate of the rotor temperature increase, and to calculate an expected rotor temperature increase by applying the correlation to the estimate of the motor temperature increase, according to any one or more of the preceding embodiments. The computer readable instructions further cause the processor to perform a corrective action by performing one or more of shutting down power to the induction motor and sounding an alarm, and the device is one of a motor overload protection device or an edge device, in accordance with any one or more of the preceding embodiments.

In general, in another aspect, embodiments of the disclosure relate to a non-transitory computer-readable medium containing program logic. When executed by operation of one or more computer processors, the program logic causes the one or more computer processors to calculate an estimate of a rotor temperature rise, and an estimate of a motor thermal state of the induction motor, based on, inter alia, an estimate of a rotor resistance of the induction motor. The program logic further causes the one or more processors to derive a correlation between the thermal state estimate and the rotor temperature rise estimate, calculate a new motor thermal state estimate for the induction motor, and calculate an expected rotor temperature rise for the induction motor using the correlation and the new motor thermal state. The program logic then causes the one or more processors to check whether the new estimate of temperature increase exceeds the expected rotor temperature increase by more than a predefined rotor temperature increase threshold for the induction machine, and to perform a corrective action in response to the new estimate of temperature increase exceeding the expected rotor temperature increase by more than the predefined rotor temperature increase threshold.

In accordance with any one or more of the preceding embodiments, the program logic further causes the one or more processors to obtain an estimate of rotor resistance from real and slip values of a complex motor input admittance of the induction motor, and further causes the one or more processors to determine the complex motor input admittance from a motor voltage and a current in a synchronous reference frame, wherein the motor voltage is oriented on a Q-axis of the synchronous reference frame. In accordance with any one or more of the preceding embodiments, the program logic further causes the one or more processors to derive the correlation by fitting a line to a plurality of points and determining a slope of the line, each point defined by an ordered pair of an estimate of the rotor temperature rise and a corresponding estimate of the thermal state of the motor. In accordance with any one or more of the preceding embodiments, the program logic further causes the one or more processors to calculate an expected rotor temperature rise value for the induction motor by applying the slope to the new motor thermal estimate and to perform a corrective action by performing one or more of shutting down power to the induction motor and issuing an alarm.

Drawings

A more particular description of the disclosure briefly summarized above may be had by reference to various embodiments, some of which are illustrated in the appended drawings. While the drawings represent selected embodiments of the present disclosure, these drawings should not be considered limiting of its scope, as the disclosure may admit to other equally effective embodiments.

Fig. 1A and 1B illustrate an exemplary motor overload protection apparatus equipped with a thermal monitor in accordance with an embodiment of the present disclosure;

FIG. 2 illustrates an exemplary thermal monitor, according to an embodiment of the present disclosure;

fig. 3A and 3B illustrate circuit models of an exemplary electric machine according to an embodiment of the present disclosure;

FIGS. 4A and 4B illustrate exemplary motor voltages and currents in a synchronous reference frame according to an embodiment of the present disclosure;

FIG. 5 illustrates a data plot showing exemplary rotor resistance estimates, according to an embodiment of the present disclosure;

FIG. 6 shows a flow chart of an exemplary method of monitoring motor temperature in accordance with an embodiment of the present disclosure;

FIG. 7 illustrates a flow chart of an exemplary method of obtaining a rotor temperature estimate, according to an embodiment of the present disclosure;

FIG. 8 illustrates a flow chart of an exemplary method of obtaining a motor thermal state estimate, according to an embodiment of the present disclosure;

FIG. 9 shows a flowchart of an exemplary method of correlating a rotor temperature estimate to a motor thermal state estimate, according to an embodiment of the present disclosure;

FIG. 10 illustrates a graph of exemplary rotor temperature rise values versus thermal conditions and corresponding line slopes, in accordance with an embodiment of the present disclosure;

FIG. 11 illustrates a graph of measured stator winding temperature rise values, exemplary rotor temperature rise values, and calculated thermal conditions for a non-blocking fan versus time, in accordance with an embodiment of the present disclosure;

FIG. 12 illustrates a graph of measured stator winding temperature rise values, exemplary rotor temperature rise values, and calculated thermal conditions for a partially blocked fan versus time, in accordance with an embodiment of the present disclosure; and

FIG. 13 illustrates a graph of measured stator winding temperature rise values, exemplary rotor temperature rise values, and calculated thermal conditions for a fan with no blockage at all versus time, according to an embodiment of the disclosure.

Identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. However, elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

Detailed Description

The specification and drawings illustrate exemplary embodiments of the disclosure and are not to be considered limiting, the scope of the disclosure being defined by the claims, including equivalents. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the scope of the description and claims, including equivalents. In some instances, well-known structures and techniques have not been shown or described in detail to avoid obscuring the disclosure. Furthermore, elements and their associated aspects, which are described in detail with reference to one embodiment, may be included in other embodiments not specifically shown or described, where practicable. For example, if an element is described in detail with reference to one embodiment, but not with reference to the second embodiment, the element may still be said to be included in the second embodiment.

At a high level, embodiments of the present disclosure provide a method of correlating a motor temperature increase value with a motor thermal state to detect abnormal motor temperature increases that may indicate impaired motor cooling. Embodiments provide systems and methods for calculating motor input admittance using a simplified motor model, estimating motor speed/slip from rotor slot harmonics, calculating rotor temperature rise values, and correlating rotor temperature rise values to motor thermal conditions. These systems and methods may then be deployed in an overload protection device to monitor and protect the induction motor from thermal overload conditions.

Referring now to fig. 1A, a functional diagram of a motor management system 100 for an ac motor 102, such as an induction motor, is shown in accordance with one or more embodiments disclosed herein. The motor 102 in this example is a typical three-phase induction motor having a stator (not expressly shown), a rotor (not expressly shown) rotatably disposed within the stator, and driven by three-phase line voltages and currents having phases "a", "b", "c". Although a three-phase motor is shown, it should be understood that two-phase or other types of multi-phase induction motors may also be used without departing from the scope of the disclosed embodiments.

As can be seen, the motor management system 100 has a number of components including a power module 104, a motor controller 106, voltage and current sensors 108, and an overload protection device 110. An ac mains power supply 112 provides voltage and current to the motor 102 through the power module 104 and an external control system 114 provides overall control of the motor 102 through the motor controller 106. This type of arrangement is commonly used in a variety of commercial and residential applications. For example, in an HVAC system, the motor 102 may be a fan motor that drives a fan to move air through an air duct, and the external control system 114 may be a thermostat that controls the HVAC system to maintain a desired temperature and humidity.

In general operation, the Power Module 104, which may be, for example, an Intelligent Power Module (IPM), converts the ac voltage and current from the ac main Power 112 to the three-phase voltages and currents required to operate the motor 102. The motor controller 106 is typically a Micro Controller Unit (MCU) programmed to control the power supply module 104 to produce the appropriate amplitude and phase angle for the phase voltages and currents. These amplitude and phase angles may be derived by the motor controller 106 using real-time or near real-time measurements of the actual phase voltages and currents in the motor 102 provided by the voltage and current sensors 108. Any suitable sensor 108 may be used, including hall effect sensors, current transformers, and the like, which are capable of measuring phase voltages and currents in the motor 102 in real time or near real time. The overload protection device 110 can be, for example, a circuit breaker, an overload relay, and the like, and the overload protection device 110 disconnects or removes the power module 104 from the motor 102 upon detection of an overload condition in the motor 102.

