Motor test bed #7- Santa Fe GP38-2

This engine has been tested as part of a series of engines to extensively to evaluate the impact of various motors & electronics modules.

This is one of a series of tests being performed on various motors with and without electronic modules. The purpose in this series is to understand the performance differences with various motors. It is also looking at the impact when an electronic module is in the circuit. These electronics can take many forms. The most obvious is a DCC decoder. It can be as simple as a light card. It can be as complicated as a sound system added to a decoder or something else. It may be one of the new DC control modules.
This is by no means a complete examination of the motors. It is intended to understand how the addition of a different motor in the overall system affects the engine performance.

The standard test activity has been performed on each variation and the results compared and discussed. The intention is to examine this over a series of engine, motor and electronic variations. It is important to verify and define the usual result and the anomaly. This will require several tests. Each of these reports will focus on a test series on one engine. The overall results will be examined as testing proceeds.
For this specific test series, the engine configuration is an Athearn Blue Box Santa Fe GP38-2 engine with the following characteristics:
1. Athearn BB Drive Parts
2. Athearn BB GP38-2 BB trucks *
3. Stock Athearn BB Wheels *
4. New Axle Gears
5. No weight added
6. Motor, Trucks Cleaned & lightly lubed
7. Wheels polished with Kadee wire brush
8. 5-Wire connections

* (except as noted)

This engine is a stock configuration showing the performance impact of adding various motors including a stock Athearn blue box rectangular motor.
In this series there were five variations tested. These configurations are:
1. Athearn BB Rectangular Open Frame Motor #6
2. Proto 1000 Can Motor #2
3. P2K Open Frame Motor #1
4. Helix Humper Can Motor #3
5. A-Line Can Motor

There are no electronic modules in place for these five engines.

This discussion will focus on the performance measurements of the unit. The basic test requirements are as follows:

1.) Powered with DC voltage with no external pulse wave modulation

2.) Running on a level surface (measured and adjusted as required weekly).

3.) The same 8-foot segment of track is used for all the basic tests.

4.) The track is cleaned before each test.

5.) The unit runs without external load for all but the max draw bar force tests.

6.) Each, track running, data point is the average of three measurements.

These discussions do not deal with accuracy of the shell, the location and nature of the details or the lettering. In fact, because it is a test bed, several details have been left off for expediency.

To appreciate the results, the question of relative to what always comes to mind. For this reason, the data is compared with the results from the full set of nearly 300 engines tested in the database.

Basic function results are presented in the following charts. The data in each category is compared to the total set of engines.

1- Scale velocity vs. voltage of the engine running alone on straight and level track. There is no external load.

This data is shown from minimum to 16 volts. The actual minimum values are included here because they show how the DCC modules are impacting the shape and position of the curves. The color code is maintained throughout these charts. The grey lines are from the data base of engines tested in this manner. These include all the engines in the data base with no attempt to segregate by some feature or era.


These results are typical of a DC no pulse signature. There is a finite voltage when sustained velocity is achieved. Below this voltage, the unit may start to move, but will stop because the resistance is greater than the motor torque can handle. The velocity level that is sustainable varies and is an interesting discriminator for the motors.
In this chart a reference line is shown (in black reference) that is intended to represent the goal of best speed voltage function. Basically, the desire is for the unit to crawl as slow as possible at low voltage and be able to replicate the full size unit at high-speed. For this purpose, 80 smph at 12 volts was chosen as the value. One could argue that it should be higher. True, but without any external load, why would it be lower? Notice a significant number of engines from the data base run slower than this goal level.
Except at low voltage, all the motors have velocity levels that are higher than this notional goal value. The motors that run closest are the Athearn blue box and the P1K can motor. While still faster, they run close to the line. The Helix Humper motor has a speed to voltage that is higher than the other motors. It is still a good candidate based on this parameter.
2- Current draw vs. voltage for the engine only operating on straight and level track.

As shown in the chart, there are three distinct levels of current draw for these motors, except at 16 volts. The P1K can motor has a very good (low) current draw signature. The P2K open frame, Helix Humper and the A-line can form a second current draw level. The Helix Humper’s draw spikes at the high voltage point. As has been seen in other tests in this series, the Athearn blue box motor tends to be a current hog.

Keep in mind that the input voltage times this current is the input power to the system. If the efficiency of the motors were the same, this would indicate how the output power would look. Fortunately, they do not have the same operating efficiency. That was indicated in the velocity chart and will show in other parameters as well.
This current draw is artificially low. There is no external load or resistance on the engine. Normally, this no load draw is indicative of the relative operation position when in a loaded operation. In this case, that changes as will be seen in the later maximum load discussions.

3- Starting velocity.

This is the minimum velocity that will sustain movement. This occurs at a discreet input voltage. This voltage varies from motor to motor and drive to drive. It seems to be a function of engine weight and motor capacity. For these charts it is shown as a function of weight.

