A control method of a three-phase asynchronous motor cooling system

Abstract

The present application relates to the technical field of motor control and thermal management, in particular to a control method of a three-phase asynchronous motor cooling system; comprising: obtaining ambient temperature, relative humidity, stator winding temperature, bearing area temperature, shell temperature, motor torque and rotating speed, calculating dew point temperature, motor comprehensive temperature, safety temperature difference and load rate; when the load rate meets the low load condition and the safety temperature difference meets the entering condition, entering the anti-condensation mode, controlling the independent cooling fan to run at low speed, and adjusting the direct-axis excitation current while keeping the quadrature-axis current to meet the load demand; when the safety temperature difference meets the exit condition, reducing the direct-axis excitation current to zero, restoring the fan to the normal heat dissipation speed, and exiting the anti-condensation mode; the present application improves the reliability and stability of the motor in long-term operation in high humidity and temperature difference fluctuation environment.

Description

Technical Field

[0001] This invention relates to the field of motor control and thermal management technology, specifically a control method for a three-phase asynchronous motor cooling system. Background Technology

[0002] Three-phase asynchronous motors are widely used in industrial transmission, ventilation and conveying and continuous operation equipment. In environments with high humidity and temperature differences that exceed the set fluctuation range, condensation is easily generated inside the motor due to heat dissipation and exchange with the humid and cold air in the environment, which affects the insulation performance and operational reliability. Therefore, effective anti-condensation control of motors is of great significance.

[0003] Existing cooling control methods mostly rely on fixed-speed heat dissipation or simple start-stop based on temperature, which makes it difficult to simultaneously meet the requirements of anti-condensation and motor thermal stability. This can easily lead to problems such as the airflow exceeding the set upper limit, causing a sudden drop in temperature, or the airflow falling below the set lower limit, failing to suppress moisture intrusion. Especially when the load changes and environmental parameters fluctuate, the control effect is unstable and it is difficult to meet the requirements of long-term safe operation. Summary of the Invention

[0004] The purpose of this invention is to provide a control method for a three-phase asynchronous motor cooling system, avoiding the problems of sudden temperature drops or moisture intrusion caused by improper airflow control in existing methods. It also balances anti-condensation and motor thermal stability, achieving precise control and smooth exit from the anti-condensation mode. The advantages of this invention are:

[0005] The three-phase asynchronous motor is driven by a frequency converter. The cooling system includes an independent cooling fan connected to the motor's heat dissipation channel, and includes:

[0006] The system acquires ambient temperature, relative humidity, stator winding temperature, bearing area temperature, housing temperature, motor torque, and motor speed.

[0007] The dew point temperature is determined based on the ambient temperature and relative humidity. The overall motor temperature is determined based on the stator winding temperature, bearing area temperature, and housing temperature. The safe temperature difference is the difference between the overall motor temperature and the dew point temperature. The load rate is determined based on the motor torque and motor speed.

[0008] When the load rate meets the preset low load condition and the safe temperature difference meets the preset entry condition, the anti-condensation mode is entered and the preset low-speed operation frequency is determined.

[0009] The independent cooling fan is controlled to operate at a low speed according to the preset low-speed operating frequency.

[0010] While ensuring that the quadrature-axis current meets the current load requirements, adjust the direct-axis excitation current output by the frequency converter;

[0011] When the safe temperature difference meets the exit conditions, first reduce the direct shaft excitation current to zero, then restore the independent cooling fan to the preset normal heat dissipation speed, and exit the anti-condensation mode.

[0012] As a preferred option, the dew point temperature is determined according to the dew point calculation relationship between ambient temperature and relative humidity.

[0013] Preferably, the overall motor temperature is determined by the highest value among the stator winding temperature, bearing area temperature, and housing temperature.

[0014] Preferably, the load rate is determined based on the motor torque and speed.

[0015] As a preferred option, the preset low-speed operating frequency is determined based on the net flow rate of the air inlet and outlet corresponding to different fan operating frequencies and the temperature drop value of the motor per unit time. The net flow rate corresponding to the preset low-speed operating frequency meets the preset net flow rate threshold, and the corresponding temperature drop value of the motor per unit time meets the preset cooling requirements.

