From EMD SD60 Maintenance Manual:

"The main generator ... is a three-phase alternator equipped with two independent and interwoven sets of stator windings and a rotating field common tot he windings. The output from each set of generator windings is supplied to an air cooled rectifier assembly in an airbox that is an integral part of the main generator. The rectifier assembly consists of banks of high-current/ high-voltage silicon diodes interconnected in three-phase, full wave rectifier circuits."

"The six locomotive traction motors are connected in parallel across the main generator. Main generator output current demand is highest when the traction motors accelerate the train from standstill and during slow drag operation."

"The output from each set of stator windings is connected to a three-phase full-wave rectifier bank. At startup, and during the ensuing low-speed high-current operation, the two rectifier banks operate operate in parallel. As locomotive speed increases, traction motor counter electromotive force (cemf) increases, reducing traction motor/ generator output current and raising traction motor/ main generator output voltage. At approximately 25 mph, main generator forward transition occurs: the control system disconnects the paralleled main generator rectifier banks, then reconnects them in series; it also acts to back off the load regulator during the parallel to series transition process. When the main generator is operating in series and locomotive speed drops below approximately 22 mph, main generator backward transisition occurs: the control system disconnects the main generator rectifier banks, then reconnects them in parallel."

"Main Generator Field Control Circuit" "The companion alternator supplies three-phase AC power to the SCR assembly when the generator field is closed and generator field contactor GFC is picked up ... The locomotive computer (Excitation portion) supplies gating signals to switch on individual SCRs in the SCR assembly."

My comments - EMD is still calling it a main generator, though it is an AC machine with a rectified output. Same with the auxiliary generator, most of those are now also thre-phase AC that is rectified. EMD does call this a transistion process. No mention of GTOs. The excitation scheme seems to be EMD's common static arrangement using SCRs. These were equipped with AR11-D18A.


From GE Locomotive Service Manual for Series 7 Road Locomotives:

"Two types of alternators have been used on Series-7 locomotives ... ; the GTA11 on locomotives with traction motor transition and the GTA24, introduced in 1980, on locomotives with alternator transition. The Types GTA11 and GTA24 Traction ALternators are three-phase, Y-connected, alernating current generators ..."

"Traction alternator output is determined by the amount of excitation current supplied to its rotating field coils by an Exciter generator ..."

My comments: GE uses separate rectifier assemblies usually mounted above the traction alternator, unlike EMD which mounts them in the end of the ARs. The manual also confirms that it uses two GY27s - one as an exciter and the other as an auxiliary generator, and that both have starting windings in them for engine cranking.

Adding to a previous thread - this shows how even though these are both Dash 7 locomotives, the two versions have very different electrical systems as they shifted transition from the traction motor connection arrangement to the traction alternator, yet they did not recieve any model designation change - much like EMD's GP35 using two completely different frames.

The whole term "transition" is a gray area though I guess it is only proper to use the term in reference to the change in motor or main generator/alternator connections. An engine can have transition without field shunting, or field shunting without transition, but a transition sequence chart will include field shunting.

The first mention I can remember of GTOs in locomotive use is for the invertors on AC drive locomotives, presumably both EMD and GE. I know the GTO made the AC drive scheme practical, but still had some drawbacks. I haven't heard of GTOs in a while, as everyone seems to have shifted to IGBTs which are more reliable and don't have some of the problems the GTOs had. The IGBTs are used in both AC invertor drives and for DC chopper controls in gen-sets.

I haven't run AC drive locos in a while, and don't have to work on them. My technical info and knowledge is limited on them, so they might very well be doing something different. I imagine IGBTs are used as part of the chopper control used in modern trolleys and electric locomotives.