The shape and size of the rotor slots have a direct impact on the rotor resistance and leakage flux, which in turn will affect the motor’s efficiency, power factor, maximum torque, starting torque and other performance indicators. The performance that is affected is of great importance to motor products.
In actual operation, it is often necessary to give up the demand for other properties for a certain performance. The old saying “you can’t have your cake and eat it too” is really appropriate here. Of course, some revolutionary technological breakthroughs in new materials and new processes will temporarily break this rule. For example, in the early stage of the application of the high-voltage motor insulation system with “mica tape with less glue powder” as the main material combined with the new process technology of “vacuum pressure immersion coating”, it once achieved the effect of “having your cake and eat it too” in terms of reducing insulation thickness and improving voltage and corona resistance. However, it still cannot get rid of the constraints of the rules and always has to face difficult-to-handle contradictions or embarrassments.
1 Performance balance between starting performance and overload capacity
In order to improve the motor overload capacity, the maximum torque needs to be increased, so the rotor leakage reactance needs to be reduced; and in order to meet the small starting current and large starting torque during the starting process, the rotor skin effect needs to be increased as much as possible, but the rotor slot leakage magnetic flux and leakage reactance must inevitably be increased.
2 Balance between efficiency and starting performance
We know that increasing the rotor resistance can improve the motor starting performance, such as reducing the rotor slot and using a double cage rotor, but due to the increase in rotor resistance and leakage current, the stator and rotor copper loss will increase significantly, resulting in reduced efficiency.
3 Checks and balances between power factor and starting performance
In order to improve the starting performance of the motor, we use the skin effect, such as using deep narrow grooves, convex grooves, knife-shaped grooves, deep grooves or double squirrel cage grooves to increase the rotor resistance during starting, but the most direct impact is to increase The rotor slot leakage is reduced, the rotor leakage inductance is increased, and the rotor’s reactive current is increased, which in most cases will directly lead to a reduction in the power factor.
4 Efficiency and power factor performance checks and balances
If the rotor slot area increases and the resistance decreases, the rotor copper loss will decrease and the efficiency will naturally increase; however, due to the decrease in the magnetic permeability area of the rotor yoke, the magnetic resistance will increase and the magnetic flux density will increase, causing the iron loss to increase and the power factor to increase. decline. Many motors with efficiency as the optimization goal will always have this phenomenon: the efficiency improvement is indeed significant, but the rated current is large and the power factor is low. Customers complain that high-efficiency motors are not as good as ordinary motors.
There are many issues of gains and losses in motor design. This article only deals with the external characteristics. To balance these performance relationships, we need to explore the internal characteristics in depth and skillfully apply the iterative thinking mode of gains and losses to resolve the so-called contradictions or embarrassment.
Post time: Aug-12-2024
