AC vs. DC

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AC vs. DC
Posted by Lithonia Operator on Tuesday, November 12, 2019 9:25 PM

What are the advantages of AC traction motors over DC ones? In what kinds of situations is an AC unit greatly preferred?

Are there some applications where DC traction motors are more desirable? Or is it the case that AC is now state of the art, and if railroads could wave a magic wand (and cast was no object) they would have all AC units?

Keep in mind that I'm not a technical guy. I'm more interested in the what and why than the how. I probably wouldn't understand a lot of the how.

 

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Posted by Erik_Mag on Tuesday, November 12, 2019 11:32 PM

Advatanges of AC over DC: Much more rugged motors; better adhesion due to much tighter control of motor speed and much faster inherent drop in torque with an increase in rotational speed; generally lighter motors for a given output; no problems with commutators getting dirty.

Advantages of DC over AC: Historically cheaper (especilly with remanufacturing of traction motors from DC trade-ins); no inverter cabinet to deal with.

I suspect whatever price advantage of DC over AC will disappear in the near future and inverters will shrink in size due to devlopments in power electronics.

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Posted by Lithonia Operator on Wednesday, November 13, 2019 10:12 AM

Thanks, Erik.

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Posted by Overmod on Wednesday, November 13, 2019 10:31 AM

Let me add very slightly:  "AC" in this context actually refers to 'synthesized-AC'; this is not at all what is produced by the "alternator" on a modern locomotive (or received/transformed from an overhead line).  (In fact, the AC output of the traction alternator has to be converted fully to DC, and relatively well-smoothed DC at that, before it can be used to produce the particular interesting flavor of alternating current sent to modern AC traction motors...)

Older designs of AC motor could be fascinating, but they didn't gain much traction (pun intended) in the railroad world due to inherent shortcomings.  If you have not read about 'universal motors' (as on some New Haven electrics, and the GG1s). or don't remember Dave Klepper's postings and at least one Classic Trains article on the subject, you could make motors that would 'run' on straight AC, but you needed an enormous number of internal taps in the main transformers to control them.  (That is the great reason among the many reasons it isn't likely that anything but a greatly-modified GG1 will ever run under its own power again)  A much more common approach -- and you can gauge some of the issues involved from the technical requirements it involved -- was to build a motor-generator (or MG) locomotive, in which a whopping synchronous AC motor ran at appropriate rpm for the AC catenary power, and drove a DC generator whose field was then adjusted in the normal way for DC traction-motor control, with all the general drawbacks of that method.

The 'cheapness' of remanufactured rewound DC motors effectively ended several years ago -- I think you can mark the time by the official NS decision to stop buying DC-motored power and start programs to rebuild units with AC drive.  It is much cheaper, as you can imagine, when standardized AC motors become 'mainstream' in the aftermarket, with a little light face machining and some new roller bearings are the principal rebuilding expense vs. winding, soldering, balancing, wrapping, and potting armatures and dealing with commutator and brush issues.  And there is considerable etc. involved, too.

I do expect the same scam to be involved with 'rebuilding' PM alternating-current motors as with other types of permag motor, should PM-AC become established in North American practice at some point.  These will probably have the same irreversible loss of 'magnetic strength and quality' due to repeated heating above the Curie point that high-power DC permag motors currently do -- and the same lies when rebuilders fail to replace the magnets completely as part of their overhaul.

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Posted by kenny dorham on Sunday, November 17, 2019 6:16 PM

Wow.

I, kind of sort of, follow most of what you are saying. I don't suppose you have a link to a video that explains this a bit.?

Thank You

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Posted by Overmod on Sunday, November 17, 2019 6:58 PM

There are many videos on YouTube that explain various kinds of AC drive, but none of the ones I've seen for locomotives really explain more than they confuse.

Look to see if you can find a comprehensible video about how AC synchronous motors work, including how they 'lock' to a particular frequency with slip.

What the AC drive does is to simulate this frequency, corresponding to a desired motor speed.  

Note that there are three 'phases' of magnetic change inducing torque in the rotor, some submultiple of 120 degrees (a third of a complete circle) apart.  These 'chase' each other like the on-and-off of the bulbs in a marquee sign, which is why we call it a 'rotating magnetic field'.  Note that the motor does not have to be physically turning to have the chasing fields induce torque in it.

Now go back to the diesel engine.  The governor that controls it is not concerned with making it run faster or slower proportional to speed, as the cruise control in an automobile would.  Instead there are fixed fuel settings, which in the absence of applied load would result in the engine taking up particular speeds that 'balance' fuel feed.  The main 'generator' in a locomotive actually is an alternator, producing alternating current, but this would be almost worthless for use in synchronous motors because its frequency effectively changes with diesel engine rpm.

