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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?

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  • \$\begingroup\$ Figure out how many PV modules (from area, weight and budget) you can use to get a total power budget. Brushless motors don't need more current than brushed (for the same Kv and torque) but don't forget you can keep paralleling PVs. And yes, gearing will probably be necessary. \$\endgroup\$ – Brian Drummond Dec 1 '15 at 14:37
  • \$\begingroup\$ For a brushed or brush-less motor stall (zero speed) torque will be proportional to current. Yes you can get more torque (for the same current) by using a gear box. \$\endgroup\$ – Icy Dec 1 '15 at 14:38
  • \$\begingroup\$ The motor you posted looks to be a 3 phase motor used for something like a quad copter. You will need to have a "speed controller" to control it's speed and convert the DC to 3 phase. \$\endgroup\$ – MadHatter Dec 1 '15 at 14:50
  • \$\begingroup\$ Brushless motors will be more complicated to drive, like @MadHatter points out. You will need a controller. However, they are more reliable and you never have to replace the brushes (or the motor if the brushes wear out and are not replaceable.) \$\endgroup\$ – John D Dec 1 '15 at 15:17
  • \$\begingroup\$ Thanks so much for your answers, -We already have a controller for the EMAX BL2212 but we just want to make sure that this motor can work on current as low as 1 or 1.5Amps and move the car with a proper speed \$\endgroup\$ – Elbehery Dec 1 '15 at 15:23
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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.

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  • \$\begingroup\$ Thanks so much for this detailed answer! So i should be looking at the No load current and try to pickup a motor that has the minimum no load current and lower KV ? something like this MX2212-920kv maybe ? \$\endgroup\$ – Elbehery Dec 2 '15 at 13:04
  • \$\begingroup\$ The MX2212-920kv looks good except for lacking a drive shaft for the pinion gear (perhaps a pulley and belt would work, or you could replace the shaft with a longer one). \$\endgroup\$ – Bruce Abbott Dec 2 '15 at 16:33
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I recommend you an brushed-dc for the first steps, because they are much easier to control. If you want to use an brushless-dc, please take care of finding a good speed controller. Most of the cheap speed controllers don't have a fieled orientated control, only BEMF. With BEMF only, it's quite complicated to start the motor. This isn't very critical in a quadcopter design, because your initial torque is nearly zero (torque is only dependent on propellor speed). If you have a car, you might have a very high initial torque, so starting the motor won't be easy without an field orientated control. With the field control, you have to measure the current of at least two motor inductances and you will need an more complicated regulator. If you have something like this, i suggest the brushless drive.

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  • \$\begingroup\$ Thanks so much for your answer, Lets assume that the initial torque problem is not that critical, But would a 1 Ampere , proper gear ratio turn on for example the MX2212 900KV brushless motor @ 300 RPM, 0.2n-m torque ? or i`ll need more amount of current for a task like this ? \$\endgroup\$ – Elbehery Dec 2 '15 at 1:52
  • \$\begingroup\$ More simply, the power system design should probably be informed by looking at large-scale radio control cars of similar size. Typically, when RC surface vehicles use brushless motors, they incorporate hall sensors as they have a need for low speed torque, in contrast propeller driven vehicles use the BEMF only setups. \$\endgroup\$ – Chris Stratton Aug 14 '16 at 15:36

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