Troubleshooting a VFD before turning it on is a recommended selection, especially after a tripping, surge, or evident damage.
How to test a VFD offline (safely) with a multimeter?
1). Safety First
• Isolate all power sources.
• Allow for DC bus capacitors to drain (this could take 5-10 minutes).
• Before touching anything, confirm that the DC voltage is 0V.
2). Input Rectifier -Diode Test
• Perform a diode test on the input rectifier by setting the multimeter to diode mode.
• Place a red probe on DC+ & a black probe on each R, S, and T input terminal individually.
• Expect to see approximately 0.3-0.7V. Reverse the probes to read OL (open line).
• Repeat for DC.
3). Output IGBT Stage -Diode Test
• Perform a diode test on the output IGBT stage using a red probe on DC+ and black probes on U, V, and W.
• You should observe around 0.3-0.7V.
• Reverse the probes - must read OL.
• Repeat with red on DC and black on U, V, and W.
4). Check for Short Circuits
• Check for short circuits by switching to resistance (or) continuity mode. Input resistance (R/S/T) to ground should be high.
• Measure the resistance between the output (U/V/W) and ground, which can be high.
• Any continuity (or) low resistance here indicates an internal short.
5). Visual Inspection
• Check for burnt components, bulged capacitors, and fractured IGBTs.
• Ensure that fans, connectors, & terminal blocks are clean & tight.
Why it is important?
Before turning on the power, perform these basic checks to discover shorted diodes, IGBTs, or grounding difficulties.
It is an important step in preventing damage to motors & electrical equipment.