[Physics query] early EMDs: transitions and the ammeter

[Kali]

One for a footplate crewman if there's any around:

Reading one of the more detailed parts of a GP9 manual, it got to the use of the ammeter as a load meter. Sure, ok, I understand that ( it's pretty obvious ), but it totally failed to mention any change in the ammeter reading after a transition. Field divert equipped diesels over here most definitely do have a rather large change in ammeter reading after a field divert, as does the 66 with it's split alternator ... but no mention of anything for a Geep. So the question is:

Is there any difference in the cab ammeter reading before and after a transition in an older EMD ( or anything else pre-AC/digital control )? or is it somehow reading generator output power rather than amps? it was my understanding that it read off one of the motors. It seems odd there's nothing in the manual though.

[_o_OOOO_oo-Kanawha]

Interesting question, Kali.

First thought that occured to me: is there still a chance of overloading after the transition from series to parallel has been made? When not, there is no use of the ammeter in preventing an overload condition and it is merely there for information.

As far as I know the GP35 was the last EMD loco to have transition, the second genearation 645 engined locos never had it.
No transition and therefor simpler lelectrical gear was one of the big selling points of GE when it introduced the first model of the Univeral line, the U25.

I can still think of roads ordering special high geared SD40-2 like Union Pacific which could do 100 MpH in express freight service to be interested in electrical systems featuring series-parallel transition. Or SP/Santa Fe which ran high speed passenger trains up till Amtrak. Perhaps over at http://www.railroad.net/ you will get better answers?

Where are those manuals you are referring to?
Here are a lot, highly technical but interesting still: http://www.rr-fallenflags.org/manual/manual.html

[Kali]

Most of my manuals are from there. What triggers an upward transition on a UK-built diesel is the load regulator running out of travel; careful choice of electrical tolerance should mean you can't cause an overload that way, old generators here came with overload cutouts, but when they refitted alternators those got removed and you had to pay attention in case of motor-destroying runaway wheelslip. EMDs at least seem to transition at a predefined amp/volt ratio ( it would be nice to know what that was ) as well as I guess generator limits. The GP9 manual mentions you should run it in notch 8 at low speed & let the WSP deal with wheelslip, which kinda implies the generator is big enough not to hit it's amp limit...

U25s have a transition; the generator was big enough that it could cope, I don't think the engine was though!

I was considering railroad.net, but thought I'd ask somewhere where people already know me first.