Re: MD-80 Electrical

Date:         16 Jun 98 02:15:31 
From:         jtarver@tminet.com (Tarver Engineering)
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On 06 Jun 98 15:39:17 , James Matthew Weber <m.weber@duc.com.kw>
wrote:

>>(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...)
>
>You can choose any speed you want, the slower you turn it, the few turns it
>will make per operating hour, and longer the bearings will last.

Hysterysis considerations prove this to be false.  If you try to run a
400 Hz synchronous generator at 60 Hz, you will get smoke.  The USAF
tried to convert their weight and balance equipment to 20 Hz from 60
on Edwards; burned every motor down.

>Maintenance costs money, so you design the equipment for the longest life
>you can get away, so that usually means the lowest RPM that will give you
>what you need. 24000 RPM is real problem.

The higher the rotating frequency, the more efficient and smaller the
generator.  If what you are claiming were true then hydro-electric
power would be more efficient than steam; it is not. (the relationship
is the square of the rotation speed)

> Even if it is high, you may find an external rotor resistor can be
>switched in to alter the speed torque characteristic to produce maximum
>torque at low RPM, in fact most servo motors are designed to produce
>maximum torque at zero RPM. Take a good look a switching locamotive some
>time. If you look carefully you will find this big piece of metal that
>almost looks like chian link fence. Its the external rotor resistor that is
>switched in when the engineer starts up the train. It alters the
>speed/torque relationship to produce very high torque at very low RPM.
>Really makes the thing run badly at High RPM, so generally it gets shorted
>out as soon as the train is moving more than few miles per hour.

More correctly, we would call that resistor a capacitor, so that it
provides the phase shift necessary to produce starting torque.

>They are also much smaller and ligher than their single phase counterparts.
>They have much lower starting currents than their single phase cousins as
>well, which means you don't need huge contactor to start them.

You are confusing DC locomotives, with AC locomotives.

John P. Tarver, PE
Electrical Engineer E14066