There are at least 2 definitions of the term efficiency when applied to an R/C car, boat, and airplane motor. The first is of course the measurement of power in vs. power out such as that measured by the Fantom Racing Dynamometer. The second definition is the one I will deal with in this paper.
I want to explain how to adjust the timing on a modified motor so that it will produce the most amount of rotational power from the energy source. Since every motor has its own characteristics, this procedure should be carried out whenever the motor is taken apart or when you change the brushes.
Here's a list of things you will need:2 - Ni-Cads wired in series. About 2 volts.
1 - pair of lead wires with alligator clips to connect the 2-cell pack to the motor.
1 - .1 ohm, 20 watt (minimum) ceramic resistor with an alligator clip on one of the leads. Alternatively, you may use approximately 10 feet of the super flexible motor & battery wire wound into a coil. Each end of the wire becomes each end of a resistor.
You'll also need a digital volt meter set to the lowest DC volts range. Alligator clips are recommended on the DVM leads.
Connect one lead from the battery pack to the motor. Connect the resistor to the other brush hood with the alligator clip. If you touch the remaining alligator clip from the 2-cell pack to the lead sticking out of the resistor, the motor should run. The batteries are charged, right? Make sure the motor is running in the correct direction.
Connect the DVM across the resistor, one lead from the DVM to each lead from the resistor. Now you can touch the lead from the 2-cell pack to the open lead on the resistor to run the motor again. Keep touching this connection while looking at the DVM to see the reading. If you let the motor run a second or two, you should be able to see the 'free running' current draw. Multiply the meter reading by 10 to get the actual current draw of the motor.
Okay, here's the fun part. We have to be able to rotate the endbell while the motor is running, so loosen the retaining screws just enough to do so. Rotate the endbell on the running motor a little bit in either direction over the zero degree timing mark. The DVM reading should change up and down as you move the endbell. Find the point in the rotation that produces the lowest DVM reading. This is the point where true magnetic zero timing occurs.
I'd like to mention here that if you put another armature in this can the true magnetic zero timing point may also change. The main reason for this is that the commutator and stack alignment may differ from on arm to another.
Once you have found zero, start advancing the timing by turning the endbell counterclockwise. For the standard R/C motor, that would be like unscrewing the endbell from the can. Watch the DVM as you turn the endbell. As you rotate the endbell the DVM reading will rise pretty much proportionately. Then as you continue to turn the endbell, the reading will start to go really high for the amount of endbell rotation. Turn the endbell back a little and try to find the point where the amps start to take off. You'll have to move the endbell back and forth several times in most cases to get the timing just right.
Disconnect the 2-cell pack and tighten the endbell retaining screws. It may not look like it physically, but electromagnetically you have set the timing to about 26 degrees of advance. The timing point of 26 degrees is the point at which this type of motor design generates the most amount of power from it's energy source. If you test this motor on the Fantom Racing Dyno, run the gear ratio/rollout the program recommends. If you've input accurate information into the dyno program, you should be very happy with the results.
This page and it's contents are Copyright 1996 ~ 2006 by Dave Clary, DBA; decCo SoftWare - June 8th, 1996