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Effects of Harmonic Currents

New technology, new challenges

A mystery is occurring in today’s Schools, office buildings and manufacturing plants.

Transformers supplying seemingly average loads are overheating. Neutral conductors in balanced circuits are overheating from excessive loads.

Circuit breakers are tripping for no apparent reason. Yet the standard troubleshooting procedures show everything to be normal. So what’s the problem?

In one word—harmonics.


Harmonics are the byproducts of modern electronics. They are especially prevalent wherever there are large numbers of personal computers, adjustable speed drives, and other types of equipment that draw current in short pulses.

This equipment is designed to draw current only during a controlled portion of the incoming voltage waveform. While this dramatically improves efficiency, it causes harmonics in the load current. And that causes overheated transformers and neutrals, as well as tripped circuit breakers.

If you were to listen to an ordinary 60-cycle power line, you’d hear a monotone hum.

When harmonics are present, you hear a different tune, rich with high notes. The problem is even more evident when you look at the waveform. A normal 60-cycle power line voltage appears on the oscilloscope as a near sine wave (Figure 1).


Figure 1. Near sine wave

When harmonics are present, the waveform is distorted (Figure 2A

and 2B).

Figure 2A. Distorted current waveform

Figure 2B. Distorted voltage waveform

These waves are described as non-sinusoidal. The voltage and current waveforms are no longer simply related-hence the term “non-linear.”

These distorted waveforms create real problems in todays Schools, office buildings and manufacturing plants.

Check out the next page on "Effects of Harmonic currents" for a detailed explanation.