Some DC and AC Motor History

[daveklepper]

The purpose of this posting is to clear up some misunderstandings.

Normal DC Traction Motors: Require brushes, which provide electrical contact to the rotating commutator that has connections to the rotating armature (on the same shaft). Permanent magnet fields may be used in model trains but the real ones have dc coil fields. High efficiency, but requires brush maintenance. They do wear out. Stalling under continuous current under load can burn out the coils through heat build up.

Induction Synchronouse Motors Used early in a few tunnel electrifications, these are constant speed motors. Not very practical, but were used in the first Cascade Tunnel three-phase electrification. No brushes required and more difficult to burn out though overloading.

AC commutator motors. This is basically the first kind of motor, with with compensating coils to reduce hysterises lags from the alternating current. Such a motor can be used both on low-frequiency (like 25 cycles per second) and DC. New Haven EP-1 through EP-4, EF-3, PRR P-5 and GG-1 had this type of motor. Not quite as efficient on ac as on dc, and does require brush maintanance.

Hysterises non-synchronouse motor. This is like a squirel cage or regular induction motor except that the rotaing copper or aluminum bars that have the induced current are slanted, rather than straight across. The development of this motor, quite efficient and resistant to stalling and burn-out, depended on computer technology and solid state switching so permit field coils to be powered in the right sequences with proper timing and current for the speed and load conditions. This is the type of motor on all current ac-motored diesels, electric locomotives, and ligh rail cars, except as follows:

The wheel motor: This is an inside-out motor, with the field coils stationary on the axle and rotating permanent magnets or slanted copper or aluminum bars, the first considered possibly as an inside-out dc motor and the second an ac, but both really require computer control of current and voltage and timing to the stationalry field coils. In use on some European low-floor light rail cars, many airport buses, the MAX guided bus line in Las Vegas. The first manufaxcturer was Magnet Motor in Germany, now there is Stored Energy Technology in Derby, and Alstom for Irisbus. My personal belief is that all autos should use this technology instead of mechanical transmission and differentials and drive shafts, etc.

[Anonymous]

daveklepper, you are boss! II love the tech talk about electricity and electric motors! Why don't they teach this in our high schools instead of social analysis and party planning!

Perhaps our European friends would enjoy hearing that, very early in the 20th Century, North America might have adopted DC as the standing household current. But DC by its nature has to have its transmission "refreshed" by means of a booster station or intermediate way current station (surely someone out there knows the correct term?) -- several times more than AC. In essence, Thomas Edison had hissy fits until AC became the American norm.

On a recent trip to Las Vegas, halfway to the southern I-15 border at Primm, NV, I saw such a "whatchamacallit" electric facility out in the middle of nowhere where there were no residents, agriculture, or industry, just I-15, Uncle Pete's banged-up diesels, and the electric wires; about 22 miles from anything else either direction.

In Europe with its, I believe, 220V standard, such "refreshers" have to appear much more often, but they're hidden by other aspects of civilization. Recall that in the greatest number of EU countries one is rarely out of sight of the next town or village -- no real Mohave Desert to cross grid wires with.

There is practically no "hinterland" left in Western Europe. But America still has a lot of hinterland. Some people believe that this (relative) isolation creates isolationists politically -- the red-state effect to use an increasingly inflammatory but nearly meaningless term.

[daveklepper]

Your "refresher" is a "substation." Actually, all long-distance transmission both in Europe and in the USA is at very high voltage, not house current voltage, and then the substations bring it down to intermediate voltages, and then transformers near or innsde homes bring the voltage down to 115V 60 cycles per second ("Hz") in the USA and 220V, 50 Hz in Europe. Until you reach the home, transmission is three phase, and there are usually five wires, the three active phase wires, a neutral, and separate from the neutral is ground. If everything is OK, there should be zero voltage difference between neutral and ground, the voltages between phase wires should be close to identacle, and ditto from phase wires to neutral. Modern electronics makes possible high voltage transmission at DC, and this has even less loss over long distances than the same voltages and currents at AC. The Finns generate some of their own power but also buy from Russia. They had all kinds of problems with fluctuating voltages and phase or frequency with the power from Russia, so they put in a high voltage DC link and have eliminated the problem.