[0016] Preferably, the direct-axis excitation current is adjusted according to the deviation between the overall temperature of the motor and the preset target temperature, and the total stator current does not exceed the preset current upper limit.

[0017] Preferably, the preset entry condition is that the safe temperature difference is less than or equal to the entry threshold, and the exit condition is that the safe temperature difference is greater than or equal to the exit threshold, and the exit threshold is greater than the entry threshold.

[0018] Preferably, when exiting the anti-condensation mode, the direct-axis excitation current is gradually reduced to zero at a preset rate, and then the independent cooling fan is restored to the preset normal heat dissipation speed.

[0019] Preferably, the net flow rate of the air inlet and the air outlet is the difference between the mass flow rates of the air inlet and the air outlet, and the temperature drop of the motor per unit time is the temperature drop value calculated based on the temperature difference of the motor at adjacent detection times and the detection time interval.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. This invention determines the dew point temperature by jointly judging the ambient temperature and relative humidity and dynamically determining the dew point temperature. It also uses the highest value among the stator winding temperature, bearing area temperature and housing temperature as the comprehensive temperature of the motor, which can accurately identify the condensation risk boundary and avoid misjudgment caused by fixed thresholds.

[0022] 2. By jointly characterizing torque and speed and combining it with low-load condition judgment, this invention ensures that the anti-condensation mode is triggered only when the motor is indeed under low load, low speed and has a risk of condensation, thereby improving the accuracy of control entry and operational safety. Attached Figure Description

[0023] The accompanying drawings are provided to further illustrate embodiments of the present invention and form part of the specification. They are used together with the following detailed description to explain the embodiments of the present invention, but do not constitute a limitation thereof; in the drawings:

[0024] Figure 1 This is a flowchart illustrating a control method for a three-phase asynchronous motor cooling system provided in an embodiment of this application. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0026] Please see Figure 1 A control method for a cooling system of a three-phase asynchronous motor, wherein the three-phase asynchronous motor is driven by a frequency converter, and the cooling system includes an independent cooling fan connected to the motor's heat dissipation channel; comprising:

[0027] The system acquires ambient temperature, relative humidity, stator winding temperature, bearing area temperature, housing temperature, motor torque, and motor speed.

[0028] The dew point temperature is determined based on the ambient temperature and relative humidity. The overall motor temperature is determined based on the stator winding temperature, bearing area temperature, and housing temperature. The safe temperature difference is the difference between the overall motor temperature and the dew point temperature. The load rate is determined based on the motor torque and motor speed.

[0029] When the load rate meets the preset low load condition and the safe temperature difference meets the preset entry condition, the anti-condensation mode is entered and the preset low-speed operation frequency is determined.

[0030] The independent cooling fan is controlled to operate at a low speed according to the preset low-speed operating frequency.

[0031] While ensuring that the quadrature-axis current meets the current load requirements, adjust the direct-axis excitation current output by the frequency converter;

[0032] When the safe temperature difference meets the exit conditions, first reduce the direct shaft excitation current to zero, then restore the independent cooling fan to the preset normal heat dissipation speed, and exit the anti-condensation mode.

[0033] The dew point temperature is determined according to the dew point calculation relationship between ambient temperature and relative humidity.

[0034] Specifically, the dew point calculation relationship can be determined by looking up and interpolating the temperature and humidity-dew point corresponding data table preset by the system, or by calculating the saturated water vapor pressure of the current ambient air based on the ambient temperature and combining it with the relative humidity to deduce the critical temperature when the water vapor reaches saturation.

[0035] In practical implementation, the system uses an empirical formula calculation model pre-written into the control program, where the formula for calculating saturated water vapor pressure is: Ambient temperature The saturated vapor pressure can be obtained by inputting this formula. Multiply by relative humidity The current actual water vapor pressure is obtained. Finally, the critical dew point temperature for reaching saturation was calculated based on the inverse functional relationship between actual water vapor pressure and temperature. This enables the system to establish a specific mapping logic from temperature and humidity to dew point temperature, dynamically calculate the condensation risk boundary, and avoid misjudgments caused by using a fixed temperature threshold. ;

[0036] In practice, ambient temperature, relative humidity, and all temperature measuring points are sampled within the same control cycle and their validity is verified first. When a measuring point experiences a jump exceeding the upper limit of physical change within two consecutive cycles, the upper limit of physical change is jointly calibrated by the maximum sampling change rate of the corresponding measuring point sensor and the maximum thermal inertia parameter of the corresponding part of the motor. The system uses the effective value of the previous cycle for calculation and marks this sampling as degraded data to avoid transient spikes affecting dew point determination and subsequent anti-condensation triggering.

