From: pierson@empror.enet.dec.com (dave pierson)
Newsgroups: rec.railroad
Subject: Re: Alcos, GG1's, etc.
Date: 22 Dec 92 22:37:17 GMT
Distribution: na
Organization: Digital Equipment Corporation

In article <Bq47VB4w165w@1776.COM>, bob@1776.COM (Robert Coe) writes...
>smb@research.att.com (Steven Bellovin) writes:
>> I think it's more complicated than that.  According to the F40PH manual
>> I have, the throttle handle settings behaves differently when head-end
>> power is being generated than when it's not.  If it's not, the throttle
>> controls the diesel engine speed directly, in a fairly intuitive manner.
>> But the HEP generator requires constant RPM, so the engine has to run
>> at the same speed all the time.  In that case, the throttle handle
>> controls the excitation of the field windings of the main generator,
>> in order to govern the speed of the engine.
> 
>Somehow that doesn't make sense.  I'd believe "minimum" RPM, but if HEP is 
>being generated by the prime mover, how could the generator possibly require 
>"constant" engine RPM?  
	It requires nominal constant, 60Hz, from a mechanically coupled
	alternator, so it requires full RPM.  The F40PH usesa unique(?) scheme
	in that the engine RPM is always at full in normal operation.

>When the train is underway, the prime mover's RPM must depend on train speed
>and weight and on the terrain being traversed (level, climbing, etc.).
	Not so long as the available engine HP range is above that required.
	Note that full RPM is NOT, repeat NOT the same as full HP.  Full HP
	is only available at full RPM, but ANY HP (down to zero) is available
	at full RPM.  Its just a slightly noisy and inefficient way to do it.
	(The inefficiencies are not bad, either.)

	MOST control schemes run the prime mover at lower RPM if lower HP is
	required.  the F40PH does not.

	The prime mover will generate "hotel power" requirements (using power
	in the strict sense) at all times.  If running, added fuel will be
	supplied, to overcome the drag due to the train.  If a hill, yet more,
	with the primemover speed not slowing.  IF a steep enough hill is
	encountered, then the loco will slow.

	(in detail, to go faster in an F40PH:
		Engineer selects more speed.
		Control gear increases excitation of traction alternator.
		The added power requirement appears as a drag which will slow
		the primemover slightly.
		The governor will pick up the injector setting and get the
		rpm back to set point.)

	Varying the excitation to the alternator will vary the power delivered
	to the traction motors (which varies the load, which causes the
	governor to....  It's two or more interlocking feedback control loops.

>Am I missing something obvious? 
	No, something subtle: There are many ways to control the system.

>Also, I thought that the throttle settings controlled the current drawn by
>the traction  motors and only indirectly the speed of the prime mover.
	Well, over historical perspective, one needs to define which control
	scheme...  Typically, in my understanding, the "notch" or throttle
	sets a target prime mover RPM (F40PH IS THE EXCEPTION....).  The
	excitation control will cause the alternator output to rise.  If it
	rises too far, the governor will see the primemover slowing, If the
	primemover is at less than max load, more fuel is added to maintain
	RPM.

	If the prime mover is at full load (based on fuel flow/rack
	position), no more fuel flows and things slow down.  As they slow,
	traction motor current rises.  If it rises far enough, either the
	control gear or the engineer has to reduce prime mover RPM (or
	transition backwards, on units that transition.).

	Diesel electric control gear has to provide several things:
		MU capability
		SMOOTH control of power
		Full selected load (without overload) on prime mover
		Same on traction motors
		Same on generator/alternator.
	All this leads to complexities.

thanks
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own.
40 Old Bolton Rd		|I am the NRA.
Stow, Mass, USA	
01775				pierson@msd27.enet.dec.com
"He has read everything, and, to his credit, written nothing."  A J Raffles