Re: MD-80 Electrical

Date:         06 Jun 98 15:39:16 
From:         wb8foz@netcom.com (David Lesher)
Organization: NRK Clinic for habitual NetNews Abusers - Beltway Annex
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James Matthew Weber <m.weber@duc.com.kw> writes:

>>Sort-of... A 3-phase motor will not start when missing a phase
>>but it may limp along if already going. Because "single->phasing"
>>is often destructive to the motor, larger units have 'loss-of-phase'
>>protection that shuts down the motor.

>I am an electrical engineer by education. A three phase motor certainly
>will start and run with a phase missing.  It is likely to overhead if you
>run it for a long period of time, and obviously there will be a substantial
>loss of power output. There are  very good reasons for using three phase,
>400Hz power in an aircraft.

{This is perhaps off base for this group, moderator....I have cc:ed
by mail...}

[Moderator's note:  We're still talking about parts of "transport-
category aircraft" so it's within the group's charter, albeit on the
fringes.  I'm learning something in the process, and hopefully others
are too.  -- Karl]

I'm also an EE, and I dug out Electric Machine Fundamentals,
Chapman, McGraw Hill, 1985... (Ghosts of Prof Klingshirn, a
required rite of graduation, echo in my ear.....)

On page 339 the author points out that 3-phase provides ..a rotating
magnetic field of constant amplitude.. It goes on to derive the
starting and running torque...

Then on 573, it talks about how single phase motors have ...no
starting torque... and goes on to discuss the auxiliary starting
windings thus needed.

I'll submit as obvious the fact that if you lose one leg of a
3-phase source, you have a single phase system. You have but 2
wires left. And with no starting winding scheme on the motor....

Large (100hp) motors frequently have loss-of-phase protection, as
the motor will sit there, not rotating, not generating counter-EMF
to reduce the running current, and soon smoking. On a big plant,
the LoF is battery-powered so it can shunt-trip the appropriate
breakers no matter what.

>400Hz transformer will need only about 14% of the core weight a 60Hz
>transformer needs. a 400hz motor will weigh a lot less than a 60Hz motor. a
>400Hz transformer will also weigh a lot less than it is 60Hz cousin.

True, but 60 vs 400 Hz is not the issue here, 3-phase vs single is.

>A three phase alternator has 3 armature  windings instead of one. It
>produces a frequency at 3 times the rotation speed, so a 400hz would mean
>the alternator has to turn at 24000RPM as a single phase, but only 8000 RPM
>for 3 phase. Big difference in bearling life, and manufacturing cost.

(The alternator in an aircraft is driven by a "constant speed drive"
-- in reality a complex hydraulic pump/motor system. Thus the
alternator can make constant freq. AC while the engine speed
changes. Ergo, the real speed of the alternator can be chosen as
desired at design time. You want it fast, fine.. slow, sure...)

But that said, I can not see any truth to your statement above. The
output frequency of a synchronous generator is: [page 380]

	f = n * P * 1/120
	 e   m

	 where n is the rotor speed, and P the # of poles
	 	m

I can't envision where came you up with the concept that poles
equates to phases. After all, I take single phase off a 3-P
generator routinely. Is it magically 1/3 the frequency when I do?

Yes, 3-P motors easily reverse. They have lots of advantages, &
disadvantages to boot. One is, they need 3 phases to start.

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