The motor test series that was started in May 2020 has taken some twists & turns. It began as a small activity that was going to look at a few motors on three different test bed engines. The initial screen to pick the most promising motors was performed by testing each of the more the fifty potential motors by it self with no external load. Here the rotational speed was measured with a laser tachometer and the current draw with various voltage settings was recorded. From these measurements, all the motor parameters were normalized to a reference motor. The Kato #1 was used as the reference. This allows a speed ratio to be defined directly from these results. Additionally, a torque ratio can be defined by assuming that all motors are running at the same efficiency for these tests. While that is clearly not the case, it was hopped to be close enough to yield an good approximation for the screening exercise. These results are shown below:
Motors of the same manufacturer are shown as a common symbol. The five initially selected motors are further highlighted with bold underlined labels & a circle around the data point for that motor. The selected motors were a combination that was thought to be a good sample of the possibilities.
Not surprisingly this process was flawed. Besides the five motors originally selected, twenty others were tested in fifteen different Athearn BB engines. One of the “all so rans” resulted in a spectacular train length at grade. That motor was then installed in one of the selected series engines with a much reduced train length capacity as shown below:
Clearly more was driving these results than just the motor.
The engines being used have many differences:
- Number of drive axles varied from four to six
- Wheel size & material varied
- Engine weight varied by at least 100 grams, 3.5 ounces
- Truck axle frame design varied from inside frame to outside frame
- Motor drive shaft varied from stock BB to dog bone
Because of these variations, a derivative approach was implemented. Here the concept was to identify the number of car impact of each of the pertinent variations.
The first of these looked at was the wheel size & material. The impact of five wheel material & size variations are shown below:
This shows a large impact. The Athearn stock BB tests were repeated with three different sets with the best show on the chart. The 42 in NWSL SS, stainless steel & the 42 in Athearn NS, Nickel Silver we’re both repeated with the axles in different locations with the same results.
There are likely several reasons for this. It may be a simple as surface roughness or as complex as material elasticity. The NS & SS differences could be because the different manufacturers leave a different surface roughness. It could also be because the Young’s modulus for the two materials differ by a factor of two. This may impact the adhesion area considerably.
Additionally, the low speed characteristics are also being affected favoring the SS material wheels.
The impact of engine weight on cars has been defined in earlier tests as shown below:
This chart is from another study that assessed grade & train length performance for specific engines discussed here: https://www.llxlocomotives.com/wp-admin/post.php?post=2238
The take away from this chart is that engine weight definitely impacts pull force, but to differing derivatives(slopes).
This activity was generating more questions than answers. For this reason, a DOE design approach was determined to be needed to help flesh out some answers in complicated results. This DOE design is discussed in: https://www.llxlocomotives.com/wp-admin/post.php?post=2433
This will allow additional verification of the derivatives and the demonstration of the various performance aspects of these motors and engines.
