Motor thermal protection based on rotor resistance

Abstract

The motor overload protection device is equipped with a thermal monitor that can determine an expected rotor temperature rise value based on a rotor resistance estimate. The thermal monitor determines the expected rotor temperature rise value by using a correlation between rotor temperature rise values calculated from rotor resistance estimates and motor thermal state estimates. The correlation can be derived by fitting a line to a plurality of points, where each point is defined by an ordered pair of a rotor temperature rise estimate and a corresponding motor thermal state estimate, and determining the slope of the line. The thermal monitor can then use the rotor temperature slope to determine a rotor temperature rise value given a rotor resistance estimate and a corresponding motor thermal state estimate. In some embodiments, the thermal monitor issues an alert and / or takes a corrective action if the rotor temperature rise value exceeds the expected rotor temperature rise value by more than a predefined threshold.

Description

Technical Field

[0001] This disclosure relates to an induction motor, and more particularly to a method and system for monitoring and protecting such a motor from thermal overload based on rotor resistance estimates. Background Technology

[0002] Induction motors are widely used in industry due to their advantages such as low cost, high efficiency, and high reliability. A typical induction motor consists of a stationary component or stator with multiple windings, and a rotating component or rotor rotatably disposed within the stator. Applying a sinusoidal or alternating voltage to the stator windings induces a rotating magnetic field, causing the rotor to rotate. Induction motors typically operate on single-phase and three-phase voltages, although two-phase induction motors are also available.

[0003] Most induction motors employ overload protection devices to protect them from overcurrent, thermal overload, and similar effects. Overload protection devices detect excessive current or heat in the motor and interrupt the motor's power supply to prevent damage. These devices can include circuit breakers, overload relays, and other types of circuit interrupters, which typically have a resistive element connected in series with the motor's power supply line. When the resistive element becomes overheated, the overload protection device cuts off the motor's power supply.

[0004] Although much progress has been made in the field of thermal overload protection for induction motors, it is easy to understand that continuous improvement is still needed. Summary of the Invention

[0005] Embodiments of this 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 rise based on a rotor resistance estimate. The thermal monitor determines the expected rotor temperature rise by using a relationship or correlation between an estimated rotor temperature rise based on the rotor resistance estimate and an estimated motor thermal state. This relationship or correlation can be derived by fitting a line to multiple points and determining the slope of the line, each point being defined by an ordered pair consisting of the estimated rotor temperature rise and the corresponding estimated motor thermal state. Given the estimated rotor resistance and the corresponding estimated motor thermal state, this slope (also referred to as the rotor temperature slope) can then be used by the thermal monitor to determine the rotor temperature rise. In some embodiments, if the rotor temperature rise exceeds the expected rotor temperature rise value by a predefined threshold, the thermal monitor issues an alarm and / or takes corrective action.

[0006] In general, in one aspect, embodiments of this disclosure relate to a method for protecting an induction motor from thermal overload conditions. The method specifically includes calculating an estimated rotor temperature rise based on an estimated rotor resistance of the induction motor, and calculating an estimated thermal state of the induction motor. The method further includes deriving a correlation between the thermal state estimate and the estimated rotor temperature rise, and calculating a new estimated temperature rise based on a new estimated rotor resistance of the induction motor. The method also includes calculating a new estimated thermal state of the induction motor, and calculating an expected rotor temperature rise of the induction motor using the correlation and the new thermal state. The method then includes checking whether the new estimated temperature rise exceeds a predefined rotor temperature rise threshold for the induction motor, and performing a correction action in response to the new estimated temperature rise exceeding the predefined rotor temperature rise threshold.

[0007] According to one or more of the foregoing embodiments, the rotor resistance estimate is obtained based on the real part of the complex admittance of the induction motor input and the slip value, and the complex admittance of the induction motor input is determined based on the motor voltage and current in a synchronous reference frame, wherein the motor voltage is oriented on the Q-axis of the synchronous reference frame. According to one or more of the foregoing embodiments, deriving the correlation includes fitting a line to a plurality of points and determining the slope of the line, each point being defined by an ordered pair consisting of a rotor temperature rise estimate and a corresponding motor thermal state estimate. According to one or more of the foregoing embodiments, calculating the expected rotor temperature rise of the induction motor includes applying the slope to a new motor thermal estimate, performing a correction action includes disconnecting the power supply to the induction motor and issuing one or more alarms, and performing the calculation of the expected rotor temperature rise of the induction motor in one of the motor overload protection devices or edge devices.

[0008] Generally, in another aspect, embodiments of this disclosure relate to an apparatus for protecting an induction motor from thermal overload conditions. The apparatus particularly includes a processor and a storage unit communicatively coupled to the processor. The storage unit stores computer-readable instructions thereon for causing the processor, in particular, to calculate a temperature rise estimate based on an estimated rotor resistance of the induction motor, and to calculate an estimated thermal state of the induction motor. The computer-readable instructions also cause the processor to calculate a expected rotor temperature rise of the induction motor and to check whether the temperature rise estimate exceeds the expected rotor temperature rise by more than a predefined rotor temperature rise threshold for the induction motor. The computer-readable instructions further cause the processor to perform a corrective action in response to the temperature rise estimate exceeding the expected rotor temperature rise by more than the predefined rotor temperature rise threshold.