[0037] The overall temperature of the motor is determined by the highest value among the stator winding temperature, bearing area temperature, and housing temperature.

[0038] By taking the highest value among multiple measuring points as the comprehensive temperature and using it as a feedback benchmark in calculating the safe temperature difference and subsequent heat compensation, local hot spots inside the motor can always be included in the safety constraints during the anti-condensation control process. Since the fan speed is reduced and current is actively injected for heat compensation in the anti-condensation mode, using the highest value as the comprehensive temperature can effectively avoid the risk of local overheating caused by excessive heat compensation, thus achieving a balance between anti-condensation and thermal protection.

[0039] The load factor is determined based on the motor torque and speed.

[0040] Specifically, the load rate is calculated by multiplying the ratio of the current motor torque to the rated torque by the ratio of the current motor speed to the rated speed. Since this product is a dimensionless parameter that characterizes the proportion of mechanical output power, if this value is used directly under stall conditions or heavy-load start-up conditions with low speed and high torque, there will be a risk of misjudging the physical law that the mechanical power is lower than the set power lower limit but the actual stator current is higher than the set current upper limit, and the heat generated by copper consumption is much higher than the set heat generation upper limit.

[0041] The calculation logic also incorporates a torque constraint check: the load rate obtained by multiplication is considered a valid calculation result only if the ratio of the current motor torque to the rated torque is lower than a set light load torque threshold; the light load torque threshold can be set according to the motor's rated parameters, for example, 10% to 20% of the rated torque;

[0042] The preset low load condition is that the calculated effective load rate is less than or equal to the first threshold. The first threshold is a calibration constant that characterizes the motor's no-load or load rate operation state when it is below the set light load threshold. When this condition is met, it indicates that the motor is indeed in a low torque and low speed operating condition. At this time, the heat generated by the copper and iron losses of the motor itself is indeed insufficient to resist the intrusion of cold and humid air in the environment, and the basic conditions for starting active anti-condensation control are met.

[0043] By combining torque and speed and supplementing them with torque cross constraints, the thermodynamic conditions of the motor can be quantified in real time, and it can be determined whether it belongs to a low-load scenario suitable for entering anti-condensation control, thereby avoiding the false triggering of supplementary heating control under high torque heating conditions.

[0044] In practice, the system sets a stability window for torque and speed respectively. Only when both change in the same direction within two consecutive control cycles and the fluctuation amplitude does not exceed the preset tolerance, which is the allowable fluctuation range set to take into account the sensor sampling noise, will the load rate of that cycle be used as the basis for entry condition judgment.

[0045] If any signal is abnormal, the effective load rate of the previous cycle is maintained unchanged and the decision is delayed. The above implementation method achieves accurate entry and safe exit of the anti-condensation mode by jointly determining environmental parameters, thermal state parameters and load parameters, thus taking into account both the anti-condensation effect and the safety of motor operation.

[0046] The preset low-speed operating frequency is determined based on the net airflow of the air inlet and outlet at different fan operating frequencies and the temperature drop of the motor per unit time. The net airflow corresponding to the preset low-speed operating frequency meets the preset net airflow threshold, and the corresponding temperature drop of the motor per unit time meets the preset cooling requirements.

[0047] The control model incorporates airflow state and temperature drop state as evaluation dimensions; the net flow rate of the air inlet and outlet is the difference between the mass flow rates of the air inlet and outlet, and the temperature drop value of the motor per unit time is the temperature drop value calculated based on the temperature difference of the motor at adjacent detection times and the detection time interval.

[0048] In practice, the main control unit pre-stores a mapping table containing multiple candidate fan frequencies and their corresponding fan static pressure-flow performance curves. The main control unit scans and matches multiple candidate fan frequencies, obtains the estimated net airflow in the duct at the corresponding frequency through the mapping table, and combines the temperature drop trend to select specific low-speed frequency points that can both suppress the continuous entry of cold and humid air from the outside and prevent the motor temperature from dropping too quickly.

