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Traction motors

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Traction motors
Posted by Enzoamps on Friday, May 6, 2022 10:00 PM

I know something about small motors, like model trains, or recording equipment (tape decks, disc drives), juke boxes, etc.   But reading about locomotives I find I know squat.  I can assume a locomotive traction motor is not a permanent magnet type, but do they use brushes?  SLip rings?  The stationary windings are easy to figure.  But how does energy get into the rotor?   And assuming the AC motors are not induction types (like an alarm clock) same questions.

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Posted by Erik_Mag on Friday, May 6, 2022 11:06 PM

DC traction motors are almost always series motors, exceptions including some European electrics had separately excited field windings (e.g. AEM-7). "Separately excited means that the current for generating the field is from a separate source of power than what supplies the armature. A large fraction of DC series traction motors had field taps or shunts to provide for reduced field (higher speed) operation in lieu of the separately excited field.

The DC motors do indeed use brushes, which has been a maintennance issue since the 1880's.

Most AC traction motors are three phase induction motors driven by a variable frequency drive. Some are synchronous motors, but those require a bt more complicated control scheme, but have the advantage of being more efficient at very low speeds.

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Posted by Enzoamps on Saturday, May 7, 2022 5:29 AM

AC motor variable frequency?  Determined by the rotational speed of the engine/alternator?  Or is it a more complex system?

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Posted by CSSHEGEWISCH on Saturday, May 7, 2022 9:56 AM

Enzoamps

AC motor variable frequency?  Determined by the rotational speed of the engine/alternator?  Or is it a more complex system?

 
It's a bit more complicated.  The main alternator feeds AC into rectifiers which convert the current to DC.  The DC current is then fed into inverters of various designs which convert it to variable frequency AC.  Somebody else can provide the particulars of how this is done.
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Posted by Overmod on Saturday, May 7, 2022 11:49 AM

The situation with AC motor control is complicated if operation with other locomotives using a form of 8-notch MU or DPU is needed.  These control the speed of the Diesel engine.  The engine governor controls fuel and hence developed horsepower as load 'in notch' increases and decreases; it is not like a typical car where engine speed is used to vary road speed.

If you were to vary engine speed in some proportional synchronous speed of an induction motor, you'd have ghastly low-end torque -- these motors self-induce a current in a conductive but non-initially-magnetic armature and then use 'rotating' fields to pull the 'induced' armature around.  A slow enough speed on the motor/alternator to do this at slow road speed would lug or stall the engine, unless you used a great multiplicity of poles as in an Alexanderson machine... sized and insulated for the peak current and protected from flashover issues between near-adjacent poles in the structure...

It might be possible to use a mechanical variable-speed drive, like the Bowes drive or a Beier gear with a great multiplicity of discs, but you would still not have the ability to utilize full engine power-in-notch at any physical road speed -- which a synthesized-AC drive can do very well.

The 'traditional' method of generating ripple-less DC from any of the alternator frequencies requires what might be considerable current and voltage storage -- inductors and capacitors, in regular DC motor control.  Erik knows more than I do about current best practices (no pun intended) in producing flexible near-synchronous torque from synthesis.

Note the difference between varying rotational speed of an alternator and accelerating a locomotive whose alternator is constantly turned at proper frequency generation for HEP (on traditional GE and EMD engines, that is frequently (again no pun intended) 720rpm.  At that speed, the engine can produce power with field excitation as quickly as the governor can get the engine to hold rpm with applied load (the engine and alternator rotor having substantial rotational inertia) so loading and subsequent amp generation for rapid acceleration is 'on tap'... just not up to the full horsepower the engine could develop at its nominal eighth-notch speed (which might be 1050rpm on a GE marketed to compete with a comparable-horsepower EMD 2-stroke)

You could approximate multiple-speed control of induction motors by changing the mechanical number of poles 'active' in the motors (as was done for some early AC electric locomotives) or in an alternator.  There are reasons this was not and is not common practice for modern power...

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Posted by Erik_Mag on Saturday, May 7, 2022 12:25 PM

[N.B. My reply was being typed about the same time that OM was replying).