The background data on this chart has been segregated by era, pre 2000 and post 2000 approximated release date.

All of these motors are starting higher than the average recent experience engines.
In this case, the stock P1K motor with the Athearn Blue box rectangular motor slightly higher achieves the lowest velocity.
The drive and trucks likely dominate these results compared to the database. While the parts were cleaned and lubricated, all but the P1K are pre dog bone designs. For The P1k option, the motor and trucks were changed as a set. On the other four variations, the original trucks and couplings were used. In those four, the drive shafts varied in length with the standard blue box couplings.

4- The voltage that is required for the sustained velocity is shown in the following chart:

For the best low speed capabilities, the starting velocity and voltage levels need to be as low as possible. These motors all are below the pre 2000 expectation. Four of these motors are at or above the post 2000 average.
The unit with the lowest starting voltage is the P1K can motor. It is distinctly below the expectation and close to the lowest measured in all tests in the database.
The second is the Helix Humper motor.

5- Starting velocity variation, implied torque wobble

In every case, the data is repeated three times. Running over the same distance. The velocity and current levels are measured digitally. Differences in these readings are an implication of the potential torque wobble of the motor. This also can be a measure of the pulsing level of a PWM impact on the motor, if there is one.

Interestingly, again the P1K can motor has the smallest starting velocity variation. All of these motors are very good in repeatability. The P2K open frame motor happens to yield the most variation. This is still a very respectable result.

6- Starting current draw


The initial current draw follows the trend indicated in the earlier current draw chart. The P1K can motor sets a very low standard. Four of the motors in the series are at or better than the post 2000 expectation for their weight. The Athearn Blue box open frame motor is clearly the highest in the set.

7- The maximum pull force of the engine is shown in the following figure.

These data are taken at 12 volts, when the engine will no longer pull the weight off the floor. Thus it is zero velocity. This may actually be just above the maximum pull force.

Here there is a large variation in the engines examined in this series. Interestingly the Athearn blue box motor is the second strongest puller. The best is the A-line can motor. The other three all fall near the current expectation average. Much lower than the other two. Considering that very little has changed on in most these configurations, beyond the motor, this is an interesting result.

8- Maximum Pull force weight function is shown in the following chart.

Here four weights increments have tested for each motor configuration. This is a relatively new test in the series, so the data base does not include as much history. The weight increments are roughly 150 grams. Totaling just over a pound of added weight. The base engine configuration is the left end of the curve.

In this case, the two Proto motors show a smaller pull increase with weight than the three motors. The result is that at high weight, the P1K cam motor is distinctly poorer than the others at the highest weight. This is the main short fall for that motor.
The A-line can motor is clearly the standard puller in this set. It is near the best measured in these tests.

9- Current draw at max pull force.

This is the more meaningful current draw level. In every case, the wheels are slipping. Therefore, there has been some current relaxation.

This shows that the Athearn blue box and draws a lot of current.
The second highest is the A-line can motor, substantially above the other three. This is the parameter where this motor falls back. The clearly the best (lowest) current draw motor is the P1K can motor. This result presents a decision. The P1K motor is the most current friendly, while being the poorest puller. The A-line is the best puller, but requires two to three times the current. One has to think about the current budget and train size in this selection process.

10- Based on the work of others, the maximum pull force can be translated into the number of 4-ounce cars that can be pulled up a 2.5 percent grade.

This assumes that the entire train is seeing an integral grade of 2.5 percent. This is a fairly sever assumption.

In this case, the force curve is mimicked by the translation constant. The beauty of this is that one can see how many cars are implied by the differences in pull force.
The A-line can and the Athearn Blue box rectangular open frame motors are exceptional train motors. This comes with a rise in current draw as indicated in the last section. Here the other three motors change capability with weight. The Helix Humper moving from last to third with the weight increase. Over most of the weight variation, the P1K can motor lags in last.

Keep in mind, all of the motors can pull well over eight cars up the grade. The real engines would be expected to pull 2 times the number of drive axles, or 2 X 4 = 8 cars.

11- Taking all of these results into account through the second performance criteria as defined on, these motors are compared in the following figure.

As a gage, a PC2 value of 10 or more is considered acceptable. Greater than 50 would be very good to excellent.

This parameter shows the A-line and the P1K can motors to be substantially better than the other three in the series. Clearly as of this writing, these would be the motors of choice. The actual selection would depend on train size and current budget requirements. Bothe motors are very respectable in this parameter.

The other three motors change relationship with weight. At the base weight, a stock Athearn motor is the best of these three. On the high weight side, the Helix Humper shows to be the best of these three. The P2K open frame motor does not show well in this test bed. This same motor tests very well in other engine options.

This engine is scheduled to be run with additional motor variations; these will be more recent production motors to see how they stack up with these.

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