[0049] During this process, the preset net flow threshold is set as the minimum difference between the inlet and outlet air flow required to maintain the air pressure in the duct at a slightly positive pressure state. According to the fluid dynamics continuity equation, under steady-state operation, this net flow is essentially the air flow that compensates for the leakage of air from the casing gaps to the external environment.

[0050] Because the external cold air expands in volume after being heated inside the motor in anti-condensation mode, if the volume flow rate of the air inlet and the air outlet are directly subtracted algebraically, the uneven distribution of fluid density in the space due to the change in thermodynamic state will lead to a physical calculation deviation that violates the law of conservation of mass. Therefore, the actual net flow rate should be the difference between the mass flow rate of the air inlet and the air outlet.

[0051] In practice, the system collects the absolute temperature of the air at the air inlet in real time. absolute temperature of the air at the air outlet Since the motor is controlled to be in a steady-state flow field with a slight positive pressure, the static pressure difference at the inlet and outlet is within the set tolerance range and can be regarded as constant. According to the ideal gas law, the airflow density is strictly inversely proportional to its absolute temperature.

[0052] The system does not require the introduction of complex density sensors; it only needs to combine the volumetric flow rate estimated from the mapping table with the standard atmospheric pressure P and the gas constant. Through formula These are converted into mass flow rates at the air inlet and outlet, respectively. Given the absolute temperature of the air at the inlet or outlet, calculate the accurate net mass flow rate that satisfies fluid dynamic continuity and correct physical dimensions. In the formula, The mass flow rate at the air inlet or outlet. The corresponding volumetric flow rate is estimated from the mapping table.

[0053] The system ensures that the aerodynamic pressure generated by the fan overcomes the flow resistance of the duct structure and maintains the net mass flow rate to meet the preset threshold. It accurately establishes a slightly positive pressure inside that is slightly higher than the atmospheric pressure of the external environment, thereby strictly preventing the backflow and infiltration of external cold and humid gas in the physical flow field.

[0054] The preset cooling requirement is set so that the absolute value of the temperature drop of the motor per unit time is less than the safe cooling rate threshold. The safe cooling rate threshold is the maximum allowable cooling rate set according to the thermal stress limit of the motor stator winding and insulation material, in order to prevent the insulation structure from being damaged by a sudden drop in temperature.

[0055] The main control unit has a built-in motor equivalent thermal impedance prediction model. For each candidate frequency, the system calculates the corresponding net mass flow rate. Specific heat capacity of air Temperature difference between inlet and outlet air and the equivalent heat capacity of the motor The temperature drop per unit time of the motor at this candidate frequency can be estimated using the following formula: ;

[0056] During the screening process, the system first removes fan frequency candidates that cause the absolute value of the temperature drop to exceed the safe cooling rate threshold. Among the remaining candidates, the system selects the operating frequency that makes the net flow rate greater than or equal to and closest to the preset net flow rate threshold, and uses it as the preset low-speed operating frequency. This frequency point puts the fan in a low-speed operating state, reduces the intensity of air exchange inside and outside the air duct, and creates a stable boundary for maintaining the temperature through the motor's own losses.

[0057] In practice, frequency filtering is performed in the following order: first, eliminate, then sort, and then lock. First, eliminate all candidate frequency points that do not meet the temperature drop constraint. Then, sort the remaining frequency points in ascending order of net flow rate and select the frequency point that is closest to the threshold and still meets the threshold. If all candidate frequency points do not meet the net flow rate threshold, then fall back to the previous stable frequency point and maintain the minimum safe fan speed to avoid frequent shaking.

[0058] Compared with traditional fan full-speed cooling or stop control, this embodiment optimizes airflow and temperature drop states together and eliminates fluid dynamics calculation errors based on temperature density compensation. This ensures that the independent cooling fan does not excessively remove heat or completely lose its airflow constraint function in anti-condensation mode, thereby reducing the risk of condensation and improving control stability.

[0059] The direct-axis excitation current is adjusted according to the deviation between the overall temperature of the motor and the preset target temperature, and the total stator current does not exceed the preset upper limit of current.

