Many RC ESC’s have an electronic brake function to slow down the motor where and when required. For RC Cars this is extremely important as the guys exceeding 100mph would be in the next city without brakes. For airplanes that can cruise back down from a high altitude with no throttle input, a drag brake stopping or slowing the propeller can help increase the glide ratio allowing the plane to glide further. The big question is if the ESC is able to take this energy and use it to charge a battery through regenerative braking.
You can watch this video below or skip down below the video for the answer and explanation.
First we will understand how the ESC is able to slow the motor down.
How the ESC slows the Motor down Applying Brakes
The best way to see for yourself how an ESC slows down a motor is to grab a brushless motor and use alligator clips to short out the 3 leads to one another. With the leads all shorted, rotate the motor by hand quickly. You will notice that there is a great amount of resistance to this motion. The faster you try and spin the motor, the more resistance you will feel. Removing the wires shorting out the motor leads will remove all of this added resistance.
The ESC does the exact same thing, shorting out the leads of the motor electrically. In order to manage the amount of braking force that is applied, the ESC must rapidly switch on/off the leads that are shorted out.
Does Regenerative Braking Occur
Can we recharge our battery as we use the Brake? As the leads of the motor are shorted by the ESC, power is in fact produced and sent back to the battery. An approximate 15-30% of the braking efforts ended up back in the battery through the test in the video above. This is not an overly impressive efficiency for charging the battery pack, however it doesn’t matter whether the process is 1% efficient or 100%, regenerative braking is still taking place.
Regenerative Braking Efficiency
What is interesting is that as more brake input is provided via the transmitter, a lesser amount of braking energy is placed back in to the battery. Braking that is done as more of a “drag brake” maximizes the efficiency using the data from the simple experiment completed in the video.
How is Power Generated and Pushed to the Battery?
The ESC shorting out the leads of the motor is the first part in the braking process. The core function responsible for generating the power to push in to the battery happens as the motor leads are opened and closed using the FETs (Field-Effect Transistor) on the ESC.
Back EMF is generated creating a voltage caused by the rotating motor. In turn, the motor leads are shorted by the ESC. Voltage within the windings quickly drops and approaches zero but current surges. When the FETs open the circuit, current has absolutely nowhere to go. The brushless motor windings act as an inductor which does not like any change in current. As a result, the rapid change in current causes the voltage to increase significantly. It is this increase in voltage that allows power to flow back through the circuit and into the battery pack.
Conclusion
Regenerative Braking does indeed occur in the ESC tested and any other ESC that uses the same principle for braking. Efficiency is not a strength within these systems, especially when a significant amount of braking force is required. Overall, the total amount of braking energy that makes its way back in to the battery is not significant. However, it is greater than zero and will extend the overall run time of an RC that uses brakes during the run.
Is that not cool or what?
Unfortunately the rest of the braking energy results in heat. Take a look at how hot the braking circuit can get when a small amount of brakes are applied.