According to an embodiment of the present disclosure, the overload protection device 110 is equipped with a thermal monitor 116, the thermal monitor 116 monitoring and protecting the motor 102 from thermal overload conditions. The thermal monitor 116 generally operates based on the observation that the rotor resistance increases with rotor temperature, and that the increase tends to be linear. Thus, the rotor temperature, or more precisely the change therein, can be estimated by estimating the rotor resistance. In particular, rotor temperature rise models under normal motor operating conditions may be established using rotor resistance estimates obtained under normal motor operating conditions. The model sets the expected rotor temperature increase or increases to be expected under normal motor operating conditions. Thereafter, the thermal monitor 116 may periodically obtain estimates of the rotor temperature rise of the electric machine and determine whether any of these estimates exceed the expected rotor temperature rise by more than a predefined threshold. If so, the thermal monitor 116 issues an alarm and/or takes corrective action as needed, including immediately shutting off power to the motor 102. The model may be established by the thermal monitor 116 within the overload protection device 110 or may be established externally (e.g., as part of a motor characterization process) and the results provided to the overload protection device 110.

Fig. 1B shows a motor management system 100 'in which the thermal monitor 116 is located in an edge device 118 connected to the system 100' rather than in the overload protection device 110. The edge device 118 provides an entry or entry point for the motor management system 100' to transmit collected data about the motor 102 to an external system, such as a supervisory control and data acquisition (SCADA) system 120. Communication may be via communication link 122, and communication link 122 may be a wired or wireless link, such as Wi-Fi, Bluetooth, GPRS, CDMA, satellite, and the like. The data may also be forwarded by the edge device 118 to other systems, including a cloud-based system 124 (which may be a private enterprise cloud) for further processing by the data analysis service 126. Any type of edge device may be used as the edge device 118, provided that the device has sufficient processing power for the purposes discussed herein. Examples of suitable edge devices include gateways, routers, routing switches, Integrated Access Devices (IADs), and various MAN and WAN access devices.

Fig. 2 illustrates an example computing system 200 that can be used to implement the overload protection apparatus 110 and/or the edge device 118. The exemplary computing system 200 has a typical system architecture including a CPU 202 communicatively coupled to a random-access memory (RAM) 204 or other dynamic storage device, and an input/output interface 206 that allows the CPU 202 to communicate with external systems or networks. A computer-readable storage device 208, such as a non-volatile memory (e.g., flash drive) disk or the like, is communicatively coupled to the CPU 202 and stores programs and data for it. These computing components 202-208 operate in a manner well known in the art and, therefore, a detailed description is omitted herein for the sake of economy.

Among the programs and computer readable instructions residing on storage device 208 are computer readable instructions for thermal monitor 116. In the illustrated example, the thermal monitor 116 has or consists of a plurality of functions or modules depicted as discrete blocks. One of ordinary skill in the art will certainly appreciate that any one block may be divided into several constituent blocks and two or more blocks may be combined into a single block as desired without departing from the scope of the disclosed embodiments.

In the example of fig. 2, the thermal monitor 116 includes a voltage and current acquisition module 210 that operates to obtain samples of motor voltage and current, and a reference frame transformation module 212 that operates to transform the motor voltage and current from a stationary reference frame to a rotating reference frame, and then to a synchronous reference frame. There is also a motor slip and speed module 214 that operates to determine motor slip and speed, and an input admittance module 216 that operates to derive a motor input admittance. The thermal monitor 116 additionally includes a rotor resistance module 218 and a motor thermal state module 220, the rotor resistance module 218 operating to obtain a rotor resistance estimate and the motor thermal state module 220 operating to obtain a motor thermal state estimate for the motor 102. There is also a rotor temperature increase module 222 and a correlation module 224, the rotor temperature increase module 222 operating to obtain an estimated rotor temperature increase value, the correlation module 224 for correlating the rotor temperature increase value with a motor thermal state. The rotor temperature increase monitor 226 operates to compare the rotor temperature increase estimate to an expected rotor temperature increase value and take one or more actions if the difference exceeds a predefined threshold.

