Re: MD-80 Electrical

Date:         06 Jun 98 15:39:17 
From:         James Matthew Weber <m.weber@duc.com.kw>
References:   1 2 3 4
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>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....

WRONG.. Each phase has a pair of wires if it is a Wye connect. The neutrals
may in fact be tied to gether, but power is still hot to netural. Analysis
of A Delta connect (which has no neutral connects) is much more complex,
however Aircraft do not use Delta connects.

The reason you have no starting torque on a single phase, is that the it
appears as a pair of phasers operating exactly 180 degrees out of phase.
The torque is literally the product of current in the windings multipled by
the rotor fields. The mirror image phasers cause the torque to cancel. Two
phases will in fact produce a phaser that does indeed have a net energy in
one direction or the other. It will be seen to rotate, although not very
nicely, and not with the nice symmetry you get from 3 phases into a 3 phase
motor.

What you have a 3 phase motor with asymetric torque, i.e. it produces much
more torque in some positions than others, but it does indeed produce
torque. The assymetric torque, causes the loss of RPM, which reduces the
back EMF, and the motor will overheat. Actually something else happens if
you don't protect it. Most motors have iron core components that run very
close to saturation to minimize the use of iron core components. As they
begin to heat, the permeability goes down, so the iron components saturate.
This greatly reduces the inductive component in the impedance, and
dramatically increases the current in the rotor windings, which greatly
increases the ohmic heating in the core. In fact the permittivity goes to
unity (Same as air)  at the Curie point if it gets hot enough, this reduces
the back EMF still further (more accurately the inductive component in the
impedance, which raises the current,  which raises the I squared R losses
(ohmic heating), and you get thermal runaway until the circuit breaker
trips out, or the windings melt.

The other problem is the assymetric load causes substantial size loads on
the bearings, which they are not designed to take, so it will probably lead
to premature bearing failure as well, but it will run.

>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.

The motor stops do to loss of torque. When one phase goes, 1/3 of the
energy, and most big electric motors are not exactly grossly oversized.
Motor do indeed have something called Stall Torque, if you exceed it, they
do indeed stall, and with only 2 phases, there will be rotor positions
where very little torque is produced. If the rotor stalls, bad things
happen and in fact the motor on the nameplate has a value called L R A,
Locked ROTOR amperage, which is what the motor will draw if the rotor
doesn't turn, and power is applied. It is typically many times run current.
(it also tells you what the motor will draw the instant you close contactor
to start the motor).

>>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...)

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.
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. Most Iron core equipment will
literally disintegrate at about 20,000 RPM. (That speed is also often
listed on the name plate. If you want to see a mess, you should see what a
building looks like after the shunt winding on a 100 HP DC motor has
opened. The Armature usually comes apart at about 20,000 RPM, and sends
200-300 pound pieces of iron sailing through the air at 300-400 mph. You
can hear the motor 'takeoff', and then you pray you are parallel to axis of
rotation, and not anywhere near it. Afterward it looks like a bulldozer has
been through the building. In my youth, in the Motor Lab at the University
of Wisconin we had a before and after picture of the Allis Chalmers Motor
lab after a big one got away..

By the way, most current aircraft has gone away from VSCF drives on the
generators. They are too complicated, It is now down with an Integrated
Drive Generator (IDG), the Frequency is controlled electronically by
altering the frequency that you drive the field windings on the alternator.
Remove Hydraulics and moving parts. The IDG is part of the engine assembly.
Driving the Field winding with AC in the proper phase relationship makes
the field appear (electrically) to be rotating, this allows you alter the
effective speed that rotor turns, without actually altering its RPM at all.
This allows you to maintain 400 Hz power, no mater what speed the engine is
turning, and does so without any complex mechanical drive or hydraulics.

>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

Poles are in two places my friend. They are in the rotor, AND in the
armature. You are correct if you assume there is only 1 pole in the field
winding. Demonstration generators are built that way, real ones are not
because it is very poor use of the space.

If you think about that for a minute, you will realize that the armature
winding will in fact produce 1 cycle each time it goes past a pole in the
field coil. If I have 3 poles in the field, the armature will in fact
produce 3 cycles as it turns through 360 degrees, hence the frequency will
be 3 times the rotation speed.

The relationship between Field poles and Armature poles is a construction
detail that perhaps should have pointed out in the original article.

 So in the real world, single pole field windings almost never exist. There
is usually a one to one relationship between field and rotor windings, so a
3 pole armature will almost always  have a 3 pole field winding as well.
Now you do get 3 times the frequency.  My guess is an Aircraft probably
actually uses a 6 pole armature and a 6 pole field winding, and that would
result in 400Hz power from only 4000 RPM.

>	 	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?

It isn't a concept, its a matter of understanding how these things are
actually built, which I didn't think was of great interest to most people
who follow this group.

Where space and weight aren't an issue, you may have many more poles, and
many more field windings. Generators turned by water turbines are typically
 24 pole. They will have usually have field windings with 24 poles as well,
and these configurations will also produce 3 phase power by properly wiring
the pole windings. A 24 pole alternator only has to turn at 150RPM to
produce 60Hz, which means you can drive it directly from the water turbine
in a hydroelectric plant, not gear drives, no transmission.

This is probably not the case in aircraft because both size and weight are
at premium.

I doubt you have ever  worked on rotating machinery.

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

Do the field analysis my friend. They will indeed start as long as the
starting torque requirement isn't very high.

 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.

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.

I don't know what you do for a living, but I doubt  your Education
included a course in Rotating  Electrical Machinery. Mine was taught by one
Professor Donald W. Novotony, who wrote the book on Rotating Machines in
the 1960's, recently retired as Professor of Electrical Engineering at the
Univerity of Wisconsin. Professor Novotony was also a consultant to a
number of motor manufacturers, generator makers, and Wisconsin Electric
Power.
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