April 2022 Propwash Web

The meters show the current through the winding and the voltage drop across the winding at that current. The voltage is in millivolts so to find the resistance divide the voltage (.0357 volts) by the current (2.19 amps). This gives a winding resistance of .0163 ohms. Lookup Ohms law for an explanation of this formula. I got readings within one millivolt across the other windings indicating a good motor so far. No Load Current Measurement The no load current measurement is more straightforward. Setup the motor, ESC, and receiver but with nothing connected to the motor shaft. Place an ammeter with a 10-amp capacity in line with a battery lead. Run the motor at full throttle and record the current. The meter shows a current of 3.7 amps on a 4S pack. The resistor bank consists of one-ohm resistors wired in series. One terminal from the battery is hooked up to the start of the series resistors while the red wire is touched to a terminal down the chain to give the desired current. Touch the red wire to the resistor bank for the shortest time needed to get a steady reading from both meters. Too long of a hookup will raise the temperature of the winding and throw off the result. Wait for everything to cool off before repeating the test on that or the other windings. PROPWASH April 2022 27 Meter setup for resistance measurement Battery and resistor bank setup Meter winding voltage (left meter) and current readings Setup for no load current measurement Motor Loss Calculation These two measurements can be used to calculate the power losses in the motor and therefore its efficiency at a particular power input. The winding resistance allows the calculation of the winding power losses. This is the winding resistance times the square of the full load current flowing through the motor according to the data logger. All other losses are indicated by the no load current. They are hysteresis and eddy current losses in the windings as well as friction losses. This loss in watts is the no load current times the full load voltage according to the data logger. Assume the motor is pulling 100 amps at 16 volts in the boat at full throttle or 1600 watts of input power. That means that the winding losses are 10,000 (the current squared) times .0163 or 163 watts. The other losses are 16 volts times 3.7 or 59.2 watts. This means the output power is 1600 minus the sum of the losses or 1600 – (163+59.2) or 1378 watts. This gives a motor efficiency of 1378 divided by 1600 or 86%. Note that because of the high importance of the winding resistance the efficiency will fall off at higher currents. Assuming the voltage is the same the winding loss will be 367 watts at 150 amps. The other losses stay the same giving a total loss of 426 watts. The input power is 16 times 150 or 2400 watts with the output power now 1974 watts for an efficiency of 82%. Note that the kV is not included in these losses. It is a factor on high kV motors since the no load current will increase as the motor runs faster if all else is equal. (Continued on page 28)