In the maintenance process, a multimeter should be used to assess the condition of electronic components based on the specific fault situation. If the measurement technique is not applied correctly, it can lead to incorrect diagnoses, complicating the repair process and potentially causing financial losses. There are two main measurement approaches: component testing and board-level testing. During a road test, the inverter power supply should be disconnected, and components on the circuit board should be tested without removing them. This method allows for quick identification of damaged parts due to breakdowns, short circuits, or open circuits. However, it's important to account for the influence of adjacent components connected in parallel, as they may affect the readings. To avoid misjudgment, proper procedures must be followed.
First, testing ordinary diodes: Using an MF47 multimeter, connect the red and black leads to the diode’s terminals. Swap the leads and compare the readings. A good germanium diode typically shows a forward resistance of 300–500 Ω, while a silicon diode has a forward resistance of around 1 kΩ or higher. The reverse resistance should be significantly larger—several tens of kiloohms for germanium and over 500 kΩ for silicon. If both forward and reverse resistances are close to zero, the diode is shorted. If they are very high or infinite, the diode is open. In such cases, the diode should be replaced.
During a road test, measuring the forward and reverse resistance of the PN junction helps determine if the diode is shorted or open.
Second, testing transistors: Switch the digital multimeter to the diode mode and measure the PN junctions. If conducting, the display will show the forward voltage drop. To identify the collector and emitter, measure the forward voltage drop of the two junctions. The emitter junction will have a higher drop than the collector. When testing, if the red lead connects to the base, it's an NPN transistor; if the black lead connects to the base, it's a PNP. After determining the type, check the forward and reverse resistance of each junction. If the measured values are similar, the transistor may be damaged. If the resistance is too low, the junction is shorted; if it's too high, the junction is open.
Third, testing three-phase rectifier bridge modules: For example, with a SEMIKRON module, set the multimeter to the diode test mode. Connect the black lead to COM and the red lead to VΩ. Measure the forward and reverse characteristics between the 3, 4, 5 and 2 poles. A good module will show a large difference between forward and reverse resistance. If the resistance is zero in either direction, the phase is shorted; if it's infinite, the phase is open. Any damage to one phase requires replacement of the entire module.
Fourth, testing MOSFETs: Connect the black lead to the D terminal and the red lead to the S terminal. Normally, the resistance should be around 500–600 Ω. Then, touch the G terminal with the red lead and measure the S terminal again. If conduction occurs, the MOSFET is functioning. Reversing the leads and repeating the test confirms its integrity. If the resistance between S and G is below 30 Ω, the MOSFET is likely damaged.
Fifth, testing IGBT modules: Set the multimeter to the diode test mode and check the forward and reverse characteristics between C1-E1, C2-E2, and G-E1/E2. For a German EUPEC 25A/1200V module, the resistance should be consistent across all phases. If there's a significant deviation, the module may need replacement. Also, check the gate-to-emitter resistance. If the multimeter shows any value, the gate might be faulty. Zero resistance indicates a short, while infinite resistance suggests an open circuit.
Sixth, testing electrolytic capacitors: Use the MF47 multimeter and select the appropriate range. Capacitors under 47μF are tested on R×1K, while larger ones use R×100. Connect the red lead to the negative terminal and the black lead to the positive. The needle should swing right and then return to a high resistance value. Reverse the leads and repeat the test. A good capacitor should show high leakage resistance. If no movement is observed, the capacitor is faulty.
Seventh, testing inductors and transformers: Use the multimeter’s resistance setting to measure inductor windings. Zero resistance indicates a short, while infinite resistance means an open circuit. For transformers, check insulation resistance using R×10K. Infinite resistance is ideal. Test winding continuity with R×1. Small resistance values suggest normal operation, while infinite resistance indicates an open circuit.
Eighth, testing resistors: Ensure the circuit is powered off and capacitors are discharged before measuring. Avoid interference from other components. Take multiple readings for accuracy.
Ninth, handling surface-mount devices (SMD): These are commonly found on modern inverter boards. They include chip resistors, capacitors, and integrated circuits. Disassembling SMD components requires precision. Use a 35W soldering iron with a fine tip. Remove components carefully, avoiding damage to the board. Soldering involves cleaning the area, applying flux, and carefully placing the component before soldering each pin.
Proper techniques ensure accurate diagnosis and effective repairs, reducing the risk of further damage and improving overall system reliability.
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