At the transformers to house voltage, the attempt is made to divide the load among the three phases. Perfect match at one street or at one home or set of homes is not required, because despite variations locally, the overall balance can be maintained by the randomness of loading producing averages that are nearly equal.

All AC diesel electrics are somewhat similar in that three-phase alternators can be used. But so can single phase, like the alternator in a car including hybrids. Note that electrification, you can have any combination of current source and motor type you want, today. The Virginian and Amtrak E-60 and New Haven EP-5 electcrics picked up high voltage AC power (except the EP-5 in New York Central third rail territory) and used dc motors. The AE-7 and Acela are all-AC. The tyypical most modern low-floor European tramcar and the latest New York subway cars pick up DC power but have AC hysterises non-synchronous motors. You could theoretically build a diesel elctric with a dc generator and ac motors, but nobody does it. Like the motors, using an ac alternator instead of a dc generator eliminates brush replacement maintenance.

[Anonymous]

Do 3 phase AC motors exist? In the US some motors, typicaly HVAC equipment can run on either 240V or 208V. If I understand correctly, 208 has 2 phases 120 degrees apart instead of 2 phases 180 apart. How do these motors compensate for that? Would a 3 phase motor produce smoother torque?

[Anonymous]

3 Phase AC motors are actually the standard for industrial use. They are very efficient. A 2 phase motor with the phases 120 degrees apart would not run very well. Generally when someone specifies a motor to operate on 208 volts instead of 220 or 240 the reason is that they live in an area that suffers from serious and frequent voltage sags. A motor can absorb slighly higher voltages much better than it can lower voltages. By purchasing a motor that will operate efficiently at the lower voltage they are protecting their operations from all but the worst voltage sags.

The AC commutator motors are also known as universal motors as they run on AC or DC. The motors in Lionel and American Flyer toy trains are this type.

[lonewoof]

Motors are specified for "208/240 volt" operation because they are designed to be used on normal house-supply voltage (240 volts, actually two 120v. circuits 180 degrees out-of-phase) OR on 208 volts, which is the phase-to-phase voltage of three-phase power that is normally supplied to (small) industrial or commercial users.
I worked at a TV station that had only 3-phase (120/208) power. To install a window air conditioner, we used a small transformer to boost the 208 v. to somewhere around 232 v.

[Overmod]

Ye gods, there's some wacky stuff out there!

208V is, by definition, a three-phase rating (that weird-looking number is the resultant). Not at all 'unusual' when you recognize the physics involved. You were right about the 120 degree phase angle... but note that there's little advantage to having traction motors -- which one assumes are intended to run smoothly -- with 'two phases 120 degrees apart'. You can indeed build a motor to use such a supply, but it wouldn't turn smoothly, as the definition of phase assumes 'per cycle', and the 2-phase 120-degree power would produce no torque for 1/3 of each rotation...

Europeans had synchronous AC systems, too; at least one of them was a three-phase system with multiple collectors. I believe the AEG high-speed electric test car in the very early 1900s (which went to speeds well over 100mph) had three separate collectors vertically spaced on a mast. This stuff is very efficient when train speed can match the motors' synchronous speed as expressed through the gearing and drive, but torque getting up to synchronous speed is not as high, and overloading that makes the motor speed drag down below a certain percentage of synchronous can result in the motor more or less abruptly dropping out and losing power. So where you saw this stuff was predominantly on mountain grades or in long tunnels, where you could start more or less on the level and get to synchronous speed, not have to go very fast or accelerate very hard, and then work the train consistently at the synchronous speed through the entire track range where efficient high power was desired.

Technically, the problem with overheated motor coils isn't that the copper in them melts, it's that the insulation on the wire melts and causes short-circuiting (which then can melt the copper, but perhaps more importantly causes significant loss of magnetic-field generation (which is what actually causes the motor to develop torque) because there are now fewer loops to produce the B-field with the shorts present... less power means more current required, more current means more I2R heating... you get the picture.

Three-phase or other alternators have nothing whatsoever to do with traction motors in any significant modern locomotive. As Randy will confirm, the current has to be rectified to DC whether or not the locomotive is equipped with AC drive! (You can design locomotives in which the alternator system is modulated to produce some of the traction waveform -- I suspect the Caterpillar MorElectric truck "generator" design can be modified if desired to do this, for example -- but I don't know of any production locomotives using traction alternators, at least in the USA, that go to the trouble and complexity of doing so).