In some embodiments, the correlation module 224 is an optional module (represented by dashed lines) that may be implemented separately from the thermal monitor 116, e.g., by the cloud-based data analysis service 128, and the correlations resulting therefrom are provided to the thermal monitor 116.

Fig. 3A and 3B illustrate circuit models 300a and 300B that help explain the basis of operation of the thermal monitor 116. Referring first to fig. 3A, a circuit model 300a represents an exemplary steady state equivalent circuit of an ac motor, such as motor 102. In this model, V₁Is the motor input voltage, R₁And X₁Representing stator resistance and leakage reactance, I₁Is the current through these components, R₂And X₂Representing rotor resistance and leakage reactance, I₂Is the current through these components, G_CRepresenting shunt resistance, X_mAnd I_mRepresenting magnetizing reactance and current, and s represents motor slip. In most motor applications, R₁、X₁And X₂And G_CRelative to the magnetizing reactance X_mAnd rotor resistance R₂The value of (c) is negligible and therefore can be ignored. The result is a simplified equivalent circuit model 300B of FIG. 3B in which the negligible component R is ignored₁、X₁And X₂And G_C. The complex admittance Y of the motor can be expressed as equation (1):

as can be seen from equation (1), the real part Y of the admittance_realEqual to the rotor resistance R₂Inverse of the ratio to the motor slip s, and the imaginary part of the admittance Y_imagEqual to magnetizing reactance X_mThe reciprocal of (c). Thus, the rotor resistance R₂Can use the motor slip s and the real part Y of the admittance_realAs shown in equation (2):

R₂＝s/Y_real (2)

real admittance Y_realAnd virtual admittance Y_imagMay be determined by motor voltage and current. However, the motor voltage and current are three-phase time-varying signals V_a,V_b,V_cAnd I_a,I_b,I_cThese signals are computationally difficult to analyze for motor control. By converting these signals into quiescent voltages and currents Vx in a quiescent reference frameVy and Ix, Iy (e.g., using clark transformation) may make the analysis of these signals computationally easier to manage, as is well known in the art. By converting the static voltage and current into quadrature-axis and direct-axis voltage and current V in a synchronous reference frame_Q、V_DAnd I_Q、I_DFurther computational simplification (e.g., using a park transformation) may be achieved. Synchronizing reference frame voltage vectors (V)_Q，V_D) Oriented to the Q-axis of the synchronous reference frame such that the D-axis voltage V_DWith a zero mean value, which allows this term to be conveniently omitted from the analysis. Fig. 4A and 4B illustrate the synchronous reference frame voltages and currents described above.

Referring to fig. 4A, a voltage waveform capture 400 shows the motor voltage (or sampled data) after conversion to the synchronous reference frame. In this example, waveform 402 is the quadrature axis voltage V_QLine 404 is the quadrature voltage V_QFIs filtered and smoothed, and waveform 406 is the direct-axis voltage V_DAnd line 408 is the direct axis voltage V_DFFiltered and smoothed versions of (a). As shown by line 408, the reference frame voltage vector (V) will be synchronized_Q，V_D) Oriented to the Q axis such that the direct axis voltage V_DWith a zero mean value. In some embodiments, a phase-locked loop (PLL) or equivalent computational mechanism is used to synchronize the reference frame voltage vectors (V)_Q，V_D) Oriented to the Q-axis.

Fig. 4B shows a current waveform capture 410 of the motor current (or sampled data thereof), the current waveform capture 410 resulting from the transformation to the synchronous reference frame and the orientation of the voltage vector to the Q-axis. In this example, waveform 412 is quadrature axis current I_QLine 414 is a filtered and smoothed version I of the quadrature current_QFAnd waveform 416 is the direct axis current I_DLine 418 is a filtered and smoothed version I of the direct axis current_DF. Real and imaginary admittance Y in equation (1)_realAnd Y_imagIt can be expressed in quadrature and direct axis voltages and currents (or filtered versions thereof), as shown in equations (3) and (4):

as for motor slip s, this amount may be determined using any of a number of known techniques. In one example, the motor slip s may be estimated by performing a Fast Fourier Transform (FFT) on the motor current to convert the current from the time domain to the frequency domain, and then locating the peak frequency corresponding to the rotor slot harmonics in the current spectrum. The frequency corresponding to the rotor slot harmonics can then be used to determine the motor slip s according to the well-known relationship shown in equation (5).

