Sep 02, 2020
Five essentials in the design of inverter control cabinet
The inverter should be installed inside the control cabinet. The following issues should be paid attention to when designing the control cabinet:
1. Heat dissipation problem
The heating of the inverter is caused by internal losses. The main circuit is the main part of the loss in each part of the frequency converter, accounting for about 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the inverter, the inverter must be dissipated. We usually use a fan to dissipate heat;
The built-in fan of the inverter can take away the heat dissipation inside the box of the inverter. If the fan fails to work normally, the inverter should be stopped immediately; for high-power inverters, a fan must be added to the control cabinet, and the air duct of the control cabinet should be designed Reasonable, all air inlets should be equipped with dust-proof nets, and the exhaust air should be unobstructed to avoid the formation of eddy currents in the cabinet and the formation of dust accumulation in a fixed position; choose the matching fan according to the ventilation rate of the inverter manual, and the fan installation should pay attention to the problem of vibration.
2. Electromagnetic interference problem
I. Due to the rectification and frequency conversion of the inverter during operation, a lot of interference electromagnetic waves are generated around. These high-frequency electromagnetic waves have a certain interference to nearby instruments and instruments, and will produce high-order harmonics. This high-order harmonics will Enter the entire power supply network through the power supply loop, thereby affecting other meters. If the power of the frequency converter accounts for more than 25% of the entire system, you need to consider anti-interference measures for the control power supply.
II. When there are high-frequency shock loads in the system such as electric welders and electroplating power supplies, the inverter itself will be protected due to interference, so consider the power quality of the entire system.
Three, protection issues need to pay attention to the following points
I. Waterproof and anti-condensation: If the inverter is placed on site, it should be noted that there are no pipe flanges or other leaks on the top of the inverter cabinet, and there should be no splashing water near the inverter. In short, the protection level of the on-site cabinet should be IP43 the above. WeChat technical training is worthy of your attention!
II. Dust-proof: All air inlets should be equipped with dust-proof nets to prevent flocculent debris from entering. The dust-proof nets should be designed to be detachable to facilitate cleaning and maintenance. The grid of the dust-proof net is determined according to the specific conditions of the site, and the joints around the dust-proof net and the control cabinet should be handled strictly.
III. Anti-corrosive gas: This situation is more common in the chemical industry. At this time, the frequency conversion cabinet can be placed in the control room.
Four, inverter wiring specifications
The signal line and the power line must be routed separately: When using analog signals to remotely control the inverter, in order to reduce the interference from the inverter and other equipment by the analog signal, please connect the signal line of the control inverter to the strong current circuit (main circuit). And sequence control loop) separate wiring. The distance should be above 30cm. Even in the control cabinet, the same wiring specifications must be maintained. The longest control circuit line between this signal and the inverter shall not exceed 50m.
The signal line and power line must be placed in different metal pipes or metal hoses: if the signal line connecting the PLC and the inverter is not placed in the metal pipe, it is very susceptible to interference from the inverter and external equipment; at the same time, due to the inverter There is no built-in reactor, so the input and output power lines of the inverter will cause strong interference to the outside. Therefore, the metal pipe or metal hose for the signal line must be extended to the control terminal of the inverter to ensure the signal line Complete separation from the power line.
1) The analog control signal wire should use twisted-pair shielded wire, the wire size is 0.75mm2. Be careful when wiring, the cable stripping should be as short as possible (about 5-7mm), and the shielding layer after stripping should be wrapped with insulating tape to prevent the shielding wire from contacting other equipment and causing interference.
2) In order to improve the simplicity and reliability of wiring, it is recommended to use crimping bar terminals on signal wires.
5. The operation of the inverter and the setting of related parameters
There are many setting parameters of the inverter, and each parameter has a certain selection range. In use, it is often encountered that the inverter cannot work normally due to improper setting of individual parameters.
Control method: namely speed control, torque control, PID control or other methods. After adopting the control method, it is generally necessary to perform static or dynamic identification according to the control accuracy.
Minimum operating frequency: the minimum speed at which the motor runs. When the motor runs at a low speed, its heat dissipation performance is very poor. If the motor runs at a low speed for a long time, it will cause the motor to burn. And at low speed, the current in the cable will increase, which will also cause the cable to heat up. WeChat technical training is worthy of your attention!
Operating frequency: The maximum frequency of general frequency converters is up to 60Hz, and some even up to 400 Hz. High frequency will make the motor run at high speed. For ordinary motors, its bearings cannot run at super-rated speeds for a long time. Withstand such centrifugal force.
Carrier frequency: The higher the carrier frequency, the greater the higher harmonic components, which are closely related to the length of the cable, the heating of the motor, and the heating of the cable and the inverter.
Motor parameters: The inverter sets the power, current, voltage, speed, and maximum frequency of the motor in the parameters. These parameters can be obtained directly from the motor nameplate.
Frequency hopping: At a certain frequency point, resonance may occur, especially when the entire device is relatively high; when controlling the compressor, avoid the compressor surge point.