Harmonics & Waveform Measurements Low Pass Filter & Sine Wave Filter for a 460V 100HP AC Motor with VFD

Published by Dranetz™ Technologies, Inc., Case Study, Website: dranetz.com.

This case study was submitted by Ing. Leopoldo Martínez Basulto, Dranetz México.


Harmonics are an important PQ issue today. Variable Frequency Drives (VFD), Uninterruptible Power Systems (UPS), computers, LED lighting, electronic ballasts, etc. are widely used in all kinds of facilities. It is well known that such devices can cause many problems at end user’s installations and utility networks, such as: Voltage distortion, damaged capacitor banks, tripping breakers, blown fuses, overheating of cables and transformers, damage to PLC’s and sensitive electronic loads, etc.


Variable Frequency Dives are becoming very popular in controlling AC motors for industrial and commercial applications. They provide speed control for processes such as ventilators, pumps, air compressors, elevators, etc. at substantial energy savings. However, VFD’s generate significant harmonic currents depending upon the power electronics technology used (number of pulses, SCR, thyristors, etc.). In many cases, they exceed IEEE-519 recommendations and cause some of the problems mentioned above. Also, Pulse Width Modulated (PWM) drives can cause additional problems in AC motors, such as premature bearing wear and short-circuits in windings due to spikes from modulation if the motor is not designed for this condition.

In many applications, harmonic mitigation techniques are required, along with proper power monitoring instrumentation to measure the harmonics spectrum and waveforms. This can be essential in assuring such solutions meet customer requirements. Passive filters have proven their effectiveness at reasonable prices.

This case study is a customer witness and performance test carried out at the laboratory of a manufacturer of two passive filters to be supplied. The test is conducted with a 460V, 100HP AC motor with a 6-pulse PWM VFD. One Low Pass Filter (LPF) is installed at the input of the VFD (load side), and one Sine Wave Filter (SWF) is installed at the output of the VFD (motor side).

Figure 1: Low Pass Filter and Sine Wave Filter Locations

Four Dranetz PowerXplorer and PowerVisa Power Quality analyzers were used to measure the effectiveness of the filters, monitoring the points shown in Figure 1 above: Low Pass Filter Input, Low Pass Filter Output, Sine Wave Filter Input, and Sine Wave Filter Output.

Figure 2: Test In Progress

Below are waveshapes captured during the test:

LPF Output (No Harmonics Filtering)

LPF Input (Harmonics Filtering)

SWF Input (VFD Output Terminals)

SWF Output (Moto Terminals)


  • LPF (low pass filter) reduced VTHD from 6.3% to 1.5% and ITHD from 70% to 6%. Corresponding waveforms were also improved as well.
  • SWF (Sine wave filter) effectively improved the voltage waveform at the 100HP AC motor terminals (as shown).
  • Dranetz Power Quality analyzers advanced capabilities were essential in measuring not only at VFD line side, but also measuring and capturing the PWM waveforms High frequency pulses at VFD output)!
  • The customer was very satisfied, not only with the filtering results, but with the Dranetz PQ analyzers. He said: “With other manufacturers like Fluke, one PQ instrument and one oscilloscope are required to be able to perform VFD & filtering tests, which implies more cost about instrumentation.”
  • The filter customer decided to buy one PowerGuide portable instrument from Arteche Inelap, an authorized Dranetz distributor in Mexico who is fully committed to Dranetz products.

Published by PQTBlog

Electrical Engineer

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