Control method for thermal overload protection of asynchronous motor

Abstract

The present disclosure relates to a control method for thermal overload protection of an asynchronous motor, wherein the asynchronous motor comprises a rotor and a stator, the method comprising: determining a current state of the asynchronous motor; determining a thermal level of the rotor according to a first formula based on the asynchronous motor being in a start-up state; determining the thermal level of the rotor according to a second formula different from the first formula based on the asynchronous motor being in a running state; determining the thermal level of the rotor according to a third formula different from the first formula and different from the second formula based on the asynchronous motor being in a shutdown state; comparing the thermal level of the rotor with a first predetermined threshold and a second predetermined threshold greater than the first predetermined threshold, issuing an overheat warning if the thermal level of the rotor is greater than the first predetermined threshold and less than the second predetermined threshold; shutting down the asynchronous motor if the thermal level of the rotor is greater than the second predetermined threshold.

Description

Technical Field

[0001] This disclosure relates to a control method for thermal overload protection of asynchronous motors. Background Technology

[0002] Traditional motor thermal overload protection does not provide a separate rotor thermal model. Alternatively, some motor thermal overload protection systems, while providing a rotor thermal model, only consider the rotor thermal model during motor startup and do not account for heat exchange between the rotor and other components of the motor. In other words, current rotor thermal models treat the rotor as adiabatic.

[0003] Without accurate rotor thermal level calculations, rotor thermal overload protection will be inaccurate. The goal is to propose a new, more accurate rotor thermal model to provide better thermal overload protection for asynchronous motors. Summary of the Invention

[0004] In response to the problems and needs mentioned above, this disclosure proposes a novel control method for thermal overload protection of asynchronous motors. This method solves the aforementioned problems and brings other technical benefits by adopting the following technical features.

[0005] On one hand, this disclosure proposes a control method for thermal overload protection of an asynchronous motor, wherein the asynchronous motor includes a rotor and a stator, and the method includes: determining the current state of the asynchronous motor; determining the thermal level of the rotor according to a first formula based on the asynchronous motor being in a starting state; determining the thermal level of the rotor according to a second formula different from the first formula based on the asynchronous motor being in a running state; determining the thermal level of the rotor according to a third formula different from both the first and second formulas based on the asynchronous motor being in a stopped state; comparing the thermal level of the rotor with a first predetermined threshold and a second predetermined threshold greater than the first predetermined threshold; if the thermal level of the rotor is greater than the first predetermined threshold and less than the second predetermined threshold, issuing an overheat alarm; if the thermal level of the rotor is greater than the second predetermined threshold, stopping the asynchronous motor.

[0006] According to the preferred embodiment, in the first formula, the thermal level of the rotor is determined based on the first rotor heat generation term; in the second formula, the thermal level of the rotor is determined based on the second rotor heat generation term and the stator thermal balance term; and in the third formula, the thermal level of the rotor is determined based on the stator thermal balance term.

[0007] According to the preferred scheme, the first formula is:

[0008] H rotor (t)=H R1 (t)+H rotor (t-Δt)

[0009] The second formula is:

[0010]

[0011] The third formula is:

[0012]

[0013] Among them, H rotor (t) and H rotor (t-Δt) represent the rotor's thermal levels at time t and (t-Δt), respectively; H R1 (t) represents the heat generated by the first rotor; H R2 (t) represents the heat generated by the second rotor; H S (t) represents the stator thermal balance heat term; Δt is the time interval for thermal calculation, τ rotor It is the rotor heating time constant during motor operation.

[0014] According to the preferred scheme, the rotor heating time constant τ rotor Calculated based on the following formula:

[0015] τ rotor =R thermal ·C thermal

[0016] Where R thermal It is the equivalent thermal resistance of the rotor, C thermal It is the equivalent thermal capacitance of the rotor.

[0017] According to the preferred scheme, both the first rotor heat generation term and the second rotor heat generation term are determined based on the rotor's equivalent thermocurrent.

[0018] According to the preferred embodiment, the heat generation of the first rotor is obtained based on the following formula:

[0019]

[0020] Among them, I eq.rotor (t) is the equivalent thermal current of the rotor at time t; I LR It is the stator current when the rotor is stalled; R N It is the rotor resistance at rated speed; R LR It is the rotor resistance under stall conditions; T cold It is the allowable rotor stall time under cold conditions.

