AC vs DC

It may seem like that, but it's not to be.  They're three pole motors, and modern can motors are at least five poles for smoother running.  Also, the old Pulmore motors are very inefficient, they consume two-three times the power that a can motor doing the same work would consume.  More power means larger and more expensive electronics.  Finally, there's no flywheel on almost all Pulmore motored engines, another negative.

The key is the conversion from DC to AC traction motors.    They replace the traction motors.

@Deere Lines posted:

Isn't it ironic that old full size trains started out as DC, but when rebuilt they are converted to AC tractive motors. Why can't the old Pulmor motors be upgraded in a similar fashion. Seems to me that it would be possible, with the right electronics.

The D.C. traction motors found in first and second (and some third) generation diesel electric locomotives were D.C. universal motors very similar in construction to Lionel's universal "Pullmor" motor. Locomotives never used can motors as can motors are just not as efficient as universal motors. The D.C. universal traction motors can be operated using A.C. current, but they are much less efficient. In many ways, Lionel's Pullmor motor could be considered a D.C. universal motor as it operates more efficiently off of D.C. than A.C. 

The A.C. drive of a locomotive displaced D.C. units because the A.C. traction motors are much more efficient than any D.C. motor. The traction motors on the ET44AC or SD70ACe are asynchronous motors (induction motors, a Tesla invention) are 90+% efficient. Compare that to ~75% for comparable universal motors or ~65% for comparable can motor.  The high efficiency of asynchronous motors means more mechanical motion performed with less heat generation which is one reason why they displaced D.C. traction motors relatively quickly. 

@prrjim posted:

The key is the conversion from DC to AC traction motors.    They replace the traction motors.

The prime mover rotates a generator that produces A.C. current. The A.C. current is rectified to D.C. As you said that is a key step. The D.C. current is then rectified back to A.C. This is the next key step. The purpose of this back-and-forth from A.C. to D.C. and back again to A.C. is that it is easier to vary the frequency of the A.C. feed to the motors. One can use, say, 60 cycle A.C. and convert it to something else such as 12 cycle AC. However, changing one frequency of AC to another frequency is more difficult than taking D.C. and converting it some frequency of A.C. 

The rotational speed of the asynchronous motor is controlled by varying the frequency of A.C. feed to the motor.  The D.C. is converted to variable frequency A.C. (VFD) which is fed to the asynchronous traction motors that drive the wheels. The video is a simple description. 

A hypothetical advantage for a heavy haul locomotive is that with a VFD is that full power can be given to a motor and have a low frequency A.C. fed to the motor so it turns very slowly. This allows full power to start a train. As the large mass begins to move the frequency can be increased which will increase the rotational rate of the motors. 

VFD's are very common in the RC hobby. They commonly use what are commonly called brushless D.C. motors which are in fact AC motors. The reason that they are called brushless DC motors is that DC from the batteries is used to feed the VFD. Using the same extension of thought the induction motors on a locomotive could be considered brushless DC motors even though an induction motor will never operate off of DC being fed directly into the motor. VFD's are certainly applicable to the model railroad hobby. Märklin created a motor and VFD for their trains. 

Lionel attempted a VFD product in their Odyssey motor, but ultimately failed and the product never made it to market. 

I'm quite sure that brushless motors haven't made it to the model trains for one simple reason.  The can motors are "good enough", and revamping all the electronics to brushless motors would be a significant R&D expense.  I used brushless motors extensively in avionics instrumentation 30 years ago, they've been around for a long time.

As for AC motors being more efficient than can motors, that clearly doesn't translate to the Pulmore vs a can motor in model trains.

My comment was not clear, what I meant was that they did not "convert" the traction motors, they had to replace them with AC motors.    I was not addressing the generating system.    Sorry for the confusion.    The starting question was why can't reconvert these toy train motors from DC to AC, and I was trying to say, you have to replace not convert.

@prrjim posted:

My comment was not clear, what I meant was that they did not "convert" the traction motors, they had to replace them with AC motors.    I was not addressing the generating system.    Sorry for the confusion.    The starting question was why can't reconvert these toy train motors from DC to AC, and I was trying to say, you have to replace not convert.

Locomotives went from universal DC traction motors to AC induction motors (using an AC-DC-AC transmission).

Lionel trains went from the AC universal "Pullmor" motor to a DC can motor that is used in today's hobby products.

On the surface the prototype change over seems analogous to the hobby change over but they really are not. 

If I interpret your question correctly, you are asking why we can't or don't go back to the universal motor of the old days. There are several reasons as to why:

1. The motor is too large to use in scale proportioned products. The Pullmor motor is a scale 7 feet across which requires hoods to be a scale 8 feet to accommodate the motor. Hoods on most locomotives are 6 feet. This is a big strike against the Pullmor motor in modern products. 

2. As G,J, wrote earlier, the Pullmor motor is inefficient. The Pullmor motor is a design that is essentially from the 1930's. It has tinny motor brushes in proportion to the size of the motor. It also has a flat faced commutator which leads to uneven wear of the motor brushes. The tiny size of the brushes in conjunction with the flat faced commutator leads to poor contact, excessive sparking, and ultimately a large heat generation (and ozone production ).  In the end, the Pullmor motor is roughly 35% efficient whereas the DC can (RS385PH) motors are roughly 60% efficient. 

3. As G.J. mentioned, the armature of the Pullmor motor has only three poles. 3 pole motors are generally not as smooth in operation as armatures with more poles. The field is important as well. Lionel made different designs of the universal motor and, not getting into much detail, the field contours greatly impact motor performance.

4. Expense of manufacture. The Pullmor motor only has utility in Lionel trains and as such had limited production runs. Limited production run translates to more expensive. DC can motors are used for many other applications and are adapted to use in model trains. They have much larger production runs and benefit from the economy of scale. Can motors are much less expensive. 

5. Speed control technologies could not be adopted for use with the Pullmor motor. 

All these are solvable with some design work. The flat faced commutator can easily be replaced with a drum commutator for better electrical continuity. In truth, nobody even uses flat faced commutators anymore (and probably the majority of industrial motors don't even use commutators anymore). With modern winding and production technologies, the number of poles can be easily be increased on both the stator and rotor. Favorable gear ratios can be used. The size of the Pullmor motor can easily be reduced in size and fit into the width of a scale hood. Inherently, universal motors are more efficient than can motors when modern designs are used. Most applications that utilize universal motors also utilize speed control to maintain speed under different loads so speed control can work with universal motors. However, I do not believe that Lionel, MTH, Atlas, 3rd Rail and the others employ engineers that are knowledgeable in motor design. If they did, something like Lionel's F40PH fiasco (the motor was too tall and cut into wires which blew out the whole thing) would not have happened.

So in short, yes it is technologically feasible to go back to universal motors and have an improved product.  

In the end, it is just easier for the manufactures to buy an off the shelf DC can motor and plop it into their product. 

As far as AC-DC-AC system like in the videos to control synchronous or asynchronous motors like what exists in locomotives, off the shelf products exist for that too that would easily fit inside O gauge/scale (even HO scale) trains.