I. Working Principle of Frequency Converter
A frequency converter is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. A frequency converter mainly consists of a rectification unit, filter, inverter, braking unit, drive unit, detection unit, and microprocessor unit. The frequency converter adjusts the voltage and frequency of the output power supply by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation.
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 DC current to AC current. Its DC circuit filtering consists of an inductor, a rectifier, and an inverter.
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.
II. What types of frequency converters are there?
1. Classified by input voltage level
Frequency converters can be classified into low-voltage and high-voltage frequency converters according to their input voltage level. Common low-voltage frequency converters in China include single-phase 220V frequency converters, three-phase 220V frequency converters, and single-phase 380V frequency converters. High-voltage frequency converters commonly use 6 kV and 10 kV transformers, and the control method is generally a high-low to high-voltage frequency converter or a high to high-voltage frequency converter.
2. Classification by frequency conversion method
Frequency converters are classified into AC-AC converters and AC-DC-AC converters according to their frequency conversion methods. AC-AC converters directly convert mains frequency AC power into AC power with controllable frequency and voltage, hence the name direct-type converters. AC-DC-AC converters first convert mains frequency AC power into DC power through a rectifier, and then convert the DC power into AC power with adjustable frequency and voltage; therefore, they are also called indirect-type converters.
3. Classification by the nature of DC power supply
In AC-DC-AC inverters, based on the nature of the DC power supply during the process of converting the main circuit power supply into DC power supply, inverters are classified into voltage-source inverters and current-source inverters.
III. Inverter Maintenance
1. A good grounding wire is required.
Factory grounding wires rarely break, but if they do, the frequency converter can easily burn out. This is because if a motor leaks current, and the factory grounding wire happens to be broken, high-voltage electricity will flow back into the frequency converter's main board via the grounding wire, causing arcing at the main board's terminals and burning out the main board. Therefore, a good grounding wire is essential.
2. Prevent the frequency converter from being interfered with.
When a frequency converter is running, it acts like a powerful interference source, with the source of the interference being the six IGBT transistors in the output module. Some frequency converters' switching power supplies can also cause interference; the power lines and motor wires act as antennas for this interference. If the grounding is faulty, interference signals can also be emitted through the ground wire connected to the casing. The longer the line, the greater the interference range.
Interference signals from frequency converters can interfere not only with surrounding electronic equipment but also with the frequency converter itself. Some frequency converters have built-in functions to prevent interference signal radiation and input, while others lack anti-interference capabilities. If the control system uses other electronic equipment that communicates via analog or pulse signals, such as computers, human-machine interfaces, and sensors, anti-interference measures should be considered when selecting the frequency converter and during wiring.
Preventing inverter interference is a complex issue that requires different approaches depending on the site conditions. Here are some methods: 1. Add reactors, filters, and ferrite cores to control lines. 2. Use shielded cables. 3. Store the inverter in a metal cabinet. 4. Enclose incoming and outgoing power lines in metal conduits. 5. Do not run control lines alongside power lines; wiring should be orderly and crisscrossed. 6. Lower the carrier frequency. 7. Ensure a good grounding wire, but note that many inverter control lines' common terminal cannot be grounded.
3. A varistor should be added to the air switch at the input terminal of the frequency converter.
Some enterprises have suboptimal power supply quality. When a power line malfunctions, the high-voltage output can easily damage the frequency converter and related electronic equipment. To effectively solve this problem, a passive protection method can be adopted: adding a varistor (821K for 380V, 471K for 220V) to the air switch at the input of the frequency converter or instrument. When high-voltage current passes through, the varistor will short-circuit, tripping the air switch and protecting the frequency converter, thus greatly reducing its failure rate.
It is important to emphasize that the varistor should not be placed too close to the frequency converter. When the varistor short-circuits and explodes, the resulting metal fragments will fly everywhere. The strong static electricity and electromagnetic waves emitted during the explosion will affect the frequency converter's function. At best, it will burn out the copper wires on the circuit board; at worst, it will burn out the rectifier module, switching power supply, CPU board, and capacitors. Therefore, it is better to add a varistor outside the frequency converter.