[0009] According to any one or more of the foregoing embodiments, the computer-readable instructions further cause the processor to obtain a rotor resistance estimate based on the real part of the complex admittance of the induction motor input and the slip value, and to determine the complex admittance of the induction motor input based on the motor voltage and current in a synchronous reference frame, wherein the motor voltage is oriented on the Q-axis of the synchronous reference frame. According to any one or more of the foregoing embodiments, the computer-readable instructions also cause the processor to derive a correlation between the thermal state estimate of the induction motor and the rotor temperature rise estimate, and to calculate the expected rotor temperature rise value by applying the correlation to the motor thermal estimate. According to any one or more of the foregoing embodiments, the computer-readable instructions further cause the processor to perform a corrective action by executing one or more of cutting off the power supply to the induction motor and issuing an alarm, and the device is one of a motor overload protection device or an edge protection device.

[0010] Generally, in another aspect, embodiments of this disclosure relate to a non-transitory computer-readable medium containing program logic. When executed by the operation of one or more computer processors, the program logic causes one or more computer processors to calculate, in particular, an estimated rotor temperature rise based on an estimated rotor resistance of an induction motor, and to calculate an estimated motor thermal state of the induction motor. The program logic further causes one or more processors to derive a correlation between the thermal state estimate and the estimated rotor temperature rise, calculate a new estimated motor thermal state of the induction motor, and calculate the expected rotor temperature rise of the induction motor using the correlation and the new motor thermal state. The program logic then causes one or more processors to check whether the new temperature rise estimate exceeds the expected rotor temperature rise by more than a predefined rotor temperature rise threshold for the induction motor, and to perform a correction action in response to the new temperature rise estimate exceeding the expected rotor temperature rise by more than the predefined rotor temperature rise threshold.

[0011] According to any one or more of the foregoing embodiments, the program logic further causes one or more processors to obtain a rotor resistance estimate from the real part of the complex admittance of the induction motor input and the slip value of the induction motor, and further causes one or more processors to determine the complex admittance of the induction motor input based on the motor voltage and current in a synchronous reference frame, wherein the motor voltage is oriented on the Q-axis of the synchronous reference frame. According to any one or more of the foregoing embodiments, the program logic further causes one or more processors to derive a correlation by fitting a line to multiple points and determining the slope of the line, each point being defined by an ordered pair consisting of a rotor temperature rise estimate and a corresponding motor thermal state estimate. According to any one or more of the foregoing embodiments, the program logic further causes one or more processors to calculate the expected rotor temperature rise of the induction motor by applying the slope to the new motor thermal estimate, and to perform a correction action by executing one or more of cutting off the power supply to the induction motor and issuing an alarm. Attached Figure Description

[0012] A more detailed description of the present disclosure, which has been briefly summarized above, can be obtained by referring to various embodiments, some of which are illustrated in the accompanying drawings. While the drawings illustrate selected embodiments of the present disclosure, they should not be considered as limiting its scope, as the present disclosure may allow for other equally effective embodiments.

[0013] Figure 1A and 1B An exemplary motor overload protection device equipped with a thermal monitor, according to an embodiment of the present disclosure, is shown;

[0014] Figure 2 An exemplary thermal monitor according to an embodiment of this disclosure is shown;

[0015] Figure 3A and 3B A circuit model of an exemplary motor according to an embodiment of the present disclosure is shown;

[0016] Figure 4A and 4B Exemplary motor voltages and currents in a synchronous reference frame according to embodiments of the present disclosure are shown;

[0017] Figure 5 A data graph showing exemplary rotor resistance estimates according to embodiments of the present disclosure is illustrated;

[0018] Figure 6 A flowchart illustrating an exemplary method for monitoring motor temperature according to an embodiment of this disclosure is shown;

[0019] Figure 7 A flowchart illustrating an exemplary method for obtaining rotor temperature estimates according to embodiments of the present disclosure is shown;

[0020] Figure 8 A flowchart illustrating an exemplary method for obtaining an estimate of the thermal state of an electric motor according to an embodiment of the present disclosure is shown.

[0021] Figure 9 A flowchart illustrating an exemplary method for associating rotor temperature estimates with motor thermal state estimates according to embodiments of the present disclosure is shown.

[0022] Figure 10 A graph illustrating an exemplary rotor temperature rise relative to thermal state and corresponding linear slope according to an embodiment of the present disclosure is shown.

[0023] Figure 11A graph showing the stator winding temperature rise over time, an exemplary rotor temperature rise, and a calculated thermal state of an unobstructed fan according to an embodiment of the present disclosure is provided.

[0024] Figure 12 The diagram illustrates a graph showing the stator winding temperature rise relative to time, an exemplary rotor temperature rise, and a calculated thermal state of a partially blocked fan according to an embodiment of the present disclosure; and

[0025] Figure 13 The diagram shows a graph of the stator winding temperature rise, an exemplary rotor temperature rise, and a calculated thermal state of a completely unobstructed fan relative to time, according to an embodiment of the present disclosure.

