1、 Classification of frequency converter
There are many classification methods of frequency converter. This paper introduces two kinds of classification methods according to the voltage level of working power supply and the nature of internal DC power supply.
According to the voltage level of working power supply, frequency converter has two categories: high voltage and low voltage. The voltage levels of high voltage inverter are 6kV and 10kV; the voltage levels of low voltage inverter are 380V, 660V and 1140V, which are also used in some special occasions.
The input and output of frequency converter are usually three-phase AC, but a small number of Low-Power Frequency Converters adopt the structure of single-phase input and three-phase output.
According to the nature of internal DC power supply, frequency converters are classified into voltage type and current type.
The circuit structure diagram of voltage source inverter is shown in Figure 1. Its middle DC link uses large capacitor C filter. The capacitor can store or release electric field energy in the process of charging and discharging, so that the voltage UD of DC link is relatively stable and the internal resistance is small, which is equivalent to the voltage source, and is often used in the occasions where the load voltage changes greatly.
The circuit structure of current mode inverter is shown in Figure 2. In the middle DC link, the reactor is used as the energy storage element for filtering, and the DC current is relatively stable. The inductance L of the filter element in this DC link will show a larger inductive reactance to the AC ripple in the DC circuit, which has a better filtering effect, and has the characteristics of approximate current source. Therefore, the inverter using this DC link is called current source inverter. It is often used in the situation where the load current changes greatly.
2、 Internal main circuit of frequency converter
At present, the commonly used inverter mostly adopts the circuit structure of “AC-DC-AC”, and its internal main power is composed of rectifier, filter and inverter, as shown in Figure 3. The three-phase AC power supply is input from the R, s and t terminals of the frequency converter, and is rectified into DC through a three-phase rectifier bridge composed of diodes d1-d6, with the voltage of UD. Capacitors C1 and C2 are filter capacitors. Six IGBT transistors V1-V6 form a three-phase inverter bridge, which reverses the direct current into three-phase alternating current whose frequency and voltage can be adjusted arbitrarily and transmits it to the load motor.
In the circuit diagram of Figure 3, two filter capacitors C1 and C2 are used in series to improve their withstand voltage. A resistor is connected in parallel at both ends of the capacitor, in which resistor R1 is connected in parallel with capacitor C1, and resistor R2 is connected in parallel with capacitor C2. These two resistors are called equalizing resistors. Their function is to make the voltage on the two capacitors basically equal and prevent the capacitors from being damaged during operation. diangon.com During the manufacture of capacitor, due to the material, technology and other reasons, it is inevitable that each finished capacitor will have leakage resistance with different resistance values. The two leakage resistances with different resistance values are in series, and the voltage UD is divided, which will make the voltage of each capacitor not equal, and even make the capacitor with higher voltage breakdown. Resistor R1 and R2 in parallel with capacitor can effectively solve this problem. The selection of equalizing resistor R1 and R2 is much smaller than the leakage resistance of the capacitor. The equalizing resistor with smaller resistance is connected in parallel with the leakage resistor with larger resistance. The value of parallel resistance basically depends on the equalizing resistor with smaller resistance. In this way, as long as the resistor R1 and R2 select the same resistance, the voltage at both ends of each filter capacitor can be approximately equal, so as to ensure the operation of the capacitor It’s safe.
A resistor R and a pair of contactor contacts km are connected between the rectifier bridge and the filter capacitor, and some inverters have a resistor R in parallel with an IGBT tube. The mechanism is the same. When the inverter is just connected to the power supply, the voltage on the filter capacitor is 0V, and the peak value of the rectifier voltage when the power supply voltage is 380V is 537V (times of 380V RMS), so there will be a large charging impulse current at the moment of connecting to the power supply, which may damage the rectifier diode; in addition, the filter capacitor with terminal voltage of 0 will make the rectifier voltage instantly reduce to 0V, forming a negative impact on the power network Interference. In order to solve the above problems, a current limiting resistor R is connected between the rectifier bridge and the filter capacitor, which can limit the charging current of the filter capacitor to a allowable range. However, if the current limiting resistor R is always connected in the circuit, its voltage drop will affect the output voltage of the frequency converter, and also reduce the power conversion efficiency of the frequency converter. Therefore, after the filter capacitor is charged, the current limiting resistor R will be short circuited by the contact of the contactor km, or the IGBT will be turned on by controlling the circuit, which can make the current limiting resistor out of operation.