A digital multimeter (DMM) is the first tool most of us reach for when troubleshooting an electrical issue. It provides a variety of measurement features that can help isolate a problem or narrow down the list of potential issues.
Troubleshoot Transient Voltages in VFDs Using a Digital Multimeter
When troubleshooting a variable frequency drive (VFD), a multimeter is an excellent tool to start with. Variable frequency drives are motor controllers that vary the frequency and voltage of power supplied to an electric motor. This gives users the ability to control the motor speed and achieve smoother starts and improved energy efficiency, which also reduces wear and tear on mechanical components. VFDs are important pieces of equipment in industrial settings and are tasked with keeping assets like pumps and fans running optimally.
While a multimeter cannot troubleshoot every issue in a VFD, it’s often the best tool to turn to when an issue arises. In this article, we’ll discuss how to detect transients, measure peak voltages, and when to employ other measurement tools, like an oscilloscope.
Understanding Variable Frequency Drives (VFDs) and Transient Voltages
VFDs work by taking the incoming AC voltage and changing it to DC, then storing it in capacitors. Then, it inverts the DC power back into AC power by turning six high-speed electrical switches fed by the DC power on and off to produce a pulse of voltage. This creates a sinusoidal AC current in a technique known as pulse-width modulation (PWM). With the six switches, users can control both the frequency and the voltage of the current.
The pulses of voltage are essentially square waves with fast-rising edges that can cause transient voltage reflections (“transients”) in the cable between the VFD and the motor. These reflections can combine with the incoming pulses and create voltage spikes. Transients are often caused by improper cabling.
Transients can cause problems with the insulation of the motor and impact cable integrity. Motor insulation is designed to withstand standard operating voltages. However, transient voltages can exceed these normal levels, causing problems like insulation breakdown, corona discharge, and thermal stress on the motor. VFD output cables can also be damaged over time by thermal degradation.
Using a Multimeter to Measure Transients in VFD Systems
Transient voltages can cause a VFD to trip and lead to various issues in the connected motor. If the motor is overheating, produces unusual noises or vibrations, or is experiencing insulation breakdowns, transient voltages could be the culprit. Here's how to use a multimeter to check for transients.
In this example, we use a Fluke 87V Industrial Multimeter. Procedures may vary slightly depending on which multimeter you use.
1. Safety Precautions
Before proceeding with any electrical measurements, ensure that you follow these safety precautions:
- Wear personal protective equipment (PPE). Always wear insulated gloves, safety glasses, and any other required PPE to protect against electrical shock and arc flash.
- Verify multimeter ratings. The Fluke 87V is rated CAT III 1000V and CAT IV 600V, making it suitable for industrial applications. Ensure your multimeter rating is appropriate for the voltage you’ll be working with.
- Inspect test leads. Make sure the test leads are in good condition, with no exposed wires or damage. Test leads should also be rated CAT III 1000V / CAT IV 600V.
2. Prepare the Fluke 87V Multimeter
- Select the correct measurement mode. Set the multimeter to the AC voltage mode (V~).
- Enable the Peak MIN MAX function. Press the MIN MAX button, then the continuity button. "PEAK MIN MAX" will appear on the display.
3. Connecting the Multimeter to Measure Peak Voltage
The power should remain on for this test. To measure the peak voltage between the phases at the motor terminals, follow these steps:
- Identify the motor terminals. Locate the terminals for the three phases of the motor, typically labeled L1, L2, and L3.
- Connect the multimeter test leads. Plug the black lead into the COM terminal and the red lead into the V terminal of the multimeter.
- Take the measurement:
- Place the black lead on terminal L1 and the red lead on terminal L2.
- The display will show the true RMS voltage.
- Press the MIN MAX button repeatedly to cycle through the peak maximum and peak minimum readings.
- Record the and max voltage readings. Repeat the measurement for other phase pairs (L2-L3, L1-L3).
4. Interpreting the Results
In a balanced three-phase 480V system, the RMS phase-to-phase voltage should be around 480V for each measurement.
Significant deviations from these values or differences between phase pairs could indicate issues with the VFD, motor, or supply wiring. High peak voltages may indicate the presence of harmful transients. Check equipment specifications for excess voltage thresholds.
5. Additional Considerations
- The Fluke 87V's Peak MIN MAX function captures peaks as short as 250 microseconds (µs), which can capture some transients, but not all.
- For more detailed analysis, consider using an oscilloscope or a motor-drive analyzer.
- Regular monitoring and trending of peak voltages can help predict potential issues before they lead to motor failure.
By using the Fluke 87V's advanced features, you can accurately measure and analyze peak voltages in VFD systems, helping to diagnose and prevent motor issues related to transient voltages.
Case Study: Real-World Application
An electrician arrives at a manufacturing plant to investigate a series of unexpected motor failures. The plant manager explains that several motors controlled by VFDs burned out over the past few weeks, despite being relatively new.
The electrician connects the multimeter to the motor terminals for each VFD. They set the device to record the minimum and maximum voltages.
As the VFDs operate, the electrician monitors the readings on the multimeter. They notice that the maximum recorded voltage is significantly higher than the nominal voltage, suggesting the presence of transient spikes.
Even though the electrician couldn’t see the exact shape or duration of the spikes, the abnormal voltage readings were enough to raise a red flag. Based on this data and their knowledge of how VFDs and long cable runs could generate transients, the electrician recommends adding dV/dt filters and checking the grounding and cable shielding to mitigate the issue.
Limitations of the Multimeter in Measuring Transients
If transients are present, the multimeter won't be able to precisely measure them. Instead, you may see the following:
- Fluctuating or unstable readings: If the transient lasts long enough, the multimeter may briefly display a higher voltage than expected. However, the multimeter will not be able to capture the full peak of the transient.
- Higher than expected voltage: If the MIN MAX function is activated, the maximum voltage reported may be higher than the steady-state voltage. However, the maximum value will not be as high as the full peak of the transient because of the multimeter's slower sampling rate.
- Inaccurate average readings: A true-RMS multimeter will display the average reading. However, since it uses peak transient voltages in its calculations, the average reading will still be higher than it should.
- Overload indication: If the transients spike higher than the voltage range selected on the multimeter, the device may display an overload warning.
Conclusion
Digital multimeters can give you an idea of whether transient voltages are present in your application. However, because of their limited bandwidth, they can’t precisely measure the peak voltage of the transient or tell you how long the transients last.
If your multimeter readings lead you to suspect transient voltages, a handheld oscilloscope can help you gather more information. An oscilloscope can provide a more detailed analysis of transient voltages and display a graphic that can tell you exactly how long the transient lasts and how high it spikes.
