What are the differences in rated current and torque between motors with the same power?

It should be noted that the speed of electric motors with the same power may vary significantly depending on the application conditions or supporting equipment. By way of illustration, the 2P and 8P motors in the industrial electric ac motor have a synchronous speed of 3000 rpm for the 2-pole motor, while the synchronous speed of the 8P motor is only 750 rpm. The objective of today’s discussion is to analyse the size relationship of the relevant parameters of three-phase asynchronous motors with the same power but different pole numbers. The rated voltage and rated frequency are the power supply conditions for the normal operation of the motor. Based on these two conditions, we analyse and discuss the size relationship of other parameters.

The size relationship of the rated current: We analyse the relationship between input power and rated voltage, rated current, efficiency and power factor based on the formula P1=√3UIcosϕ.

It should be noted that the efficiency and power factor of low-speed motors are relatively small, which results in a larger corresponding rated current.

The relationship between the rated torque and the size of the component in question is as follows: In accordance with the correlation between the rated torque and rated power of the motor, T=9.55P/n, it can be observed that as the speed decreases under identical power motor conditions, the corresponding torque increases in magnitude. Torque is defined as the ability of the motor to pull the load. Low-speed motors are distinguished by their ability to deliver high torque at low speeds. Similarly, high-speed motors are distinguished by low torque and high speeds.

From this data, we can ascertain that for motors with the same power but different pole numbers, the rated current of the low-speed motor is slightly larger than that of the high-speed motor. However, there is a notable discrepancy in the rated torque, which is inversely proportional to the speed. This also provides insight into why the shaft diameter of the low-speed motor is relatively large, and the corresponding bearing specifications are also larger.