[0060] The preset target temperature can be obtained by superimposing the preset maintenance safety temperature difference on the dew point temperature. The preset maintenance safety temperature difference is a temperature margin set to ensure that the actual temperature at any location inside the motor is always safely higher than the dew point temperature, and is used as the temperature reference for motor heat compensation control.

[0061] While ensuring that the quadrature axis current meets the current load requirements, the stator copper loss and iron loss are changed by adjusting the direct axis excitation current, so as to replenish the heat inside the motor on demand. At the same time, the motor is prevented from entering the overcurrent state by the current upper limit constraint.

[0062] Specifically, the direct-axis excitation current is adjusted using proportional-integral-derivative closed-loop control, with the deviation used as the controller input to calculate the target adjustment amount. In addition, the preset current upper limit is dynamically calculated based on the inverter's maximum allowable output current and the current quadrature-axis current. That is, the preset current upper limit is equal to the square root of the square of the inverter's maximum allowable output current minus the square of the current quadrature-axis current, thereby maximizing the use of the remaining current margin for heat compensation within the safe capacity range.

[0063] In practice, the system reads the direct-axis current feedback value and the quadrature-axis current feedback value in each control cycle, and performs three-stage processing on the direct-axis target value: limiting, integral anti-saturation, and output holding. When the temperature deviation is positive and has not reached the current upper limit, the output of the proportional-integral-derivative controller increases the direct-axis current.

[0064] When the temperature deviation turns negative, the direct-axis current target is immediately compressed and gradually reduced; when the calculated upper limit of the current is less than the current quadrature-axis current requirement, the direct-axis current target is directly set to zero, and only the quadrature-axis current is retained to maintain the load torque, so as to avoid the conflict between the heat compensation command and the load command.

[0065] The preset entry condition is that the safe temperature difference is less than or equal to the entry threshold, and the exit condition is that the safe temperature difference is greater than or equal to the exit threshold, and the exit threshold is greater than the entry threshold.

[0066] By setting a hysteresis range between the entry and exit thresholds, the system can avoid frequently switching between anti-condensation states near the critical temperature difference, thus improving control stability.

[0067] When exiting the anti-condensation mode, the direct-axis excitation current gradually decreases to zero at a preset rate, and then the independent cooling fan is restored to the preset normal heat dissipation speed.

[0068] The preset rate is adaptively set based on the difference between the motor's overall temperature at the initial moment of exit and the current dew point temperature. The specific judgment logic is as follows: when the difference is greater than or equal to the first set temperature difference, the rate decreases according to the preset full base descent rate.

[0069] The first set temperature difference is greater than the second set temperature difference; when the difference is between the second set temperature difference and the first set temperature difference, it is reduced by multiplying the total base descent rate by a linear interpolation ratio coefficient between 0 and 1. The linear interpolation ratio coefficient is specifically defined as the ratio of the current difference minus the second set temperature difference to the first set temperature difference minus the second set temperature difference.

[0070] When the temperature difference is less than or equal to the second set temperature difference, the temperature will decrease at a preset minimum safe rate, which is defined as the total base descent rate. ,Right now This establishes a clear segmented mapping relationship, achieving the control effect that the smaller the difference, the lower the preset rate.

[0071] Specifically, to address the severe time scale asymmetry between the millisecond-level instruction execution cycle of the electrical control link and the minute- to hour-level physical hysteresis of the thermodynamic response of the motor stator core and housing, the preset full-base descent rate is strictly defined as the initial stator heat generation power corresponding to the exit initial moment. Divide by the system-calibrated motor thermal time constant ;

[0072] Through this definition, the system establishes a direct correspondence between the rate of decrease in heat generation power and the physical heat dissipation characteristics of the motor, thus solving the problem of transient cold spots caused by the disconnect between the sudden drop in control commands and actual heat conduction.

[0073] Since the heat generation power of the motor under the Ohm Joule heating effect is proportional to the square of the direct-axis excitation current, if the direct-axis excitation current is reduced linearly at a preset rate during the withdrawal phase, the heat generation power will drop sharply in a non-linear quadratic parabola at the initial withdrawal stage, which violates the thermodynamic requirements of smooth withdrawal.

