Compared with industrial frequency motors, variable frequency motors are characterized by the change in speed. When the speed is adjusted, the motor's input voltage, current, torque, power and other parameters will also change. Some netizens have raised the question of the relationship between current and speed during motor speed regulation. Today, we will make a qualitative analysis with you through typical load characteristics and specific formulas.

In order to maximize the efficiency of the variable frequency motor, whether it is constant torque operation speed regulation below the rated speed or constant power operation speed regulation above the rated speed, the current size always changes slightly near the rated current, or the current size remains basically unchanged. If the motor is overloaded, that is, it is operated at a state greater than the rated torque or exceeding the rated power, the actual current will be greater than the rated current, and the thermal load will exceed the range allowed by the design, which may cause the motor to heat up seriously or even burn out the winding. On the contrary, when the speed is reduced, the torque also becomes smaller, and when the speed is increased, the torque also increases, which means that the load is light at low speed and heavy at high speed. The current increases and decreases in a positive correlation with the change of speed, such as the load of fans and water pumps. Based on the above situation, the operating characteristic curve of the variable frequency motor is usually shown to the user in low speed constant torque operation and high speed constant power operation. For fan and water pump working conditions, the service factor must be given to ensure the safety and reliability during the actual operation process.

How can we estimate the current during the actual operation of the variable frequency motor? We can use the following formula (1) to calculate with high probability:

I=T/C _MΦ =P/UηcosΦ.............(1)

In formula (1), C _M is the motor constant, T is the output torque, η is the motor efficiency, and Φ is the magnetic flux density. When Φ is maintained at the rated load, the motor can exert its maximum efficiency. Therefore, at low speed, Φ remains unchanged, and I also remains unchanged. At high speed, P remains unchanged, and U is fixed. The efficiency η and cosΦ tend to increase but do not change too much, so the current remains basically unchanged.