So instead, what we do is take the "AC" produced by the traction alternator and rectify it to DC, then 'smooth' its 'ripple' to get what is called the DC-Link voltage.  The circuits called 'inverters' then make the correct frequency AC, at the correct phase, to apply rotating field to the induction traction motor -- look for videos on how inverters do this; you may find it as fascinating as Erik and I do.

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Posted by kenny dorham on Sunday, November 17, 2019 7:10 PM

"So instead, what we do is take the "AC" produced by the traction alternator and rectify it to DC, then 'smooth' its 'ripple' to get what is called the DC-Link voltage. The circuits called 'inverters' then make the correct frequency AC, at the correct phase, to apply rotating field to the induction traction motor -- look for videos on how inverters do this; you may find it as fascinating as Erik and I do."

Oh Wow.....OK. That certainly clears up some of my confusion. I just have a basic knowledge of AC-DC, but even that is enough to make this fascinating for me. Probably, most of us, just think of a diesel locomotive as some kind of Giant Generator that provides electricity....like you would for a home or business, if you know what i mean.?

I suppose with the changing of speeds and loads, and the balancing of the fuel supply, that makes a loco a much more complicated scenario than i realized :-)

I will do some more looking on Youtube.

Thanks Again

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Posted by Overmod on Sunday, November 17, 2019 7:20 PM

Of course, you'll notice I left out precisely how the inverters 'know' what the correct frequency and phase angle is supposed to be.  That's a bit harder -- and is typically done by 'computer' taking the locomotive throttle position, speed, and load as some of the inputs.  

It is interesting to look at why modern locomotives (since the '60s) use alternators instead of generators in the first place.  You should look at why alternators have come to be used in almost all modern automobiles in place of generators, even though historically everything on a car uses straight DC.  You may find it interesting that alternators 'need electricity to make electricity' -- it's not uncommon to find a small DC generator called an 'exciter' whose function is precisely to supply the initial current an alternator requires to sustain its electrical generation.

Amusingly enough, you CAN think of a locomotive as a giant generator: they have been used as such (see the case of the one in Canada that was actually run down an asphalt street under its own power on its own flanged wheels!) with special hookups to produce single-phase 60Hz domestic North American power.  If you use the signal off the 'grid' for excitation, the developed current will be in correct phase without having to synchronize it externally with a clock signal!

Of course you'll be familiar with applications of HEP that use the traction alternator rather than gensets -- and that run the diesel engine at a precise rpm to get reasonable frequency.  That also makes for remarkably quick potential loading and acceleration with minimal smoke, as the engine is at a high percentage of peak rpm already, needing only fuel feed to make power, and the turbo is reasonably well spooled up at that rpm just on the exhaust corresponding to the lighting load...

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Posted by Erik_Mag on Sunday, November 17, 2019 8:18 PM

There are two reasons for the switch from traction generators (inherently DC) to traction alternators (AC converted to DC) in the mid-60's. Note that the traction motors were DC series motors. One was that traction generators capable of handling more than 2500-2800HP would be too wide to fit in the carbody. The other was that large silicon rectifiers became available to rectify the AC to DC. The traction alternators were set up to mimic traction generators with Lemp contorl, that is the output voltage would drop with increasing current draw to cause the electric power produced to remain roughly constant, which makes for a continuously variable transmission.

The use of AC induction traction motors dates back to the electrification of the original Cascade Tunnel, and the N&W electrification shortly thereafter. Getting efficient continuously variable speed control had to wait for advances in power electronics to provide the needed variable voltage variable frequency drive.

 

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Posted by IbanezGuiness on Sunday, November 17, 2019 9:05 PM

Overmod

Amusingly enough, you CAN think of a locomotive as a giant generator: they have been used as such (see the case of the one in Canada that was actually run down an asphalt street under its own power on its own flanged wheels!) with special hookups to produce single-phase 60Hz domestic North American power.  

 

Any more info on this or a link to an article about it? This sounds really interesting!

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Posted by kenny dorham on Sunday, November 17, 2019 10:19 PM

And it generated the residential power at 60Hz.? :-)

I won't ask how that was achieved.  I need to watch some more videos.

Thanks Again (again)

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Posted by M636C on Sunday, November 17, 2019 11:12 PM

kenny dorham

And it generated the residential power at 60Hz.? :-)

I won't ask how that was achieved.  I need to watch some more videos.

