Analysis Of Temperature Field Of Submerged Brushless Motor Based On Lumped Heat Network Method

In recent years,with the rapid development of power electronics technology and rare earth permanent magnetic materials,Brushless DC propulsion motor has been widely used and studied scientifically.The brushless motor is changed from the mechanical commutation of the original brush motor to the electronic commutation using the power transistor switch,so it has the advantages of simple structure and high reliability.In addition,Brushless DC motor has the advantages of simple control,low manufacturing cost,high power density and so on.It is widely used in underwater unmanned equipment,marine resource exploration,and other fields.The motor is a coupling and crossing nonlinear multivariable complex systems with multiple physical fields,such as electric field,magnetic field,temperature field,etc.Compared with the study of electromagnetic field of motor,the analysis of the temperature field started relatively late,and the causes of heat source in the working process of motor is more complex.It is not only a one-dimensional heat transfer process,but also involved in motor science,heat transfer,fluid mechanics and other aspects of interdisciplinary knowledge.The thermal design of motor is an important part of motor design.As the submerged brushless propulsion motor,there are strict requirements on weight and volume,because of its special working environment.On the one hand,over temperature can cause the insulation ability of armature windings decreased and the resistance increased when the power density of the motor is high,and the working heat is serious after a long continuous period.When the motor is working at high current,the skin effect is obvious,which will cause the copper loss of winding increased and the efficiency decreased,thus affecting the output of the rated power of the motor.On the other hand,the high temperature can lead to the decrease of the rotor permanent magnetic material performance,and even the irreversible demagnetization phenomenon when serve,which seriously affect the smooth output of motor torque and motor operating reliability.The non-uniformity of local temperature can also cause the thermal stress concentration in a certain part of the motor,which will affect its safety performance.Therefore,It is necessary to analyze the temperature field of brushless DC motor accurately andquantitatively.It is of great significance to the optimized design and efficiency improvement of the motor.In this paper,the lumped thermal network method and Motor-CAD software are used to calculate the loss and to analyze the simulation and optimization of temperature field of submerged brushless DC motor.And explored the existence of shell heat sinks under circumstance of natural convection,and aiming at the cooling structure of different shells,the influence on the temperature field of the motor under different flow velocities and the inhibition of temperature rise under circumstance of forced convection.The main contents of this article are as follows:(1)The working principle of brushless DC motor is introduced.The heat generationmechanism of the main heat loss(copper loss,iron loss,eddy current loss of permanentmagnet)of brushless DC motor is expounded and calculated and analyzed.Thesteady-state temperature fields at the axial flow speeds of 2kn and 3kn were investigatedrespectively.(2)The basic principle of lumped thermal network method is discussed.The simulationsoftware Motor-CAD is introduced.According to the basic theoretical knowledge of heattransfer,the temperature field analysis model of the brushless DC motor immersion isestablished,and the thermal resistance(conductive thermal resistance,convective thermalresistance,contact thermal resistance)in the model was calculated with different coolingconditions.(3)The steady-state temperature field and transient temperature rise of the brushless DCmotor under the rated conditions are numerically simulated by using the Motor-CADsoftware and lumped thermal network method and the following results are obtained:(1)When the motor housing is forced to be submerged and cooled,the flow rate on the surface of the casing greatly affects the steady-state temperature field.(2)If change the current density of the motor by changing the wire gauge,the copper consumption of the motor will change significantly,and it will have a great impact on the winding temperature rise.(3)When the cooling condition of the shell is changed from immersion forced coolingto natural convection air cooling,the heat transfer resistance of convection is obviously increased,and the radiation heat transfer effect is enhanced.On this basis,the heat dissipation fins are added on the surface of the shell.It is helpful for the heat exchange between the shell and the outside.(4)When the shell convection heat transfer cooling is changed to the spiral channel way,the inlet flow has a greater influence on the maximum steady-state temperature of the windings,and the size of the channel itself has less affect on the temperature rise of the motor.(5)According to the analysis of steady state temperature field under the rated working condition of the motor,the temperature field is distributed along radial gradient basically,all parts of the motor are in the same radius axis,and the temperature difference is very small and the temperature distribution is basically uniform.(6)According to the requirements of different working conditions of the motor,the transient temperature rise of the motor is calculated and analyzed.When the motor is immersed in water,the internal and external heat exchange of the motor will be balanced after 50 minutes,and the temperature rise of the motor winding will reach the steady state.(4)The temperature field of immersion motor was experimental studied by using PROVAthermometer and FLIR infrared thermal imager after Motor-CAD simulation.Thetemperature rise curve at the end of the motor winding and the imaging cloud image ofthe temperature field were measured.The temperature rise curve of the end part of thewinding and the distribution of the temperature field of the motor obtained by theexperiment are in good agreement with the simulation results,the error is reasonable.Thisproves the validity of using thermal network method to analyze the model.In this paper,an immersion brushless DC motor for a certain type of underwater carrier is taken as an example.The main heat loss of motor is calculated,the temperature field simulation model is simplified,and temperature field of the housing under different cooling conditions is numerically simulated by using the Motor-CAD software and lumped thermal network method.And the experiment on transient temperature rise of windings and the temperature field test of the motor are also carried out under the immersion environment.According to the results of the experiment,to compare and analyze the the previoussimulation results,the two data results are consistent.Verified the accuracy of the calculation results of this method.The thermal analysis and thermal optimization of brushless DC motor have made some explorations.