Last several days have been focused on engine health issues.
Additional tests on more modern engines seem to raise more questions.
The issues are as follows:
1- as has been indicated earlier, there is a relationship between current level and torque. Torque translates into velocity on the test track.
2- As the motor wears out, that relationship changes, such that for a given voltage level, it takes more current to achieve a given speed. As things further deteriorate. The speed starts to fall off.
3- additionally, the minimum voltage to keep the motor running also increases
4- the pulling power also gets lower, it takes more energy to get the motor to run the engine, less is available to pull a train.
These characteristics are similar to those of excessive internal load due to friction or lack of proper maintenance.
The question is how do you know if the motor is worn out?
The testing has shed some light on these topics. Examining a relative efficiency at several operating points has indicated some interesting results.
1- examining engines of the same type motor, if the relative efficiency is below 50% then the motor is suspect. It is possible there is binding in the gears or drive. So that needs to be verified.
Efficiency is the classic physics form of a ratio of power out over the power in.
2- the current measurements are very unstable on suspect motors. Not uncommon to see tenths of an amp swings, where the healthy motor will only swing in the hundredths of an amp before stabilizing.
3- the draw bar pull is weak relative to the motor type. A motor needing maintenance will still have a healthy draw bar pull.
4- the velocity at voltage will be lower because the torque is low.
The sense is that generally any deterioration of performance is gradual initially. After a loss of around five percent, then the motor will run erratically at low-speed. The starting voltage will rise.
These conditions will progress at a rapid rate until the engine does not run at any power setting.
More often than not this is the condition I see when I first receive the engine. It just won’t run, or the space for good running is at very high power.
Clearly the engine needs to be tuned up.
Even a sick motor will improve with tuning.
This process usually take these steps in some form:
1- clean and polish electrical contact surfaces
2- clean and lubricated the truck gears and bearings
3- balance the motor rotor
4- clean and lubricate the motor bearing surfaces.
The value of this tuning the engine has many benefits.
1- initiate a window of operation of the engine.
2- provide capability to pull a reasonable train.
3- eliminate the erratic behavior in this region.
This seems obvious, but once the source of the erratic behavior is minimized, further tune up will show only modest additional improvement in operational capability.
At this point, the tuning is working up that gradual trend indicated earlier. The work is worth doing, but the benefits are small.
This is the point when the relative health of the motor can be finalized.
What is needed is additional tests of “tuned” engines with similar drive systems. The range of variation for healthy engines can the be described and the “sick” motors will be apparent.
As discussed in the revised engine health post, a performance parameter has been developed that clarifies the good, poor and sick engines. Thus parameter also can quantify the benefits of the various tuning features. The spirit of the original post remains, the parameter of interest has changed.