To ensure the accuracy of the calculations, it is necessary to analyze and verify the electromagnetic performance of the drive motor . Here, the finite element method is used to analyze and calculate the drive motor under no-load, torque overload, high-speed field weakening, and short-circuit demagnetization conditions. In a permanent magnet motor, the interaction between the magnets and the slotted armature core alters the air gap permeability, inevitably generating cogging torque, leading to torque fluctuations, noise, and vibration, further affecting the control accuracy of the entire system. Many methods have been proposed to reduce cogging torque, such as skewed slots, skewed poles, optimized slot openings, optimized pole arcs, and optimized magnet shapes. Among these, the skewed slot method is not only technologically mature and simple to manufacture, but also produces a highly sinusoidal back electromotive force waveform. Figure 1 compares the cogging torque of the drive motor before and after skewed slotting. Before skewed slotting, the cogging torque accounts for 2% of the total rated load electromagnetic torque; after skewed slotting, the cogging torque is significantly reduced. The calculated back electromotive force of the drive motor at 1500 r/min is shown in Figure 2. Because skewed slotting makes the back electromotive force more sinusoidal, its harmonic content is significantly reduced.
Figure 1: Cogging torque diagram of drive motor
Figure 2: Calculated back electromotive force
High torque overload ratios enable electric vehicles to achieve better climbing ability and acceleration performance. However, high torque overload can easily saturate the motor core, preventing the output of peak torque even with peak current input. Since the motor's no-load back EMF is proportional to its speed, the higher the speed, the greater the back EMF. Therefore, the motor terminal voltage is also higher in the absence of field-weakening current. However, under constant DC bus voltage, the controller output voltage has an upper limit. This means that high-speed output requires increasing the d-axis current to weaken the main magnetic field, keeping the air gap synthesized back EMF essentially constant.
Irreversible demagnetization of the magnets will weaken the performance of the motor, including rated voltage and rated power, thus affecting its normal operation. If the motor continues to operate under rated or overload conditions, the demagnetizing armature magnetomotive force and temperature rise will exacerbate the demagnetization, accelerating this vicious cycle. Therefore, it is necessary to check the maximum demagnetization operating point during motor design. When a permanent magnet motor experiences a short circuit, the magnetomotive force generated by the armature reaction is almost a pure demagnetizing direct-axis magnetomotive force; therefore, magnetomagnetization analysis should focus on this situation. From the magnetic flux density distribution on the magnet surface, it can be seen that the magnets experience varying degrees of demagnetization in both cases. The demagnetization area is largest during an asymmetrical three-phase short circuit, but the maximum demagnetization area is less than 0.2%.
The entire drive motor testing system mainly consists of a DC power supply, drive motors, a brushless controller, a cooling water system, a torsion sensor, a power analyzer, and an oscilloscope. The DC power supply's primary function is to rectify the three-phase AC power from the grid into DC, which is then input to the drive motor controller for use in the testing system. During motor testing, two drive motors were used in a counter-drive configuration, with one motor acting as the electric motor and the other as the generator. Cooling water for the drive motors was supplied by a cooling water tank. The cooling water was first pumped into the drive motors by an external pump to cool them before circulating back into the cooling water tank. The controller was cooled using forced air cooling.
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