What is a frequency converter? This article will teach you how to understand frequency converters
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Figure 1
Buffer circuit: suppresses the impact current at the moment of power-on. According to the working principle of electrolytic capacitors, when the inverter is powered on, the voltage at both ends of the capacitor will not change suddenly, but the current at both ends of the capacitor will change suddenly, and the two ends of the capacitor are equivalent to a short circuit. If there is no buffer circuit (charging resistor), the rectifier bridge will be damaged due to excessive current. The buffer circuit plays a role in protecting the rectifier bridge.
Filter circuit: The withstand voltage of a general electrolytic capacitor is 400V; and the theoretical value of the DC voltage of a three-phase 380V AC power after rectification is about 537V. Therefore, the filter capacitor can only be made up of two electrolytic capacitors in series. Since the capacity of the electrolytic capacitors cannot be absolutely the same, the voltage distribution on the two electrolytic capacitors after series connection is unbalanced, which will result in different service lives of the two electrolytic capacitors. In order to solve the problem of voltage imbalance, equalizing resistors with the same resistance value need to be connected in parallel at both ends of the two electrolytic capacitors.
What are the functional features of frequency converter?
When the motor is started directly with the mains, the starting current is 5-7 times the rated current of the motor. In this case, the direct start of the motor will cause the grid voltage to drop, affecting the normal operation of other electrical equipment.
Using inverter soft start, the starting current is generally 1.2-1.5 times the rated current, which effectively reduces the starting impact current and reduces the transformer occupancy (Figure 2).
2. Variable frequency speed regulation
Traditional motor speed regulation is variable pole speed regulation; according to the same speed of three-phase asynchronous motor:
By changing the number of pole pairs of the motor stator winding, its synchronous speed can be changed, thereby changing the rotor speed. Through different combinations of winding connections, two-pole, three-pole, and four-pole speeds can generally be obtained. The speed regulation range is relatively narrow, generally two-speed (Figure 3).
Variable frequency speed regulation is based on the relationship that the motor speed is proportional to the input frequency of the working power supply:
The purpose of changing the motor speed is achieved by changing the working power frequency f of the motor. The speed regulation method is convenient and can give the motor different speeds according to different processes (Figure 4).
Figure 4
3. Energy saving effect
In various fans, water pumps, and oil pumps, the resistance generated by air or liquid is roughly proportional to the square of the motor speed, and the power required is proportional to the cube of the motor speed.
When the required air volume and flow rate decrease, the air volume and flow rate can be adjusted by using the frequency converter to adjust the speed, which can greatly save electricity. Figure 5 is the theoretical basis for energy saving of pump loads.
Figure 5
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