Re: MD-80 Electrical

Date:         17 May 98 00:42:47 
From:         "Donald Mc Lean" <>
Organization: Mittagong Mania -
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Stein1111 wrote in message ...
>Can someone help explane " 3 phase" when it comes to a 40KVA  115 volt and
>3 phase output generator.  What does 3 phases do for you?

Answer from DOWN UNDER:
I presume that it actually is rated as a 40KVA 115/200V 3 phase generator.

This a pilots explanation and no doubt a 'plumber' will point out the errors
of my ways.

The stator in the generator has three arms aligned 120degrees apart.Each arm
is wired to be an electro-magnet. Imagine a "Y " as the three arms.  In the
centre driven from the engine via the CSD is a rotating 'wire/s' called
naturally, the rotor. As the rotor cuts the magnetic flux and passes a
particular arm it produces an EMF of 115V as a single phase. We could see it
on a cathode ray tube as a simple sign wave. As it rotates past the next arm
it starts another sign wave off as another phase. We could see three waves
generated with one 360 degree revolution of the rotor ,each one 120 degrees
late on the other. As I said each arm produces 115V. (If thats what the
manufacturer wants, here we presume it does)
Look now at either top arm of the ' Y '.( Lets say the right hand one) We
can resolve with a vector triangle the component of that arm that acts
vertically in series with the bottom one. (Like working out the head wind
component of a cross wind on the runway) This will be 85V. The generator can
sum any two arms to get
115V + 85V = 200V. So, in the avionics suite of the aeroplane the engineers
have available a direct source of 115v  or  200v  power, single or three
phase. This is AC power. If they want higher or lower voltage AC they run it
through either a 'step up' or ' step down ' transformer. They do that a  lot
on big jets. As they want DC as well they actually run it through aTRU
(Transformer/Rectifier Unit.) Rectification is the process of converting AC
into DC.

Pontification ceases.That turned out to be bloody long winded didn't it.
Pilots hate electrical stuff and diagrams, give us a good clean (dirty)
hydraulic system any day.