Re: AC sine wave: What does increasing the frequency do?

From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 11/27/04 

Date: Fri, 26 Nov 2004 18:02:46 -0800

On Fri, 26 Nov 2004 19:42:05 -0600, John Fields
<jfields@austininstruments.com> wrote:

>On Fri, 26 Nov 2004 17:23:14 -0800, John Larkin
><jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:
>
>>On Fri, 26 Nov 2004 08:56:34 -0800, John Larkin
>><jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:
>>
>>>On Fri, 26 Nov 2004 09:51:32 -0600, "Commander Dave"
>>><cmdr-dave@spamcop.net> wrote:
>>>
>>>>Thanks for the answer... it is exactly what I needed. I was really
>>>>looking to see if increasing the frequency increased power. From what I
>>>>gather, while it makes it incompatible with things that run on 60 Hz, it
>>>>doesn't change the available power... it just cycles faster.
>>>>
>>>
>>>Right. Resistive loads (heaters, light bulbs) won't care; they'll use
>>>the same current and power independent of frequency (except at the far
>>>extremes.) Reactive loads, like motors and transformers, will behave
>>>differently at different frequencies.
>>>
>>
>>Which brings up the concept that an incandescent lamp appears to have
>>a capacitive component of impedance, which is itself a function of
>>frequency.
>
>---
>It may seem that it does if you're referring to the inrush current,
>but put a resistor in series with the lamp and the voltage and current
>will be in phase across and through them both, I believe, since all
>that changes is the resistance of the lamp filament.

The filament has a substantial 120 Hz temperature cycle (you can hear
it with a photocell) and the tungsten has a positive TC. So the
resistance varies with time. The thermal lag results in the filament
resistance peaking later than the voltage peak. So the current leads
the voltage, which looks like a capacitive component.

There are also harmonics in the current, for the same reasons. GR once
made a line-voltage regulator that used a motorized variac; the
voltage sensor was an incandescent bulb, and they sensed the second
harmonic current (somehow) to servo on.

John

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