[0021] According to the preferred embodiment, the heat generation of the second rotor is obtained based on the following formula:

[0022]

[0023] Among them, I eq.rotor(t) is the equivalent thermal current of the rotor at time t; I LR It is the stator current under rotor stall conditions; R N It is the rotor resistance at rated speed; R LR It is the rotor resistance when the rotor is stalled.

[0024] According to the preferred scheme, the stator thermal balance heat term in the second and third formulas is determined based on the following formula:

[0025]

[0026] Where α is the rotor thermal level when the motor stabilizes at its rated operating temperature; k is the motor overload coefficient; H stator (t) represents the stator thermal level at time t.

[0027] According to the preferred scheme, the equivalent thermal current I of the rotor eq.rotor (t) is determined based on the following formula:

[0028]

[0029] Among them, R pos (t) The positive sequence rotor resistance at time t; R neg I1(t) is the negative sequence rotor resistance at time t; I2(t) is the positive sequence current in the stator at time t; and I2(t) is the negative sequence current in the stator at time t.

[0030] According to the preferred scheme, the positive sequence rotor resistance R pos (t) and negative sequence rotor resistance R neg (t) is obtained based on the following formula:

[0031]

[0032] Where s(t) is the real-time slip of the asynchronous motor at time t.

[0033] The preferred embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings so that the features and advantages of the present disclosure can be readily understood. Attached Figure Description

[0034] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings of the embodiments of this disclosure will be briefly described below. The drawings are merely illustrative of some embodiments of this disclosure and are not intended to limit all embodiments of this disclosure to them.

[0035] Figure 1 This is a flowchart of the control method for thermal overload protection of asynchronous motors proposed in this disclosure. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this disclosure clearer, the technical solutions of this disclosure will be clearly and completely described below in conjunction with specific embodiments. The same reference numerals in the accompanying drawings represent the same components. It should be noted that the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0037] Compared to the embodiments shown in the accompanying drawings or described in the text, feasible embodiments within the scope of this disclosure may have fewer steps, different steps, steps in a different order, etc. Furthermore, two or more steps described below may be implemented in a single step, or a single step shown or described may be implemented as multiple separate steps.

[0038] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure pertains. The terms “first,” “second,” and similar terms used in this patent application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, “an” or “a” and similar terms do not necessarily indicate a quantity limitation. Terms such as “comprising” or “including” mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0039] See Figure 1 This diagram illustrates the basic flowchart of the main steps of the motor thermal overload protection control method proposed in this disclosure. This method is particularly applicable to asynchronous motors that include both a rotor and a stator.

[0040] The thermal overload protection and control method proposed in this disclosure mainly includes the following steps.

[0041] First, determine the current state of the asynchronous motor. Here, the current state of the asynchronous motor is divided into three types: starting state, running state, and stopping state. Among them, in this field, the starting state can also be referred to as the rotor-locked state.

[0042] After determining the current state of the asynchronous motor, different thermal models (different formulas) are selected based on the different states to determine the rotor's thermal level.

[0043] Specifically, based on the determination that the asynchronous motor is in a starting state, the thermal level of the rotor is determined according to the first formula. Based on the determination that the asynchronous motor is in a running state, the thermal level of the rotor is determined according to the second formula, which is different from the first formula. Based on the determination that the asynchronous motor is in a stopped state, the thermal level of the rotor is determined according to the third formula, which is different from both the first and second formulas.

[0044] After determining the rotor's heat level, it is compared with a predetermined threshold to determine if overheating is occurring and whether intervention is necessary. Specifically, the rotor's heat level is compared with a first predetermined threshold and a second predetermined threshold that is greater than the first predetermined threshold. If the rotor's heat level is greater than the first predetermined threshold but less than the second predetermined threshold, an overheat alarm is issued. If the rotor's heat level is greater than the second predetermined threshold, the asynchronous motor is stopped. If the heat level is less than the first predetermined threshold, the motor is operating normally and no overheat alarm or shutdown operation is required.

[0045] As can be seen, this method monitors the heat at each stage of motor operation, thus providing full protection for the motor and improving safety.