A more detailed description is that the latest locomotive traction inverters act pretty much like a Class-D audio amplifier. That is the outputs are rapidly switched between the positive and negative "rails" (or buses) of the DC supply with varyiing intervals of being connected to the positive and negative rails.

The "switches" are a key point of inverter technology as a locomotive inverter needs to supply at least a good fraction of a megawatt. The original Siemens inverters used on the EMD AC locomotives used Gate Turn Off thyristors (GTO's) that were incapable of switching at much more than line frequency, at higher motor speeds the inverters were switching at the drive frequency. The gate drive circuitry for GTO's is very substantial, so EMD shared the inverter with the three motors on the truck. In the mid to late 1990's, high power IGBT's (Insulated Gate Bipolar Transistors, a modification to FET's) with MUCH simpler gate drive requirements. This allowed GE to use an inverter per motor, which gives finer control over the traction motors.

For line frequency induction motors, there is a tradeoff between starting torque and efficiency (one work around was wound rotor induction motors as used on GN's three phase Cascade tunnel electrification, the N&W and VGN phase splitter locomotives. With a variable frequency drive, it is possible to design the induction motor for high efficiency and still have high starting torque. In fact, the high efficiency design helps with adhesion as motor torque drops rapidly when the wheels start slipping.

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Posted by timz on Saturday, May 7, 2022 3:45 PM

Speaking of AC motors --

Does any RR in the world still use straight-AC locomotives, that run over a normal speed range sending constant-frequency AC to the traction motors, like a GG-1? I assume no one builds such locomotives now; when was the last one built? Did such engines always need 25 Hz or less in the catenary?

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Posted by Enzoamps on Saturday, May 7, 2022 4:42 PM

Thanks.  I totally do understand class-D, and so on as my career was in electronics, and specifically professional Audio.  Just I rarely encountered power amplifiers in excess of 4000 watts.   Never occurred to me they would have power oscillators driving the motors.

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Posted by M636C on Saturday, May 7, 2022 6:52 PM

timz

Speaking of AC motors --

Does any RR in the world still use straight-AC locomotives, that run over a normal speed range sending constant-frequency AC to the traction motors, like a GG-1? I assume no one builds such locomotives now; when was the last one built? Did such engines always need 25 Hz or less in the catenary?

 

Although it is some time since I've been to Europe, I think the last locomotives with AC commutator motors built in Germany were the Br 111 and in Switzerland were the Re6/6. Both these classes may have different classifications now, in Germany post the absorption of East Germany and the DR and in Switzerland due to the adoption pf six digit numeric classes. I think these would have last been built in the 1980s. I think both these types and some older locomotives are still in use in Germany and Switzerland. There may be similar locomotives in service in Norway but possibly not in Sweden now.

All the locomotives described here worked on 16.66 Hz, 15kV.

The Germans and French built locomotives that could operate using commutator motors on the grid frequency of 50Hz, at 20kV or 25kV respectively. These were experimental and not adopted for general use (although the French tried).

The main problem involved the commutators on the motors as at higher frequencies the current tended to "flash over" causing a short circuit and damaging the commutator. There was more deterioration at the commutator with the higher frequency, even if flashovers were avoided.

Peter

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Posted by Erik_Mag on Saturday, May 7, 2022 10:32 PM

Enzoamps

Never occurred to me they would have power oscillators driving the motors.

Do note that the switching frequency for locomotive inverters is a few kHz at most, where class D audio amplifiers are running 500 kHz or higher. The inverters provide a variable voltage variable frequency drive, as the motors want a relatively low voltage and high current at low frequencies, but higher voltages at high frequencies - where low frequency may be under 10 Hz and high frequency may be 60 Hz.

The Siemens inverters relied on varying the DC Link (rails) voltage when operating where the output frequency was the switching frequency, thus preventing use of pulse width modulation to vary the output voltage.

I first ran across Class-D amplifiers in a 1965 issue of Electronics World that I read in fifth grade.

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Posted by M636C on Monday, May 9, 2022 4:18 AM

Erik_Mag

 

 
Enzoamps

Never occurred to me they would have power oscillators driving the motors.

