RE: MD-80 Electrical

Date:         29 May 98 02:43:56 
From:         James Matthew Weber <m.weber@duc.com.kw>
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>>ehahn@mitre.org (Edward Hahn) writes:
>>The reason for having three-phase electrical equipment is two-fold.  1) it
>>allows three-phase heavy-duty motors to be driven (such as in the stabilizer
>>trim on a B727), and 2) redundancy: if one of the phases shorts out, a
>>three-phase motor will still operate.

>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.

Motors, generators  and power supplies use 'iron' core components. The
amount of Iron you need is directly proportional to the frequency. Hence a
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's 60Hz cousin.

Weight is money in an Aircraft.

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.

A three phase alternator operating at 400hz will also weigh a lot less, and
occuply a much smaller volume than a single phase alternator of simliar
output. (better use of the interior volume for windings, and less Iron in
the field winding core).

Any polyphase electric motor has starting torque. Single phase motors are
quite troublesome because by definition they have no starting torque. You
do various things to them, like using a capacitor to introduce a phase
shift (effectively another phase) to get them started. In general, the more
things you do to make a single phase motor start well, the worse (less
efficient it is) when you actually run it. Three phase motors are also much
more efficient than their single phase relatives. You will notice if you
ever look that you will never find a single phase motor rated at more than
about 2 horsepower. They just don't work very well. Single phase motors
also have very large starting currents, another problem...

The other issue is many Aircraft motors have to run backwards and fowards.
On a three phase motor this is easy. Just exchange any two phases, and the
direction reverses. This is a much more difficult problem on a single phase
motor.

--

James Matthew Weber
Service Delivery Manager

Diyar United Trading and Contracting Co.
P.O. Box 44240
Hawalli 32057
State of Kuwait
PH +965 434 0560 x 230  FAX +965 431 5107 Mobile +965 971 2069