[0026] Where possible, the same reference numerals are used to denote the same elements in the drawings. However, elements disclosed in one embodiment may be advantageously used in other embodiments without specific description. Detailed Implementation

[0027] This specification and accompanying drawings illustrate exemplary embodiments of this disclosure and should not be construed as limiting. The claims define the scope of this disclosure, including equivalents. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the scope of this specification and claims (including equivalents). In some cases, publicly known structures and techniques have not been shown or described in detail to avoid obscuring this disclosure. Furthermore, elements and related aspects thereof described in detail with reference to one embodiment may be included in other embodiments where they are not specifically shown or described, whenever feasible. For example, if an element is described in detail with reference to one embodiment but not with reference to a second embodiment, that element may still be declared as included in the second embodiment.

[0028] At a high level, embodiments of this disclosure provide a method for correlating motor temperature rise with motor thermal state to detect abnormal motor temperature rises 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 based on rotor slot harmonics, calculating rotor temperature rise, and correlating rotor temperature rise with motor thermal state. These systems and methods can then be deployed in overload protection devices to monitor and protect induction motors from thermal overload conditions.

[0029] Now for reference Figure 1AAccording to one or more embodiments disclosed herein, a functional diagram of a motor management system 100 for an AC motor 102, such as an induction motor, is shown. The motor 102 in this example is a typical three-phase induction motor having a stator (not explicitly shown), a rotor (not explicitly shown) rotatably disposed within the stator, and driven by three-phase line voltages and currents having phases "a", "b", and "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.

[0030] As can be seen, the motor management system 100 has multiple components, including a power module 104, a motor controller 106, voltage and current sensors 108, and an overload protection device 110. The AC mains power supply 112 provides voltage and current to the motor 102 through the power module 104, and the external control system 114 provides overall control of the motor 102 through the motor controller 106. This type of arrangement is commonly used in various commercial and residential applications. For example, in an HVAC system, the motor 102 could be a fan motor that drives a fan to pass air through air ducts, and the external control system 114 could be a thermostat that controls the HVAC system to maintain the desired temperature and humidity.

[0031] In normal operation, power module 104, such as an Intelligent Power Module (IPM), converts the AC voltage and current from AC mains power supply 112 into the three-phase voltage and current required to operate motor 102. Motor controller 106, typically a microcontroller unit (MCU), is programmed to control power module 104 to generate appropriate amplitudes and phase angles for the phase voltages and currents. These amplitudes and phase angles can be derived by motor controller 106 using real-time or near-real-time measurements of the actual phase voltages and currents in motor 102, provided by voltage and current sensors 108. Any suitable sensor 108 can be used, including Hall effect sensors, current transformers, and the like, capable of measuring phase voltages and currents in motor 102 in real-time or near-real-time. Overload protection device 110, such as a circuit breaker, overload relay, and the like, disconnects or removes power module 104 from motor 102 upon detecting an overload condition in motor 102.

[0032] According to embodiments of this disclosure, the overload protection device 110 is equipped with a thermal monitor 116 that monitors and protects the motor 102 from thermal overload conditions. The thermal monitor 116 typically operates based on the observation that rotor resistance increases with rotor temperature, and that this increase tends to be linear. Therefore, the rotor temperature, or more precisely, its variation, can be estimated by estimating the rotor resistance. Specifically, a rotor temperature rise model under normal motor operating conditions can be established using rotor resistance estimates obtained under normal motor operating conditions. This model sets the expected rotor temperature rise or anticipated temperature rise under normal motor operating conditions. Subsequently, the thermal monitor 116 can periodically obtain estimates of the motor's rotor temperature rise and determine whether any of these estimates exceeds the expected rotor temperature rise by a predefined threshold. If so, the thermal monitor 116 issues an alarm and / or takes corrective actions as needed, including immediately disconnecting the power supply to the motor 102. The model can be established by the thermal monitor 116 within the overload protection device 110, or it can be established externally (e.g., as part of the motor characterization process) and the results can be provided to the overload protection device 110.

[0033] Figure 1B A motor management system 100' is illustrated, in which a thermal monitor 116 is located in an edge device 118 connected to the system 100', rather than in an overload protection device 110. The edge device 118 provides an entry point for the motor management system 100' to transmit collected data about the motor 102 to external systems, such as a supervisory control and data acquisition (SCADA) system 120. Communication can be via a communication link 122, which can be a wired or wireless link, such as Wi-Fi, Bluetooth, GPRS, CDMA, satellite, etc. Data can also be forwarded by the edge device 118 to other systems, including a cloud-based system 124 (which could be a private enterprise cloud), for further processing by a data analytics service 126. Any type of edge device can be used as 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.

[0034] Figure 2An exemplary computing system 200 is shown that can be used to implement overload protection device 110 and / or edge device 118. The exemplary computing system 200 has a typical system architecture including a CPU 202 communicatively coupled to random-access memory (RAM) 204 or other dynamic storage device, and an input / output interface 206 allowing the CPU 202 to communicate with external systems or networks. A computer-readable storage device 208, such as non-volatile memory (e.g., flash drive), disk, etc., is communicatively coupled to the CPU 202 and stores programs and data therein. These computing components 202-208 operate in a manner well known in the art, and therefore, detailed descriptions are omitted herein for economic reasons.

