Re: MD-80 Electrical

Date:         18 Apr 98 00:49:18 
From: (David Lesher)
Organization: NRK Clinic for habitual NetNews Abusers - Beltway Annex
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  or MIME structure (Stein1111) writes:

>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?

Three phase power means [grossly simplified] there are 3 output
leads, not just two. The combination of them provides a steady
power output, not one cycling 400 times per second. [sort of...]

But that is less important than the fact that while phase A is
going down, B is going up; and when B is then headed down, C is
increasing. {Geek-speak -- they are 120 degrees out of phase...}
When graphed, you get *_a rotating magnetic field_* that is ideal
for driving motors.

The motors need no starting winding scheme, and they are reversable
by just swapping 2 wires. They tend to be lighter than single phase
units of the same size. THAT is why 3 phase is used; starting
windings and switches for same are trouble spots, the motors
themselves run forever.

So the air conditioning compressors will have 3-phase motors. I'd
not guess if things like the hydraulic packs do; it would make sense
except they'd not work off a DC backup battery.

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