AC sine wave: What does increasing the frequency do?

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OK. I get it.

Thanks,

Yes. And all without any name-call "a. For a sinusoidal source, a time-varying resistive load can have a load current with a non-zero fundamental phase shift, hence a reactive load component. This load component can be expressed as an equivalent inductance or capacitance.

b. For a sinusoidal source, a time-varying reactive load can have a load current with a non-quadrature phase shift, hence a real load component. This real component can be expressed as a positive or negative equivalent resistance. This is why a varicap can be used as a parametric amplifier.

In case a, it takes no power to vary the resistance (as say moving a pot wiper or switching resistors in or out) because the synthesized reactance doesn't dissipate power. In case b, power must be involved in varying the reactance (spinning the shaft of a variable cap, or pumping a varactor) because we're synthesizing a real resistance."

I love this newsgroup because a simple, one sentence question can generate days of technical exchange that teaches quite a few of us things we knew but didn't realize. :-)

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John Popelish

It amazes me that my simple question on power spawned this much discussion! I followed a little of it, but for the most part it made me realize how much I have to learn... :-)

Cheers!

-Commander Dave

Tons of theoretical work has been done on time-varying capacitances, mostly in the 60's and such when two-terminal devices (varactors, tunnel diodes, step-recovery diodes) were the rage.

I like to try to avoid equations until I can really feel what's going on. Being able to do the math doesn't mean you understand it, just that you can push some symbols around. This is risky of course, because instincts are often wrong about stuff like this. But the guys who just do the math can make ghastly blunders, too, and they sometimes don't have the instincts to recognize an absurd result when they see it.

Things like Fourier transforms can be visualized and sort of done by inspection, but not many EE courses try to track that along with the math.

To a creature that was sufficiently intelligent, everything would be intuitively obvious.

John

The wheels are turning.

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John Popelish

You too?

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John Popelish

The classes of gyrators here, has made for an interesting and

*understandable* thread. Sometime this kind of stuff will be written up in depth, (maybe already has!). I'll bet that during the process 'understanding' will be #1 item to fall by the wayside :-) regards john

This is a mechanical problem. At the most basic level the generators are unable to maintain their rpm while heavily loaded.

'scuse my ignorance, but doesn't an incandescent lamp behave more like say a triac or a set of zeners after the startup cycle? this means, the gas ignites at a specific voltage (think it was about 80V somewhere) thus lowering the lamps impedance from that point on. that's also the reason of the coil in the circuit, the impedance reduces lamps current, it allows the lamps voltage to drop to the burning point. the gas stays ignited until again a specific voltage where it stops conducting, and re-ignites again at a specific voltage the other half of the cycle. only other thing in the circuit is a coil, so i don't see the "capacitor" anywhere (view drawing in notepad using fixed font)

|mains | . . | . . |. . .-----------.------------.------ | . . | . . | . . | | .. | . . | . . | . . |....----- .....-------.......-- | . . |lamp . . |current . . | .. |

What you are talking about is not an incandescent lamp, it is a fluoroscent lamp.

An incandescent lamp has a thin wire inside which glows, it is the most common lamp in the world. But fluoroscents with their higher efficiency are taking increasing parts of the market.

It is difficult to keep all these technical terms apart in english if you are not born in an english-speaking country.

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Roger J.

oops, sorry, my mistake :-( indeed I'm dutch, thereof the mistake

So, could a motor-driven variable capacitor, paralleled with an inductor, become an oscillator? Seems like it.

John

Wasn't it Tesla that proposed the 60 Hz. standard? And probably because there are 60 minutes in an hour, and 60 seconds in a minute, and therefore 60 cycles in a second, and 60 was high enough to avoid flicker, and maybe some other reasons.

What was the reasoning for 50 Hz, other than slightly better transmission efficiency?

-Bill

and

--------- Of course, this much variation only applies to small independent generators. In a grid system, a 2Hz frequency deviation is unacceptable. A change in the order of 0.05 Hz is a fairly extreme transient .

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Don Kelly
dhky@peeshaw.ca
remove the urine to answer

and

--------- The difference between 50 and 60 Hz will make some gain in a reduction of iron in machines at 60 Hz and, on the other side, some increase in transmission capability (not necessarily efficiency) at 50 Hz. However, it appears that the areas which originally had longer transmission distances went to 60 Hz. so where's the logic. Probably a choice of "the Brits chose 50Hz so we will choose 60Hz" (or the opposite with Yanks substituted for Brits ).

As for flicker at 25 Hz ( Not 24) with incandescents. It exists and can be noticed. I have seen it. Subtle but there- somewhat similar to a computer monitor refresh rate of 55 to 60Hz. It can be annoying if you are not used to it..

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Don Kelly
dhky@peeshaw.ca
remove the urine to answer

At the intuitive level, a Fourier series answers the question "how much does this waveform look like a 60 Hz sine wave? How much like a

120 Hz sine wave?...". The answers are of the form "2 volts, 45 degrees" and such, one answer for DC and one for each harmonic. The Fourier transform is a math operation that gives these answers. It produces the same results you could get using a bandpass filter bank at f, 2f, etc (plus the DC term, the zero frequency Fourier term, which you'd get using a lowpass filter).

You can do an eyeball Fourier by printing the waveform on a piece of paper. Suppose some waveform has a basic frequency of 60 Hz. Now plot a 60 Hz sinewave on another piece of paper and hold it next to the original waveform. Slide it horizontally until you see the best match, so that the input waveform "helps" the sinewave template, pushing it up and pulling it down in the best places. Now make a rough estimate of how much it helps (amplitude), and how far you shifted the papers to get the best match (phase.) Repeat for higher harmonics, one at a time. Fourier!!

The SCR phase control waveform at 50% on has power on the load in the last half of each half-cycle. That shifts the center-of-gravity of the waveform later in time from the line voltage wave, so the fundamental component, the 60 Hz Fourier line, lags. By something like 32 degrees, some people have calculated in other posts. That does look inductive.

John