Engine health testing rev 11-22-14

The research, see favorite links, indicates that engine motor health is related to the motor magnet strength. The following two equations show how motor torque, RPM, voltage and current are related to the magnet strength.

TORQUE = K * CURRENT * FLUX * turns * poles

RPM = K * Voltage * FLUX / turns / poles

Torque can be related to draw bar force and RPM can be related to engine speed.

Torque, speed and current are response variables when the voltage is set. As magnet strength diminishes, At constant load, speed drops while current increases. This will also happen when the load is increased due to added friction in the engine drive or train.

To understand the relative health, similar weight models can be compared at fixed voltage, external load and slope.

The goal here is to make a comment about the relative magnet strength between similar models. Understanding the normal distribution of magnet strength on new models will allow the establishment of an indication relative health due to current draw.

The following figure potentially is implicit of this health issue. The torque issue shows in the current at max load data point.


This chart raises a number of questions.

1- while the later vintage units have a lower running current, do they actually have as much deterioration margin?

2- Are the new systems more sensitive to dirt and grime? do they require more frequent lubrication and maintenance?

3- two of the engines on this chart have identical Athearn BB rectangular motors. The data indicates these motors have significantly different levels of stall current. That means the motor’s resistance follows this same trend. How does this happen?

Is it constant with time?

How much variation is typical?

With these questions and likely more, a plan to get some answers is needed.

This plan will start with the following:

1- where ever possible, groups of like engines should be tested.

2- Different manufacturers and design eras should be included.

3- a set of new and old should also be included.

Clearly, this will require time, an open mind and a fair amount of pondering.

This activity will be on going for a while.

(Revision 11/22/14)

The last few weeks have been spent doing additional testing and examining the results. Parameters such as relative efficiency, velocity variation between readings and pull force to name a few. Individually, nothing made much sense. In fact, the benefit of tuning seemed to be lost in the noise of the data.

because of this, a combination parameter that is highlighting the positive aspects needed to be described. If done properly, the clear winners engines will stand out and the poor or sick engine will also be apparent.

The parameter needs to be generated from the attributes that a modern modeler is looking for.
As far as I can determine these are as follows:

(For engine only testing on a straing and level test track.)
1- Low starting voltage. This is the voltage where the motor will start the engine through sticktion with out a puulse wave. Just on full DC.

2- Low minimum starting voltage velocity in SMPH. If the DC starting velocity is low, then to really make the engine crawl a very small pulse is required.

3- Low minimum starting voltage current draw.

At the same time the engine has to have adequate pulling power. Adequate will differ from modeler to modeler. In this case, The pulling power is sufficient to pull a train of 2 times the engine axles up a 2.5 percent grade with ten percent margin. It seems like if it can not at least do that, then the modeler will be restricted to short trains or minimum grades or both. Being short on power will limit the time between maintenance because draw bar force goes down with deteriorated conditions. Engines that do not acheive this minimum force will show up on the negative side of this performance criteria

an initial definition of the performance criteria parameteris as follows:

PP = (DBFmax – DBFreq) / (V * I * Vel)min

Where: DBFmax is the maximum measured draw bar force in grams
DBFreq is 10.5 * n axles, again the units are grams
V is the minimum starting voltage, Volts
I is the operating current draw at this minimum starting voltage, AMPS
Vel is the operating velocity at the mimimum voltage, SMPH

I & vel are the average of three runs at this minimum voltage level.

At this time I haven’t sought to make this parameter deminsionless. It is a means to indicate which units meet the criteria the best and which appear to be poor or sick.

This parameter for the engines tested as of this date, which now are at 53, are shown in the following figure:

performance criteria 1

In this reguard, engines with a parameter value over 50 should be considered exceptional and those below 5 or 10 are suspect. Below 5 would be poor and possible sick. That determination will become apparent as more engines of a similar design with the same type motor are examined.

It is interesting that the ennine that falls the lowest, because it does not provide sufficient draw bar force is a recent vintage production engine, relaesed in the last two years. It is DCC ready, and these data are right out of the box. This engine with some others are part of my collection and will be worked to see how the performance cqan be improved.

This form of the data will be discussed for decreat types of engines in other posts. the value of this type parameter will be more apparent in those posts. For now, I feel like a means to discriminate the really good and the poor or sick engines has been identified.

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