Selection skills of VFD (Variable-frequency Drive)

The device that converts alternating current with constant voltage and frequency into alternating current with variable voltage or frequency is called "VFD (Variable-frequency Drive)". To generate variable voltage and frequency, the device first converts the alternating current from the power supply to direct current (DC). The device that converts direct current (DC) into alternating current (AC), its scientific term is "inverter" (inverter). Since the main device that generates changing voltage or frequency in VFD (Variable-frequency Drive) equipment is called "inverter", the product itself is named "inverter". The VFD (Variable-frequency Drive) used for motor control can be Changing the voltage can also change the frequency.

The number of poles of the motor is determined by 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 at the front of the motor and supplied to the motor so that the rotation speed of the motor can be freely controlled. Therefore, the VFD (Variable-frequency Drive) for the purpose of frequency control is the preferred device as a motor speed control device.

At present, the AC power supply used in China, whether it is used in households or factories, its voltage and frequency are 400V/50Hz or 200V/60Hz (50Hz) and so on.

Changing the frequency and voltage is the optimal motor control method. If only the frequency is changed without changing the voltage, the motor will be out of overvoltage (overexcitation) when the frequency is reduced, which may cause the motor to burn out. Therefore, VFD (Variable-frequency Drive) 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 VFD (Variable-frequency Drive) from 50Hz to 25Hz, then the output voltage of the VFD (Variable-frequency Drive) needs to be changed from 400V to about 200V. The correct selection of VFD (Variable-frequency Drive) is very critical for the normal operation of the control system. When choosing a VFD (Variable-frequency Drive), it is necessary to fully understand the characteristics of the load driven by the VFD (Variable-frequency Drive). 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 VFD (Variable-frequency Drive) according to their actual process requirements and application occasions. Pay attention to the following points when selecting VFD (Variable-frequency Drive): When selecting VFD (Variable-frequency Drive), the actual motor current value should be used as the basis for VFD (Variable-frequency Drive) selection, and the rated power of the motor is only can be used as a reference.

In addition, it should be fully considered that the output of VFD (Variable-frequency Drive) contains rich high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, compared with using VFD (Variable-frequency Drive) to supply power to the motor, the current of the motor will increase by 10% and the temperature rise will increase by about 20%. Therefore, when selecting the motor and VFD (Variable-frequency Drive), this situation should be taken into consideration, and an appropriate margin should be left to prevent the temperature from rising too high and affecting the service life of the motor.

If the VFD (Variable-frequency Drive) needs to run 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 VFD (Variable-frequency Drive). Therefore, the VFD (Variable-frequency Drive) should amplify one or two gears or install an output reactor at the output end of the VFD (Variable-frequency Drive).

For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., the VFD (Variable-frequency Drive) will be derated at this time, and the VFD (Variable-frequency Drive) needs to be enlarged by one gear. When VFD (Variable-frequency Drive) is used to control high-speed motors, more high-order harmonics will be generated due to the small reactance of high-speed motors. And these higher harmonics will increase the output current value of VFD (Variable-frequency Drive). Therefore, when choosing a VFD (Variable-frequency Drive) for a high-speed motor, it should be slightly larger than a VFD (Variable-frequency Drive) for an ordinary motor. When using a VFD (Variable-frequency Drive) to drive a geared motor, the application range is limited 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 exceeding the rated speed, there may be a danger of running out of lubricating oil. Therefore, do not exceed the maximum allowable rotational speed.

When VFD (Variable-frequency Drive) drives the wound rotor asynchronous motor, most of them use the existing motor. Wound motors have a lower impedance in the windings of a wound motor than a normal squirrel cage motor. Therefore, overcurrent tripping due to ripple current is easy to occur, so a VFD (Variable-frequency Drive) with a slightly larger capacity than usual should be selected. Generally, wound motors are mostly used in occasions where the flywheel torque GD2 is relatively large, and more attention should be paid when setting the acceleration and deceleration time.

When a VFD (Variable-frequency Drive) drives a synchronous motor, compared with a commercial frequency power supply, the output capacity will be reduced by 10% to 20%. The continuous output current of the VFD (Variable-frequency Drive) is greater than the rated current of the synchronous motor and the synchronous pull The product of the per-unit values of the currents.

For loads with large torque fluctuations such as compressors and vibrators and peak loads such as oil pressure pumps, if VFD (Variable-frequency Drive) is selected according to the rated current or power value of the motor, it may occur due to peak current. Overcurrent protection action phenomenon. Therefore, you should understand the power frequency operation and choose a VFD (Variable-frequency Drive) with a rated output current greater than its maximum current. When choosing a VFD (Variable-frequency Drive), be sure to pay attention to whether its protection level matches the situation on site. Otherwise, the dust and water vapor on site will affect the long-term operation of VFD (Variable-frequency Drive). For more VFD (Variable-frequency Drive) application knowledge, please consult Sunwoele VFD (Variable-frequency Drive) home, and participate in VFD (Variable-frequency Drive) learning and selection skills.