Re: MD-80 Electrical

Date:         06 Jun 98 15:39:15 
From:         "S.L." <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
References:   1 2 3 4
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James Matthew Weber wrote:

> I am an electrical engineer by education.

So am I, and I'm going to send you back for a review course! ;-) (I know
you just typed ahead of your thinking and confused *poles* with
*phases*, but others probably would misunderstand this)

You said:
> A three phase alternator has 3 armature  windings instead of one.

Correct for a "single pole" alternator, but very few are built that way.
An alternator may have any number of poles per phase (n-poles means that
the armature has n*3 windings for 3-phase, n windings for single phase),
so it can operate at integral divisions of the "one-pole" speed to get
the same frequency at a lower shaft RPM. a 4-pole motor or alternator
(whether 3 phase or single phase) will turn half as fast as a 2-pole
motor or alternator. That is how hydroelectric alternators produce the
same frequency as steam turbine powered alternators while spinning at a
far lower speed- they have many more poles per phase.

>It  produces a frequency at 3 times the rotation speed,

INCORRECT!!!  Single phase and 3-phase alternators or motors having the
same number of poles per phase will rotate at exactly the same speed for
the same frequency excitation. Recall that we don't have separate
alternators in our powerplants to produce 60Hz single phase and 60 Hz
3-phase power, which would be the case if your statement were true! Any
one "leg" of a 3-phase 60 hz line can be used to drive a single-phase

Think of it this way: a 3-phase alternator is simply 3 single-phase
alternators of the same frequency rating, mounted 120 degrees apart on
the same frame and shaft.

Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin (Remove the extra 'x' to mail me)