 

 

Do note that the switching frequency for locomotive inverters is a few kHz at most, where class D audio amplifiers are running 500 kHz or higher. The inverters provide a variable voltage variable frequency drive, as the motors want a relatively low voltage and high current at low frequencies, but higher voltages at high frequencies - where low frequency may be under 10 Hz and high frequency may be 60 Hz.

The Siemens inverters relied on varying the DC Link (rails) voltage when operating where the output frequency was the switching frequency, thus preventing use of pulse width modulation to vary the output voltage.

I first ran across Class-D amplifiers in a 1965 issue of Electronics World that I read in fifth grade.

 

The "Siemens Inverters" referred to were those in the earliest EMD locomotives. These used Gate Turn Off (GTO) Thyristors, and in electric locomotives these had a distinctive sound that might not have been audible in EMD diesel locomotives.

EMD later used Mitsubishi Insulated Gate Bipolar Transistors (IGBT), and these are being used in some rebuilds of earlier EMD AC locomotives.

Of course Siemens now make IGBT devices and EMD's choice was presumably based on lower purchasing costs rather than availability.

The actual motors can be driven by either type of inverter, so traction motors can be retained if GTO inverters are replaced by IGBT or later devices.

Peter

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Posted by JayBee on Tuesday, May 10, 2022 1:23 PM

The Siemens GTO Inverters used in EMD SD70MAC locomotives, use four different modes depending on locomotive speed. The first mode is "Asynchronous or Free Running" mode and is used from 0 to 14 mph, the switching frequency increases from 1 to 15 Hz. The second mode is called "Sinusoidal Switching" and operates from 14 to 28 mph, with output frequencies of 15 to 31 Hz. The third mode is ""Rectangular" for speeds from 28 to 35 mph using output frequencies from 31 to 38.5 Hz. Above 35 mph the Inverters are in "Block" mode and use Pulse Width Modulation, and frequencies from 38.5 to 120 Hz.

Power output from the Inverters is 2025 VAC, the full power rating of the diesel is available above 9.3 mph. Below that it is limited to control torque and wheelslip.

All of the above is from the EMD "SD70MAC Locomotive Service Manual 2nd Ed"

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Posted by daveklepper on Wednesday, May 11, 2022 4:44 AM

An AC commutator motor, as used in nearly all PRR and NYNH&H electric locomotives, exceptions being the New Haven's EF-2s (rotary converter and DC motors), and New Haven (ex-Virginian-N&W) and PRR post-WWII freight electrics and New Haven's "Jets" (EP-5s) which were all rectifier-DC motor types, is basically a DC motor and will work OK on DC as done on the New Hacvewn into GCT, but separate control equipment is usual (transformer taps for AC, with or without series-parallel transition, resistor insetion for DC, always inclkuding series-parallel transition). Cleverly desigmed "copmpensating coils" mitigate the loss of efficiencu from the alternating polarity.  Model train "universal motors," as typical for classic Lionel, are small variations of the same idea.  But no locomotives using such motors have been built since the early 1950s.  (Europe and North America)  25Hz is about the highest frequency such a motor built for heavy duty will tolerate.  If fed 50 Hz or 60 Hz, it will sit and hum and heat up. possdibly to self denstruction.  Small motors at 60Hz are possible.

Today's locomoitives, rapid-transit and  commuter and high-speed MU cars usually employ a variation of the "squirrel-cage" induction motor that is the non-synchronous hysterisis induction motor, where the rotating bars  are slanted with respect to parallel to the center shaft.  As with the sychronous motor, speed is determined by the frequency of the power to the field coils which number three or multiples of three.  But the slant-bar armeture does not rotate exactly at the speed of the rotating field but slips slightly, the  degree of slip depending on the load.  At overload, it simply won't turn before it destructs, unlike a brush-commtator motor or an AC slip-ring motor.  (If already turning before overload, it will simply not provide power and slow-down.)

Variations include rotating permanent magnets instead of rotating aluninum or copper bars.  Efficiency is greater, but so is weight.  I don't believe any North American builder uses this approch.