In equation (5), f_shIs the peak frequency corresponding to the rotor slot harmonic, k is an integer representing the order of the slot harmonic (e.g., 1, 2, 3, etc.), R represents the number of slots (or bars) in the rotor, p represents the number of poles in the motor, f represents the number of poles in the motor, and₁representing the ac mains frequency. The motor slip s can be determined from this equation and then compared to the actual admittance Y of the motor_realUsed together to obtain an estimate of rotor resistance according to equation (2).

Fig. 5 shows a plot 500 of rotor resistance estimates obtained from equation (2) for a particular 10 horsepower induction machine over a measured stator winding temperature range. In the figure, the horizontal axis represents measured stator winding temperature in degrees celsius and the vertical axis represents rotor resistance in ohms. The data point labeled 502 represents the estimated rotor resistance obtained at 70% motor load, the data point labeled 504 represents the estimated rotor resistance obtained at 90% motor load, and the data point labeled 506 represents the estimated rotor resistance obtained at 110% motor load. It can be seen that the rotor resistance estimate generally increases with increasing stator temperature, and that this increase tends to be linear. Although the stator temperature is not completely correlated with the rotor temperature (e.g., due to thermal lag between different parts of the motor), linear tracking between the rotor resistance estimate and the stator temperature strongly suggests that the rotor resistance estimate can be effectively used to obtain at least a change in the rotor temperature.

In some embodiments, the change in rotor temperature, in particular the rotor temperature rise T_riseThe relationship shown in equation (6) can be used to estimate:

in equation (6), R_initIs an initial or "cold" rotor resistance estimate obtained shortly after the motor is started and before the motor begins to heat up, and α is the temperature coefficient of resistance of the particular metal of the motor (e.g., aluminum, copper, etc.). The thermal monitor 116 may thus be used to obtain an estimate T of the rotor resistance at any given point during operation of the motor_riseTo obtain an estimate of the rotor temperature rise. If the estimated rotor temperature rise exceeds the expected rotor temperature rise by more than a predefined threshold, the thermal monitor 116 may automatically issue an alarm and/or take corrective action. In some embodiments, multiple predefined thresholds may be used, each indicating a progressively higher level of thermal overload and resulting in a different corrective action. In this way, the thermal monitor 116 may provide a more elaborate method to monitor and protect the motor from thermal overload conditions.

Turning now to FIG. 6, a flow diagram of a method 600 that may be used in conjunction with the thermal monitor 116 is shown. The method 600 generally begins at block 602 where a rotor temperature increase estimate is calculated based on a rotor resistance estimate under normal motor operating conditions at block 602. At block 604, a motor thermal state estimate is obtained (calculated) under normal motor operating conditions. At block 606, a correlation is derived or otherwise obtained between the thermal state estimate and the rotor temperature rise estimate. This correlation can then be used as a basis for a rotor temperature rise model under normal motor operating conditions for the motor. The thermal monitor 116 then compares the new rotor temperature increase estimate to an expected rotor temperature increase estimate that is continuously predicted by the model to monitor and detect a potential thermal overload condition, as shown in block 608.

At block 608, a new rotor temperature increase estimate is calculated or obtained based on the rotor resistance estimate, and at block 610, a new motor thermal state estimate is calculated. The new thermal state estimate and correlation or model are then used to calculate an expected rotor temperature rise value at block 612. At block 614, it is determined whether the new estimate of rotor temperature increase exceeds the expected rotor temperature increase by more than a predefined threshold. If not, the method 600 returns to block 608 to continue monitoring for potential thermal overload conditions. If so, one or more corrective actions are taken at block 616, such as cutting power to the motor, sounding an alarm, and the like. The method 600 may then continue further processing or return to block 608 to continue monitoring for potential thermal overload conditions depending on the particular application.

