Re: Problem with common leads in dual channel scopings?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

At 03:30 PM 9/1/2003 -0700, you wrote:

>The reactance current that a 15 uf cap might draw at
>60 hz might be deemed small, and too small to use as a
>typical DC pulse filter. However recall that I am
>working at 480 hz, where the same cap would have 8
>times the reactive draw at AC, so the ability of the
>15 uf cap to smooth out the ripple of the AC would be
>equivalent to a 15*8= 120 uf cap acting at 60 hz,
>which is 1/8th the the frequency I am obtaining at the
>alternator.

15uF at 480Hz is 22.1 ohms.  However, if you are using a three phase bridge 
and all. the smoothing gets a whole lot better!

>Yesterday I did a DC cap experiment, to
>see how much capacity that needs to be added to smooth
>out the DC pulsing through a large induction coil.
>Here the scope problems are becoming very evident, as
>the measuring instruments stated 1 mgOhm internal
>capacity, should be 8 times less at 8 times the
>standard frequency and not more as I had formerly
>mistakenly reasoned.  The coil that the probes are
>hooked to sees a pulsing DC of 480 *2 DC pulses per
>second. Without doing the calculation, I am assuming
>that the stated 30 pf internal capacity would be
>equivalent to the stated 1 meg ohmat 60 hz.

Xc = 1 / (2 x pi x f x C)

1 / (2 x 3.14159 x 480 30e-12) = 11 Meg ohm.  They just pulled the 30pF out 
of the air and it became sort of a standard scope input capacitance.  As 
good as any...


>The internal capacity of the scope is actually storing
>a charge and giving it back to the circuit, making a
>DC pulse measurement appear as a strange AC signal!

Unlikely that the scope's tiny stored energy would affect your powerful 
circuit at all!


>I have a Delco Remy Car Alternator that
>I have converted to three phase AC, with pulley
>arrangements made to procure 480 hz I just put this
>thing through some grueling tests, not a wise thing to
>do since the amount of internal heat generated at the
>end of these tests was probably enough to start
>melting the insulation, and I could smell that
>starting to happen. At the end of these tests I was
>making 46.7 volts with some 9 amps. (which might be
>considered about 420 watts output!)

Most alternators can do 60 amps..  12V x 60 = 720 watts.  I would think 
your alternator would run cool!!



>Variac Volts/DC field Amps/ Stator Volts
>
>10 .04 A 2.55 Volts
>15 .21 A 5.28 Volts
>20 .41 A 9.11 Volts
>25 .60 A 13.4 Volts
>30 1.04 A 21.6 Volts
>35 1.32 A 26.5 Volts
>40 1.63 A 30.3 Volts
>45 1.94 A 34.7 Volts
>50 2.28 A 38 Volts
>55 2.58 A 40.6 Volts
>60 2.92 A 42.8 Volts
>65 3.2 A 44.2 Volts
>70 3.52 A 45.3 Volts
>75 3.8 A 46 Volts
>80 4.05 A 46.7 Volts

Interesting data!


>I think for now this thread has become very off topic,
>and Terry may be getting the ax out, so when it
>becomes more relevant by the building of an alternator
>based tesla coil, more posting of alternator work will
>be made.


Cool!

Cheers,

         Terry




>Sincerely HDN