Finally I am sick of mowing my lawn, and am starting to plan a robotic lawn mower.
I would like to use one of these batteries (12V Li-ion) on the mower.
I am trying to decide on which of these electric motors to use for the main blade movement. Here is a table of the different values the different options have.
I was thinking of the 9T option, due to the higher KV (revolutions per volt), but am not sure of the Resistance option. Would a higher resistance be a good thing if the blade was to hit a large object?
Thanks for any help.
Edit. This will be for grass cutting and am thinking the blade dimension would be between 25-30cm. And would be some kind of blades such as these.
Quite a task
These motors are no good for a lawn mower.
I used to race RC cars in competition quite some time ago, before brushless motors even existed. That means that my knowledge is outdated, but for chosing a motor that doesn’t matter. The fastest most powerful motor is more than often not the best choice, that’s true for RC racing and even more true when you want to use a motor for a totally different task. The motor you mentioned is designed to give a whole lot of speed for a short period. I don’t know what the duration for a race is today, but it used to be 5 minutes and motors got really hot. So here we have the first problem: you can’t mow a lawn in 5 minutes so you have to find a way to cool the motor to avoid burning it out.
Then we have the design problem: speed is something you don’t need. Imagine a blade spinning at 50.000rpm. (if you’re able to reach that speed) First of all I wouldn’t want to be near a blade that spins that fast, but besides that it won’t cut much grass because the drag the blades create at high speed is pushing the grass flat to the ground. So you have to gear the motor down to the appropriate speed at which the blades should turn. In the “old days” a 9 turns motor was designed for high speed with low torque, I can’t imagine that a 9T brushless motor can deliver both and torque is what you do need the most. Ever tried cutting wet grass with a petrol mower? It will stall eventually because of the sticky cutted grass clogging up everything.
Even if you’re able to maintain the appropriate speed with enough power, which I highly doubt, you’ll have a motor that runs hot, isn’t very efficient because of the gearing and it will drain the 3Ah battery quicker than the robot can mow your lawn
What you want is a massive motor with lots of torque that is able to drive the blades at the appropriate speed, preferable with a belt drive so that when the blades hit something they shouldn’t hit, the belt will slip instead of damaging something
Such a motor won’t be very cheap, together with the blades, the battery and everything else you’ll need for a mower, it’s cheaper to buy a used petrol powered mower. The benefits of a petrol powered mower are that it’s powerful enough without draining batteries and it can even move on it’s own power, so you only have to make a steering mechanism and of course use servos to control engine speed and movement. (If you want the mower to move backwards then you’ll need to figure out how to reverse the movement mechanism or just use an electric motor to power the wheels. A windscreen wiper motor will do fine, probably.)
Awesome Information.
And a very big thank you for such a detailed answer there.
What you want is a massive motor with lots of torque that is able to drive the blades at the appropriate speed, preferable with a belt drive so that when the blades hit something they shouldn’t hit, the belt will slip instead of damaging something
That’s the information I think I needed to know. The lawn mower I want to build will be a run every day or every second day automated one, so that long grass shouldn’t be an issue (so a petrol engine will hopefully be over kill). And electric motors will be much quieter than petrol if the mower is running that often. I realise I haven’t stated that in my original question, but it’s more of a FYI for anyone interested.
Thanks again for the help everyone on picking motors, I shall continue the project and report back on its progress.