Allen, thanks very much to you and your correspondent for that posting, which makes very interesting reading and does answer some of the questions that have come up. I do have some comments and observations, below.
Allen Hazen wrote:FF2 regeneration -Trains Magazine June 1958 has a 5 page article by Frederick Westing on the purchase and conversion of these for helper service. On pg 47 he says "But the regenerative braking apparatus was removed, for the work involved made its use unnecessary".
Good, so we now know the answer to that question about the FF2.
Allen Hazen wrote:Other AC regeneration -Let us branch a bit here. In broad terms DC machines are equally good as generators and motors. In detail however it is only the shunt field machines that easily generate, series field machines are almost hopeless to control and keep stable. So if we look first at traction (which was almost universally series motors) and then at diesel dynamic brakes; while you already know that the control circuits reconnect the fields in a low voltage DC loop and energize them from the idling diesel generator, the switching to shunt field generation turns out to also be an important step in getting the system to work. This will apply for electric locomotive dynamics on DC roads too, there we will find that the fields are reconnected for shunt generation and then supplied from some separate source. For the CMStP&P EP-3's this was an auxiliary generator mounted on the pilot truck.
Yes, I’d say that that shunt (separately excited) machines were the primary type used for dynamic brakes, and very probably the only type used for diesel-electric locomotive dynamic brakes, for DC electric locomotive regenerative brakes, and for AC-DC electric locomotive dynamic brakes. But perhaps not to be overlooked is that self-excited series machines were used for dynamic braking in streetcars (including the PCC), heavy rail transit equipment, and in some DC electric locomotives. So the evidence is that series excitation could in fact be made to work stably enough for braking purposes, albeit within boundaries.
Allen Hazen wrote:Coming forward to locomotives with AC series traction motors however an extra problem occurs as these are commutator machines. If you find and put a DC supply on their field, while they will create AC in the armature, the commutator will chop it to DC which you then have to figure out how to get into the catenary. If you put AC on the field I'm not sure what you would get, I've never seen anybody try to reason it out. I have also read enough about AC series motor design to have grave doubts about managing and commutating the inductive kicks involved too. The issues involve the brushes shorting one coil of the armature as they bridge from one commutator bar to the next while the AC field is trying to power the coil as if it was a transformer winding. The resulting short circuit limits coil design voltages to about 2 volts each. So I think AC regeneration on past series motored locomotives was likely unfeasible, AC commutator motors don't seem to back generate AC.
Well, that’s what I thought too until I read about single-phase commutator motor regenerative locomotives being used in Switzerland. Even then I initially saw it as an easily dismissed oddity, whereas in fact it was an established practice going back to the early days of single-phase motors, with initial development work being done by Behn Eschenburg, who was a major player involved in bringing the low-frequency single-phase motor to a workable form for traction purposes. There certainly were difficulties as compared with DC regeneration practice, but they were overcome by various means. And regenerative braking was used by the SBB right through until its final design of single-phase commutator locomotive, the Re6/6, first built in the 1970s, with construction continuing into the 1980s. I’ll follow up with some book excerpts that provide an overview of single-phase regeneration techniques.
Allen Hazen wrote:For more reading I imagine your University access lets you into IEEExplore. Some papers of interest are:
-PRR GE E2B AC series motored locos were built with dynamic braking as you have mentioned. GE claimed this was the first dynamic applied on an electric. The motors were reconnected from series to shunt for braking and an AC-DC MG set excited the fields, so the braking mode functioned in DC. This makes me think GE knew about the difficulties of series motor AC regeneration 60 years before my writing above. AIEE Transactions Jan 1952 pg 27-36
Certainly this was the first dynamic brake applied to a North American AC electric locomotive, and possibly to any North American electric locomotive; I can’t think of any DC examples that had dynamic rather than regenerative braking. I imagine that GE was referring to North American practice in claiming a “first”. Worldwide, dynamic braking (usually referred to as rheostatic braking in Europe when used on electric locomotives) was well-established going back to the 1920s at least, and for both DC and AC (single-phase commutator motors) locomotives. In the AC case, both AC and DC excitation were used. I imagine that one advantage of AC excitation was that it avoided the need for a heavy current DC machine. In Switzerland, whereas SBB favoured regeneration, BLS used dynamic braking on its single-phase electric locomotives, with AC excitation on some of the older types but DC excitation for the Ae4/4 and later. From the apparent SBB and BLS dichotomy, one might reasonably infer that the choice between regenerative and dynamic braking was quite situational, with no clear winner in the abstract. Possibly then GE’s choice for the E2b was more situation-based than feasibility-based. I am not sure that it did, but if the E2b had employed standard diesel-electric locomotive dynamic braking units, that could have been another reason for GE’s choice.