When a matching inverter is selected for a variable frequency motor, the following two verification tests should be carried out on the basis of a detailed analysis of the load characteristics under the actual working conditions of the motor: 1) the electromagnetic compatibility of the inverter itself; 2) no-load, load, adjustment performance characteristics such as vibration and noise during speed.
1 Constant torque load
When the frequency conversion speed regulation is performed under the constant torque load, the resistance torque on the motor output shaft will remain unchanged during the process of increasing or decreasing the speed, but the maximum value of the increase speed is not allowed to exceed the rated speed, otherwise the motor will be burned due to overload operation. In the process of speed increase, there is not only resistance torque, but also inertia torque to prevent the speed change, so that the torque on the motor shaft exceeds the rated torque of the motor, and various electrical faults may be triggered due to shaft breakage or overheating of the windings. The so-called constant torque speed regulation actually refers to the constant torque on the output shaft of the motor when the speed is adjusted to any speed for stable operation, and it has the ability to drive a constant torque load. In the process of motor acceleration or deceleration, in order to shorten the transition process time, within the allowable range of the mechanical strength of the motor and the temperature rise of the motor, the motor shaft should be able to generate a large enough acceleration or braking torque, so that the motor can quickly enter a constant rotation speed. torque running state.
2 Constant power load
The torque-speed characteristic of constant power refers to the fact that the power provided by the motor needs to be constant when the equipment or machinery changes in operating speed. The characteristic requirements of high torque and high speed, that is, the motor should have the ability to drive variable torque and constant power loads.
When the voltage of the motor increases with the increase of frequency, if the voltage of the motor has reached the rated voltage of the motor, it is not allowed to continue to increase the voltage with the increase of frequency, otherwise the motor insulation will be broken down due to overvoltage . For this reason, after the motor reaches the rated voltage, even if the frequency increases, the motor voltage remains unchanged. The power that the motor can output is determined by the product of the rated voltage and rated current of the motor, and the current no longer changes with the frequency. It has achieved constant voltage, constant current and constant power operation.
Except for constant power and constant torque loads, some equipment consumes power that varies dramatically with operating speed. For equipment such as fans and water pumps, the resistance torque is proportional to the 2nd to 3rd power of the running speed, that is, the square torque reduction load characteristic, only need to select the energy-saving inverter according to the rated point; If the motor is used, the performance requirements of the motor during the entire starting process from standstill to normal running speed should be considered more seriously.
Post time: May-13-2022