[0035] The program and computer-readable instructions residing on storage device 208 include computer-readable instructions for thermal monitor 116. In the example shown, thermal monitor 116 has or is composed of multiple functions or modules described as discrete blocks. Those skilled in the art will readily understand that any block can be divided into several constituent blocks, and two or more blocks can be combined into a single block as needed, without departing from the scope of the disclosed embodiments.

[0036] exist Figure 2 In the example, thermal monitor 116 includes a voltage and current acquisition module 210, which operates to acquire samples of motor voltage and current, and a reference frame transformation module 212, which operates to transform the motor voltage and current from a stationary reference frame to a rotating reference frame, and subsequently to a synchronous reference frame. There is also a motor slip and speed module 214, which operates to determine the motor slip and speed, and an input admittance module 216, which operates to derive the motor input admittance. Thermal monitor 116 further includes a rotor resistance module 218 and a motor thermal state module 220, the rotor resistance module 218 operating to acquire an estimated rotor resistance value, and the motor thermal state module 220 operating to acquire an estimated motor thermal state value for motor 102. There is also a rotor temperature rise module 222 and a correlation module 224, the rotor temperature rise module 222 operating to acquire an estimated rotor temperature rise value, and the correlation module 224 used to correlate the rotor temperature rise value with the motor thermal state. The rotor temperature rise monitor 226 operates to compare the estimated rotor temperature rise with the expected rotor temperature rise, and takes one or more actions if the difference exceeds a predefined threshold.

[0037] In some embodiments, the correlation module 224 is an optional module (indicated by dashed lines) that can be implemented separately from the thermal monitor 116, for example through a cloud-based data analytics service 128, and the resulting correlation is provided to the thermal monitor 116.

[0038] Figure 3A and 3B Circuit models 300a and 300b are shown to help explain the operating basis of the thermal monitor 116. First, refer to... Figure 3A Circuit model 300a represents an AC motor, such as the exemplary steady-state equivalent circuit of motor 102. In this model, V1 is the motor input voltage, R1 and X1 represent the stator resistance and leakage reactance, I1 is the current through these components, R2 and X2 represent the rotor resistance and leakage reactance, I2 is the current through these components, and G... C X represents the shunt resistor. m and I m Representing magnetizing reactance and current, s represents motor slip. In most motor applications, R1, X1, and X2, as well as G... C The value relative to the magnetizing reactance X m The value of the rotor resistance R2 is negligible, therefore it can be ignored. The result is... Figure 3B The simplified equivalent circuit model 300b is given, in which negligible components R1, X1, and X2, as well as G, are ignored. C The complex admittance Y of the motor can be expressed as equation (1):

[0039]

[0040] It can be seen from equation (1) that the real part of the admittance Y real It is equal to the reciprocal of the ratio of rotor resistance R2 to motor slip s, and the imaginary part of admittance Y imag Equal to magnetizing reactance X m The reciprocal of the summation. Therefore, the rotor resistance R2 can be expressed as the real part of the motor slip s and admittance Y. real To represent, as shown in equation (2):

[0041]

[0042] Real admittance Y real And virtual admittance Y imag It can be determined by the motor voltage and current. However, the motor voltage and current are three-phase time-varying signals V. a V b V c and I a , I b , I c These signals are computationally difficult to analyze for motor control. It is well known in the art that the analysis of these signals can be made computationally more manageable by converting them into stationary voltages and currents Vx, Vy and Ix, Iy in a stationary reference frame (e.g., using the Clarke transform). This conversion of stationary voltages and currents into quadrature-axis and direct-axis voltages and currents Vx, Vy and Iy in a synchronous reference frame... Q VD and I Q I D This allows for further computational simplification (e.g., using the Parker transform). The synchronous reference frame voltage vector (V...) Q V D Oriented to the Q-axis of the synchronous reference frame, such that the D-axis voltage V D It has a zero mean, which allows this term to be conveniently omitted from the analysis. Figure 4A and 4B This explains the voltage and current of the aforementioned synchronous reference system.

[0043] refer to Figure 4A Voltage waveform capture 400 shows the motor voltage (or sampled data) after being converted to the synchronous reference frame. In this example, waveform 402 is the quadrature-axis voltage V. Q Line 404 is the quadrature axis voltage V. QF The filtered and smoothed version, while waveform 406 is the direct-axis voltage V. D And line 408 is a direct-axis voltage V DF The filtered and smoothed version. As shown in line 408, the synchronous reference voltage vector (V Q V D Oriented to the Q-axis, making the direct-axis voltage V D It has a zero average value. In some embodiments, a phase-locked loop (PLL) or equivalent computer mechanism is used to transmit the synchronous reference voltage vector (V0). Q V D Oriented to the Q-axis.

