In article , betwys1
@sbcglobal.net says...

On 5/10/2011 8:25 AM, Mark Borgerson wrote:

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

Mark Borgerson


Interesting inputs. If voltage and current doesn't drop below 10 volts
(and presumably peaks at 14 volt or more?) then the power variation
might be 10X10 / 14X14 - a ratio of 1:2 possibly??

Its more complex than that, unfortunately. The peak voltage will be
limited by the battery voltage and internal charging resistance. The
minimum voltage will be determined by the battery.

Only if the battery is very discharged, will the voltage drop down
to 10V. The maximum voltage in that case will be limited by the
alternator output capacity and wiring and battery internal resistance.

If the battery is nearly charged, the minimum will be about
13.2V and the maximum something over 14V---as set by the regulator
in the alternator system.

I think the plot in the last cited reference would apply only to
an alternator connected to a pure resistive load. Add in a battery
and things would look quite different. With a battery, the current
output from the alternator will actually drop to zero when the
alternator output drops below the battery voltage. However, the
voltage falls only to the level of the battery.

Mark Borgerson