Instead, your traction alternator produces EMF via rotating "coils inside coils", with the resulting power being smoothed to a fair approximation of DC. This is then supplied to the traction inverters (2 for EMDs, 6 for GEs IIRC) which synthesize the appropriate waveforms and frequencies to drive the traction motors, and supply the appropriate phases to the windings in those motors -- since the inverter system does not 'predict' the momentary power going to each of those phases and use the information to modulate the alternator with sufficient intelligence, it's necessary to have DC, or close to DC, available at the inverter input to be assured of sufficient power over the allowable range of motor speeds for a given alternator output...

Tiny nit-pick: many designs of AC machine do not ELIMINATE brush maintenance; they only reduce it greatly. When any kind of power needs to be fed to the rotating parts of AC machines, by definition you use a brush. It runs on a slip ring, rather than a commutator, and it does not handle the high impulse loads, making and 'breaking' arcs at comm segments, etc. that represent the major maintenance issues with carbon brushes on 'hard' commutated motors. But you still have wear and oxidation issues, and to a certain extent bounce issues that can cause sparking/arcing at the slip rings -- which is why the 'pure' induction motor that uses induced eddy currents in a 'bar' armature is so much more desirable a solution in railroad service than the old 627-style motors. Look here:

http://www.crisny.org/not-for-profit/railroad/gg1.htm

for a pretty good analysis of the traction system in the GG1.

smalling, you need a major course in American technical history! ;-}
Edison was ALWAYS a major DC guy; it was Tesla, and Westinghouse, that worked out the AC distribution system (and, be it added, three-phase power motors). Edison had the hissy fits about AC being selected for power transmission -- he's also responsible for the coining of the term 'electrocution' to characterize the use of electricity -- AC electricity, let's not forget -- to perform executions at Sing Sing. Gabe et al. will recognize the nifty courtroom tactic -- associate your opponent's technology with something bad.

Dave, you might explain to the audience why the bars in modern induction traction motors are slanted...

And, of course, we should probably mention the flavors of linear induction motor (LIM) as brought up over on the monorail-top-speed thread: these have an operating principle very similar to induction traction motors, but usually involve putting the active part of the motor (the windings) on the moving railcar, with the stator being a 'reaction rail' which is usually placed between the running rails. (The LIM system I was researching in the early '70s inverted this, with sets of fixed coils being spaced periodically along the track and interacting with (superconducting) magnets -- required instead of passive metal to give the required high flux interaction and relatively low tare weight I then thought would be needed for very high speed operation -- to produce motor action).

Be interesting, I think, to see if there is indeed a future for permanent-magnet traction motors in some railroad applications -- I know of at least two efforts currently underway to commercialize them...

[Anonymous]

[8)]

Overmod, I deserved the good whoopin' you gave me. Guity as charged about being really, really ignorant about "American technical history"; in my high school -- mind you, this was 30 years ago -- there was no course in engineering, nor even pre-calc; just physics that had to do almost entirely with the Newtonian system, next to nothing about electromagnetism, and nothin' at all about electronics, elec. engineering and so on. (If you think this thwarted would-be engineers and architects, by gum you're right--the error was eventually eliminated). But in my case, I left high school thinking that "Dynamo" was the name of a liquid laundry detergent.

Anywho, I bet you (and a couple of others on this post) can tell me (us) why the Chicago CTA's "L" cars exhibit almost no brake screech when they come to a halt at the station. In my dark past of READER reading, I think I remember the term "linear induction," in the sense that the some motors impelling forward motion could be used (in reverse??) to slow the train down to about 4 or 5 m.p.h., leaving the brakes free for relatively light duty, which I guess would mean not having to overcome so much inertia (impetus), the brakes just didn't wear out as quickly or in the same way--or perhaps a better Newtonian way to say that is that there was almost no need for the brakes to convert energy/momentum into energy/friction heat (??)

But since every time I make a suppository remark (?) I get busted[:0], and rightly so. Is there truth in the above paragraph, notwithstanding the fact that I probably abused about half a dozen terms? [:(]

Here's another thought for food--in railwaying, or interurbaning (CSS&SB), or light rail, does third-rail or "shoe" tend to make use of differing currents whether it draws from catenary or shoe; or whether it's AC or DC? (Is there a clever term that means "whether current is AC or DC"? "Modes," or something like that.)

