The Basics
What is a 3 Phase AC Brushless Motor?
An AC Brushless motor is a type of motor where the current passes through the motor and causes a magnetic field to rotate and the rotor to turn [1]. Since there is no brush to direct current flow, brushless motors work with AC current. The lack of brushes, meaning less physical contact between moving components, means brushless motors have less wear and tear, allowing them to have longer life span. A 3 Phase motor has a three-phase power supply in order to convert electric energy into mechanical energy and is more efficient in energy conversion compared to single phase motors. Many 3 Phase motors are reversible, which allow them to work as generators, which is how they are being used in our project [2]. This means that they can turn mechanical energy into electrical energy, which we are then harnessing through our Energy Storage System.

Visual of a common 3 Phase AC Brushless Motor
Why is this being used in our project?
Small AC motors are being used in our project due to their widespread availability, low cost, and compact size. These attributes make rapid prototyping and replication in the classroom or in future experiments much easier.
Further on this webpage, we will describe other motor ideas to consider for generator usage in the future.
Design Decisions
The final component of the project, converting mechanical energy to electrical current, is the generator. We selected a small AC motor (a2212/13t 1000kv) as our generator.
Experimental Process
2 types of AC Motors (a2212/13t 1000kv & a2212/6t 2200kv) were tested for better understanding of correlation between their rotation speed and power output. The two motors have similar power input ratings, both range between 2 – 3 cells. The 2200kv motor can produce higher speed while the 1000kv produces stronger torque at the same voltage rating.
- A total number of three motors are used in the test, setting up two test configurations.
- For each configuration, an a2212/13t 1000kv motor serves as the driver. The driver is supplied with a 11.1V DC current using a power supply.
- An electronic speed controller (ESC) is combined with an Arduino Uno to control signal pulse width and thus speed of the rotor.
- The shaft of the driver is then coupled to the driven motor.
- For the first configuration, an a2212/13t 1000kv motor is driven.
- For the second configuration, an a2212/6t 2200kv motor is driven.
- The driven motor outputs to a 3-phase bridge rectifier circuit.
For the 3 Phase Bridger Rectifier, we used a breadboard, 6 1N4001 diodes, a variable resistor box, a variable capacitance box, and a Moku Go attached to the resistor to read output voltage. The outputs of the driven motor go into the 3 phase bridge rectifier and then through the rest of the circuit.
Additional instruments used in the tests that are not shown in the picture below include a digital multimeter for reading the load resistor current, and a tachometer for reading the driver motor angular velocity.
Shown below is the full testing set-up.

Motor Testing Set-Up
Two sets of testing were conducted in total.
The first series of testing is done to compare the voltage output of the two different driven motors under the same input RPM. The testing results are shown below:

Motor Testing Results
As shown in the results table, when supplied with the same mechanical energy, the 1000kv motor can generate a higher voltage.
A second series of testing was done to draw correlations between input rotational speed, circuit load (resistance), and power output across the load for the 1000kv motor. The power output across the load is estimated as the product of the voltage and current across the resistor, read by the oscilloscope and the digital multimeter. The testing results are below:

Power as a Function of RPM for Different Resistances
It can be observed that, with higher RPM and smaller resistance, a larger amount of power can flow through the entire circuit system. While raising the angular velocity of the driven motor increases power output, the correlation between power/angular velocity rises much faster at a lower resistance.
For example, comparing the 500Ohm resistance and the 25Ohm resistance, both power plots start at a similar value at a low RPM (1400), but quickly diverge when rotation becomes faster. However, regardless of the resistance, the selected motor requires a high rotation velocity to generate meaningful power output, and for actual application, this could mean stepping up to over 7000 rpm, while further lowering the load resistance.
The former requires a better designed transmission system with better quality material for the much higher cyclic loads, while the latter is determined by internal resistance of batteries implemented, which is usually much lower than 25Ohm.
Future Experiments
Additional investigations on this aspect of the project could involve implementing storage units, such as batteries, into the circuit for further experimentation and analysis. For different types of batteries, the internal resistance could be estimated, and a dummy load resembling that resistance may be used in finding the actual output received by the battery. Further testing could also be done using other motors of different ratings, or of different designs to further optimize power output.
References
[1] “AC Brushless Motors.” Automate. [Online]. Available: https://www.automate.org/products/ac-brushless-motors#:~:text=AC%20brushless%20motors%20pass%20current,and%20the%20rotor%20to%20turn.
[2] “Single-Phase vs. Three-Phase Motors Guide.” Gainesville Industrial Electric. [Online]. Available: https://www.gainesvilleindustrial.com/blog/single-three-phase-motors-guide/#:~:text=A%20three%2Dphase%20electric%20motor,need%20a%20capacitor%20for%20startup.