We are trying to build a small Solar car
the car's dimensions (Length :100CM, Width 60CM)
the car mass is not going to exceed 2KG
The car speed can be up to 2.5 or 3 m/s
We`ll be using PV modules that each can provide up to 6Volts & 250mA
Those modules have dimensions 65mm x 165mm X 3.0mm along the car surface area
We can connect all those modules in paralle or every 2 in series to obtain a 12V
Wheels will be of a diameter 12CM or 15CM depending on the motor
yet we are confused, what type of motors should we use
A brush-less DC motor ? or a normal brushed motor
i think i`m misunderstanding Brush-less motors as i think they do need huge amount of current to work and in our case we are working with a current limited source [1~1.5Amps] (The PV cells) is this correct ?
Should we go for a brush-less or a brushed motor?
we bought the EMAX BL2212 1400 KV Brushless motor we think we can use gears to obtain more torque and decrease this RPM will this method work?
Best Answer
Brushless motors are generally more efficient and so can make better use of the limited power available. However you still need to match the motor to the power source and load to get best efficiency. A few basic calculations can tell you what motor specs to look for:-
First calculate what rpm the wheels must do to get the speed you want. A 12cm diameter wheel has a circumference of 0.377m. To get 2.5m/s linear velocity it needs to turn at ~400rpm.
Your motor has a Kv of 1400rpm/V, so if powered by 12V it should spin at ~12*1400 = 16800rpm without a load. Under load its speed will drop due to voltage lost in the resistance of the windings. Loading depends on a lot of factors such as rolling resistance, aerodynamic drag, bearing friction etc. however assuming you can get the motor running at peak efficiency its speed may drop to ~85% of no-load or 14000 rpm. Therefore you need a gearbox ratio of about 14000/400 = 35:1.
The 1~1.5A maximum output current of your solar panels could be a problem with this motor because it draws over 1A just to turn over. Under acceleration it will try to draw even more current, but the solar panel may not be able to supply it so its voltage will drop and the speed controller might cut out.
You should use a lower Kv motor that has smaller no-load current, eg. Scorpion SII-2212-885Kv which draws less than 0.5A at 12V. This motor only does about 9000rpm (12V * 885rpm/V * 85%) so for it you would need a 23:1 gearbox.