Understanding how power is created in an RC power system will allow you to build or operate a more reliable power system. Power is a difficult parameter for us to understand as we can not see it or hear it. The only way we can determine its presence is by measuring it. To answer our question, does a brushless motor pull power from the ESC, let’s first look at the task of the ESC.
How the Motor receives Power from the ESC
When we are operating our radio control vehicle, we use a radio to communicate with the model. Upon a control input to the radio, the receiver onboard the model sends the appropriate signal out to the ESC. When the ESC receives a signal to start the motor, it sends a pulse to the motor from the battery pack. The battery pack would be of a specific voltage and the ESC sends this to the motor. The pulse sent would be at a voltage and a specific frequency. The frequency is dependent on the RPM depending on the amount of throttle input provided. The voltage is dependant on the main battery pack.
Is the Brushless Motors Responsibility to Pull Power?
Firstly we must assume that we have a brushless motor hooked up to a power system and there is no load applied. Let’s also assume that the brushless motor does not consume power to rotate at constant speed. Newton’s First Law of motion states that an object in motion will stay in motion provided there are no external forces acting on it. Reality tells us that life is never 100% efficient hence in reality the motor would indeed have a load applied. For simplicity of explanation, let’s ignore this.
Here is where it gets interesting. The mechanical load applied to the power system is directly related to the current drawn from the battery pack. If we have a 4 cell LiPo battery pack operating a motor at a constant speed of 1000 RPM, and zero load, that motor will draw 0 amps. This equates to a 4 cell LiPo, ( 14.8v x 0 amps = 0 watts ) producing 0 watts of power. As we begin to place a load on the motor, current changes.
Current Drawn vs Brushless Motor Load
The load placed on the motor forces the motor to draw current. As the load on the motor increases, we can assume the current the motor requires to turn the load increases. The motor indeed pulls power from the battery pack. Power is equivalent to the wattage that the motor is running at. Wattage is the product of voltage and current. As we load the motor, the motor will draw more current resulting in an increase of the total amount of power drawn.
Knowing that the motor is ultimately responsible for consuming power in an electric RC model allows us to better select power system. Many DIY power systems that fail are related to this exact thing! Modelers select a power system that draws too much current which can ultimately destroy a power system component. If current is directly related to the load on the motor, simply reducing the load can save our power systems. Keep this in mind when you build and select your next power system.