10-25-14 Five wire tests and some realizations

Working up more details on the engines listed in the previous post.
While pondering over the data, several things have come to the surface.

1- for every test the first activity is to run the engine on rollers to get a stabilized reading for current at a given voltage. Keep in mind that there is additional voltage loss to the wheels because of the track to roller case and roller base to roller electrical contacts. What has been occurring is the current drops with the voltage until the motor stops running. When this happens, the current increases to a value that is greater than the operating current just before the motor stopped running. It dawned on me that this is the stall current at the voltage level. At stall the motor becomes a resister and the current will vary with voltage by the Ohm’s law, V = I R . With this result. The stall voltage at 16 volts can be determined.

2- the obvious relationship that had not occurred to me is that the current level can not exceed the stall current at any given voltage. This reinforces the notion that the worry current draw level needs to change as well. The question is that will a given level of dirt and corrosion yield the same delta current rise or the same percentage current rise. It is probably a percentage issue, but the lower current draw modern motors could be more sensitive to dirt and corrosion.

These issues are shown in the following figure:
roolingcurrent1

The dash lines are the stall current estimates for the five engines depicted in data. The solid lines are the current levels from the roller tests. The data points are the maximum load current levels.

3- For every case three data points are taken with the loco running on the track. One of the issues that every motor has is torque wobble. Better motors initially have less of it, but as the motor ages and or needs lubrication, this torque wobble will grow in amplitude. In these tests, the torque wobble will manifest in the velocity data taken at each point. Care is taken the position and start technique for all points are the same. What is likely not the same is the motor rotor position with the brushes. Thus the position on the torque wobble curve will be different. The variation of the velocity at any voltage level for a given engine is indicative of a torque wobble. Improvements due to tune up features can be evaluated by the change in this variation. For the first cut look the variation is normalized by the average velocity at the point. The smaller the percentage, the smaller the torque wobble.

This velocity variation is shown in the following figure for the thirty one engines tested as of this writing.
cumdveloc1

Hope to finish the 5-wire test series in a day or so.

Leave a Reply