FIG. 7 illustrates a flow chart of a method 700 that may be used to calculate the above-described rotor temperature increase estimate in conjunction with the thermal monitor 116. The method 700 generally begins at block 702, where a motor phase voltage V is captured or otherwise obtained at block 702_a、V_b、V_cAnd current I_a、I_b、I_cThe waveform of (2). In some embodiments, the waveform is captured at a sampling rate of 5000 samples/second over a sampling interval of 15 seconds, although other sampling intervals and sampling rates may of course be used. In general, the sampling interval and sampling rate should be high enough to allow identification of the rotor slot harmonics from the motor voltage and current (see block 708). In block 704, the time-varying voltage and current waveforms (or sampled data thereof) are transformed to a stationary reference frame using clarke transformation or similar techniques.

At block 706, it is determined whether the waveform has stabilized to a steady state condition. If not, the sample is discarded and the method 700 returns to block 702 for another waveform capture. If it has stabilized, at block 708, the ac mains frequency and the motor slip are calculated. In some embodiments, motor slip is obtained by performing an FFT on the motor current to locate the peak frequency corresponding to the rotor slot harmonics and then resolving equation (5) to determine motor slip s.

At block 710, a PLL or similar computing device is run or otherwise applied to the stationary reference frame voltage and currents Vx, Vy and Ix, Iy, which has the effect of locking the stationary voltage Vx to the ac main power frequency. As a result, when the stationary reference frame voltage and currents Vx, Vy and Ix, Iy are converted to the synchronous reference frame at block 712, the voltage vectors (VQ, VD) are oriented to the Q-axis of the synchronous reference frame. This makes the average value of the D-axis voltage VD zero as shown in fig. 4A. There are many definitions of the transformation from a three-phase variable to a stationary two-phase variable and then to a synchronous system variable. One such set is shown in equations (6) and (7):

synchronous reference frame voltage and current V_Q、V_DAnd I_Q、I_D(see fig. 4A and 4B) are then used to calculate the real and imaginary parts of the complex motor input admittance at block 714 according to equations (3) and (4). In some embodiments, only the second half of the waveform (or its sampled data), and preferably only its last four seconds, is used to calculate the real and imaginary admittances, thereby avoiding extraneous noise or interference, which may sometimes appear in front of the waveform, for example, due to filter settling times.

At block 716, a rotor resistance estimate is calculated from the real part of the input admittance according to equation (2). Then, at block 718, a rotor temperature increase estimate may be calculated from the rotor resistance estimate according to equation (6). The rotor temperature increase estimate obtained under normal motor operating conditions can then be paired with the motor thermal state estimate obtained under normal operating conditions to derive a correlation therebetween that can be used to model the rotor temperature increase. As previously described, the modeling may be performed by the thermal monitor 116 within the overload protection apparatus 110, or may be performed externally as part of the motor characterization process, and the results then provided to the overload protection apparatus 110.

FIG. 8 illustrates a flow chart of a method 800 that may be used to calculate or otherwise obtain a motor thermal state estimate in conjunction with the thermal monitor 116. Most motor overload protection devices employ a thermal model, typically a first order model, which can be used to estimate the motor thermal state. Such models are well known in the art. The thermal state is obtained using the square of the motor RMS current, and is typically provided on a normalized unit (p.u.) basis, e.g., from 0 to 1p.u. or from 0% to 100%.

