locomotive drawbar pull and digital drawbar pull meters

I've done a lot of draw bar pull testing. In my experience max pull, at least as told by the pull meter, is with moderate wheel slippage - too little or too much and the reading drops. YouTuber "ericstrains" says he uses 40 smph for his pull test (I only run conventional but that sounds about like what I've seen). The taxing point is current draw for a particular locomotive's motor(s). It can be a bit of chore to determine which motor a particular locomotive uses, and even more so finding the motor's technical specs. From my research the Pullmor and Atomic motors are rated at 2.5 amps max continuous and the Mabuchi RS-385PH (used in many newer locomotives) is rated 1.5-2.0 amps continuous.

I will put a Fluke meter in series with the transformer to measure current draw. Virtually all locomotives I've tested are well below their max current rating (though I will say there can be significant variation between the same model locomotive, depending on gear train fitment, wear and even wheel condition). I've only seen one factory stock locomotive exceed its max motor current draw during the pull test, and that was a 1950 issue 773 (it's heavy, all six drivers contact the rail, and the gear train is high reduction worm drive (= inefficient = power hog)).

The below is my test rig. It's portable so I can just plop it down anywhere there is a transformer. I have mostly postwar stuff but much of my track is nonmagnetic, ergo, the two track types to gauge performance "now vs. then." As to what various locomotives pull, again check out "ericstrains" YouTube channel - he's reviewed dozens of modern locomotives and he includes his pull test results.

The maximum drawbar pull should be about when the wheel speed is about 15% faster than the locomotive speed.   SAL9000 I am interested in how you determined that maximum allowable current for the Pullmor and Aromic motors.  The maximum current in a motor is usually determined by embedding thermocouples  in the windings to determine when the winding temperature exceeds the insulations temperture rating.  What is your criteria?  

Thanks,

David Johnston

I think this can be a little tough to ascertain with extreme accuracy  and that is probably why most mag reviews today talk about scale MPH with a particular lode on it, lets say ten cars.

What I think we should be interested in is the current draw at what speed. This will tell us what lode is being developed and your best transformers, like the Z4K have built in volt and amp meters.

As for the Micro Mark meter I have one and it works pretty good. When trying to get a pull force reading we should consider the cleanliness of the track and wheels, because any oil or grease will obviously cause early slippage.

Oh yea I think the true pull effort is where just before the wheels slip at the slowest speed possible.

David Johnston posted:

The maximum drawbar pull should be about when the wheel speed is about 15% faster than the locomotive speed.   SAL9000 I am interested in how you determined that maximum allowable current for the Pullmor and Aromic motors.  The maximum current in a motor is usually determined by embedding thermocouples  in the windings to determine when the winding temperature exceeds the insulations temperture rating.  What is your criteria?  

Thanks,

David Johnston

I saw the 2.5A printed somewhere official. I can't find it now of course. Lionel also quotes a single motor locomotive as drawing 25-35 watts. Anecdotally, having had a Fluke on many a postwar locomotive that is about right - anytime draw is more than 2.5A/motor there's usually been something wrong and I've melted field coil bobbins at 3-3.5A in a frankentrain that had a bunch of weight and traction tires.

I have done a few Tractive Force or Effort tests using a trigger pull gauge, but my only interest has been the Starting Tractive Effort; i.e., the ability to get a train in motion.

Usually, the maximum tractive effort is the same as the starting tractive effort, and whether it occurs when the motor stalls or when the wheels slip, that is the point at which the gauge records the maximum force - I prefer not to allow the wheels to slip to at best gain a few extra ounces of pull.

If the wheels slip, we could add weight to the locomotive and increase its tractive effort. If the motors stall and the current supply is not maxed out, then that is the maximum tractive effort.

Alex

Thanks to all of you, Sal9000, David J., GG1 Man, and Ingeniero No1 for your insightful feedback and advice. Most appreciated.

Very interesting topic; thanks for posting.

Rod