Investigating harmonic distortion in motors

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A Fluke power quality analyzer display harmonics data.
 

Investigating harmonic distortion in motors

Simply stated, harmonics are any unwanted additional source of high frequency ac voltages or currents supplying energy to the motor windings. Harmonics decrease motor efficiency resulting in added cost and an increase in operating temperatures.

This additional energy is not used to turn the motor shaft but circulates in the windings and ultimately contributes to internal energy losses. These losses dissipate in the form of heat, which, over time, will deteriorate the insulation capability of the windings.

Some harmonic distortion of the current is normal on any part of the system serving electronic loads.

First, identify equipment suspected of producing the bulk of the harmonic currents. Most often these are variable frequency drives (VFD's).

To start investigating harmonic distortion, use a power quality analyzer to monitor electrical current levels and temperatures at transformers, or the point at which the VFD connects to other system loads (often called the Point of Common Coupling) to be sure that they are not overstressed.

Each harmonic has a different acceptable level of distortion, which is defined by standards such as IEEE 519-1992.

Use quality test and measurement instruments, such as the Fluke 435-II Three-Phase Power Quality Analyzer, to analyze the frequencies and amplitudes of the harmonics. A best practice in industrial plants is to use a power quality recorder to capture data over 24 hours in order to monitor equipment during one complete plant cycle.

CAUTION: Troubleshooting often requires working on live circuits; be sure to follow all safety requirements of NFPA 70E for electrical safety in the workplace.

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