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The frequency converter allows the motor drive system to achieve two wishes. One is to allow the motor to run with higher efficiency; the other is to make the motor controllable in working conditions and avoid the problem of big cows pulling small cars. But the question before engineers is: There are so many types of motor loads, and the performance requirements of the inverters are also very different. How to choose the right inverter for the motor?
The English translation of the inverter is VFD (Variable Frequency Drive), which may be one of the few instances where modern technology reversely translates from Chinese to English. Frequency converter is used in frequency conversion technology and microelectronics technology to control the power transmission components of AC motors by changing the frequency and amplitude of the motor's working power supply.
Choosing the right frequency converter for the entire motor motion system is already a headache for engineers.
In general, the selection of the frequency converter should be considered in accordance with the type of the controlled object, speed range, static speed accuracy, starting torque, etc., so that it can meet the process and production requirements while being easy to use and economic .
The general experience is:
Choose the inverter as large as the motor, and sometimes it can be one size larger.
The power factor of the high-power inverter is lower, it is better to install an AC reactor at the incoming line of the inverter. One is to improve the power factor, and the other is to suppress high-frequency harmonics. If you frequently start and brake frequently, install a brake unit and a brake resistor.
If you need to reduce noise, you can choose a water-cooled inverter;
If braking is required, a braking chopper and braking resistor are required. Or you can choose four-quadrant products, which can feed back energy to the grid and save energy;
If there is only DC power supply on site, you can choose a pure inverter product (using DC power supply) to drive the motor.
Here are some practical issues that we need to pay attention to when choosing an inverter.
1. The purpose of using frequency conversion; constant voltage control or constant current control, etc.
2. The load type of the inverter; such as vane pump or positive displacement pump, etc., pay special attention to the performance curve of the load. The performance curve determines the method of application.
3. The matching problem between the inverter and the load;
Voltage matching; the rated voltage of the inverter matches the rated voltage of the load.
Current matching: For ordinary centrifugal pumps, the rated current of the frequency converter matches the rated current of the motor. For special loads such as deep water pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
Torque matching; this situation may occur under constant torque load or with a deceleration device.
4. When using a frequency converter to drive a high-speed motor, due to the low reactance of the high-speed motor, the increase in high-order harmonics causes the output current to increase. Therefore, the capacity of the inverter used in high-speed motors is slightly larger than that of ordinary motors.
5. If the inverter needs to run with a long cable, measures should be taken to suppress the impact of the long cable on the ground coupling capacitance to avoid insufficient output of the inverter. Therefore, in this case, the inverter capacity should be enlarged by one level or Install output reactor at the output end.
6. For some special applications, such as high temperature and high altitude, the inverter will be derated at this time, and the inverter capacity should be enlarged by one level.
For some occasions with strict requirements for the motor motion control system, it is necessary to accurately detect the effect of the inverter selection. The direct method is to test through the motor test system. But in order to complete the overall test of the inverter and the motor system, higher requirements are put forward for the motor test system, such as high-bandwidth, high-precision electrical parameter measurement, and multi-channel synchronous testing.
