Re: Power consumption

Mike Benedict wrote:

> > He (of course) guessed they would dim. WRONG!
> > They got quite a bit brighter. Why? Because
> > they were on the other "phase" and the voltage
> > dropped in the neutral when the saw was turned
> > on got added to the other phase.
>
> Roger,
>
> Could you elaborate on this please? I understand what you're saying, we've
> all seen the effect, but what I'm not following is how / why does the
> voltage drop in the neutral when the load on the opposite phase increases?
> Something isn't sinking in here.
>
> Thanks,
> Mike Benedict

When you apply a load to any power source you get voltage drops in the wiring
in between the load (in this case a bank of video games) and the power source
(in this case the transformer out in the back yard). Some of this loss is
resistive, some is inductive. A light bulb running would be an example of a
resistive load, where wiring loses are nice and well defined. So let's start
with that and my farm shed example.

There is a LOT of wire between the transformer and the shed in my example and
about 900 watts of bulbs (3 300 watt bulbs) across the ceiling. Let's say, for
example (because we don't really have hard data here), that my ideal 120 volts
is really 110 volts by the time I got to the shed due to losing 5 volts in EACH
of the hot line and neutral. I doubt it's that bad, but play along please.
Now I turn on the radial arm saw connected to the other phase. Now IF it drew
exactly the same current as the lamps there would effectively be no longer any
current draw in the neutral (they cancel each other) which make the neutral
drop go away and the bulbs get an extra 5 volts. And that's why they got
brighter. And there's a good chance that the saw motor actually pulled double
the current of the light bulbs very briefly while starting. If that had
happened the neutral would have dropped the same loss as before the saw was
turned on (same current, just opposite polarity) and instead of the bulbs
getting an extra 5 volts (due to no drop in the neutral) they now get an extra
10 volts (due to the drop in the neutral switching polarity).

If this happening to an AC power line makes you head hurt, just do the math
assuming a DC source instead. The neutral is the common, the bulbs are getting
+120 volts, and the saw -120 volts. With only the lights on the +120 drops to
+115 through the hot power line, the neutral goes to +5 through the neutral
line resistance, and the result is 115-5=110 volts. In the case of the motor
drawing twice the bulb load the neutral ends up at -5 volts and the lights get
+115-(-5)=120. Since they only were getting 110 before the motor was turned on
and now that went to 120 they got considerably brighter.

But a electronic power supply isn't as simple as that. When first turned on
the filter capacitors must get charged. This draws a large surge current. And
to make matters worse, when manually flipping a switch or circuit breaker, you
can't guarantee that the power is applied when it's crossing zero. What
happens if instead the power line is at it's peak when the contacts of whatever
you're using as a switch connect? All of a sudden you have a better than 150
volt peak voltage connected to an effective dead short with the surge current
limited only by wiring, transformer, and whatever other resistance. A helluva
surge current flows, if only for one cycle, but it's still there. A whole bank
of games switched on at the same time? The peak current could be literally
dozens of amps, if not over 100. But it's a one cycle thing, so fuses and
breakers can usually ignore it.

In businesses like arcades you can sort of get by since your connection to the
power company is VERY stiff (you aren't sharing a transformer with 5 other
houses, you don't have that long underrated wire from your house to the
transformer) and power distribution is very good (not that many outlets per
breaker and wiring runs are short). You don't have an old house with 60 amp
service running power through knob-and-tube 16 gauge wiring (all right, an
extreme example again, in this case the HOUSE at the farm).

In fact, at my Real Job the power comes in as 13,200 volts, goes to IN-PLANT
substation transformers, is distributed via buss bars to breakers mounted
almost at the equipment that using the power. That place can switch a 300 amp
load and not even blink the lights! Just plain scary-stiff wiring.

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Received on Fri Oct 13 16:17:43 2000