# DC Brushless Motor For Electric Rover

I'm an industrial designer, and am working on building a small electronic rover,

however I've been doing some research and I feel a little bit lost in terms of what motor is best for me,

The device consist on a 4wd electric vehicle, (a dc brushless motor on each wheel)

Wheel diameter: 7cm

Weight carried: 100kg (mass 980N)

Top speed: 20km/h (results in about 2000rpm)

Acceleration: 0.4m/ss (value may vary)

After doing some research I plugged numbers in equations and figured out the following:

Total torque: 40N

Torque in each motor: 0.350Nm

Power required by each motor: 73 watts

Now, this means I need a dc motor which gives me 2000rpm and a 0.350Nm torque As I have little experience with these motors, I can't seem to find any that match these settings,

The question is: Am I looking for something feasible by a dc brushless motor?

Follow-up question: What Voltage/Amperage values am I to expect from a motor of these characteristics?

• If this is a hobby project, surplus wheel chair motors are relatively inexpensive. – BSEE Jan 10 '14 at 0:55
• OP is asking how to understand motor parameters and match his needs to them. Clearly a question on design. – Passerby Jan 10 '14 at 5:13
• You appear to have reversed weight and mass in the above : mass is measured in kg, force is measured in N) – user_1818839 Jan 10 '14 at 13:08
• Small but v important quibble: ALWAYS write values < 1 with a leading zero eg 0.40 m/s/s and NOT .40 m/s/s, and 0.350 Nm and NOT .350 Nm. Otherwise the leading decimal point can VERY easily be missed and nasty things can happen. (I change .40 and .350 in text. If not intended as shown please re-edit) – Russell McMahon Apr 16 '14 at 12:53
• Newtons is a unit of force, which can be used for weight, and kg is a unit of mass. – Olin Lathrop Apr 16 '14 at 14:06

This is a bit off-the-cuff mathematically just to see if things stack-up: -

Your mass is 100 kg and your top speed is 20 kmph (5.56 m/s).

You want to accelerate at 0.4 m/$s^2$ - this means it'll take about 14 seconds to get to full speed.

The energy attained by 100 kg at 5.56 m/s is $\dfrac{mV^2}{2}$ = 1546 J and this equates to a power during acceleration of $\dfrac{1546}{14}$ = 110 watts or about 28 watts per motor.

Am I looking for something feasible by a dc brushless motor?

It definitely sounds feasible - I don't see why you shouldn't find a brushless motor like this but it'll be easier to find a brushed one for sure.

What Voltage/Amperage values am I to expect from a motor of these characteristics?

I think you can be looking at 80% efficiency so each motor will consume about 35 watts. 4 x 12V, 3A full-load rated motor should work but maybe I'd play conservative and go for something like 50 watts: -

I know these aren't cheap but it was virtually the first hit when I searched for "50W brushless motor". There should be plenty that are a bit cheaper.

Try this too - maxon are the supplier.

This is definitely in the realm of possibilities, you just need to know where to look. I would google "fractional horsepower brushless DC motor" and you'll get multiple manufacturers.

As far as voltage/current, typical voltages for this size motor would be 12, 24, and sometimes 48 V. And Andy aka is correct in his assumption of about 80% efficiency for a motor this size. That is a pretty typical value. Assuming 73 W output power, this would give you currents of 7.6 A, 3.8 A, and 1.9 A for 12 V, 24 V, and 48 V, respectively. If you assume 90% efficient control, you are looking at currents closer to 8.4 A, 4.2 A, and 2.1 A, respectively.