[0046] Preferably, in the first formula, the rotor's thermal level is determined based on the rotor heating element. Hereinafter, the rotor heating element in the first formula is referred to as the first rotor heating element, to distinguish it from the rotor heating element in the second formula (referred to as the second rotor heating element). Specifically, the first formula can be as follows.

[0047] H rotor (t)=H R1 (t)+H rotor (t-Δt)

[0048] Among them, H rotor (t) and H rotor (t-Δt) represent the rotor's thermal levels at time t and (t-Δt), respectively; H R1 (t) represents the heat generated by the first rotor.

[0049] Preferably, in the second formula, the thermal level of the rotor is determined based on the second rotor heat generation term and the stator thermal balance heat generation term. Specifically, the second formula is as follows.

[0050]

[0051] Among them, H R2 (t) represents the heat generated by the second rotor, H S (t) represents the stator thermal balance heat term.

[0052] Preferably, in the third formula, the rotor's thermal level is determined based on the stator thermal balance heat term. Specifically, the third formula is as follows.

[0053]

[0054] Where, τ rotor This is the rotor heating time constant during motor operation. This rotor heating time constant can be obtained based on the motor model. More preferably, this rotor heating time constant is calculated based on the following formula: τ rotor =R thermal ·C thermal , where R thermal It is the equivalent thermal resistance of the rotor, C thermal This is the equivalent thermal capacitance of the rotor. The equivalent thermal capacitance can be based on the rotor resistance R at rated speed. N The rotor resistance R is the rotor resistance under stall conditions. LR To calculate, that is:

[0055]

[0056] The equivalent thermal resistance R of the rotor thermal Rotor current I based on stalled rotor LR The allowable rotor stall time T under cold conditions cold And the allowable rotor stall time T under hot conditions. hot Calculation, that is:

[0057] R thermal =I LR 2 ·(T cold -T hot )

[0058] It can be seen that this method uses different rotor thermal models for the motor at different stages. For the starting state, the rotor thermal level is determined based on the first rotor heat generation term. For the running state, the rotor thermal level is determined based on the second rotor heat generation term and the stator thermal balance term. For the shutdown state, the rotor thermal level is determined based on the stator thermal balance term, so that each stage has more accurate thermal model protection.

[0059] In particular, this disclosure incorporates the stator thermal balance heat term into the model, making the established thermal model and the implemented thermal overload protection control method more accurate, and greatly reducing the risk of accidental tripping and overheating damage.

[0060] According to this disclosure, the first rotor heat generation term and the second rotor heat generation term in the first formula and the second formula are both determined based on the rotor's equivalent thermocurrent.

[0061] Specifically, the heat generation of the first rotor can be obtained based on the following formula.

[0062]

[0063] Among them, I eq.rotor (t) is the equivalent thermocurrent of the rotor at time t, I LR It is the stator current R when the rotor is locked. N R is the rotor resistance at rated speed. LR It is the rotor resistance T under the rotor stall condition. cold It is the allowable rotor stall time under cold conditions.

[0064] Specifically, the heat generation of the second rotor can be obtained based on the following formula.

[0065]

[0066] Among them, I eq.rotor (t) is the equivalent thermocurrent of the rotor at time t, I LR It is the stator current R when the rotor is locked. N R is the rotor resistance at rated speed. LR It is the rotor resistance when the rotor is stalled.

[0067] For the stator heat balance term in the second and third formulas, it is preferably determined based on the following formula.

[0068]

[0069] Where α is the rotor thermal level when the motor is stable at its rated operating temperature, k is the motor overload coefficient, and H... stator (t) represents the stator thermal level at time t.

[0070] Substituting the first and second rotor heat generation terms and the stator heat balance term into the first, second, and third formulas, we obtain the following more specific formulas.

[0071] The first formula is:

[0072]

[0073] The second formula is:

[0074]

[0075] The third formula is:

[0076]

[0077] According to the preferred embodiment of this disclosure, in the above algorithm, the equivalent thermal current I of the rotor eq.rotor (t) is determined based on the following formula:

[0078]

[0079] Among them, R pos (t) The positive sequence rotor resistance at time t, R neg I1(t) is the negative sequence rotor resistance at time t, I2(t) is the positive sequence current in the stator at time t, and I2(t) is the negative sequence current in the stator at time t.

