Electrical Faux Pas

[z_p_e]

The purpose of this thread is to identify and correct any errors and assumptions being made regarding basic electrical theory.

It is hoped that this thread may act as place newbies and non-electrical types can ask questions on this topic as well.

Cheers,
z_p_e

[z_p_e]

Below are two errors that seem to occur on a regular basis, yet I have seen no one yet correct them.

These are fundamental faux pas and should not be propagated any further. They are:

1) Square waves contain all frequencies.

2) Transformers do not faithfully pass square waves. A 50% duty-cycle square wave applied to the primary of a transformer, will only yield a "short pulse" on its secondary. This is because most of the square wave time is spent on a steady voltage, either +V, or 0V... i.e. the wave form is hardly ever "changing" over time.

Both notions are absolutely false!

1) True square waves contain only odd order (integer) harmonics. A sawtooth wave form does contain all integer harmonics (one example).

2) All transformers have a finite bandwidth. Any wave form can be disassembled into its constituent sine wave components, a la Fourier. A square wave can be reasonably constructed using the first 5 sine waves, i.e 1, 3, 5, 7, and the 9th harmonics. The more harmonics applied, the more exact the resulting square wave shape will be. It comes down to transformer bandwidth, and what square wave frequency is applied to it. Too high an applied frequency, and the corners will start to become more rounded. In addition, the flat portions will begin to show "ripple" until finally, only the fundamental sine wave will appear. Too low a square wave frequency, and the flat portions will begin to tilt. At no time however between these two extremes, does the secondary exhibit a "short pulse-like" output. The transformer does not arbitrarily convert the square wave duty cycle to something other than what it is at the primary. Transformers therefore can pass square waves, and in fact any wave form faithfully, as long as the input frequency lies reasonably within the bandwidth of the transformer. If this were not the case, Audiophile tube amplifiers would not be possible.

[hansvonlieven]

Good boy z_p_e,

This sort of thing is sorely needed here. Thanks for starting this sort of thread.

Hans von Lieven

[BEP]

Excellent!

What would be a correct description of rotating magnetic fields? This is something that irks me every time it is mentioned but I've learned to shut up about it.

[z_p_e]

Here is something I posted almost a year ago. I think it still has relevance.

OK, first, I have not seen a practical down-to-earth explanation of what exactly a rotating magnetic field is. We all have said it, but what is it?

The purest example in my opinion, is to take a bar magnet polarized at the ends, and with a hole milled through the mid-point between the ends, the magnet is spun on an axis formed by this hole. This constitutes a constant magnetic field that is not only stable in magnitude, but one that is rotating as well. The distance between the ends of this bar magnet represent the diameter of a circle or toroid it would circumscribe, and it "splits" this toroid in half by virtue of its existence.

Of course we must envision the field as a ball-like shape, and remember that the poles can be rotated in any plane.