Brazilian B-B-B-B SD40-2 Transition

[Engineer Spike]

With these units having 8 traction motors instead of 6, are more steps of transition needed? Does the use of smaller motors, due to narrow gauge negate that need?

[Pneudyne]

Good question, and one to which I can’t find the answer. For EMD, the issue would have first arisen with the DDM45 D-D locomotives around 45 years ago.

My best guess is that the traction motor groupings are 2S4P and 8P, with field shunting in 8P, perhaps also in 2S4P. Also possible is fixed 8P with field shunting. For example, EMD used fixed 6P with field shunting on some narrow-gauge 6-motor locomotives, even in the pre-alternator days.

On that basis the 8-motor locomotives at interest would not have more transition steps than “normal”. And as you say, that is because of their use of narrow-gauge motors. The current draw of 8P narrow-gauge motors in any situation is probably similar to that of 6P standard-gauge motors.

But the foregoing is speculation, so we need some “hard” information.

Cheers,

[Allen Hazen]

I don't know what the current draw of the EMD narrow-gauge motor is, but…
I think it is designed for metre gauge (and wouldn't fit on a 3 foot gauge wheel set): like the GE 761 as opposed to the 3-foot-gauge 764.
From the weights, it seemed to me that the GE 761 was probably about as large in diameter as the 752, so the amount of copper in the two designs would be proportional to their widths (roughly: you probably have to allow a bit for the end plates of the motor housing), and I would assume that EMD would have followed the same design principle.
Standard gauge is 1.435 times as wide as metre gauge, so, allowing for the motor housing end plates, it seems like a good guess that the metre gauge motor would have about three quarters, more or less, as much copper as the standard gauge. Which ought to be a good indicator of how much current they can handle.
Which is just a long-winded way of saying Pneudyne's "The current draw of 8P narrow-gauge motors in any situation is probably similar to that of 6P standard-gauge motors" seems plausible to me.

(Pneudyne: Apologies for not really adding very much to the discussion. Let's say I posted just to show my appreciation for your characteristically thoughtful and well-informed comment!)

[Engineer Spike]

I agree with Allan, always an informative answer, as in my field loop thread. Thanks Pneudyne.