Methods for Testing Variable Frequency Drives (VFDs) Using a Multimeter
Testing the Rectifier CircuitLocate the positive DC terminal P and negative DC terminal N inside the variable frequency drive (VFD). Set the multimeter to the ×10 resistance range.Connect the red probe to terminal P, and touch the black probe to terminals R, S, T one by one. Each should show a resistance of approximately several tens of ohms, with roughly balanced values across phases.Reverse the probes: connect the black probe to P, and the red probe to R, S, T in sequence. The resistance should be nearly infinite.Connect the red probe to N and repeat the above steps; the results should be identical.
Abnormal conditions indicating circuit faults:A. Unbalanced three-phase resistance → rectifier bridge failure.B. Infinite resistance when the red probe is connected to P → rectifier bridge or starting resistor failure.
Testing the Inverter CircuitConnect the red probe to terminal P, and the black probe to U, V, W terminals one by one. Each should show a resistance of several tens of ohms, with similar values across phases; reverse readings should be infinite.Connect the black probe to N and repeat the steps. Identical results are expected; otherwise, the inverter module is faulty.
II. Dynamic Testing
Dynamic testing (power-on operation) may only be performed after passing static testing.Observe the following before and after power-on:
Before powering on, verify the input voltage. Connecting 380V to a 220V-class VFD will cause component burnout (capacitors, varistors, power modules, etc.).
Check that all VFD connectors are properly attached and secure. Loose or abnormal connections may cause faults or even burnout.
After power-on, check the fault display to preliminarily identify the fault type and cause.
If no fault is displayed, first check for abnormal parameters. After resetting parameters, start the VFD without load (no motor connected) and measure the three-phase output voltages U, V, W.Phase loss, unbalanced three-phase voltage, etc., indicate faults in the inverter module or drive board.
With normal output voltage (no phase loss, balanced three phases), perform loaded testing, preferably under full load.
