How to Test a Variable Frequency Drive, VFD or Invertor

In this guide, we’ll delve into the process of testing an Inverter or a Variable Frequency Drive (VFD). Many engineers are unaware of how to test these devices, even though they are very common in Factory machinery. Let’s break down the process step by step.

This blog post started life as a video on our Engineering youtube channel follow the link for more engineering training content. 

1. Understanding the Basics

      • An inverter or VFD, like the one discussed in the video, is a single-phase to three-phase variable frequency drive.

      • Inside the inverter, there’s a set of diodes. Specifically:

      • A bridge rectifier on the AC side that rectifies to DC.

      • Diodes in the IGBTs on the output side.

    2. The VFD Structure

        • Input Side:

        • AC power source enters.

        • There’s a bridge rectifier:
              • On a single-phase inverter, there are four diodes.

              • On a three-phase inverter, there are six diodes.

          • A smoothing capacitor is present to smooth out the waveform, converting AC to DC.

          • Output Side:

          • Contains IGBTs (Insulated Gate Bipolar Transistors) that typically have a diode.

          • There are three of these IGBTs for three-phase output.

        3. Testing the Diodes

            • The primary test involves checking the diodes on both the input and output sides.

            • A diode will allow a voltage drop across one side and block it on the other side.

          Steps:

              1. Use a Multimeter with Diode Test Function: Most multimeters come with this function.

              1. Test the Input Side:

                • Connect the multimeter to the input terminals for the single-phase AC.

                • Check for a voltage drop (e.g., 0.437 volts).

                • Swap the multimeter probes and ensure there’s no reading on the other side.

                  1. Test the Output Side:

                    • Connect the multimeter to the motor terminals, which are the three-phase rectified or inverted three-phase waveform for the motor drive.

                    • Check for a voltage drop and ensure the readings are balanced across all three phases.

                    • Swap the multimeter probes and ensure there’s no reading on the other side.

                  4. Identifying Issues

                      • If readings are present on both sides of the diode, there’s an issue with the power supply internally.

                      • In the video, a faulty inverter was tested, and it showed readings on both sides, indicating a problem.

                      • Upon inspection, some components inside the faulty inverter were visibly damaged.

                    5. Potential Causes of Damage

                        • Inverters are sensitive to water. In industries like the food sector, panels often get sprayed, and water can damage the electronic components inside the inverter.

                      Conclusion

                      Testing an inverter or VFD is a straightforward process that can quickly indicate whether the device is functioning correctly or not. Regular checks can prevent potential issues and ensure the longevity of the device.

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