April 2022 Propwash Web

The kV determines the no load rpm of the motor at a specific voltage. Knowing these three things lets you know the details of how a motor makes power. This is valuable if you want to change any of these factors. Below is how to actually measure these things and what effect they have on motor output power. Dual Meter Setup for Winding Resistance Measurement The Dark Side of Model Boating (Continued from page 25) The clock was stopped for the “injury”, but that was pure stupidity. Clowns! Obviously, it’s much more fun to win, than lose, but at what expense? Your integrity? When my son was a senior in high school, he took the SAT’s and a friend of his sat close by. During the exam, the proctor left the room and the kid took out a cheat-sheet paper with the answers. We later found out that he scored a 1510 on that SAT and was accepted into a prestigious college. However, after his freshman year, he had to drop out because his grade point average was too low. Niceeeee! You can put lipstick on a pig but it’s still a pig! So why cheat? We race model boats and strive to win because we want to prove that we are the fastest racer in the district or in the country? We race model boats and strive to win because we want to expand a model boating business or engine business? We race model boats and strive to win because of our egos? Whatever the reason, we all should be straight forward and race legitimately. Could you imagine if we were racing for money? Some of these guys would go nuts with the antics. I was always under the impression that we help every other boater into the water and just beat them after the horn sounds. At the end of the day, when sitting home with your first place trophies, do these cheaters feel a sense of accomplish- ment? All in all, 95% of the racers in NAMBA are stand-up people. However, it only takes a few to ruin things and soil the name and reputation of themselves and the organization. How many tombstones have “The Greatest Model Boater” carved under the name??? None! So, let’s just go out and have fun; win or lose. Editors Note: I have known Dave for over 30 years and he is a great story teller but his stories are true. If you cross paths with him, ask him about the time one boater threatened another boater with an Exacto knife on the driver’s stand or if you remember the days where frequency clips were used on radios, a boater attaching a clothes pin clip to a fellow drivers ear because he did not like how close he was driving around his dead boat. It does happen. Please remember. This is a hobby and there are spectators and children in the area. Brushless Motor Theory By Lohring Miller NAMBA Safety Chairman Brief Theory Electric motors are fundamentally different from internal combustion (IC) engines. IC engines are approximately constant torque power plants over a useful rpm range. An electric motor is a constant power powerplant plant with the torque highest close to zero rpm. In order to do this an electric motor draws more and more current with increasing load until the smoke gets out. An IC engine is smarter and stalls when you overload it. The power and speed of an electric motor depends on several factors. See my Propwash article for a comparison of electric and IC model boat power plants at http://namba.com/content/ library/propwash/2018/April/18/ The input power in watts is the volts times the current as shown on the data logger in the electronic speed control (ESC). One horsepower is 746 watts, though this isn’t particularly useful for comparing the output power of an electric motor to an IC engine. The output power is that number minus the various losses in the motor. The measurements to determine motor efficiency are the winding resistance and the no load current. PROPWASH 26 April 2022 Winding resistance diagram Measuring winding resistance is a little difficult because the winding resistance is really low. To measure this, you need to run a current through the winding and measure the voltage drop across the winding. The resistor in the diagram is actually the resistance of a current limiting resistor plus the motor resistance. Since the motor winding has near zero resistance you can test the setup with only the resistor bank. You want the current through the winding to be around 1 or 2 amps. That means you need a current limiting resistor in the circuit if you use a normal 2S or 4S pack. Around 5 amps will work with 2S and 10 amps will work with 4S. These need to be 10-watt power resistors since they will be dissipating significant power. You can use single resistors, but I had a resistor bank so that’s what I used. The picture below shows the two meters hooked up to one of the motor windings. The red millivolt meter is hooked across the winding while the grey ammeter has one lead hooked into one side of the winding and its red lead connected to the battery. This will measure the current flowing in the circuit. The other red lead is hooked to the other side of the motor winding and will go to the resistor bank. This connects the battery into the circuit through the current limiting resistor bank. The other battery lead is connected to the start of the resistor bank. Actual wiring for resistance measurement