Looking for Harmonic Content

There are two scenarios behind the need to understanding and then look for harmonic content. The first is heat damage to components and/or wiring while apparently carrying current well within the capability of the damaged parts. The second is ensuring a proposed non-linear load won't damage a system when the load is placed on it.

When it comes to large items, such as motor speed controllers, then harmonic profiles are usually available from the manufacturers allowing the electrical engineer to plan the wiring accordingly. However, it would appear that low current items, and this includes the PC, do not come with such profiles or even have them available from the manufacturers.

The fact is we are slowly becoming more and more reliant on the switch mode power supply. They are small, convenient, efficient, low heat (the list goes on) devices. "But they draw such low currents why are you worried?" This statement is true, but many small drops make an ocean. When hi-tech devices form the majority of the load structure then harmonic profiles of these devices become exceedingly important.

Before one thinks that this "hi-tech load" lark is being played a bit far, lets consider the typical work desk/station in a general office. There is almost undoubtedly a PC with accompanying monitor. Optionals will include a desk light and at least one charger for some device (cellphone, camera, organiser, etc.). Added to this is the fair share of printers, fax machines, and scanners each probably hitting as high as 1 in 5. This makes the average number of switch mode power supplies per modern desk around 5.

Now quickly count how many people work in your company and multiply that by 5. Yes, even in a medium concern of 40 employees we have already hit 200 SMPSs! Can you imaging if that was fair-sized office block with 2000 people i.e. 10000 hi-tech power supplies! Note that we have not counted all general common use items such as video equipment, TVs, display unit etc.

If you really want to shudder think of a specialist location like a share dealing area where there could be 2 PC's and 4 monitors per workstation. Now, do you still think the hi-tech stuff is not the major portion of the load?

We have taken a few low-energy items and graphed the harmonic profile of the current of each. Low-energy lighting (7W), PC, 17" monitor, laptop, and TV.

Before we do...



In the strive to understand more about harmonics, many turn to harmonic profiles of various devices as a means to identify site specific problems or (after ruling out the known culprits) as a means to locate the remaining causes of harmonic content.

This is exactly what gets folk into serious trouble. One client did this and landed up paying over 100k correcting their mistake (in fact, this figure was before a transformer blew up too). Learn from their mistake! You simply cannot approach harmonic content in this manner.


Harmonic Content of Low-energy Lighting (7W)

We start with the most surprising. Let's just say, they're asking that people switch to these in their homes!


Harmonic Content of the Modern Desk

Below is a graph of the harmonic content of the current of a typical modern desk with a PC and 17" monitor.

typical harmonic content of a SMPS current curve

The PC is in RED, the Monitor in GREEN, with the overall harmonic content of the desk in BLUE. The level of harmonic above the 40th was so low it was not displayed.


Harmonic Content of the Portable Modern Desk (Laptop).


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© 05.05.02