Principles for Selecting a Frequency Inverter
1.When selecting a frequency inverter, the actual motor current should be used as the basis, while the rated power of the motor should only be considered as a reference. Furthermore, the output of the frequency inverter contains high harmonic content, which can deteriorate the power factor and efficiency of the motor. Therefore, when supplying power to the motor with a frequency inverter instead of the power grid, the motor current will increase by 10% and temperature rise will increase by about 20%. Thus, when selecting the motor and frequency inverter, this situation should be taken into account, allowing for some margin to prevent excessive temperature rise and impact on the motor’s lifespan.
2.If the frequency inverter is to be used for long cable operation, measures should be taken to suppress the impact of the coupling capacitance between the long cables and the ground. This avoids insufficient output of the frequency inverter, requiring the selection of a higher-rated frequency inverter or the installation of an output reactor at the output end of the inverter.
3.For some special applications, such as high environmental temperatures, high switching frequencies (especially in applications with strict noise restrictions, such as building automation), and high altitudes, the capacity of the frequency inverter will be reduced. In such cases, a higher-rated frequency inverter should be selected.
4.When the frequency inverter is used to control several parallel-connected motors, the total length of the cables from the inverter to the motors must be within the permissible range of the inverter. If it exceeds the specified value, the frequency inverter should be one or two sizes larger. Additionally, in this situation, the control mode of the frequency inverter can only be V/F control. The inverter cannot provide motor protection against overcurrent or overload, so fuses should be added to each motor for protection.
5.When using a frequency inverter to control a high-speed motor, the small reactance of the high-speed motor results in increased output current harmonic values. Therefore, when selecting a frequency inverter for a high-speed motor, it should be slightly larger than the one used for a regular motor.
6.When using a frequency inverter for a variable-pole motor, it is vital to carefully select the capacity of the inverter so that the maximum rated current is below the rated output current of the inverter. Additionally, when performing pole conversions during operation, the motor should be stopped first to avoid motor coasting, which can lead to severe damage to the frequency inverter.
7.When driving explosion-proof motors, the frequency inverter does not have an explosion-proof structure, so it should be set up outside dangerous areas.
8.When using a frequency inverter to drive gear reduction motors, the range of use is limited by the lubrication method of the rotating part of the gears. In the case of oil lubrication, there are no restrictions in the low-speed range, but there is a risk of oil depletion in the high-speed range exceeding the rated speed. Therefore, do not exceed the maximum allowable speed value.
9.When using a frequency inverter to drive a wound rotor asynchronous motor, it is often based on an existing motor. Compared to a regular squirrel cage motor, the winding impedance of the wound rotor motor is smaller. Hence, there is a higher possibility of overcurrent trip caused by ripple currents, necessitating the selection of a slightly larger capacity frequency inverter. Wound rotor motors are commonly used in occasions with large flywheel torque GD2, so extra attention should be paid to setting acceleration and deceleration times.
10. When the frequency converter drives the synchronous motor, compared with the industrial frequency power supply, the output capacity is reduced by 10% to 20%. The continuous output current of the frequency converter is greater than the product of the rated current of the synchronous motor and the nominal value of the synchronous pull-in current.
11. For loads with large torque fluctuations such as compressors and vibrators and peak loads such as oil pressure pumps, if the inverter is selected according to the rated current or power value of the motor, the overcurrent protection may occur due to the peak current. action phenomenon. Therefore, you should understand the power frequency operation and choose a frequency converter with a rated output current greater than its maximum current. When the frequency converter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than the rated current of the normal motor, when selecting the frequency converter, its rated current should be greater than the rated current of the submersible pump motor.
12. When the frequency converter controls the Roots blower, due to its large starting current, it is necessary to pay attention to whether the capacity of the frequency converter is large enough when selecting the frequency converter.
13. When selecting a frequency converter, you must 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 the inverter.
14. When the load of the motor is very light, even if the load current of the motor is within the rated current of the inverter, the inverter whose capacity is much smaller than that of the motor cannot be used. This is because the reactance of the motor varies with the capacity of the motor. Even if the load of the motor is the same, the larger the capacity of the motor, the greater the pulsating current value, which may exceed the current allowable value of the inverter.
15. If the power supply of the inverter is self-contained, it is better to add a line reactor.
Post time: Aug-03-2023