Other types are "inside-out" versions, the "wheel-motor." where the rotaring slanted bars or magnets are on the perimeter, and the multiple-of-three field coils form a non-rotating drum in the center.  Manfacturers include Alstom, Stored Energy Systems (Derby, England), and Magnet Motor (Pressberg, Germany).  They are used by most of the low-floor terminal-to-aircraft airport buses in Europe and by most Eropean trolleybuses.

All these locomotives and other vehickles take DC directly or rectiftify AC from catenary or alternator (AC generator) to DC and then use inverters to prioduce the 3-phase AC  at the right freqency and voltage to drive the motors.

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Posted by timz on Wednesday, May 11, 2022 12:30 PM

daveklepper
An AC commutator motor, as used in nearly all PRR, NYNH&H, and GN electric locomotives...

Not GN.

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Posted by Overmod on Wednesday, May 11, 2022 2:41 PM

The W-1s were motor-generator and used GE 746 motors; the Z-1s were motor-generator and also used DC motors.  I remember the PRR FF2s as being motor-generator too.

These motors all had 'commutators' but were not the AC 'universal' type that could operate from multiple transformer taps like the GG1s... 

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Posted by Erik_Mag on Wednesday, May 11, 2022 10:45 PM

timz

 

daveklepper
An AC commutator motor, as used in nearly all PRR, NYNH&H, and GN electric locomotives...

Not GN.

 

FWIW, GN as using a few of the former Spokane & Inland Empire locomotives on the Cascade tunnel line and those were most assuredly using AC commutator motors. OTOH, all of the locomotives built specificaly for the new Cascade Tunnel electrification were M-G equiped with DC traction motors.

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Posted by Erik_Mag on Wednesday, May 11, 2022 10:53 PM

M636C

The actual motors can be driven by either type of inverter, so traction motors can be retained if GTO inverters are replaced by IGBT or later devices.

One issue with designing motors to be driven by inverters is dealing the with the ripple currents induced by the switching. These currents can be a pain to deal with as proximity effect rears its ugly head. One possible advantage of using SiCFET's is that it will be easier to filter the ripple currents allowing more conventional wiring to be used in the motors.

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Posted by M636C on Monday, May 16, 2022 5:16 AM

On an earlier post, the subject of directly fed AC traction motors was raised:

I found this French Documentary, with an english commentary that deals with early French 50 Hz electrification. Sadly the colour is quite faded but it gives an excellent view of the operation of both steam and electric locomotives.

In particular, it describes a class of locomotives, the BB 13000, that used 50Hz commutator motors. These locomotives were all (53 total) still in use in 1977. The majority of passenger locomotives (BB 12000) were built with ignitron rectifiers, and these were probably converted to solid state rectifiers later. Most Freight locomotives were motor generator units with DC motors (CC14100) but there were a few (CC14000) units that had three phase motors driven by rotary converters.

Anyway, this is the video:

https://www.youtube.com/watch?v=7l6v2qLYg4Q

Even dyed in the wool steam fans should enjoy this...

Peter

 

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Posted by daveklepper on Wednesday, May 18, 2022 11:06 AM

I see a requirement to add to the discussion bof transition.  Transition was no longer required , at least in many cases, when three-phase alternators replaced  DC genrators, even while retaining DC motors.  By carefull control of field currect, the right output voltage and thus power to the motors cold be assres at the right RPM of the diesel for efficiency at the specific throttle (controller) setting and the trains speed.

A parallel development in DC traction for light-ral, rapid-transit, and MU cars, but unknown to me for locomotive applications, was chopper control.  Instead of  wasfing power in grid resistors and asdding the complexity of series-to-paralltl transition, the current from a fixed voltage source was interrupted, with the on time increasing and  the off time decreasing as more power was desired and as speed increased.  I believe the Baltimore Light Rail system still uses raii cars with this technology.

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Posted by timz on Wednesday, May 18, 2022 11:25 AM

daveklepper
Transition was no longer required , at least in many cases, when three-phase alternators replaced  DC genrators, even while retaining DC motors.

