In order to determine how much current the motor pulls, we had to make it turn with a load and monitor its consumption. The perfect load was in hand, we took last year’s LED pannel, as ours will have approximatly the same weight and dimension, therefore giving a good estimate on our power consumption.
We took the same brushless motor as last year, a MEGA Acn 22/20/4. Which is connected to a motor controller, a EZRUN Max 10 SCT. It is rated to be able to supply up to 120A, so we could not take this value to size our power supply. So I tested the whole system to see how much would it drain.
First I had to understand how the controller worked. The datasheet is not very helpful, as it only says to put a RC receiver on the input wires. So I emulated one with an arduino. A RC receiver uses PPM signal, and is waiting for a 1 to 2ms pulse every 20ms. 1ms being the command for neutral throttle and 2ms for full throttle.
In order for the motor controller to acknowledge that you sent a RC signanl, you need to start by sending neutral throttle command. It is a security measure to prevent the motor turning on when it has just been plugged in.
After came my first attempt with no load on the motor. It ran fine and the controller pulled around 1.5 to 2A. Then Itried with last year’s pannel, and it would not start rotating at the lowest speed, or be all jittery. My teacher told me that last year, in order to start the rotation, they sent a high speed command and backed down to lower one when it started. I did so and now the whole structure is rotating.
I filmed it slow motion to be able to determine the speed it’s rotating at. With the almost lowest setting it is rotating just under 20rps. At this speed it is pulling 4A under 12V. I tried making it rotate faster, so I just left it at the high speed command I use to start it up. I did not go well. Leaving a high speed command made the controller pull more than 20A because it was unable to match the requested speed. So the power outlet shut down.
In order to get the whole system to high speed I proceeded gradually. Letting it establish at low speed for seconds and increasing the speed slowly.
An other problem rose up, at low speed it was not that wobbly. The structure was only a bit unstable. At high speed it is a whole different story. The metal structure would move around the table and produce more noise. We will have to be very carefull about the placement of the component and plan to be able to put masses and the lower rotating part to balance the whole structure, as you would on a car tire.