Accurate Measurement of VFD Output Voltage to Motor

Measuring the output voltage from a Variable Frequency Drive (VFD) to a motor requires careful attention due to the high voltages and frequencies involved. Here is a step-by-step guide on how to do it safely and accurately, ensuring that your readings are reliable and informative.

Key Equipment Needed

Multimeter: A true RMS multimeter is recommended for accurate readings of AC voltages since VFD outputs are often non-sinusoidal. Oscilloscope: Optional but useful for a more detailed analysis of the waveform. Safety Gear: Insulated gloves and safety goggles.

Steps to Measure Output Voltage

Safety First

Wear Safety Gear: Ensure that you are wearing appropriate safety gear including insulated gloves and safety goggles. Understand VFD Configuration: Make sure the VFD is properly configured and that you understand its operation.

Set Up the Multimeter

Select Appropriate Settings: Set the multimeter to the appropriate AC voltage setting. If your instrument has a true RMS setting, use that for more accurate readings.

Identify Measurement Points

Locate Output Terminals: Identify the output terminals of the VFD that connect to the motor. These are usually labeled U, V, and W or R, S, and T.

Take Measurements

Phase-to-Phase Voltage: Measure the voltage between two of the three output terminals, for example, U to V, V to W, or W to U. This will give you the line-to-line voltage. Phase-to-Neutral Voltage: If you need to measure phase-to-neutral voltage, measure from one output terminal to the ground, if applicable. However, this is less common for VFD applications.

Record the Values

Document Readings: Write down the voltage readings for each combination you measured.

Optional: Analyze the Waveform

Connect Oscilloscope Probes: If using an oscilloscope, connect the probes to the output terminals to visualize the waveform. This can help you identify issues like harmonics or distortion.

Important Considerations

Frequency: The output frequency of a VFD can vary widely based on the motor's speed settings so be aware that the voltage may change. Harmonic Distortion: VFDs can produce waveforms that are not purely sinusoidal, leading to harmonic distortion. A true RMS multimeter is crucial for accurate readings. Voltage Ratings: Ensure that your measuring equipment can handle the expected voltage levels. Avoid Direct Connection: Do not connect the multimeter directly across the output terminals without proper isolation. This can damage the multimeter or cause electrical hazards.

Conclusion

Measuring the output voltage from a VFD requires careful setup and safety precautions. Using the right tools and techniques will ensure accurate readings and help you maintain the equipment effectively. If you're ever uncertain, consult the VFD's manual or a qualified technician.