[0044] Figure 4B A current waveform capture 410 of the motor current (or its sampled data) is shown, generated by switching to a synchronous reference frame and orienting the voltage vector to the Q-axis. In this example, waveform 412 is the quadrature-axis current I. Q Line 414 is a filtered and smoothed version I of the quadrature-axis current. QF Waveform 416 is the direct-axis current I. D Line 418 is a filtered and smoothed version I of the direct-axis current. DF The real and imaginary admittances Y in equation (1) real and Y imag It can be expressed using quadrature-axis and direct-axis voltages and currents (or their filtered forms), as shown in equations (3) and (4):

[0045]

[0046] As for the motor slip s, this quantity can be determined using any of a variety of known techniques. In one example, the motor slip s can 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. Then, according to the well-known relationship shown in equation (5), the frequency corresponding to the rotor slot harmonics can be used to determine the motor slip s.

[0047]

[0048] In equation (5), f sh This corresponds to the peak frequency of the rotor slot harmonics, where k is an integer representing the order of the slot harmonics (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, and f1 represents the AC main power supply frequency. The motor slip s can be determined by this equation, and then compared with the actual admittance Y of the motor. real Used together, to obtain the rotor resistance estimate according to equation (2).

[0049] Figure 5 A graph 500 shows the estimated rotor resistance values ​​obtained according to equation (2) for a specific 10 hp induction motor over the measured stator winding temperature range. In the graph, the horizontal axis represents the measured stator winding temperature in degrees Celsius, and the vertical axis represents the rotor resistance in ohms. Data point 502 represents the estimated rotor resistance value obtained at 70% motor load, data point 504 represents the estimated rotor resistance value obtained at 90% motor load, and data point 506 represents the estimated rotor resistance value obtained at 110% motor load. It can be seen that the estimated rotor resistance value generally increases with increasing stator temperature, and this increase tends to be linear. Although the stator temperature is not perfectly correlated with the rotor temperature (e.g., due to thermal hysteresis between different parts of the motor), the linear tracking between the estimated rotor resistance value and the stator temperature strongly suggests that the estimated rotor resistance value can be effectively used to at least obtain the variation in rotor temperature.

[0050] In some embodiments, the change in rotor temperature, particularly the increase in rotor temperature T rise The relationship shown in equation (6) can be used to estimate:

[0051]

[0052] In equation (6), R initThis is an initial or "cold" estimate of the rotor resistance obtained shortly after the motor starts and before it begins to heat up, where α is the temperature coefficient of resistance of the motor's specific metal (e.g., aluminum, copper, etc.). The thermal monitor 116 can therefore obtain an estimate of the rotor resistance T at any given point during motor operation. rise The thermal monitor 116 obtains 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 can automatically issue an alarm and / or take corrective action. In some embodiments, multiple predefined thresholds can be used, each indicating a progressively increasing level of thermal overload and resulting in different corrective actions. In this way, the thermal monitor 116 can provide a more refined method to monitor and protect the motor from thermal overload conditions.

[0053] Now go to Figure 6 The diagram illustrates a flowchart of method 600, which can be used in conjunction with thermal monitor 116. Method 600 typically begins at block 602, where a rotor temperature rise estimate is calculated based on a rotor resistance estimate under normal motor operating conditions. 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 serve as the basis for a rotor temperature rise model under normal motor operating conditions. Thermal monitor 116 then compares the new rotor temperature rise estimate with a predicted rotor temperature rise estimate continuously predicted by the model to monitor and detect potential thermal overload conditions, as shown in block 608.

[0054] At box 608, a new rotor temperature rise estimate is calculated or obtained based on the rotor resistance estimate, and at box 610, a new motor thermal state estimate is calculated. Then, at box 612, the new thermal state estimate and a correlation or model are used to calculate the expected rotor temperature rise. At box 614, it is determined whether the new rotor temperature rise estimate exceeds the expected rotor temperature rise by a predefined threshold. If not, method 600 returns to box 608 to continue monitoring for potential thermal overload conditions. If so, one or more corrective actions are taken at box 616, such as disconnecting the motor power, issuing an alarm, or the like. Method 600 may then proceed with further processing or return to box 608 to continue monitoring for potential thermal overload conditions depending on the application.

[0055] Figure 7 A flowchart of method 700 is shown, which can be used in conjunction with thermal monitor 116 to calculate the aforementioned rotor temperature rise estimate. Method 700 typically begins at block 702, where the motor phase voltage V is captured or otherwise acquired. a V bV c and current I a I b I c The waveform is captured. In some embodiments, the waveform is captured at a sampling rate of 5000 samples / second within a 15-second sampling interval, although other sampling intervals and sampling rates can certainly be used. Generally, the sampling interval and sampling rate should be high enough to allow identification of rotor slot harmonics from the motor voltage and current (see box 708). In box 704, the time-varying voltage and current waveforms (or their sampled data) are transformed to a stationary reference frame using a Clarke transform or similar technique.

[0056] At box 706, it is determined whether the waveform has stabilized to a steady-state condition. If not, the sample is discarded, and method 700 returns to box 702 for another waveform capture. If it has stabilized, at box 708, the AC mains power frequency and motor slip are calculated. In some embodiments, the 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 analyzing equation (5) to determine the motor slip s.

