In article .com,
llid says...

"Mark Borgerson" wrote in message
...
In article .com,
llid says...

Some Rube posted that current from an alternator never goes to zero.

WRONG.

1) an alternator produces alternating current.

2) alternating current alternates directions.

3) every time the current reverses, it MUST stop and go to zero.

4) if it did not stop and go to zero it could not reverse - it could not
alternate.


When you rectify this stop-and-go current through a bridge setup of
diodes,
all that happens is the direction of the current is is made to flow the
same
direction via the correct placement of diodes which allow current one way
but not the other. However, re-routing the current to all the same
direction
(DC) does not and cannot eliminate those finite periods of time when the
original alternating current stops and reverses directions. This
alternating
current is produced by the north and south poles lines of flux cutting
through a coil of wire. It goes one way, stops between poles, and then
goes
back the other way.

Walk forward, then walk backward. There will be a finite period of time
when
you're not moving because you have completely stopped when you reversed
directions. You can never eliminate the stop.

What you've said is true for a single-phase alternator. Most auto and
marine alternators are at least 3-phase systems that combine the output
from the three phases through multiple diodes. In those systems one
phase is always providing current through the diodes and the output
voltage never drops to zero.


Mark Borgerson


I disagree! Three-phase means three current stoppages. Remember, I'm
talking current (amperage) not voltage. Voltage is just another word for
pressure. I'm talking the two-part electrical concept of amperage where you
have both pressure and flow. Without voltage, of course, you will not have
any flow. Without a circuit you will not have any amperage. When you say the
output voltage never goes to zero that's incorrect. Sure you can put a volt
meter in parallel but no volt meter in the world has a fast enough needle
(analog type) or readout (digital type) that can respond fast enough to
alternator-produced stoppages. It reads an average over a finite period of
time.

But, we are discussing amperage which is flow. I say even in a three-phase
alternator the flow MUST stop every time the current reverses in the
alternating current section prior to the rectification process. Three-phase
simply means that the current reverses three times as much per revolution.
Three stops that don't go away when the current passes through the
rectifier. Even if the phases are overlapped there still will be a finite
period of time when the current stops so it drops the overall amperage. Done
right, you are correct, it won't go to zero but it will show a reduction
provided your amp meter is responsive enough to measure it. Since none are,
they necessarily measure an average over the sampling time period.

In a three-phase system, there are three separate windings that each
generate a sinusoidal waveform. They are arranged so that when one
winding is a zero voltage, the other two are not. Thus, the total
rectified voltage never drops to zero and the current also never drops
to zero. The three phases each have their own pair of diodes, so that
the three waveforms are summed together.

"Some Basic factoids about 3 phase.... Most of the electric power in
the world is 3 phase. The concept was originally conceived by Nikola
Tesla and was proven that 3 phase was far superior to single phase
power. 3 phase power is typically 150% more efficient than single phase
in the same power range. In a single phase unit the power falls to zero
three times during each cycle, in 3 phase it never drops to zero. The
power delivered to the load is the same at any instant. Also, in 3
phase the conductors need only be 75% the size of conductors for single
phase for the same power output."

http://www.windstuffnow.com/main/3_phase_basics.htm

"To provide direct current with low ripple, automotive alternators have
a three-phase winding. In addition, the pole-pieces of the rotor are
shaped (claw-pole) so as to produce a voltage waveform closer to a
square wave that, when rectified by the diodes, produces even less
ripple than the rectification of three-phase sinusoidal voltages."

http://en.wikipedia.org/wiki/Alternator

There's a plot of the output voltages at:

http://www.alternatorparts.com/under...lternators.htm

You can see that the voltage, and thus, the current, never
drops below about 10V.


Reading through those references will improve your understanding
of modern 3-phase alternators.



The gauss in the magnetic field around the wire is what clamp-type amp meter
measures. It will not measure the peaks and valleys because it's just not
responsive enough. So it measures an average somewhere in the middle of the
peaks and valleys. Consequently it might read five amps when ten are really
being produced from a peak standpoint. Use the same clamp meter on a load
from a 12V battery and it will read correctly as there is true and steady DC
current to measure.

The ripple on a three phase alternator output is only about 15%.

Mark Borgerson