A frequency converter is a power control device that uses frequency conversion technology and microelectronics technology to control an AC motor by changing the frequency of the motor's power supply.
A frequency converter mainly consists of a rectifier (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit.
The frequency converter adjusts the voltage and frequency of the output power supply by switching the internal IGBTs , and provides the required power voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation.
Inverter working principle
Frequency converters can be divided into two types: voltage-type and current-type frequency converters.
A voltage-type inverter converts DC voltage from a voltage source into AC voltage, and the DC circuit is filtered by a capacitor.
A current-source inverter converts direct current (DC) to alternating current (AC), and its DC circuit filtering is done using an inductor. It is a rectifier, inverter, or converter.
The main circuit of the frequency converter consists of three parts: a rectifier, a smoothing circuit, and an inverter. The "rectifier" converts the industrial frequency power into DC power, and the "smoothing circuit" absorbs the voltage ripples generated in the converter and inverter.
Inverter wiring diagram
The image above is a wiring diagram for a frequency converter. There are several points to note during the installation of a frequency converter. For example, the frequency converter itself has strong electromagnetic interference, which can interfere with the operation of some equipment. Therefore, we can add cable sleeves to the output cables of the frequency converter. Also, the control lines inside the frequency converter or control cabinet should be at least 100mm away from the power cables, etc.
Inverter wiring method
1. Wiring of the main circuit
1. The power supply should be connected to the R, S, T terminals of the inverter input. It must not be connected to the inverter output terminals (U, V, W), otherwise the inverter will be damaged.
After wiring, all loose wire ends must be thoroughly cleaned up. Loose wire ends can cause abnormalities, malfunctions, and failures; therefore, the inverter must always be kept clean. When drilling holes in the control panel, care must be taken to prevent debris or dust from entering the inverter.
2. Do not connect anything other than the recommended braking resistor option between terminals + and PR, or short-circuit it under any circumstances.
3. Electromagnetic interference: The inverter's input/output (main circuit) contains harmonic components, which may interfere with nearby communication equipment. Therefore, installing optional radio noise filters FR-BIF, FRBSF01, or FR-BLF line noise filters can minimize interference.
4. When laying long-distance cables, the parasitic capacitance charging current of the cable can reduce the fast response current limiting function, causing instruments connected to the secondary side to malfunction. Therefore, the maximum cable length should be less than the specified value.
If the wiring length is unavoidably exceeded, Pr.156 should be set to 1.
5. Do not install power capacitors, surge suppressors, or radio noise filters on the output side of the frequency converter. Otherwise, it will cause frequency converter failure or damage to the capacitors and surge suppressors.
6. To keep the voltage drop within 2%, use appropriate wires for wiring. When the wiring distance between the frequency converter and the motor is long, especially under low-frequency output conditions, the motor torque will decrease due to the voltage drop in the main circuit cable.
7. After operation, any changes to the wiring must be made only after the power has been disconnected for at least 10 minutes and the voltage has been checked with a multimeter. For a period of time after power is off, dangerously high voltage may still be present on the capacitors.
2. Wiring of the control circuit
The control circuits of frequency converters can be broadly divided into two types: analog and digital.
1. Shielded or twisted-pair cables should be used for wiring the control circuit terminals, and they must be wired separately from the main circuit and high-voltage circuit (including 200V relay program circuit).
2. Since the frequency input signal of the control circuit is a small current, in the case of contact input, in order to prevent poor contact, two parallel nodes or double contacts should be used for the small signal contacts.
3. For the wiring of the control circuit, cables with a diameter of 0.3 to 0.75 square meters are generally selected.
3. Ground wire wiring
1. Due to leakage current in the frequency converter, the frequency converter and motor must be grounded to prevent electric shock.
2. Use a dedicated grounding terminal for the frequency converter. For grounding wire connections, use tin-plated crimp terminals. When tightening the screws, be careful not to damage the screw threads.
3. Tin plating does not contain lead.
4. Use a thicker wire diameter for the grounding cable, which must be equal to or greater than the specified standard. The grounding point should be as close as possible to the frequency converter, and the grounding wire should be as short as possible.
The function of frequency converter
1. Frequency converters can adjust the power of a motor, enabling variable speed operation and thus saving energy. Examples include centrifugal fans and water pumps. When frequency converters are used in centrifugal fans and water pumps, operators can easily control the flow rate as needed by adjusting the speed, thereby saving energy.
2. Frequency converters can reduce voltage fluctuations in power lines, preventing equipment from tripping or malfunctioning due to abnormal voltage.
3. Frequency converters can reduce the impact on the power grid, thereby effectively reducing reactive power loss and increasing the effective power of the power grid.
4. Variable frequency drives can also reduce wear between transmission components in machinery, thereby reducing costs and improving system stability to some extent.
5. In addition, the transformer has a very complete set of control functions, which can be well integrated with other control equipment to achieve centralized monitoring and real-time control, solving many problems such as system compatibility for users.
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