Your point about AC in W. Europe is well taken -- my technical knowledge may have been okay at one time, but it stemmed from the "Magic Mountain" era of traction technology. [:I]

Certainly in Chicagoland the trend has been away from catenary wires whenever possible. The South Shore must have its overhead because it runs down the main streets of a couple of Indiana towns, but the sidings went catenary-free when the "Little Joe" 1947 motors were scrapped a number of years ago. And our beloved "Skokie Swift" -- actually just a tag of the old Chicago North Shore line to Milwaukee -- has gone totally third-rail in its Skokie domain, matching that of Evanston. (Alas, will I ever see "pan up" on the fly again?)

Other than the convenience factor--third rail doesn't blow down in a stiff wind or ice storm--is it really possible to make third-rail as safe as catenary? The Swift runs almost entirely at ground level in Skokie (it's mostly trenched in Evanston and no grade crossings). But Skokie is a built-up suburb with LOTS of grade crossing wig-wag + gates.

Ignoramus that I am, I myself would worry that my kid (or my neighbor's kid, or my dog) would contact the third-rail and perish. There are some swinging-gates at the pedestrian (usually sidewalk) level that close off human access to the tracks when the gate comes down, but the track is always energized, isn't it, even with no varnish in sight?. . . .

Also, several times a year someone falls (or sometimes is pushed) onto CTA third-rail and the results ain't pretty. Several years ago, in what must be the ultimate combination of stupid and pathetic, one young man arced in just the right way to fry himself while p***ing!

And finally, thanks for the link to GG-1 stats. Nothing makes learning more fun and fast than a compelling topic.

Now I think if I can learn how I'll post my own thread and stop mooching off the formidable Mookie's domain . . . [8]

al-in-chgo (4 blocks from L, 2 from commuter train).

[eolafan]

Oh my gosh, I have such a headache since reading this thread as my brain has tripped out (no, not that kind of trip!) on information overload....seriously, you guys are really wonderul and full of great information on this subject. Thanks, Jim

[daveklepper]

Again, both the regular "comstant speed" induction motors and the nonsynchronous hysterises motors do not require bruishes or sliprings, because the current is induced in their rotating bars by the magnetig field of the field coils which are fixed and which have the computer controled waveform described, which is derived from dc or semi-dc, when then comes from usually single-phase ac from the catenary or the three-phase ac provided by the alternator on the shaft of diesel. Why are the bars slanted? A simple, but possibly not extremely accurate answer, is to point out one bar is then capable of slipping between one field coil and another at a different rate than the magnetic field rotation produced in the field coils. The hysterises effect can be roughly defined as magnetism continuing to exist for a while after the current producing the field ends. If this seems like gobbldygook to you, it may reflect my still imperfect understanding of the precise physics involved in this effect. We all know what a permanent magent is (and for the sake of Stored Emergy Technology's and Magnet-Motor's Wheel-Motors or "Hub-Motors", I hope their permanent magnets are permanent", but even ordinary iron/steel field magnet cores have a bit of pemanency, they don't just switch off the magnetic field when the current stops. The rest is matter of geometry, this effect can be exploited by the slanting bars, but not the straight across ones.

[Anonymous]

Originally posted by lonewoof

Motors are specified for "208/240 volt" operation because they are designed to be used on normal house-supply voltage (240 volts, actually two 120v. circuits 180 degrees out-of-phase) OR on 208 volts, which is the phase-to-phase voltage of three-phase power that is normally supplied to (small) industrial or commercial users.
I worked at a TV station that had only 3-phase (120/208) power. To install a window air conditioner, we used a small transformer to boost the 208 v. to somewhere around 232 v.[/quote

It is possible that the small transformer was installed for the purpose you state, but it is much more likely that it was installed as an isolation transformer to protect the motor in the air conditioner from the radio frequency "noise" that the TV station equipment puts on the power. Unprotected motors tend to burn out the first winding of the stator if there is a lot of RF harmonics in the power. Isolation (otherwise known as smoothing) transformers are very common where there is equipment like transmitters, variable frequency drives and the like.

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