The digital multimeter (DMM) is a mainstay of electrical troubleshooting and the tool that most of us reach for first. They are versatile devices that can measure electrical properties including voltage, current, and resistance. Multimeters can also quickly identify faults, shorts, and open circuits, as well as test components like capacitors and resistors.
They’re especially helpful when troubleshooting issues in variable frequency drives (VFDs). VFDs are a type of motor controller that adjusts the voltage and frequency supplied to an electric motor. This allows users to control the motor speed, resulting in smoother starts, improved energy efficiency, and reduced wear and tear on mechanical components. They play an important role in industrial settings, ensuring that pumps and fans run at optimal speeds for the use case.
When a fault indicator light comes on, it's important to diagnose the issue and get it fixed quickly. In this article, we’ll walk through the steps for troubleshooting a VFD with a multimeter and when to utilize other tools for further diagnosis.
What is a VFD Fault?
When the VFD has a problem, it will display a fault code. Specific fault codes vary depending on the drive manufacturer. It is important to determine their meaning as soon as possible to troubleshoot the problem.
For example, say a VFD displays fault code F4, which indicates it is “under voltage” — a common error with VFDs. This code will be caused by one of the following three conditions:
- A fault in the drive itself
- A fault in the motor the drive is running
- A distorted supply voltage
From there, you can troubleshoot the drive, motor, and supply voltage to pinpoint the problem.
How to Troubleshoot with a Multimeter
First, we’ll check the input line voltage using a multimeter. Before testing, ensure you’re wearing the proper PPE for the environment and that the multimeter you’re using has the correct category safety ratings. For this test, the power needs to be on.
- Turn on your digital multimeter and select AC voltage. The symbol is a V with a wavy line (ṽ or V ~). Keep your meter in auto-ranging, or manually range your meter to the highest range or higher than the expected input voltage (e.g., 600V AC if you expect 480V AC input).
- Measure the voltage between each pair of input terminals. First, place the black lead into the COM jack and the red lead into the VΩ jack. Connect the black lead first, then the red lead, to the following terminals:
- L1 to L2
- L2 to L3
- L3 to L1
When removing the leads, remove the red lead first. The voltage between each pair of terminals should be roughly the same.
- Record the measured values and compare them to the expected input voltage. Ensure they are within the acceptable range specified in the VFD manual.
When troubleshooting a VFD with a multimeter, it’s important to know that different multimeters may display different voltages depending on how they calculate them. A regular multimeter displays the average value of the AC waveform and then multiplies it by a factor to estimate the root mean square (RMS) value. However, the voltage displayed by the multimeter may be up to 40% off from the actual value if the AC waveform is non-sinusoidal, which is often the case with VFDs.
A true-RMS multimeter measures the actual shape of the waveform and calculates the exact RMS value, providing more correct readings for the non-sinusoidal waveforms commonly found in VFDs.
With a digital multimeter, you can check the line input voltage, but depending on the type of distortion, a voltage reading alone may not reveal whether a problem exists. Noise, distortion, or transient failures may simply not be detectable with a multimeter.
If the voltage is within the specified range, the next step is to check the VFD DC bus voltage.
Checking the VFD DC Bus Voltage
To check the DC bus voltage, set your multimeter to test DC voltage. The symbol is a V with a straight line and a dotted line (V ⎓).
- Plug the black lead into the COM jack and the red lead into the VΩ jack.
- Connect the test leads across the + and - terminals.
- The voltage reading should be equal to approximately the RMS line voltage x 1.414. For example, for a 480V drive, the DC bus should be a little over 678V DC.
Check for any distortion or error in peak amplitude of the line voltage which can cause an over- or under-voltage error.
Let’s say that you check the drive's DC bus voltage and find that it's about 20% under the nominal value. That's a problem. But do you replace the drive controller, the motor, or both?
You need more information. The limitations of the multimeter mean that you’ll need a different tool to continue troubleshooting the problem.
When to Engage an Oscilloscope
Oscilloscopes are an essential tool for the next steps in troubleshooting a VFD. They provide real-time visualizations of electrical signals over time on a graph, revealing additional information that isn’t available while using a multimeter.
In addition to displaying the electrical signal, oscilloscopes can identify other problems that can cause VFDs to stop working. They can detect high-frequency noise that can cause interference with sensitive equipment. They also detect harmonic distortion in the AC waveform, which can cause overheating or reduced efficiency. Finally, oscilloscopes can detect transient failures like voltage spikes, surges, or dips, providing deeper insight into the cause of the VFD failure.
Multimeters provide a single reading of the average or RMS voltage being measured. And while that can be important in the first few steps of VFD troubleshooting, multimeters aren’t capable of capturing the waveform shape or detecting interference.
Conclusion
Digital multimeters are an excellent tool to get a quick voltage reading, and can be used in the first steps in troubleshooting a VFD. However, additional tools like oscilloscopes are needed for more complex issue detection. The additional information an oscilloscope can provide is essential for identifying other issues like interference and pulse-width-modulation signal integrity.
You can use a multimeter to get started with VFD troubleshooting. But you may need to move on to more specialized instruments if necessary when a problem presents itself.