Thanks Again (again)

 

From

https://www.trainorders.com/discussion/read.php?2,1104024

This is an excerpt from a long and detailed report Bryce Lee made on the 1998 ice storm. I believe it was posted in the TrainsCan newsletter:

"The mayoress of Boucherville Quebec, Mme Francine Legault suggested borrowing diesel locomotives from Canadian National for use as emergency generators. She recalled that locomotives had been used as a source of power some years ago in Fermont, at the north end of Cartier Railway in Quebec. CN initially delivered M420W 3502, to the town on the south shore of the St. Lawrence River. The locomotive was lifted off of the tracks by a crane, and placed on de Montarville Street, and allowed to proceed 1000 feet down the street under its own power. The locomotive initially rolled on the edge of the wheel flanges on top of the pavement; the weight of the locomotive eventually cut flange-deep grooves into the asphalt of the street. This locomotive was used to generate electricity for the various local municipal buildings.

On January 14, M420W 3508 was delivered, and this time the locomotive was moved down the same street although not as far as the first locomotive. When it became apparent that M420 3508, loaned to the town by CN to generate electricity for use at a high school shelter, could not be utilized as envisaged, it was decided to leave the locomotive parked on de Montarville Boulevard just up the street from sibling 3502. CN 3502 and 3508 were not needed after Saturday January 17, since electricity had returned to the area. The two units were moved back to Taschereau Yard shop, and were shown into the shop for damaged gear cases (I wonder why...). 3502 is now stored serviceable, but may be back at work soon. 3508 is back on the road working. The StL&H (CP) offered CP Rail SD40 5417 to the city of Lacolle Quebec and the all-white ex-Kansas City Southern unit was modified and running January 14, 1998. The 5417 had been previously been supplying power at the St. Luc diesel shop and had to be chiseled out from its location it was so heavily embedded in ice."

I never got over to Boucherville to photograph the locomotives personally (I was too much in survival mode as I was without power for 7 days), but I was able to buy good-quality 8 x 10 prints at a trainshow a couple of years later.

Ray

I've seen photos somewhere

Using an MLW meant that since the alternator is separate from the rectifier, three phase AC could be taken directly from the locomotive. I assume transformers were used to get 220/110 volts at the appropriate engine speed for 60Hz...

Peter

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Posted by daveklepper on Tuesday, November 19, 2019 2:01 AM

AC hsyterisis non-synchronis (synchronis with some slip, depending on load) slant-bar motors are pretty much standard today on straight electrics, diesel-electrics, mu commuter cars, subway cars, and light-rail and streetcars.  And electric buses.  1.  Muxh less problem with heating from current overload and insulation break-down because the copper or aluminum bars rotate instead of wound wire-coil armatures, and the wound wire field coils, stastionary, can be oversize without a real problem.  2.  No brushes and commutatore  bars to wear out and require replacement.

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Posted by blue streak 1 on Tuesday, November 19, 2019 6:57 PM

An acs-64 has a naximum HP of 8600 and continous 6700 HP.  That is 2150 and 1925 HP for each traction motor.  Can you imagine the size of a DC traction motor on the ACS-64 ? Don't believe it would have been possible.

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Posted by beaulieu on Tuesday, November 19, 2019 6:59 PM

One point not mentioned is that a six-axle locomotive with AC traction motors will have six sets of 3 inverter modules for a total of eighteen, one set for each axle. EMD built locomotives, all SD70MACs, and most SD70ACe locomotives have one set of Inverter Modules per truck, this was less expensive, but is requires the whole truck to be cutout if any traction motor develops a problem. Starting with the last two orders of SD70ACes, and all SD70ACe-P4s, EMD switched to separate  Inverter packs for each axle.

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Posted by JOHN BARRY on Tuesday, November 19, 2019 10:04 PM
I'm very surprised that nobody's yet mentioned the biggest advantage of AC alternators and motors, in general: the load current goes through fixed windings, not rotating ones. Except in kludges like universal motors. So that obviates commutators also. Much easier too, to cool stator than armature.
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Posted by Erik_Mag on Tuesday, November 19, 2019 10:26 PM

beaulieu

One point not mentioned is that a six-axle locomotive with AC traction motors will have six sets of 3 inverter modules for a total of eighteen, one set for each axle.

You're describing a set of three half-bridges per axle, with one half-bridge per phase to make one three phase inverter, which leaves one inverter per axle.

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Posted by daveklepper on Monday, November 25, 2019 9:01 AM

John Barry:  Please read my latest post on this thread that is prior to your "I'm very surprised..." comment.

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