To clarify: all EMD and GE C-Cs had transition until ... guess the SD50 was the first one to try to do without it?

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Posted by Erik_Mag on Wednesday, May 18, 2022 4:45 PM

Dunno about theGEC-Cs, but the later EMDs used alternator transition instead of traction motor transition.

There could be some benefit from using AC motor transition, connecting two windings in series at low speeds and in parallel at high speeds. This would ease component requirements for the inverters.

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Posted by M636C on Thursday, May 19, 2022 12:15 AM

Erik_Mag

Dunno about theGEC-Cs, but the later EMDs used alternator transition instead of traction motor transition.

There could be some benefit from using AC motor transition, connecting two windings in series at low speeds and in parallel at high speeds. This would ease component requirements for the inverters.

 

The change from using traction motor groupings was required by EMD's adoption of Super Series wheel slip control. It always amused me that "Super Series" required that all motors be in parallel, but that arrangement allowed single motors to be isolated when they slipped, rather than cutting power to all motors when an axle slipped. The "Series"name referred to series wound motors. I understand that ASEA was involved in the development and they had a system which was used with separately excited motors.

In the cab of the lead unit with conventional wheelslip control with 37 000 long tons on the drawbar, you could tell when the power cut on a trailing unit long before the remote wheelslip bell rang. I never rode on such a train after they started using SD50s but I'm sure they worked better than the old Alcos.

I recall reading in material from the time of the introduction of Super Series that GE had been using internal switching in alternators before EMD introduced the AR11 to replace the AR16. One of the reasons quoted for the change was greater fuel economy, but I don't understand why changing an alternator would save fuel, at least, not on its own...

I was disappointed that nobody commented on my earlier post about directly fed 50Hz motors.  The demonstration model in the video suggested that the motors were larger in diameter than the driving wheels, and were located well up in the bogie structure. I wonder if larger diameter commutators would reduce the chance of flashover with a high frequency AC current.

Peter

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Posted by daveklepper on Thursday, May 19, 2022 4:14 AM

MS:  I didn't comment because such large 50Hz commutator motors are entirely new foir me.  At the MIT EE Department, 1949 - 1957, EMD Summer 1952, they were considered impracticaql, except in small applications, such as Lionel and Gilbert American Flyer.   This  is now old technology, but new for me.

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Posted by Pneudyne on Wednesday, June 22, 2022 1:41 AM

50 Hz single-phase traction motors were certainly larger than their low-frequency counterparts, which in turn were larger than the DC types.  As compared with low-frequency motors, the required lower flux density (to manage the transformer EMF problem) meant more poles and so larger commutators, etc.

Here are some comparative numbers from a 1977 SBB (Switzerland) paper, for nominally 1200 kW motors:

 

DC Motor:

 

8 poles

Armature diameter 760 mm

Weight 3640 kg

 

Single-Phase 16 Hz motor:

 

10 poles

Armature diameter 820 mm

Weight 3860 kg

 

Single-Phase 50 Hz motor:

 

20 poles

Armature diameter 990 mm

Weight 4600 kg

Power 1050 kW

 

Note that even with its larger armature diameter, the 50 Hz motor is of lower power (1050 kW) than the others.  Presumably a 1200 kW 50 HZ motor would have been too large.

 

 

Re the SNCF (France) BB12000 (DC motors) and BB13000 (50 Hz motors) types, the diagrams on hand show that the BB13000 motors, at around 1215 mm outside diameter, whereas the BB12000 motors were around 820 mm diameter.  In both cases the driving wheel diameter was 1250 mm.

 

The 50 Hz motors of the BB12000 were to an ACEC (Belgium) design.  ACEC was a Westinghouse licensee, and for its 50 Hz motors (initially built for use in the Congo), it chose the resistance lead approach that Westinghouse had adopted for its 25 Hz motors post-WWII.  Actually it was an old idea that although discarded early on, was thought to be workable with modern, more heat-resistant materials and techniques.  Such motors were used for some EMUs of the time, and were originally specified for the PRR prototype locomotives, before Westinghouse requested the change to using Ignitron rectifiers and DC motors.  GE was on the record as being antithetical to the resistance lead technique, and stayed with improved conventional techniques for its late 25 Hz motors.

