How to power on and overhaul the main circuit of VFD (Variable-frequency Drive)
VFD (Variable-frequency Drive) maintenance personnel must establish such a concept: the inverter module and the drive circuit have a strong connection between faults. When the module bursts and is damaged, the drive circuit is bound to be damaged by impact; the damage of the module may also be caused by the failure of the drive circuit. Therefore, regardless of whether it is a fault in the drive circuit or the inverter output circuit, the inverter output circuit and the drive circuit must be thoroughly checked together. The power-on test machine of the main circuit must be carried out under the premise of confirming that the drive circuit is normal——can output six excitation pulses normally. For the maintenance of the drive circuit, see Chapter 4 of this book.
After checking that the drive circuit is normal, replace the damaged inverter module with a new one before powering on the test machine.
The power-on test machine after the whole machine is assembled is an event that must be done carefully. Corresponding measures must be taken to ensure that the newly replaced IGBT module will not be damaged when an abnormal situation occurs. During the test run, the VFD (Variable-frequency Drive) start-up moment is the most "dead moment". A hasty power-on without any protective measures will damage the newly replaced expensive module in an instant. The previous maintenance efforts were not only in vain, but also caused greater losses, possibly expanding the scope of the fault. Some maintenance personnel are discouraged from VFD (Variable-frequency Drive) maintenance after bombing the module several times. Taking corresponding power-on test measures can basically prevent the occurrence of damage to the inverter module of the power-on test machine. As long as you are careful, there is basically no problem.
VFD (Variable-frequency Drive) main circuit power-on maintenance method 1: Disconnect the power supply of the inverter module. In fact, the circuit is connected to the copper bar. Remove a section of the connected copper bar, which is the positive power supply end of the three-phase inverter circuit. disconnect. Note: The disconnection point must be after the energy storage capacitor! Assuming that it is disconnected before KM, the energy stored on the energy storage capacitor will release enough energy to blow up the inverter module when the inverter circuit fails! The connection diagram is as follows:
Figure 1 Wiring diagram of the power-on inspection circuit of the VFD (Variable-frequency Drive) inverter circuit
Connect two 25W AC 220V light bulbs in series at the disconnection, because the VFD (Variable-frequency Drive) DC voltage is about 530V, and the withstand voltage of one light bulb is not enough (in case of failure), two must be connected in series to meet the withstand voltage requirements . Even if there is a short-circuit fault in the inverter circuit, the supply current of the inverter circuit is limited to within 100mA due to the voltage-reducing and current-limiting effect of the bulb, and the inverter module will no longer be in danger of being damaged.
VFD (Variable-frequency Drive) has no load, and U, V, W terminals are not connected to any load. First cut off the module OC signal output circuit of the drive circuit to prevent the CPU from taking a shutdown protection action and interrupt the test machine process (see the blog post "Maintenance of the drive circuit" for the specific operation method). The following situations may occur after power-on:
1. When the VFD (Variable-frequency Drive) is in shutdown state, the light bulb is on. The three modules have an upper and lower arm IGBT leakage, such as Q1 and Q2. This kind of leakage is not easy to be exposed under low voltage conditions. For example, a multimeter cannot detect it. However, after the DC high voltage is introduced, a large leakage occurs, indicating that there is a serious insulation defect inside the module. This is sometimes the case with purchased disassembled modules. It can be repaired by elimination method. If the bulb does not light up after the U-phase module (Q1, Q2) is removed, it means that the module is damaged.
2. After the power is turned on, the light bulb does not light up, but after receiving the running signal, the light flickers and lights up synchronously with the increase of the frequency, indicating that in the three-phase inverter module, one phase of the upper or lower arm IGBT is damaged. For example, when Q1 is turned on by the excitation signal, the damaged Q2 and the turned-on Q1 form a short circuit to the power supply. Two series-connected bulbs withstand 530V DC voltage to emit light.
3. After power on, the light bulb does not light up, and after receiving the running signal, the light bulb still does not light up; use the AC 500V gear of the pointer multimeter to measure the output voltage of U, V, W terminals, which rises uniformly with the increase of frequency, and the three-phase output voltage balance. It shows that the inverter output module is basically good, and it can be tested with some loads.
4. After power on, the light bulb does not light up, and after starting the VFD (Variable-frequency Drive), the light bulb still does not light up. However, when measuring the three-phase output voltage, it is unbalanced and seriously out of phase. Causes of failure: a. The IGBT tube of a certain arm has been damaged in an open circuit; b. The internal resistance of the IGBT tube of a certain arm has become larger and is close to an open circuit state. The way to detect this fault is as follows:
(1) Let us master the method of measuring the output voltage of VFD (Variable-frequency Drive) U, V, W terminals with DC voltage file. When the VFD (Variable-frequency Drive) output terminal outputs a three-phase balanced AC voltage, it means that the output voltage does not contain a DC component. In other words, at this time, the DC voltage value measured by the DC 500V range of the pointer multimeter is zero. When the output is out of phase, the essence is that a certain arm of the inverter output circuit IGBT is poorly conducted or is in an open state, resulting in the positive or negative half-wave output of the phase output, or the asymmetry of the positive and negative half-wave output of the phase , a DC component appears in the output voltage. When one arm IGBT is in an open circuit (open circuit) state, it is a pure DC component. At this time, use a multimeter to measure the DC 500V range, and the following results can be obtained: Assume that there is no DC voltage between U and V, but there is a DC voltage value between W, V and W, U, indicating that the W phase module is defective. If the red pen is connected to the W phase, the needle deflects positively, and the measurement indicates that the lower arm IGBT (Q6) of the W phase is poorly or not conducted; ) poor or no conduction.
It is also possible to change a measurement method and directly measure the voltage value between the three output terminal pairs P and N of U, V and W. Still use the DC 500V file. From the analysis, it can be concluded that when the upper and lower arm IGBT tubes Q1 and Q2 of the U phase are fully normally and symmetrically turned on, an "equivalent" partial voltage of 530V for the DC power supply is formed at the U terminal, and the U terminals P, N two points can measure one-half of the 530V DC voltage, that is, a DC voltage of about 260V. In an abnormal state, such a measurement result can be obtained, such as the voltage measured between P and U is much higher than 260V or even equal to 530V, indicating that the internal circuit of Q1 is broken or poor conduction; if the voltage measured between U and N Far higher than 260V or even equal to 530V, it means that there is an open circuit or poor conduction between C and E inside Q2, and the "normal voltage division" to 530V cannot be formed to increase the U-phase DC voltage.
(2) The following measurement method is also an effective method. Repair a 37kW TECO VFD (Variable-frequency Drive), check that the inverter module is damaged, the model is CM100DU-24H. Purchased a module of the same model, went through all the "programs" of offline measurement, and after confirming that the module had no problems, installed and powered on the test. The three-phase output voltage is very unbalanced. After thoroughly checking the drive circuit and confirming that there is no fault, measure the conduction internal resistance of the newly replaced module according to the wiring method shown in Figure 2 (simplified diagram), and the fault is eliminated after the new module is replaced.