Question about AC/DC propulsion on modern Diesel-Electric se

Hi all! I'm an electrician (work on 15KV/5KV/450/120 AC systems and 12-28VDC control power systems), and I have a question about AC/DC propulsion system on diesel electric trains. For DC vs AC machines, does the DC sets have mechanical/electrical communication to actually provide the rotation out?

For AC machines, are there static exciters/brushless exciters, or do they still use brushes and slip rings to provide excitation to the field? I assume like most normal medium duty switchgears, it is 440/450 VAC 3phase output? Sorry if these questions are a bit technical, and thank you!

Ryand-Smith wrote:For DC vs AC machines, does the DC sets have mechanical/electrical communication to actually provide the rotation out?

Yes - DC locomotive traction motors still use mechanical commutation (brushes).

Ryand-Smith wrote:For AC machines, are there static exciters/brushless exciters, or do they still use brushes and slip rings to provide excitation to the field?

Not certain but I believe slip rings/brushes are used. They are simple and relatively maintenance-free.

Ryand-Smith wrote: I assume like most normal medium duty switchgears, it is 440/450 VAC 3phase output?

600 VAC (nominal) 3 phase. Frequency is of course dependent on engine speed (around 80 hz at maximum RPM.)

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The above is what I recall from a casual trackside discussion with a CSX locomotive electrician several years ago.

...FB

Ryand-Smith wrote:Hi all! I'm an electrician (work on 15KV/5KV/450/120 AC systems and 12-28VDC control power systems), and I have a question about AC/DC propulsion system on diesel electric trains. For DC vs AC machines, does the DC sets have mechanical/electrical communication to actually provide the rotation out?

As Farmall Bob notes DC motored locomotives use Brushes which are significant maintenance items as they are only semi-suspended which means they take a pounding from the track and can be subject to arcing if the Engineer fails to reduce the throttle for switches and diamonds.

For AC machines, are there static exciters/brushless exciters, or do they still use brushes and slip rings to provide excitation to the field? I assume like most normal medium duty switchgears, it is 440/450 VAC 3phase output? Sorry if these questions are a bit technical, and thank you!

AC motored locomotives use 3-phase asynchronous induction motors (aka Squirrel Cage) so there is no need for that.

Power output from the traction alternator varies considerably and can be as high as 1290 V when both machines are in series. The companion alternator which provides for the auxiliaries and excitation for the main alternator has 240 V output. These figures are for newer EMD locomotives, but GE should be very similar.

The AC traction motors have slip rings and small carbon brushes for exitation

DutchRailnut wrote:The AC traction motors have slip rings and small carbon brushes for exitation

My SD70MAC shop manual doesn't show any slip rings or brushes in the traction motors. What would the purpose be anyway? The TA-16 Traction Alternators require slip rings and brushes obviously.

just because your shop manual for one locomotive does not show it, does not mean ALL ac propulsion units are same.

DutchRailnut wrote:just because your shop manual for one locomotive does not show it, does not mean ALL ac propulsion units are same.

Very true Dutch. At Septa we have three different pieces of rail equipment with AC motors and none of them use slip rings or exciters. That being said our ex-Reading and ex-PRR MUs had AC motors with commutators similar to DC traction motors.

DutchRailnut wrote:just because your shop manual for one locomotive does not show it, does not mean ALL ac propulsion units are same.

Very true.

The AC traction motors have slip rings and small carbon brushes for exitation

But this statement by you makes it sound like this applies to all ac traction motors. In fact for all production Diesel-electric locomotives produced in North America by GE and EMD, none use brushes or slip rings in the traction motors. Transit equipment built using early Synchronous AC traction motors before the advent of efficient and reliable VVVF drives, would by necessity require slip rings and brushes.