[0074] Therefore, the underlying timing adjustment logic of the exit command in the control system is configured as follows: the system adjusts the timing according to the determined current preset descent rate. Construct a linear decreasing trajectory for the target heat production power, that is, on the... Each control cycle, combined with the control cycle duration Calculate the target power: ;

[0075] Within each timing cycle of the control link, the system adjusts the current overall motor temperature. Real-time compensation and calculation of the corresponding stator equivalent resistance By using this target power value Perform the following calculations to solve the problem in real time and output the corresponding direct-axis excitation current control quantity: ;

[0076] This exit mechanism causes the direct-axis excitation current to decrease substantially along a smooth convex curve trajectory that opens to the left, thereby truly ensuring that the physical decreasing gradient of the actual heat generation power is linearly reduced and precisely matched to the thermal time constant of the motor structure.

[0077] After the target value of the direct-axis current drops to zero, the system needs to confirm for two consecutive control cycles that the feedback value is within the zero current tolerance. The zero current tolerance is the detection noise dead zone range of the inverter current sensor before the fan is allowed to switch to the preset normal cooling speed to prevent secondary condensation caused by immediately increasing the cooling before the supplementary heating has completely ended.

[0078] The aforementioned exit sequence and nonlinear current mapping algorithm ensure a smooth termination of the heat replenishment process, avoiding secondary condensation caused by a sudden drop in heat. At the same time, it enables a smooth switch between the motor's normal heat dissipation and anti-condensation states after the fan returns to normal speed. The above implementation method ensures transient thermal stability and operational safety during the anti-condensation process through coordinated control of current heat replenishment, threshold hysteresis, and timing exit matching physical thermal inertia.

[0079] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A control method for a cooling system of a three-phase asynchronous motor, wherein the three-phase asynchronous motor is driven by a frequency converter, and the cooling system includes an independent cooling fan connected to the motor's heat dissipation channel, characterized in that, include: The ambient temperature, relative humidity, stator winding temperature, bearing area temperature, housing temperature, motor torque, and motor speed are obtained. The dew point temperature is determined based on the ambient temperature and relative humidity. The overall motor temperature is determined based on the stator winding temperature, bearing area temperature, and housing temperature. The safe temperature difference is the difference between the overall motor temperature and the dew point temperature. The load rate is determined based on the motor torque and motor speed. When the load rate meets the preset low load condition and the safe temperature difference meets the preset entry condition, the anti-condensation mode is entered, and the preset low-speed operation frequency is determined. The independent cooling fan is controlled to operate at a low speed according to the preset low-speed operating frequency. While ensuring that the quadrature-axis current meets the current load requirements, adjust the direct-axis excitation current output by the frequency converter; When the safe temperature difference meets the exit condition, the direct-axis excitation current is first reduced to zero, and then the independent cooling fan is restored to the preset normal heat dissipation speed to exit the anti-condensation mode.

2. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The dew point temperature is determined according to the dew point calculation relationship between ambient temperature and relative humidity.

3. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The overall temperature of the motor is determined by the highest value among the stator winding temperature, bearing area temperature, and housing temperature.

4. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The load rate is determined based on the motor torque and speed.

5. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The preset low-speed operating frequency is determined based on the net flow rate of the air inlet and outlet corresponding to different fan operating frequencies and the temperature drop value of the motor per unit time. The net flow rate corresponding to the preset low-speed operating frequency meets the preset net flow rate threshold, and the corresponding temperature drop value of the motor per unit time meets the preset cooling requirements.

6. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The direct-axis excitation current is adjusted according to the deviation between the overall temperature of the motor and the preset target temperature, and the total stator current is not greater than the preset upper limit value of the current.

7. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, The preset entry condition is that the safe temperature difference is less than or equal to the entry threshold, and the exit condition is that the safe temperature difference is greater than or equal to the exit threshold, and the exit threshold is greater than the entry threshold.

8. The control method for a three-phase asynchronous motor cooling system according to claim 1, characterized in that, When exiting the anti-condensation mode, the direct-axis excitation current gradually decreases to zero at a preset rate, and then the independent cooling fan is restored to the preset normal heat dissipation speed.

9. The control method for a three-phase asynchronous motor cooling system according to claim 5, characterized in that, The net flow rate of the air inlet and outlet is the difference between the mass flow rates of the air inlet and outlet, and the temperature drop of the motor per unit time is the temperature drop calculated based on the temperature difference of the motor at adjacent detection times and the detection time interval.

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