Maintenance principle and safety precautions of VFD (Variable-frequency Drive)
Maintenance principle and safety precautions of VFD (Variable-frequency Drive)
The basic principle of AC-DC-AC frequency conversion: First, filter the three-phase AC power and convert it into DC power through a bridge rectifier circuit, and then convert the DC power into three-phase AC power output with different frequencies by the bridge inverter circuit after filtering.
1. Determine the fault range of VFD (Variable-frequency Drive)
In actual experience maintenance, VFD (Variable-frequency Drive) is mostly caused by damage to power electronic components of the main circuit when there is no VFD (Variable-frequency Drive) circuit schematic diagram. For the main circuit part, first judge the fault range, power on the VFD (Variable-frequency Drive), and measure whether the DC bus voltage value is equal to 1.35 times the effective value of the input voltage. If the voltage is normal, it can be judged that the inverter part is faulty. Otherwise, the rectifier power element, pre-charging circuit or filter capacitor may be damaged.
For a small number of VFD (Variable-frequency Drive) with a contactor inside, the contactor is the pre-charging part of the DC bus. ” signal to activate the contactor. If the contactor does not start and there is no DC bus voltage, the fault range cannot be judged. First, simulate the "no fault" feedback signal and external start signal of the given inverter part, artificially let the contactor pull in, and measure the DC bus voltage, and judge the fault range according to the magnitude of the DC voltage, the method is the same as above. Note that before starting the pre-charging contactor, the given signal is sometimes a pulse trigger signal rather than a level signal.
2. Static detection of rectifier unit
The method of judging the damage of a certain power element in the rectification part is to use the one-way conductivity of the rectification element. Under static conditions, the positive and negative resistance values should be different when they are normal. The specific method is as follows:
The three-phase bridge rectification circuit in the rectification part may be diode rectification, thyristor half-controlled rectification, thyristor full-control rectification or igbt rectification. No matter which method is used, the three-phase rectifier circuit is symmetrical, so the static test resistance value should conform to the principle of symmetry, that is, the three-phase input or output terminal should be symmetrical with respect to the positive and negative positive and negative test values of the DC bus under static conditions. Select the "diode" file of the multimeter.
(1) In the first step, connect the red test lead to the positive pole of the DC bus, and the black test lead to the power input three-phase terminal respectively. The three test values should be the same. Conversely, connect the black test lead to the positive pole of the DC bus, and the red test lead to the three-phase wiring of the input power supply respectively. The three test values should also be the same. If a diode rectifier bridge is used for rectification and conduction, the multimeter will display 0.4~0.6v, and it will display infinity when the reverse is cut off. If the deviation of the three-phase measured values is large, or the positive and negative measured values of a certain phase are similar or the same, the diode element is damaged.
(2) In the second step, connect the red test lead to the negative pole of the DC bus, and the black test lead to the three-phase connection of the input power supply respectively. The three test values should be the same. Conversely, connect the black test lead to the negative pole of the DC bus, and the red test lead to the three-phase wiring of the input power supply respectively. The three test values should also be the same. For the pre-charging circuit designed after the rectifier bridge, this operation can be judged in the same way The quality of the three rectifier components in the negative half cycle of the rectifier bridge (the test method for the 12-pulse rectifier bridge is the same as above).
Note: For the design of the pre-charging circuit before the rectification circuit, it adopts the thyristor half-controlled or full-controlled bridge rectification, and the test result should be that one phase is different from the positive and negative resistance test values of the other two phases, that is to say, one phase is actually The resistance value of the tested diode pre-charge loop.
3. Static detection of the inverter unit
The principle of the three-phase inverter bridge triggered by 6 pulses is also to use a freewheeling diode connected in parallel in each inverter igbt module. There is unidirectional conductivity under static conditions. The measurement method is the same as the rectifier bridge detection method, that is, the DC bus positive, Compare the test values of the three-phase output points with the negative pole, and the three-phase test values should be the same. The multimeter displays 0.3~0.4v when the component is turned on in one phase, and infinity when it is reversed. The short-circuit fault of the main circuit may also be caused by the abnormality of the varistor protecting the power component, which often damages the power component.
4. Control circuit detection
The detection method of the control circuit takes acs800-04 as an example. After the VFD (Variable-frequency Drive) is powered on, observe that the signal light v204 on the aint motherboard is green, indicating that +5v is normal; The green light indicates that the igbt gate drive is normal, the red light on the rmio external signal interface board indicates a fault, and the green light indicates that the power supply +24v is normal. Finally, use an oscilloscope to detect whether there is a trigger signal at the trigger pole of each power element. Generally, there is a 5v voltage trigger. Circuit boards without signal lights (rint main circuit interface board, rrfc filter board, rvar piezoresistor board, etc.)