In the example of FIG. 8, the method 800 generally begins at block 802, where at block 802, mtrThermState is initialized, for example, by setting a motor thermal state, mtrThermState, to zero, and a heating thermal time constant, τ, of the motor is defined or otherwise set_THAnd rated motor current I_n. At block 804, a current waveform of the motor is captured or otherwise acquired. In some embodiments, three consecutive waveforms are captured at a sampling rate of 5000 samples/second, although of course a different number of waveforms and sampling rates may be used. At block 806, an average motor RMS current I is calculated over a 100ms time interval_RMS. At block 808, the average RMS current is divided by the nominal current I, for example_pu＝I_RMS/I_nTo calculate the unit current I_pu. In some embodiments, the average RMS current I_RMSApproximately every 100 ms. At block 810, mtrThermState is updated as follows:

if the motor is heating:

if the motor is cooling:

in equation (10), coolTCRatio may be 0.25 (to reflect the fact that heat transfer efficiency is low when the motor is stopped), mtrThermState (i) is the current thermal state, and mtrThermState (j) is the previous thermal state. Alternatively, the motor thermal state mtrTermState may be defined as mtrThermState (j +1) ═ mtrThermState (j) + …, where item (j) represents the previous motor thermal state and item (j +1) represents the updated motor thermal state. Generally, because the cooling time constant of the thermal model may be different from the heating time constant, the thermal state should be updated approximately every 100ms to reflect whether the motor is heating (running) or cooling. These thermal state estimates may then be used to derive a correlation to the rotor temperature rise estimate from the above.

FIG. 9 illustrates a flow chart of a method 900 that may be used to derive or calculate a correlation between a thermal state estimate and a rotor temperature rise estimate in conjunction with thermal monitor 116. The method 900 generally begins at block 902 by calculating a rotor temperature increase estimate based on a rotor resistance estimate under normal operating conditions at block 902. At block 904, it is determined whether the rotor temperature increase estimate shows that the rotor temperature is increasing relative to a previous rotor temperature increase estimate (i.e., the motor is getting hotter). If not (i.e., the motor does not get hotter), the method 900 returns to block 902 to calculate another rotor temperature increase estimate. If so, at block 906, a motor thermal state estimate is calculated and at block 908, an ordered pair is formed using the motor thermal state estimate and the rotor temperature rise estimate.

At block 910, a determination is made whether the ordered pair satisfies one or more acceptance criteria or is valid. The acceptance criteria may include, for example, whether the rotor temperature rise estimate is less than a predetermined minimum value (e.g., 40 kelvin) and whether the thermal state estimate is less than a predefined minimum value (e.g., 0.4 p.u.). If either or both criteria are not met, meaning that the motor is not significantly heating, depending on the particular application, the method 900 returns to block 902 to calculate another rotor temperature increase estimate. If either or both criteria are met, again depending on the particular application, then at block 912, the above process is repeated until a predefined number of ordered pairs have been accumulated. In some embodiments, at least 25 ordered pairs are required (i.e., N ≧ 25), but fewer or more ordered pairs can be used depending on the particular application.

At block 914, a correlation between the thermal state estimate and the rotor temperature rise estimate is derived or calculated by plotting ordered pairs and determining the slope of the fitted straight-line segment from the plotted pairs. The slope may then be used to model the rotor temperature rise of the motor under normal operating conditions at block 916. For example, for a given thermal condition, the expected rotor temperature rise value for that thermal condition may be calculated using the following standard linear equation, y ═ mx + b, where m is the slope of the straight line and b is the y-intercept:

T_{rise,expected}＝slope×mtrThermState+offset (11)

FIG. 10 illustrates a graph 1000 illustrating an exemplary correlation between a thermal state estimate and a rotor temperature rise estimate. In the figure, the horizontal axis represents the thermal state, and the vertical axis represents the estimated rotor temperature increase. Data points 1002, 1004, and 1006 represent ordered pairs of thermal states and rotor temperature rise estimates for motors with fully blocked, partially blocked, and unblocked fans, respectively, while lines 1008, 1010, and 1012 are lines passing through the fully blocked, partially blocked, and unblocked data points. Clear separation can be seen between the different lines 1008, 1010, and 1012 without any crossing, thus demonstrating the feasibility of using the slope of the lines to predict rotor temperature rise values for a given thermal condition. The slope of line 1012 fitted with unblocked data points can then be used to model the expected rotor temperature rise under normal motor operating conditions. In a preferred embodiment, the data points should not be clustered together, but should span at least 0.2p.u. along the thermal state axis to provide a more accurate slope.

