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At present, AC power supply used in our country, whether for home or for factories, the voltage and frequency are 400V/50Hz or 200V/60Hz (50Hz), and so on. A device for converting alternating voltage and frequency to alternating voltage or frequency alternating current is called “ transducer ”. To produce a variable voltage and frequency, the device first transforms the alternating current of the power source into direct current (DC). A device that converts a direct current (DC) into an alternating current (AC); the scientific term is “ inverter” (inverter). Because the main device that changes the voltage or frequency in the frequency converter equipment is called “ inverter” therefore, the product itself is named “ inverter. The frequency converter used for motor control can change the voltage and change the frequency. The structure and working principle of the motor determine that the number of poles of the motor is fixed. The speed of the motor is generally not adjusted by changing the value. The frequency can be adjusted in front of the motor and then supplied to the motor, so that the speed of the motor can be controlled freely. Therefore, the frequency converter with the purpose of controlling the frequency is the preferred equipment for the motor speed regulation equipment.
Changing the frequency and voltage is the best way to control the motor. If the frequency is changed without changing the voltage, the motor will be burned out when the frequency is lower than the voltage (Guo Lici). Therefore, the inverter must change the voltage while changing the frequency. When the output frequency is above the rated frequency, the voltage can not continue to increase, and the maximum is only equal to the rated voltage of the motor. For example: in order to make the motor rotation speed of the half, the inverter output frequency changed from 50Hz to 25Hz, when the output voltage of the inverter will need to be changed from 400V to 200V about the correct choice of motor and inverter is very important for the normal operation of the control system. When selecting a converter, it is necessary to fully understand the load characteristics of the inverter. In practice, the production machinery is often divided into three types: constant torque load, constant power load and fan, pump load. Users can choose different types of inverters according to their actual process requirements and application occasions. In selecting frequency converter, pay attention to the following points: when choosing frequency converter, the actual motor current value should be chosen as the basis of frequency converter, and the rated power of the motor can only be used as reference. In addition, full consideration should be given to the output of the converter, which is rich in higher harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, compared with the power frequency power supply, the current of the motor will increase by 10%, while the temperature rise will increase by about 20%. Therefore, in the selection of motors and inverters, should take into account this situation, appropriate margin, to prevent temperature rise is too high, affecting the service life of the motor.
If the frequency converter wants long cable operation, at this time, measures should be taken to restrain the influence of long cable to ground coupling capacitor, so as to avoid the inverter output insufficient. Therefore, the frequency converter should enlarge the one or two gear selection or install the output reactor at the output end of the inverter. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the inverter to reduce capacity, and the frequency converter needs to enlarge the first gear selection. When the high speed motor is controlled by a transducer, the higher reactance is produced due to the small reactance of the high speed motor. These higher harmonics will increase the output current of the inverter. Therefore, when choosing a converter for a high speed motor, it should be slightly larger than that of a conventional motor. When the gear reducer is driven by a frequency converter, the range of use is limited by the lubrication of the gear rotation part. When lubricating oil is lubricated, there is no limit in the low speed range; the danger of oil light consumption may occur in the high speed range above the rated speed. Therefore, do not exceed the maximum speed allowable value. When the inverter drives the wound rotor asynchronous motors, most of them use existing motors. Compared with the conventional squirrel cage motor, the winding motor has less resistance to winding. Therefore, it is easy to cause the over current tripping due to ripple current, so a converter larger than the usual capacity should be selected. Generally, winding motors are used more frequently for flywheel moment GD2, and more attention should be paid when setting acceleration and deceleration time.
Inverter driven synchronous motor, compared with the power supply, will reduce the output capacity of 10% ~ 20%, the product of continuous output current of inverter is greater than the rated current of synchronous motor and synchronous in current p.u.inductance. For compressor, vibration torque ripple large load and hydraulic pump with peak load, if the motor according to the rated current or power value selection of a frequency converter, may occur due to peak current the overcurrent protection phenomenon. Therefore, it is necessary to understand the operation of power frequency and select the frequency converter whose rated current is greater than its maximum current. Choose frequency converter, must pay attention to its protection level and whether the scene match. Otherwise, the scene dust, water vapor will affect the long-term operation of the inverter. Please use the knowledge more inverter inverter Huanmao Ogilvy consulting, together to participate in the learning and skills of using frequency converter.