[0080] According to a further preferred embodiment of this disclosure, the positive sequence rotor resistance R pos (t) and negative sequence rotor resistance R neg (t) is obtained based on the following formula:

[0081]

[0082] Where s(t) is the real-time slip of the asynchronous motor at time t. When the motor speed can be measured, the slip s(t) can be estimated based on the following formula.

[0083]

[0084] Where Ω(t) is the motor speed at time t, Ω s It is the synchronization speed. For example, it can be 3000 / N for a 50Hz system and 3600 / N for a 60Hz system, where N = 1, 2, ...

[0085] The control method for thermal overload protection of asynchronous motors proposed in this disclosure has been described above. This method establishes targeted rotor thermal models for each stage of motor operation, thereby monitoring the heat throughout the entire operation of the motor and providing more accurate thermal model protection for each stage.

[0086] The exemplary implementation of the solution proposed in this disclosure has been described in detail above with reference to preferred embodiments. However, those skilled in the art will understand that various modifications and alterations can be made to the above specific embodiments without departing from the spirit of this disclosure, and various combinations can be made to the various technical features and structures proposed in this disclosure without exceeding the protection scope of this disclosure, which is determined by the appended claims.

Claims

1. A control method for thermal overload protection of an asynchronous motor, wherein, The asynchronous motor includes a rotor and a stator, and the method includes: Determine the current state of the asynchronous motor; Based on the asynchronous motor being in the starting state, the thermal level of the rotor is determined according to the first formula; Based on the asynchronous motor being in operation, the thermal level of the rotor is determined according to a second formula, which is different from the first formula. Based on the asynchronous motor being in a stopped state, the thermal level of the rotor is determined according to a third formula that is different from both the first and second formulas; The rotor's heat level is compared with a first predetermined threshold and a second predetermined threshold that is greater than the first predetermined threshold. If the rotor's heat level is greater than the first predetermined threshold but less than the second predetermined threshold, an overheat alarm is issued. If the rotor's heat level is greater than the second predetermined threshold, the asynchronous motor is stopped. in: In the first formula, the thermal level of the rotor is determined based on the first rotor heat generation term; In the second formula, the thermal level of the rotor is determined based on the second rotor heat generation term and the stator thermal balance heat generation term; In the third formula, the rotor's thermal level is determined based on the stator thermal balance heat term.

2. The control method as described in claim 1, wherein, The first formula is: The second formula is: The third formula is: in: and At time t and respectively The rotor's thermal level at any given time; This refers to the heat generated by the first rotor. This refers to the heat generated by the second rotor. This is the heat term for the stator thermal balance; The time interval for thermal calculations; It is the rotor heating time constant during motor operation.

3. The control method as described in claim 2, wherein, Rotor heating time constant Calculated based on the following formula: Where Rthermal is the equivalent thermal resistance of the rotor. C thermal It is the equivalent thermal capacitance of the rotor.

4. The control method as described in claim 2, wherein, The first rotor heat generation term and the second rotor heat generation term are both determined based on the rotor's equivalent thermocurrent.

5. The control method as described in claim 4, wherein, The heat generation of the first rotor is obtained based on the following formula: in: It is the equivalent thermocurrent of the rotor at time t; It is the stator current when the rotor is locked. It is the rotor resistance at rated speed; It is the rotor resistance when the rotor is stalled; It is the allowable rotor stall time under cold conditions.

6. The control method as described in claim 4, wherein, The heat generation of the second rotor is obtained based on the following formula: in: It is the equivalent thermocurrent of the rotor at time t; It is the stator current when the rotor is locked. It is the rotor resistance at rated speed; It is the rotor resistance when the rotor is stalled.

7. The control method as described in claim 2, wherein, The stator heat balance term in the second and third formulas is determined based on the following formula: in: It is the rotor thermal level when the motor is stable at its rated operating temperature; It is the motor overload coefficient; Let be the stator thermal level at time t.

8. The control method as described in claim 5 or 6, wherein, Equivalent thermocurrent of the rotor Determined based on the following formula: in: The positive sequence rotor resistance at time t; It is the negative sequence rotor resistance at time t; It is the positive sequence current in the stator at time t; It is the negative sequence current in the stator at time t.

9. The control method as described in claim 8, wherein, Positive sequence rotor resistance and negative sequence rotor resistance Obtained based on the following formula: in: It is the real-time slip rate of the asynchronous motor at time t.

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