Learn the skills of selecting frequency converter

At present, the voltage and frequency of the AC power supply used in my country, whether for home or factory, are 400V/50Hz or 200V/60Hz (50Hz), etc. The device that converts the AC power with fixed voltage and frequency into the AC power with variable voltage or frequency is called "inverter". In order to produce variable voltage and frequency, the device must first convert the AC power of the power supply into direct current (DC). The scientific term for the device that converts direct current (DC) into alternating current (AC) is "inverter". Since the main device that produces variable voltage or frequency in the inverter device is called "inverter", the product itself is named "inverter". The inverter used for motor control can change both voltage and frequency. The number of poles of the motor is fixed due to the structure and working principle of the motor. It is generally not suitable to adjust the speed of the motor by changing this value. The frequency can be adjusted in front of the motor and then supplied to the motor so that the rotation speed of the motor can be freely controlled. Therefore, the inverter for the purpose of controlling frequency is the preferred device as a motor speed control device.

Changing frequency and voltage is the best motor control method. If you only change the frequency without changing the voltage, the motor will be over-voltage (over-excitation) when the frequency is reduced, which may cause the motor to burn out. Therefore, the inverter must change the voltage at the same time as changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be equal to the rated voltage of the motor. For example: in order to halve the rotation speed of the motor, change the output frequency of the inverter from 50Hz to 25Hz. At this time, the output voltage of the inverter needs to be changed from 400V to about 200V. The correct selection of motors and inverters is very critical for the normal operation of the control system. When selecting an inverter, you must fully understand the load characteristics driven by the inverter. In practice, people often divide production machinery into three types: constant torque load, constant power load and fan and water pump load. Users can choose different types of inverters according to their actual process requirements and application occasions. When selecting an inverter, pay attention to the following points: When selecting an inverter, the actual motor current value should be used as the basis for inverter selection, and the rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the frequency converter contains a lot of high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, compared with the power supply from the industrial frequency grid, the motor current will increase by 10% and the temperature rise will increase by about 20% when the frequency converter is used to power the motor. Therefore, when selecting the motor and the frequency converter, this situation should be taken into consideration and a proper margin should be left to prevent the temperature from rising too high and affecting the service life of the motor.

If the inverter is to be operated with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance to avoid insufficient output of the inverter. Therefore, the inverter should be enlarged by one or two gears or an output reactor should be installed at the output end of the inverter. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., the inverter capacity will be reduced at this time, and the inverter needs to be enlarged by one gear. When using the inverter to control the high-speed motor, due to the small reactance of the high-speed motor, more high-order harmonics will be generated. And these high-order harmonics will increase the output current value of the inverter. Therefore, when selecting the inverter for high-speed motors, it should be slightly larger than the inverter for ordinary motors. When using the inverter to drive the gear reduction motor, the scope of use is restricted by the lubrication method of the gear rotating part. When lubricating oil is lubricated, there is no limit in the low speed range; in the high speed range above the rated speed, there is a risk of running out of lubricating oil. Therefore, do not exceed the maximum allowable speed. When the inverter drives the wound rotor asynchronous motor, most of them use the existing motor. Compared with ordinary squirrel cage motors, the winding impedance of winding motors is smaller. Therefore, it is easy to cause overcurrent tripping due to ripple current, so a frequency converter with a slightly larger capacity than usual should be selected. Generally, winding motors are mostly used in occasions with large flywheel torque GD2, so more attention should be paid when setting acceleration and deceleration time.

When the inverter drives a synchronous motor, the output capacity will be reduced by 10% to 20% compared with the industrial frequency power supply. The continuous output current of the inverter must be greater than the product of the rated current of the synchronous motor and the per-unit value of the synchronous pull-in current. For loads with large torque fluctuations such as compressors and vibrators, and peak loads such as hydraulic pumps, if the inverter is selected according to the rated current or power value of the motor, the overcurrent protection may be activated due to the peak current. Therefore, the industrial frequency operation should be understood, and the inverter with a rated output current greater than its maximum current should be selected. When selecting an inverter, be sure to pay attention to whether its protection level matches the on-site conditions. Otherwise, the dust and water vapor on the site will affect the long-term operation of the inverter. For more inverter application knowledge, please go to Huanmao Ogilvy Inverter Home for consultation, and participate in the study and selection skills of inverters.