FIG. 11 illustrates a graph 1100 of measured stator winding temperature rise values, exemplary rotor temperature rise values, and calculated motor thermal conditions for an unblocked fan in accordance with an embodiment of the present invention. In the figure, the horizontal axis represents time (hours), the left axis represents a temperature increase value (degrees celsius), and the right axis represents a thermal state (p.u.). Line 1102 represents the stator temperature rise, line 1104 represents the motor thermal state, and line 1106 represents the rotor temperature rise. Data were obtained over 18 hours. It can be seen that the line 1106 representing the increase in rotor temperature substantially tracks the line 1104 representing the thermal state of the motor.

Fig. 12 shows another graph 1200 similar to the graph 1100 of fig. 11, except this time the fan is partially plugged. Line 1202 represents the stator temperature rise, line 1204 represents the motor thermal condition, and line 1206 represents the rotor temperature rise. In this example, line 1204, which represents the thermal state of the motor, is unchanged, but line 1206, which represents an increase in rotor temperature, is increased relative to line 1106 in FIG. 11, due to a decrease in cooling air on the rotor. For example, if the difference is greater than a first predefined threshold, the motor may be experiencing the onset of a thermal overload condition. In this case, the thermal monitor 116 may simply sound an alarm.

Fig. 13 shows another graph 1300 similar to the graph 1100 of fig. 11, except this time the fan is completely blocked. Line 1302 represents the stator temperature rise, line 1304 represents the motor thermal state, and line 1306 represents the rotor temperature rise. Again, line 1304, representing the thermal state of the motor, has not changed, but since the cooling air on the rotor has decreased significantly, line 1306, representing the increase in rotor temperature, has increased significantly relative to line 1106 in FIG. 11. For example, if the difference is greater than a second predefined threshold, the motor may be in a thermal overload state. In such a case, the thermal monitor may take one or more corrective actions, including powering off the motor, sounding an alarm, and the like.

In the foregoing discussion, reference has been made to various embodiments. However, the scope of the present disclosure is not limited to the specifically described embodiments. Rather, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice the contemplated embodiments. Moreover, although embodiments may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment does not limit the scope of the disclosure. Accordingly, the foregoing aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).

Various embodiments disclosed herein may be implemented as a system, method or computer program product. Accordingly, aspects may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to as a "circuit," module, "or" system. Furthermore, aspects may take the form of a computer program product embodied in one or more computer-readable media having computer-readable program code embodied thereon.

Any combination of one or more computer-readable media may be utilized. The computer readable medium may be a non-transitory computer readable medium. A non-transitory computer readable medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the non-transitory computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a Random Access Memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present disclosure may be written in any combination of one or more programming languages. Further, such computer program code may be executed using a single computer system or by multiple computer systems in communication with each other, e.g., using a Private Area Network (PAN), a Local Area Network (LAN), a Wide Area Network (WAN), the internet, etc. While the various features above are described with reference to flowchart illustrations and/or block diagrams, it will be understood by those skilled in the art that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer logic (e.g., computer program instructions, hardware logic, combinations of both, etc.). Generally, computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus. Furthermore, execution of such computer program instructions by a processor results in a machine that is capable of performing the functions or acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the figures illustrate the architecture, functionality, and/or operation of possible implementations of various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although this disclosure describes specific examples, it should be recognized that the systems and methods of the present disclosure are not limited to the examples described herein, but may be practiced with modification within the scope of the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.