NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/rec/railroad/10178

Text File | 1992-12-23 | 4.3 KB | 83 lines

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Subject: Re: Alcos, GG1's, etc.
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Date: Wed, 23 Dec 1992 22:47:58 GMT
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OOPS, I made a couple of errors in my previous response to this thread.
Just goes to show you how the old memory can fade.

The traction  alternator for -2 style EMDs is an AR10 which is a 10 pole
3 phase  AC  machine  (not 14 pole  as I  stated).  That  would  put the
layover  engine  speed at 720 RPM (not  870).  That is  compared  to the
normal  notch 8  speed  of 900 rpm  (not  1200)  and an  idle of  around
300-315.  I don't  think 750 is a normal  speed for EMDs  since it is in
between the usual notches 5 and 6 so there may be some special  governor
settings for F40PHs.

Coupled to the AR10 is an AR14 alternator  which is a 16 pole 3 phase AC
machine.  This  puts  out 120 Hz 215 V AC at 900  RPM to  drive  cooling
fans, blowers, and run part of the  excitation  system for the AR10 (are
you confused yet?).

There is also a DC  generator  to  provide 74 VDC power for the  control
relays and lights so these units have 3 power generation systems running
off the diesel (4 on an F40PH with HEP).

Thanks to Dave Pierson for his explanation which is quite accurate.  The
actual  control system is a bit more complex as he hints (there are more
than two interlocking  feedback loops).  The engineers throttle does two
things  simultaneously.  It signals the engine  governor what rpm to set
(set up loop 1) and it sets up a target power output  reference  for the
generator  excitation  control  (loop 2).  The fields  are given  enough
excitation  to  make  the  output  match  this  reference.  The  primary
function of the  governor is to maintain  engine  speed (not  locomotive
speed) but it also has a "load  regulator"  interface to the  excitation
system so it balances  both fuel and load to  maintain  set RPM and fuel
rack positions (loop 3).  I can attempt to explain how all this works if
anyone is really  interested.  There  are also  loops  for  wheel  slip,
altitude power deration, the "performance control" option, rate control,
transition, dynamic braking, and some others.  Whew!

At any rate, on an F40PH  running in  HEP/road  mode the RPM  setting is
always 900 RPM and the traction power output (and  therefore  locomotive
speed) is regulated by the setting of the excitation  reference  levels.
In HEP/layover  mode the AR10 provides hotel power at 720 RPM (I think).
There  may also be a  non-HEP/road  mode  which  funtions  like a normal
freight unit.

BTW, This is not the only  application  for this  "intervention"  in the
governor  operation.  It is common  for units  used in hump  yards to be
equipped with "hump  control".  This is basically  just a rheostat which
allows the engineer to fine tune the  horsepower  output by reducing the
excitaion  reference  voltage.  The  engine  then is running at a higher
than necessary notch (only 1 notch higher  normally) and the train speed
can be dialed in very accurately.  Oh yeah.  This is also common on coal
unit trains for pulling through the loader at just the right speed.

One last  thing,  I think  all  second  generation  diesels  incorporate
automatic transition controls so this will happen automatically as train
speed drops and current goes up in the motors.  Once the unit is in full
parallel  though, the engineer  will have to back off on the throttle if
the old ammeter gets into the red ....  but I'll save  "transition"  for
another thread.


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Guy Burnham, Boise Surface Mount Center, Hewlett Packard Co. Inc.
[e-mail]     208(323-3355)
Disclaimer: Don't you be the only one to take me seriously!
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