About the structure of FGI inverter main circuit
The main circuit of different series of AC - DC - AC inverter is basically the same, and many phenomena in the process of frequency conversion speed regulation can be analyzed through the main circuit.
1.Ac-dc conversion circuit
The AC-DC-AC frequency converter is composed of rectifier circuit, filter circuit, current limiting circuit and power supply indicating circuit.
Ac-dc conversion circuit is a rectifier and filter circuit, its task is to convert the three-phase (or single-phase) alternating current of the power supply into a smooth direct current. Because the DC voltage after rectification is high, and it is not allowed to be reduced, it has its particularity in the circuit structure.
(1)Full wave rectifier
In the SPWM frequency converter, most of the bridge full wave rectifier circuit is used, and in the medium and small capacity frequency converter, the rectifier device uses uncontrollable rectifier diode or diode module, as shown in the following figure, VD1~ VD6 in the AC/DC converter circuit. When the three-phase line voltage is 380V, the peak voltage after rectification is 537V, and the average voltage is 515V.
(2).Filter circuit
In the figure above, the filter circuits refer to CF1 and CF2. Due to the limitation of the capacitance and voltage resistance of the electrolytic capacitor, the filter circuit is usually composed of a number of capacitors and a group, and is composed of two capacitor banks CF1 and CF2 in series. Because the capacitance of electrolytic capacitors is relatively discrete, the capacitance of capacitor banks CF1 and CF2 cannot be exactly equal. As a result, the voltage UD1 and UD2 of each capacitor bank are not equal, which makes the capacitor bank with higher voltage easily damaged. To make UD1 and UD2 equal, equalizing resistors RC1 and RC2 with equal resistance are connected to CF1 and CF2 respectively.
(3) Current foldback circuit
In the figure above, the current limiting circuit refers to a parallel circuit that is connected in series between the rectifier bridge and the filter capacitor and consists of a current limiting resistor RL and a short circuit switch SL.
The role of the current limiting resistor RL is: before the inverter is connected to the power supply, the DC voltage UD=0 on the filter capacitor CF(formed by CF1 and CF2 in series). Therefore, the moment the inverter is just connected to the power supply, there will be a large impact of electricity flowing through the rectifier to the filter capacitor, which may damage the rectifier bridge. If the capacity of the capacitor is large, it will also cause the power supply voltage to drop instantaneously and form interference to the power grid. The current limiting resistor RL is connected in series between the rectifier bridge and the filter capacitor in order to weaken the impulse current.
The role of the short-circuit switch SL is: if the current limiting resistor RL is connected in the circuit for a long time, it will affect the size of the DC voltage UD and the output voltage of the inverter. Therefore, when UD increases to a certain extent, the short circuit switch SL is switched on, and the RL is cut out of the circuit. SL is mostly composed of thyristors, and often consists of relay contacts in low-capacity inverters.
(4).Power indicating circuit
In addition to indicating whether the power supply is switched on, the power indicator HL also has a very important function, that is, after the frequency inverter cuts off the power, it indicates whether the charge on the filter capacitor CF has been released.
Due to the large capacity of CF, and the power must be cut off in the inverter circuit to stop working state, so CF has no fast discharge circuit, and its discharge time is often as long as several minutes. Due to the high voltage on CF, if the power is not discharged, it will pose a threat to personal safety, so in the maintenance of the inverter, you must wait for HL to be completely extinguished before contacting the conductive part of the inverter. Therefore, HL also has the role of prompt protection.
2.Dc-ac conversion circuit
(1).Three phase inverter bridge circuit
The function of inverter bridge circuit is to convert direct current into three-phase alternating current. The inverter bridge circuit is composed of the switching devices V1~V6 in the figure below. At present, most of the switching devices in medium and small capacity frequency converters use IGBT tubes.
(2).Continuous current circuit
The continuous current circuit is composed of VD7~VD12 in the figure above. Its functions are as follows:
Provide a path for the reactive current of the motor winding to return to the direct current circuit.
When the frequency drops and the synchronous speed drops, it provides a path for the motor's regenerative energy to feed back to the DC circuit.
Provide a path for the parasitic inductance of the circuit to release energy during the inverter process.
(3).Snubber circuit
When the inverter tube is turned off and switched on, its voltage and current change rate is very large, which may cause the inverter tube to be damaged. Therefore, each inverter tube should also be connected to the buffer circuit to slow down the rate of change of voltage and current. The structure of the buffer circuit varies greatly due to the characteristics and capacity of the inverter tube. The following figure shows a typical buffer circuit (composed of R01~R06, C01~C06, VD01~VD06).
The functions of each component are as follows:
C01~C06Capacitors C01 to C06
Each time the inverter tube V1~V6 is converted from the on-state to the cut-off state, the voltage between the collector and the emitter UCE will rise very quickly from nearly 0V to UD. In this process, the voltage growth rate is very high, and it is easy to cause damage to the inverter tube. The function of C01~C06 is to reduce the voltage growth rate of V1~V6 when it is turned off.
Resistance R01 to R06
Each time V1~V6 changes from the cut-off state to the on-state, the voltage charged on C01~C06 (equal to UD) will be discharged to V1~V6. The initial value of the discharge current is very large, and will be superimposed on the load current, resulting in V1~V6 damage. Resistance R01~R06 is used to limit the discharge current of C01~C06 to V1~V6.
Diodes VD01~VD06
The access of current limiting resistance R01~R06 will affect the effect of C01~C06 limiting the voltage growth rate when V1~V6 is off. After VD01~VD06 is connected, R01~R06 will not work during the shutdown of V1~V6.
