I happened on to a number of N scale Bachman engines. Most run poorly. This group is the 1981 version as defined in Spookshow.com. Actually three different motor variations. This discussion is a subset of the larger examination on improving the performance of N scale Bachmann engines. I know why bother. They can be strong performers for a very reasonable cost.
Clearly the engine performance starts with the motor. Here we focus on the motor no load test results.
The process is to compare the performance of a number of test motors with a reference motor that is considered to be a typical healthy motor. All the motors that I get my hands on are used & abused to some extent. The reference motor achieves an engine performance that is typical of the “best” performing Bachmann engines. All of the test motors were from poor performing engines.
Additionally, steps will be taken on the noticeably poor motors to attempt to quantify how close to the reference motor can be achieved. The best version of each motor will be rerun in its drive to define how this improvement translates to the engine performance.
The elements of this discussion are:
1. Definition of the test motors.
2. Test plan for each motor
3. Analysis techniques.
4. Initial test results discussion
5. Speculation of why the performance is down & the techniques to bring the motors back to viability
6. Results of these improvements
7. Conclusions of the activity
1. Test motorsAt this writing there are ten viable test motors to compare with the reference.
These test motors come from engines that either did not run or have a very narrow range of operation. The motors all operate when voltage is applied & have no broken parts.
The test motors are shown in the following charts. As indicated earlier there are three variations in these motors. Two major, motor 4, 6 & 10 have a white plastic brush assembly shell with a brass hex bolt for contact.
The others have metal outer shells. Here motor ref,2,3, 5, 7, 8 & 9 have a plastic sleeve that loosely fits around the motor. The rest have insulation material permanently attached to the top and bottom of the metal outer shell. These are variations to prevent the motor shell from shorting the opposite polarity engine mounts(weights)
At least two of the motors (8&10) were fouled so the would not run. The both ran with a little nudge. What ever was on the commutator burned off and they appear to run now. That may be worked later.
Motor 2 ran when selected, however when it was set up to test it will not turn at any voltage, will work some more. May have to give up on it.
2 Test plan for each motor
The specific plan is to measure no external load current draw & motor speed, rpm at various supply voltage settings. The voltage levels to be tested start at the minimum to achieve rotation moving up from zero. Additional data will be taken at 2, 4, 6, 8, 10 & 12 volts. Data will be taken after 30 seconds of operation at each setting. Finally at the measuring voltages, the stall current will be measured. This will be momentarily current readings.
The tools to be used to collect this data are a XXX lab power supply. Here you set the prescribed DC voltage. This is a square wave DC signal with no PWM. The supply also measures the current draw by the motor. The power supply is shown in the following chart.
The the motor rotational speed will be measured with YYY Tachometer. It is a hand held laser device. The motor shaft has to be darkened with black tape. A sliver of reflective tape is glued to the black tape surface. The tape adhesive alone was not sufficient to hold in place at high speed. Super glueing the sliver in place proved successful.
The Tachometer & the shaft tereatment are shown in the following charts.
3 Analysis techniques
The data measured for the reference & the ten test engines are shown in the following two charts. The first shows the current draw results, both the no load operating current and the motor stall current as they vary with voltage.
For each motor, the difference between these represents the maximum power capacity of the motor.
The second chart shows the no load motor speed as it varies with the supply voltage.
In both charts, the reference motor is shown as a black line. The initial visual assessment indicates that it is common with the other motors of the same model. These
are the eight that are the same shape no matter the insulation technique.
The motors with the white plastic brush assembly shell have higher stall current. The speed characteristics are common with the other motors.
These charts show variation between the motors, but do not readily indicate which are better or poorer.
4 Initial test results discussion
To better understand the motor capabilities the normalized values of Torque capacity & speed need to be examined. Here the reference engine is used to normalize the test motors. This will help show which motors should be better than the reference and which should be poorer. This includes how they compare over the voltage range.
A magnitude of this difference is also described.
The Torque is described by the following:
After the normalization process the following two charts are defined. The first shows the normalized Torque variation with Voltage (Power). The second shows the normalized speed with Voltage.
The Torque data is the most revealing. Probably not surprising, this indicates that the amount that the capacity excess & short fall varies with voltage. Why this matters is
that once the load exceeds the capacity of the motor it stalls. Once it stalls, it is unlikely to recover without a load reduction & a voltage reset.
The speed data is indicative but can be misleading. There is a speed change with load. As load increases, the speed goes down. However, if there is indicated to be a significant speed change most of that will show up in the loaded application.