[0057] At box 710, a PLL or similar computing device is operated or otherwise applied to the quiescent reference frame voltages and currents Vx, Vy and Ix, Iy, which has the effect of locking the quiescent voltage Vx to the AC mains power frequency. As a result, when the quiescent reference frame voltages and currents Vx, Vy and Ix, Iy are transformed to the synchronous reference frame at box 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... Figure 4A As shown. There are multiple definitions for the transformation from three-phase variables to stationary two-phase variables, and then to synchronous system variables. Equations (6) and (7) show such a set:

[0058]

[0059] Synchronous reference frame voltage and current V Q V D and I Q I D (See Figure 4A and 4B The real and imaginary parts of the complex admittance of the induction motor input are then used to calculate the real and imaginary parts of the admittance at box 714 according to equations (3) and (4). In some embodiments, only the latter half of the waveform (or its sampled data), and preferably only the last four seconds, are used to calculate the real and imaginary admittances to avoid external noise or interference, which may sometimes appear at the beginning of the waveform due to filter set time.

[0060] At box 716, the rotor resistance estimate is calculated based on the real part of the input admittance according to equation (2). Then, at box 718, the rotor temperature rise estimate can be calculated based on the rotor resistance estimate according to equation (6). The rotor temperature rise estimate obtained under normal motor operating conditions can then be paired with the motor thermal state estimate obtained under normal operating conditions to derive the correlation between them, which can be used to model the rotor temperature rise. As previously mentioned, the modeling can be performed by the thermal monitor 116 within the overload protection device 110, or it can be performed externally as part of the motor characterization process, and the results can then be provided to the overload protection device 110.

[0061] Figure 8 A flowchart of method 800 is shown, which can be used in conjunction with thermal monitor 116 to calculate or otherwise obtain an estimate of the motor's thermal state. Most motor overload protection devices employ a thermal model, typically a first-order model, which can be used to estimate the motor's thermal state. Such models are well known in the art. The thermal state is obtained using the square of the motor's RMS current, typically provided on a standardized unit basis (pu), for example, from 0 to 1 pu or from 0% to 100%.

[0062] exist Figure 8 In the examples, method 800 typically begins at box 802, where mtrThermState is initialized, for example, by setting the motor thermal state mtrThermState to zero, and the heating time constant τ of the motor is defined or otherwise set. TH and rated motor current I n At box 804, the motor current waveform is captured or otherwise acquired. In some embodiments, three consecutive waveforms are captured at a sampling rate of 5000 samples / second, although different numbers of waveforms and sampling rates can certainly be used. At box 806, the average motor RMS current I is calculated over a 100 ms time interval. RMS At box 808, for example, by dividing the average RMS current by the nominal current I. pu = I RMS / I n To calculate the unit current I pu In some embodiments, the average RMS current I RMS Update approximately every 100ms. At box 810, update mtrThermState as follows:

[0063] If the motor is heating:

[0064]

[0065] If the motor is cooling:

[0066]

[0067] In equation (10), coolTCRatio might be 0.25 (to reflect the lower heat transfer efficiency 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 can be defined as mtrThermState(j+1) = mtrThermState(j) + …, where (j) represents the previous motor thermal state and (j+1) represents the updated motor thermal state. Generally, because the cooling time constant of the thermal model may differ from the heating time constant, the thermal state should be updated approximately every 100 ms to reflect whether the motor is heating (running) or cooling. These thermal state estimates can then be used to derive the correlation with the rotor temperature rise estimate based on the above.

[0068] Figure 9 A flowchart of method 900 is shown, which can be used to derive or calculate the correlation between a thermal state estimate and a rotor temperature rise estimate in conjunction with thermal monitor 116. Method 900 typically begins at block 902, where a rotor temperature rise estimate is calculated based on a rotor resistance estimate under normal operating conditions. At block 904, it is determined whether the rotor temperature rise estimate indicates that the rotor temperature is increasing relative to a previous rotor temperature rise estimate (i.e., the motor is getting hotter). If not (i.e., the motor is not getting hotter), method 900 returns to block 902 to calculate another rotor temperature rise estimate. If it has increased, a motor thermal state estimate is calculated at block 906, and at block 908, an ordered pair is formed using the motor thermal state estimate and the rotor temperature rise estimate.

[0069] At box 910, it is determined whether the ordered pairs meet one or more acceptance criteria or are valid. Acceptance criteria may include, for example, whether the estimated rotor temperature rise is less than a predetermined minimum (e.g., 40 Kelvin) and whether the estimated thermal state is less than a predefined minimum (e.g., 0.4 pu). Depending on the application, if either or both criteria are not met, meaning the motor is not generating significant heat, method 900 returns to box 902 to calculate another rotor temperature rise estimate. If either or both criteria are met, again depending on the application, then at box 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 depending on the application, fewer or more ordered pairs may be used.

