Robert:
There are some important physical discriminators between OF, can, coreless, and square motors.
(1) In all cases I’m aware of, OF motors use more current to develop the same torque as the other motors, for a given width of motor or torque of motor. So long as the stall current on your OF motor is lower than the rated current of
your decoder, that’s OK. But as many people have found out, the larger OF motors typically draw more than an amp at stall, and kill HO decoders. In smaller models, where N scale decoders are the only option that fits, current ratings are in the 0.5 amp region,
and even the smaller OF motors can typically kill them.
(2) Generally, the OF motors imported in early brass had 5 windings on the rotor, and two magnetic poles. (Some of the very early ones had only 3 windings on the rotor.) This meant that the motor imposed 10 (6) cogging events on the
drivetrain per revolution of the armature. Cogging is when the static field from the magnet and flux plates imposes a preferential angular orientation with each steel armature leg. Even with a fairly high gear ratio between the worm and worm gear, this causes
noticeable hesitations of the model at low speeds. I suspect that the Tenshodo 48:1 gearbox was used in some models to reduce the amplitude of the driver hesitations compared to what was being seen with 27:1 gearboxes. Can motors exhibit way less cogging,
and cogging is essentially eliminated in coreless motors. The square motors people have been using in models lately have 4 magnetic poles and 6 armature windings, which yield 24 cogging pulses per armature revolution—more than twice as many as an OF motor
with 5 windings, and each pulse is lower in amplitude than those of an OF motor.
(3) The magnet location behind the armature in an OF motor is one of its major detractions. The lesser issue with it is that the magnetic flux must be ducted forward to the armature. This makes the OF motors much taller than they are
wide (the minimum width being established by the diameter of the armature.) But by far the bigger issue is that the rear location of the magnet makes OF motors very
long for their torque output. Since I have a lot of trouble owning a steam loco model that doesn’t have a backhead, this issue alone makes it worthwhile to me to get rid of OF motors. Substituting in a can or coreless motor, or especially a square
motor, often shortens the space required for the motor to a length that allows correct placement of a backhead in the cab. Sadly, this isn’t always enough; I have paid to have the gearbox shifted forward a driver axle in a few models (such that the motor
could be moved forward, and a backhead installed), and that has always ended up requiring a LOT of modification and fabrication to make work. That extraordinary effort was required on a Westside LIRR G-5 4-6-0, and a PFM A&LM 4-6-0 that I had modified into
a North Western Pacific T-49. Any model that lacks a trailing truck seems to be challenging to get a backhead into, even with modern motor technology. But the modern motor technology is usually a
required part of the solution.
(4) The power density (inch-ounces of torque per cubic inch of motor volume) in the various types of motors runs from square to coreless to can to OF, in decreasing order. To cite an admittedly oddball example of how this, and the shorter
length, of a square motor beats the tar out of OF motors, Jan was able to install a square motor entirely into the water/fuel bunker of a PFM Vulcan Duplex model for me, whereas the OF motor it replaced had taken up the entire water/fuel bunker
and the cab interior below the window line and the rear part of the boiler. Now the only powerline intrusion through the cab is an 0.080” dia shaft, and I have an otherwise open cab area and a fully detailed backhead in the model. A very beautiful
thing. Oh, and I didn’t give up any pulling power, though I did lose some top-end speed. I doubt that any of the Vulcan Duplex prototypes ever saw the sunny side of 30 mph, though, so it’s all good. At the other end of the scale, I have Faulhaber coreless
motors in an Alco Models N&W Jawn Henry and an NJCB Virginian AE 2-10-10-2, and both significantly out-pull stock models with their original can motors.
(5) Some of the coreless motors are brushless (but you have to shop carefully for this), which makes brush wear a non-issue. Furthermore, most coreless motors have outstanding shaft bearings. A top-end coreless motor should provide a
lifetime of continuous operation of a model, far out-lasting the main and side-rod useful life.
I guess the point I’m working toward is that smoothness of operation and compatibility with available decoders aren’t the only reasons for swapping out OEM motors. Whether the other reasons are significant enough to compel a modeler to
do a motor swap is, obviously, a matter of personal choice. I’m aware that my mania for cab interior detailing is way off the norm. (Although, if one is going to have a $400 -$500 paint job thrown on a model, it seems hard to argue against the proposition
that it should be the best model it can be before-hand.) Increased pulling power seems a more arguable virtue to me. The VGN routinely pulled 160 car trains with the 2-10-10-2’s in the flatlands of Virginia between Roanoke and Sewell’s Point; that was limited
by the length of the passing sidings on the VGN’s single track main line, rather than by the loco’s tractive effort. So one could assert that a model 2-10-10-2 should be able to pull 160 model cars. But I doubt there’s a model railroad in existence that
would allow successful (let alone interesting) operation of 160 car trains. So one could assert (with more justification, IMHO) that a model 2-10-10-2 should pull as many cars as the longest train that could reasonably be run on its owner’s layout (or his/her
club layout). Realistically, that’s probably closer to 40 cars. Maybe the OEM motor in my NJCB model would have been good enough?
Model well!
-Eric
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With respect to old brass steam locos and open frame motors, Mark Guiffre at All Brass Workshop left the original open frame motor in my NPP K-2 because he said it ran just fine and he is a stickler. NPP was using can motors by the time
they imported the R-1 Prairie so no conversion was needed. Mark installed DCC sound and lights in both of them and tuned them up, e.g. requartering if necessary. I cant tell the difference in their running abilities. The lesson learned is to have a qualified
brass mechanic check the mechanism before ordering unnecessary work.
My humble opinion and your mileage may vary, of course.
Bob Herrick
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