Brushless drive motor electromagnetic performance analysis
2018/12/18 18:04:23
In order to ensure the accuracy of calculation, it is necessary to analyze and check the electromagnetic performance of the driving motor. In this paper, the finite element method is used to analyze and calculate the demagnetization of the driving motor under the conditions of no-load, torque overload, high-speed magnetic weakening and short-circuit. The air gap permeability of permanent magnet motor is changed due to the interaction between magnet and slotted armature core, which inevitably results in cogging torque, torque fluctuation, noise and vibration, and further affects the control accuracy of the whole system. Many methods to reduce cogging torque have been proposed, such as inclined groove, inclined pole, optimizing slot opening, optimizing pole arc and magnet shape, etc. The oblique groove method not only has mature driving technology, simple production process and good effect, but also obtains a very sinusoidal back EMF waveform. Fig. 1 is a comparison of the cogging torque before and after the inclined slot of the driving motor. The cogging torque before the inclined slot accounts for 2% of the total rated load electromagnetic torque. After the inclined slot, the cogging torque basically weakens. At 1500r/min, the back EMF calculation results of the driving motor are shown in Fig. 2. Because the oblique groove makes the back EMF more sinusoidal, its harmonic content decreases greatly.
Figure 1: Alveolar Torque Diagram of Driving Motor
Figure 2: Back EMF calculation
High torque overload ratio can make the electric vehicle obtain better climbing ability and acceleration performance. However, it is easy to saturate the core of the motor when the motor is under high torque overload, so that the peak torque can not be output when the peak current is input. Because the no-load back-EMF of the motor is proportional to the speed, the higher the speed, the greater the back-EMF. Therefore, the higher the motor terminal voltage without weak magnetic current. However, in the case of constant DC bus voltage, the output voltage of the controller has an upper limit, which means that the output of high speed needs to depend on increasing d-axis current to weaken the main magnetic field and keep the air gap synthetic back-EMF basically unchanged.
The irreversible demagnetization of magnet steel will weaken the performance of the global motor, including rated voltage and rated power, thus affecting its normal use. If the motor still works according to the design requirements of rated or overload conditions, the demagnetization armature magnetic potential and temperature rise will make the demagnetization of magnets more serious and accelerate the vicious circle. Therefore, it is necessary to check the maximum demagnetization point for motor design. When the permanent magnet motor is short-circuit, the magnetic potential produced by armature reaction is almost a straight-axis magnetic potential of pure demagnetization. Therefore, the analysis of magnetic steel demagnetization should focus on this kind of situation. From the distribution of magnetic density on the surface of magnets, it can be seen that the magnets have different degrees of demagnetization in both cases. When asymmetrical three-phase short circuit occurs, the area of demagnetization of magnets is the largest, but the area of demagnetization of maximum magnets is less than 0.2%.
The test system of driving motor includes DC power supply, driving motor, brushless controller, cooling water system, torsion sensor, power analyzer and oscilloscope. The main function of DC power supply is to convert three-phase AC rectifier in power grid into the controller of DC input drive motor for testing system. Two driving motors are used to test the motor. One of them acts as the motor and the other acts as the generator. The cooling water of the driving motor is supplied by the cooling water tank. The cooling water is pumped into the driving motor by an external water pump to cool the motor, and then flows into the cooling water tank for circulation. Controller uses forced air cooling for cooling