[0070] At box 914, the 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 using the plotted pairs. Then, at box 916, this slope can be used to model the rotor temperature rise of the motor under normal operating conditions. For example, for a given thermal state, the expected rotor temperature rise corresponding to that thermal state can 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:

[0071]

[0072] Figure 10 Graph 1000 is shown, illustrating an exemplary correlation between the thermal state estimate and the rotor temperature rise estimate. In the graph, the horizontal axis represents the thermal state, and the vertical axis represents the rotor temperature rise estimate. Data points 1002, 1004, and 1006 represent ordered pairs of thermal state 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. A clear separation can be seen between the different lines 1008, 1010, and 1012, with no intersections, thus confirming the feasibility of using the slope of the lines to predict the rotor temperature rise value for a given thermal state. The slope of line 1012, fitted through the 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 cluster together but should instead span at least 0.2 pu along the thermal state axis to provide a more accurate slope.

[0073] Figure 11 A graph 1100 is shown illustrating the measured stator winding temperature rise, exemplary rotor temperature rise, and calculated motor thermal state of an unblocked fan according to an embodiment of the present invention. In the graph, the horizontal axis represents time (hours), the left axis represents the temperature rise (degrees Celsius), and the right axis represents the thermal state (pu). Line 1102 represents the stator temperature rise, line 1104 represents the motor thermal state, and line 1106 represents the rotor temperature rise. The data was obtained over an 18-hour period. It can be seen that line 1106, representing the rotor temperature rise, essentially tracks line 1104, representing the motor thermal state.

[0074] Figure 12 It shows something similar to Figure 11Another curve, 1200, represents the stator temperature rise, except this time the fan is partially blocked. Line 1202 represents the stator temperature rise, line 1204 represents the motor's thermal state, and line 1206 represents the rotor temperature rise. In this example, line 1204, representing the motor's thermal state, remains unchanged, but due to the reduced cooling air on the rotor, line 1206, representing the rotor temperature rise, is relatively... Figure 11 Line 1106 in the sensor is increased. For example, if the difference exceeds a first predefined threshold, the motor may be experiencing the onset of a thermal overload condition. In this case, thermal monitor 116 can simply issue an alarm.

[0075] Figure 13 It shows something similar to Figure 11 The other curve, 1300, is shown in graph 1100, except this time the fan is completely blocked. Line 1302 represents the stator temperature rise, line 1304 represents the motor's thermal state, and line 1306 represents the rotor temperature rise. Again, line 1304, representing the motor's thermal state, remains unchanged, but due to the significant reduction in cooling air on the rotor, line 1306, representing the rotor temperature rise, is relatively higher. Figure 11 The line 1106 in the image shows a significant increase. For example, if the difference exceeds a second predefined threshold, the motor may be in a thermal overload state. In this case, the thermal monitor can take one or more corrective actions, including cutting off the motor power, issuing an alarm, and so on.

[0076] In the foregoing discussion, reference has been made to various embodiments. However, the scope of this disclosure is not limited to the embodiments specifically described. Rather, any combination of the described features and elements, whether or not associated with different embodiments, is contemplated for implementation and practice of the contemplated embodiments. Furthermore, while embodiments may achieve advantages over other possible solutions or prior art, whether a particular advantage is achieved by a given embodiment does not limit the scope of this disclosure. Therefore, the foregoing aspects, features, embodiments, and advantages are merely illustrative and should not be considered as elements or limitations of the appended claims unless expressly stated in the claims.

[0077] The various embodiments disclosed herein can be implemented as systems, methods, or computer program products. Therefore, aspects can take the form of entirely hardware embodiments, entirely software embodiments (including firmware, resident software, microcode, etc.), or embodiments combining software and hardware aspects, which are generally referred to as “circuit,” “module,” or “system.” Furthermore, aspects can take the form of computer program products contained in one or more computer-readable media having computer-readable program code contained thereon.

[0078] Any combination of one or more computer-readable media may be used. 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, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, apparatuses, or devices, or any suitable combination thereof. More specific examples of non-transitory computer-readable media (a non-exhaustive list) may include the following: electrical connections having one or more wires, portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, compact disc read-only memory (CD-ROM), optical storage devices, magnetic storage devices, or any suitable combination thereof. Program code contained on a computer-readable medium may be transmitted using any suitable medium, including but not limited to wireless, wired, fiber optic cable, RF, etc., or any suitable combination thereof.

[0079] Computer program code used to perform the operations of various aspects of this disclosure can be written in any combination of one or more programming languages. Furthermore, such computer program code can be executed using a single computer system or multiple computer systems communicating with each other, such as using a private area network (PAN), local area network (LAN), wide area network (WAN), the Internet, etc. While the various features described above are with reference to flowcharts and / or block diagrams, those skilled in the art will understand that each block of the flowcharts and / or block diagrams, and combinations of blocks in the flowcharts and / or block diagrams, can be implemented by computer logic (e.g., computer program instructions, hardware logic, combinations of both, etc.). Typically, computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device. Furthermore, executing such computer program instructions using a processor produces a machine capable of performing the functions or actions specified in the blocks of the flowcharts and / or block diagrams.

