Our first BOM:
A123 Battery 2S2P
2S2P battery configuration gives us 24x80=1.92KW, while 3 batteries in series gives us 36x`40=1.44KW. So we'll go with four batteries and pay 75$ for the extra one. Both Jiawei and I agreed that motor is the most important part, so we were looking for those motors that can supply a super large amount of power. I was shopping around hobbyking, then I found some powerful TURNIGY ROTOMAX motors that claims to provide 2KW+ of power. We were very happy and decided to go with that. But Charles pointed out that the power would be a total waste for our go kart cause the maximum power our go kart can provide is only 1.92KW. And although on its data sheet says the maximum current is only 70A, in reality it would suck hundreds of amperes of current and just blow the fuse in the battery. So no ROTOMAX - -. Charles suggests SK3 line, we previously thought that it was not powerful enough, but it turns out that it is actually enough according to our calculation later. For our calculation.....both of us read the dcmotor pdf file so we tried to solve the problem in a systematic mathematical approach. We first wrote a nice EXCEL sheet and tried to fit everything in, and all the mathematical equations as constraints, like solving an optimization problem you know. BUT I think we complicated things too much, all the graphs, analysis, complicated equations...... Two days scratching our head, we decided to take the easy way out, for a rough estimation, we didm't take in the frictions and air resistance. Kv and Kt are negatively proportional to each other, so lower Kv gives us a higher acceleration for a fixed gear ratio. Cause gear ratio should be between 3 and 5 to keep things simple, so we set it as 5. With a system voltage of 24 V and a 149rpm/V motor, motor speed with no load =3576 rpm. Our wheel speed is 715.2 rpm with a gear ratio of 5. The wheel we choose has a radius of 0.1 meters, that would give us a vehicle speed of around 7.5m/s < 9.7 m/s. Gear ratio X maximum torque X rotational speed of the wheel = F X speed of the wheel. F=ma and maximum torque= Kt X maximum current. With the 1 minute current 120 A controller, our maximum torque became 7.68, approximate total mass to be 90 kg, we get an acceleration of 4.3 m/s2. Not bad. But with all the friction, we're still not sure how fast it will go, but it's the best we can get within the budget. So...But we 're still not sure about whether to use four batteries or three batteries, cause it seems that 3 batteries in series has higher voltage: 36v, and the max current would be 40A. That way we'll have a higher speed of 11.2 m/s, well that's a much higher speed! But we're still not sure how the power part is playing out > < For controller we spend 149$ on the 120A KBS line, cause we want to increase our max current, then we still have space to adjust if it's too high and the fuse blows.
Wheels! To simplify the searching process, we decided to use the same wheel given to us for free for all the three wheels. Razor E150, and it turns pretty smoothly so.... The wheels on Mcmaster are super expensive by the way. We chanced upon this rear wheel assembly, which includes a band brake and a sprocket for chain drive, and it's only 3$ more expensive than a single wheel. PS: we are still sticking to the chain drive plan.
The batteries and controller would be mounted on a aluminum plate under the seat, with the motor stabilzied behind the controller. We've started doing bits and pieces in solid works. Most of the parts are done already but still need to assemble them. And we still need more aluminum frame to build our chassis, planning on ordering coming Tue. and we'll ahve the 2D drawings of the parts to be cut on aluminum plate ready by tomorrow's class so we can get the stuff back this Friday and start building things next week ;)