What are the typical characteristic harmonics for electrical loads?

What are the characteristic harmonics or the common, typical harmonics for various different loads in a power system?

To understand how harmonics are determined and what the typical harmonics are on a power system, you first have to understand some equations. Drives, whether 6-, 12-, 18- or 24-pulse, follow the equation h equals np plus minus 1. This means that the harmonics that you see are n, which is an integer, times p (pulse), plus or minus 1. If you have a 6-pulse drive, you’re going to have 6 plus or minus 1, 5th and 7th, 2 times 6 plus or minus 1, 11th and 13th, and so on. A 24-pulse drive will give 24 plus or minus 1 and 48 plus or minus 1, and so forth.

EC motors have harmonics that look a lot like 6-pulse drives. They have a lot of the 5th and 7th harmonics that are even more substantial in terms of magnitude because the EC motors don’t have line reactors or DC chokes inside of them. However, the characteristic harmonics are the same.

Computer power supplies, fluorescent lighting, LED lights, and arcing devices have all the odd harmonics. A single-phase drive or a single-phase input on a three-phase drive would give the same characteristic harmonics for switch mode power supply or the LED lights. 

Loads such as a transformer have short-term harmonic spectrums. For example, when you turn on and energize a transformer, you hear a humming sound—that is 2nd, 3rd and 4th order harmonics.

One of the important things to recognize is that the magnitude of the harmonics typically decreases as you go up in frequency. So, if you have a 6-pulse drive, the 5th harmonic is the most predominant and then it decreases as the frequency goes up.

Waveforms are going to show the current harmonics you have on the system. A characteristic waveform for a 6-pulse drive is a double hump current waveform. The current is asymmetrical so the top half and the bottom half of the waveform—the positive and negative half cycles—are the same. But if you look at a transformer’s waveform, it is going to be different—the top half and the bottom half of the waveform are not the same.

On a waveform, the even harmonics are going to be asymmetrical—the 2nd, 4th and 6th. But when you look at characteristic waveforms for loads that produce 2nd, 3rd, 4th, 5th harmonics, the even harmonics a lot of times will look a little different and depict a short-term waveform, such as from the humming of a transformer.  

Transformers,  variable frequency drives, computer power supplies, everything that converts AC to DC in terms of rectifiers—any nonlinear loads—are going to produce characteristic or typical harmonics. And, those waveforms are pretty similar across the board. The magnitude of the harmonics depends on the quality of the rectifier or the system that’s built in terms of the current.