Brushless motor winding resistance is a motor constant that is directly related to the efficiency of the motor. The winding resistance is most likely the easiest parameter in brushless motors to understand. There are no secrets here! Motor winding resistance is just that, the resistance in the motor winding itself. Now I know defining a term using the words within the term is not a very good way of explaining the definition, however there are not that many ways to describe this motor constant. Perhaps we can say that resistance is the restriction of flow in electricity and the windings are the copper wires found in our motors.
What creates Motor Winding Resistance?
Unfortunately nothing in life provides 100% efficiency. Well, it is the same for wires and it is ideal for a motors resistance to be zero ohms. Ohms are the unit of measurement for resistance in electricity. Every wire used in any application has a specific resistance per foot. When power flowing through the wires are resisted due to the wire itself, it provides wasted energy. The wasted energy is converted to heat and this energy does not provide any form of work that contributes to our power output of the brushless motor in our RC vehicles. However, we can determine the motor using inexpensive household tools.
Measuring Brushless Motor Winding Resistance
We can measure the resistance of the motor winding by introducing another resistance component to a circuit. The reason we would to do this is common multimeters do not have the sensitivity to measure milliohms. In this case we add an additional load in to the circuit to reduce the current to an acceptable level. We then use ohms law, V = I x R to determine the resistance value as we will know the voltage across the motor and the current will be the same in the entire circuit. Where it becomes challenging is where we introduce an increase in temperature to the circuit. When placing a current on a motor phase we waste electricity to the form of heat. This heat increases the resistance property of the windings. However, if we know the temperature of the winding at the time the reading was taken, we can normalize this result to a standard 20 degrees Celsius.
The following tools are required:
- 2x Multimeters. One will be used to measure current and the other is used to measure voltage.
- 4 Cell LiPo or 3 Cell LiPo (4 cell preferred for more accurate results)
- 25 Watt – 10 ohm Resistor
- Wire to connect battery to resistor and motor leads.
- Temperature gun (optional but highly recommended)
The procedure is as follows:
- Wire the multimeter in series with the power resistor.
- Wire 2 out of the 3 wires on the brushless motor in series with the power resistor and multimeter.
- Take the second multimeter and measure t he voltage across the motor leads.
- Turn on the ammeter and set it to the 10A setting. Turn the multimeter on and set it to read millivolts.
- Lastly, place the 4 cell LiPo in series with the rest of the circuit.
- Take a reading of both meter simultaneously. Make certain both reading of each multimeter are within a second of each other. Any delay in the reading will skew the results.
- Using a temperature gun, obtain the most accurate temperature reading possible of the winding in the phase that you have ran the current through. Skip this step if you do not have a temperature gun.
- Use the formula 0.0038 (20- Temperature)(V / I) + (V / I) to compute the motor winding resistance.