The type and size of the rotor slot shape have a great impact on the performance of the motor, and sometimes directly determine the application characteristics of the motor. From a geometric perspective, the tooth width and yoke height of the rotor slot must match to ensure that the saturation level of each section of the magnetic circuit is basically the same, while taking into account the processing technology and the mechanical strength of the core and the stiffness requirements of each part. Specific actual parameter settings:
1) The shape or type of the slot is related to the application characteristics of the motor, such as the change ratio of the width of the rotor slot along the slot height direction and the combination of the slot height will directly affect the overall performance level of the motor; 2) The size of the slot depends on the conductor current, and the slot size parameters ensure that the magnetic flux density of each part of the magnetic circuit is within a reasonable range.
Taking an asynchronous motor as an example, the effective area of the rotor slot is large and the current density is small, which means the rotor resistance is small. The motor has high efficiency and low heating level during stable operation, but the starting torque is small; the rotor slot width-to-height ratio is small or the selected cross-section has a sudden change. When using convex or knife-shaped grooves, the skin effect can be used to maximize the rotor resistance during starting and increase the starting torque, while ensuring that the rotor resistance is small enough and the efficiency is high during stable operation. In fact, the reason why the groove shapes of motor rotors under different application conditions are very different is that based on the above theory, the application characteristics tend to be optimal. Comparative analysis of two extreme design solutions The two more extreme design solutions can clearly reflect the relationship between the rotor slot shape and the overall performance of the motor.
First, the double squirrel cage motor generally has a small cross-section in the upper cage and a large cross-section in the lower cage. When starting, the skin effect is significant, and the current is mainly guided by the upper cage. The leakage flux of the lower cage turns is very large, and the current flowing is very small, resulting in a very large rotor resistance, so the starting torque is also very high; during stable operation , because the frequency of the rotor current is very small, the skin effect is negligible, and the double cages share the current carrying role, so the rotor resistance is small, the loss and heat generation are small, and the motor efficiency is improved. Although the double-cage structure can make up for the shortcomings in operating performance to a certain extent, the efficiency and power factor of this type of motor are still relatively low. Except for heavy-duty starting equipment such as mine boring machines, this design scheme is generally rarely used.
Second, single squirrel cage pear-shaped slot rotor. Among all types of rotor slots, the cage pear-shaped slot rotor has the best operating characteristics, but the worst starting performance. However, due to the development of power electronics technology, variable frequency motors powered by inverters are becoming more and more common, because the starting performance of single squirrel cage pear-shaped slot rotor motors can be compensated by inverter soft start, which can meet most application conditions. In summary, the overall performance of the motor is closely related to the rotor slot shape, and how to choose the rotor slot shape is often adjusted appropriately depending on the actual application goals.