 

The 50 Hz motor was a quickly passing phase, last used for new locomotives c.1958, and for EMUs a couple or so years after that.  Notwithstanding the difficulties associated with mercury arc rectifiers in the traction environment, on balance they seemed to have been preferred over 50 Hz motors.  Also, towards the end of the 1950s, the near term promise of solid-state rectifiers – and the possibility of converting existing locomotives from mercury arc to solid-state, would also have swayed choices.  One potential advantage of single-phase motors was that regenerative braking, if required was relatively easy to do.  But very few 50 Hz motor locomotives were so-fitted.  (Whereas it was more-or-less standard for the SBB 16 Hz fleet.)  It would have been difficult to do regenerative braking with ignitron-type rectifiers, but SNCF did it with the excitron type, and I think continued to build excitron regenerative locomotives after moving over to silicon rectifiers for its non-regenerative requirements.  Of course, rheostatic (dynamic) braking could be done with equal ease for both single-phase and DC motors.

 

 

Cheers,

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Posted by Overmod on Wednesday, June 22, 2022 9:14 AM

It's always nice to get good information from actual experts in a technology.  We're fortunate to have them actively posting here.

M636C
One of the reasons quoted for the change was greater fuel economy, but I don't understand why changing an alternator would save fuel, at least, not on its own...

I suspect this had to do with better control of the field so that the Woodward governor could be commanded to make better power at a lower nominal notch.    

I was disappointed that nobody commented on my earlier post about directly fed 50Hz motors.  The demonstration model in the video suggested that the motors were larger in diameter than the driving wheels, and were located well up in the bogie structure.

You'd have to use 'monomoteur' or body-mounted motors -- and these are certainly a 'thing' -- but you'd need additional room in the carbody for the motors and associated support.  American practice has generally used some variant of wheelbarrow suspension with the smaller wheel sizes.

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Posted by jeffhergert on Wednesday, June 22, 2022 9:51 AM

timz

 

 
daveklepper
Transition was no longer required , at least in many cases, when three-phase alternators replaced  DC genrators, even while retaining DC motors.

 

To clarify: all EMD and GE C-Cs had transition until ... guess the SD50 was the first one to try to do without it?

 

 

The SD70m still make transition.  Ours around 25 and 40 mph.  I've noticed foriegn power sometimes have a different setting, a few mph difference.

That way going up a stiff grade with them down on their knees you get to feel the lunge, has they momentarily drop the load and then start pulling again, twice.  All the time hoping it doesn't break a knuckle or yank out a drawbar.  

Jeff

 

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Posted by bogie_engineer on Wednesday, June 22, 2022 9:58 AM

Pneudyne,

Great to see you posting here, always enjoy reading your well researched and informative posts on that other RR forum.

Dave

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Posted by Pneudyne on Saturday, June 25, 2022 5:50 PM

bogie_engineer

 

Pneudyne,

Great to see you posting here, always enjoy reading your well researched and informative posts on that other RR forum.

Dave

 

 

Hi Dave, thanks very much for the welcome and the kind words.  There are some very interesting thread topics active here!

 

 

Cheers,

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Posted by Pneudyne on Saturday, June 25, 2022 5:53 PM

daveklepper

 

A parallel development in DC traction for light-ral, rapid-transit, and MU cars, but unknown to me for locomotive applications, was chopper control.

 

 

 

Chopper control was used for DC locomotives.  As far as I know, aside from experimental units, the first series-produced type was the SNCB, Belgium 20 class of 1975, for its 3 kV DC system.  These were six-motor units (C-C) with sepex motors.  Other major 3 kV DC systems who used chopper locmotives included FS, Italy and SAR, South Africa.  The interval between the advent of chopper controls and the arrival of inverter/AC motor techniques seems to have been relatively short as technology epochs go.

 

 

Cheers,

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Posted by JayBee on Sunday, June 26, 2022 2:37 PM

I would like to add that the newest French TGV powercars do use permanent magnet AC traaction motors.

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