[0080] The flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and / or operation of various implementations of this disclosure. In this regard, each block in a flowchart or block diagram may represent a module, code segment, or code portion, which includes one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions mentioned in the blocks may not occur in the order shown in the figures. For example, two blocks shown consecutively may actually be executed substantially simultaneously, or these blocks may sometimes be executed in reverse order, depending on the functions involved. It will also be noted that each block illustrated in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented by a system based on dedicated hardware or a combination of dedicated hardware and computer instructions that performs the specified function or action.

[0081] It should be understood that the above description is intended to be illustrative and not restrictive. Many other embodiments will become apparent from reading and understanding the above description. Although specific examples have been described in this disclosure, it should be recognized that the systems and methods of this disclosure are not limited to the examples described herein but can be implemented with modifications within the scope of the appended claims. Therefore, the specification and drawings are to be considered illustrative and not restrictive. Accordingly, the scope of this disclosure should be determined by reference to the appended claims and the full scope of their equivalents.

Claims

1. A method for protecting an induction motor from thermal overload conditions, comprising: The estimated rotor temperature rise is calculated based on the estimated rotor resistance of the induction motor. Calculate the estimated thermal state of the induction motor; Derive the correlation between the thermal state estimate and the rotor temperature rise estimate; Calculate the new temperature rise estimate based on the new rotor resistance estimate of the induction motor; Calculate a new estimate of the thermal state of the induction motor; The expected rotor temperature rise of the induction motor is calculated using the correlation and the new motor thermal state. Check whether the difference between the new estimated temperature rise and the expected rotor temperature rise is greater than the predefined rotor temperature rise threshold for the induction motor; and A correction action is performed in response to the difference between the new estimated temperature rise and the expected rotor temperature rise being greater than a predefined rotor temperature rise threshold.

2. The method according to claim 1 further includes obtaining an estimated rotor resistance value based on the real part of the complex admittance of the induction motor input and the slip value of the induction motor.

3. The method according to claim 2, further comprising determining the complex admittance of the induction motor input based on 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 the slope of the line, each of the plurality of points being defined by an ordered pair consisting of an estimated rotor temperature rise and a corresponding estimated motor thermal state.

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

6. The method of claim 1, wherein performing the correction action includes one or more of the following: cutting off the power supply to the induction motor and issuing an alarm.

7. The method of claim 1, wherein the calculation of the expected rotor temperature rise of the induction motor is performed in one of the motor overload protection devices or edge protection devices.

8. A device for protecting an induction motor from thermal overload conditions, comprising: processor; A storage unit communicatively coupled to the processor, the storage unit storing computer-readable instructions thereon for causing the processor to: The estimated temperature rise is calculated based on the estimated rotor resistance of the induction motor. Calculate the estimated thermal state of the induction motor; Calculate the expected rotor temperature rise of the induction motor; Check whether the difference between the estimated temperature rise and the expected rotor temperature rise is greater than the predefined rotor temperature rise threshold for the induction motor; and A correction action is performed in response to the difference between the estimated temperature rise and the expected rotor temperature rise being greater than a predefined rotor temperature rise threshold.

9. The apparatus according to claim 8, wherein, The computer-readable instructions also enable the processor to obtain the rotor resistance estimate based on the real part of the complex admittance of the induction motor input and the slip value of the induction motor.

10. The apparatus according to claim 9, wherein, The computer-readable instructions also cause the processor to determine the complex admittance of the induction motor input based on 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.

11. The apparatus according to claim 8, wherein, The computer-readable instructions also enable the processor to derive the correlation between the thermal state estimate of the induction motor and the rotor temperature rise estimate.

12. The apparatus according to claim 11, wherein, The computer-readable instructions also enable the processor to calculate the expected rotor temperature rise by applying the correlation to the motor thermal estimate.

13. The apparatus according to claim 8, wherein, The processor performs the correction action by executing one or more of the following: cutting off the power to the induction motor and issuing an alarm.

14. The apparatus according to claim 8, wherein the apparatus is one of the following: a motor overload protection device or an edge protection device.

15. A non-transitory computer-readable medium comprising program logic, which, when executed by operations of one or more computer processors, causes the one or more computer processors to: The estimated rotor temperature rise is calculated based on the estimated rotor resistance of the induction motor. Calculate the estimated thermal state of the induction motor; Derive the correlation between the thermal state estimate and the rotor temperature rise estimate; Calculate a new estimate of the thermal state of the induction motor; The expected rotor temperature rise of the induction motor is calculated using the correlation and the new motor thermal state. Check whether the difference between the new estimated temperature rise and the expected rotor temperature rise is greater than the predefined rotor temperature rise threshold for the induction motor; and A correction action is performed in response to the difference between the new estimated temperature rise and the expected rotor temperature rise being greater than a predefined rotor temperature rise 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 the real part of the complex admittance of the induction motor input and the slip value of the 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 admittance of the induction motor input based on the motor voltage and current in a synchronous reference frame, wherein the motor voltage is oriented on the 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 and determining the slope of the line, each of the plurality of points being defined by an ordered pair consisting of a rotor temperature rise estimate and a corresponding motor thermal state estimate.

19. The computer-readable medium of claim 18, wherein the one or more processors calculate the expected rotor temperature rise of 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 correction action by executing one or more of the following: cutting off the power to the induction motor and issuing an alarm.

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