This is a classic problem when dealing with Variable Frequency Drives (VFDs) and power measurements, especially at lower frequencies. Here's a breakdown of potential causes and some recommended meters

Why the Discrepancy at Low Frequencies?

The most likely reason for the disagreement between the power loggers at lower frequencies is their handling of the non-sinusoidal waveforms produced by the VFD. Here's a deeper dive:1. Harmonic Distortion



VFD Output

VFDs don't output pure sine waves. They create the variable frequency and voltage by rapidly switching transistors (typically IGBTs). This results in a waveform with significant harmonic distortion. The lower the output frequency, the more pronounced these harmonics become relative to the fundamental frequency.

Logger Sensitivity

Different power loggers use different methods to calculate current, voltage, and power. Some are more susceptible to being "fooled" by the harmonics than others.

True RMS vs. Average-Responding

This is the critical difference.

True RMS Meters

Accurately measure the RMS (Root Mean Square) value of any waveform, regardless of shape. This is essential for VFD outputs. They account for the heating effect of all the harmonics.

Average-Responding Meters (Calibrated to RMS for Sine Waves)

These meters measure the average value of the waveform and then apply a scaling factor assuming it's a perfect sine wave. When the waveform is distorted (like from a VFD), they give inaccurate readings. The higher current readings you're seeing likely comes from this type of meter.2.

Sampling Rate and Filtering



Sampling Rate

To accurately capture the waveforms, a power logger needs a sufficiently high sampling rate. Lower sampling rates can miss the rapid changes caused by the VFD switching.

Filtering

Meters often employ filtering to remove high-frequency noise. If the filter design isn't optimal, it might not properly handle the harmonics from the VFD, leading to inaccurate readings. Poor filtering can allow noise and harmonics to be interpreted as part of the fundamental current, resulting in inflated measurements.3.

Current Transformer (CT) Accuracy



Saturation

If you're using external CTs (Current Transformers), they can become saturated at lower frequencies, especially if the current is high. Saturation means the CT core can't accurately transform the current, leading to distorted and inaccurate readings. This is less likely to be the primary cause if both loggers use the same CTs and only one is reporting the issue.

CT Frequency Response

Some CTs have limited frequency response. While they might work fine at 50/60 Hz, their accuracy can degrade at lower frequencies and with harmonic content.4.

Phase Angle Errors



Reactive Power

VFD outputs can have complex reactive power components, especially at lower speeds. Some meters struggle to accurately calculate power factor and reactive power with these waveforms, which can indirectly affect the current readings.

What to do to Troubleshoot

1.

Identify Meter Types

Find the model numbers of both power loggers and research their specifications. Crucially, determine if they are True RMS or Average-Responding. The one that is Average-Responding is likely the culprit.2.

Check CT Specifications

If using external CTs, verify their frequency response and burden rating. Ensure they are appropriate for the VFD output.3.

Inspect the Waveforms (If Possible)

If either logger allows you to view the voltage and current waveforms, examine them closely at both high and low frequencies. This will visually show the harmonic distortion. An oscilloscope can be useful here if the loggers can't show waveforms.4.

Compare Power Factor (PF)

Examine the Power Factor readings from both loggers. A significant difference in PF might indicate one meter is struggling with the reactive components.5.

Consult Manufacturer Support

Contact the manufacturers of both power loggers and explain the problem. They might have specific recommendations or firmware updates to improve accuracy with VFDs.

Recommended Power Loggers for VFD Outputs

When selecting a power logger for VFD applications, the key is to ensure it's designed to handle non-sinusoidal waveforms accurately. Here are some recommendations, keeping in mind that specific model availability and pricing can vary:

Fluke 1738 Advanced Power Logger

Considered an industry standard. Features excellent harmonic analysis and data logging capabilities. Specifically designed for power quality studies, including VFD applications. It is a true-RMS meter.

Hioki PW3198 Power Quality Analyzer

Another excellent choice known for its accuracy and ability to capture transient events. Offers detailed harmonic analysis and is well-suited for VFD troubleshooting. It is a true-RMS meter.

AEMC Instruments PEL 103

A more affordable option that still provides true RMS measurements and decent harmonic analysis. Good for general-purpose VFD monitoring. It is a true-RMS meter.

Amprobe DM-III Power Quality Recorder

A good option if you need a durable and portable power quality recorder with true RMS measurement capabilities.

Key Features to Look For in a Power Logger for VFDs



True RMS Measurement

Absolutely essential for accurate readings of non-sinusoidal waveforms.

High Sampling Rate

The higher the sampling rate, the better it can capture fast transients and harmonics. Aim for at least 128 samples per cycle or higher.

Harmonic Analysis

The ability to measure and display individual harmonics (THD - Total Harmonic Distortion) is extremely valuable for understanding the VFD output.

Current Transformer (CT) Compatibility

The logger should be compatible with a wide range of CTs and allow for accurate scaling. Ensure the CTs themselves are suitable for the frequencies and currents you are measuring.

Data Logging Capacity

Sufficient memory to store data for the required duration.

Software and Connectivity

User-friendly software for data analysis and reporting. Connectivity options (USB, Ethernet, Wi-Fi) for easy data transfer.

Voltage Transient Capture

Capable of capturing voltage transients due to VFD switching for power quality analysis.

Important Considerations



CT Placement

Ensure the CTs are placed on the motor side of any harmonic filters that might be installed with the VFD.

Safety

Always follow proper safety procedures when working with electrical equipment. Use appropriately rated PPE (Personal Protective Equipment).

Calibration: Regularly calibrate your power loggers to ensure accuracy.By understanding the potential causes of the discrepancy and choosing a power logger specifically designed for VFD applications, you can obtain reliable and accurate measurements of motor performance. Remember to investigate the specifications of your existing loggers to pinpoint the source of the problem. Good luck! Flag for review