I plan (one day) to build an autonomous mower. This page is dedicated to the motor sizing. To size the motors, I need the following data:

I plan to build a four-wheel drive mower, so I'll need **four motors**.

I don't know yet, but I current have a Worx Landroid and
we'll assume the wheels will be the same size, about **21 centimeters**:

As for the wheel diameter, I'll assume my mower will have approximatively the same weight
than my Landroid mower. Let's say about **12kg**:

Not easy to estimate the maximum velocity of my future mower. As for the weight and wheels diameter, I check on my mower. I'll take this opportunity to do the acceleration.

Here is the result. Blue points are the point measured on my mower, the red line is the modelization.

The cruising speed of the mower is close to 0.4 m/s, let's take some margin, our goal will be 0.5m/s or 1.8km/s after conversion.

We don't need a strong acceleration to avoid wheel slip. On the previous model, the acceleration is 0.0125 m/s². Let's take some margin: 0.015 m/s² will be great.

Let's be ambitious! **45 degrees** seems a great goal.

Thank's to this wonderfull online motor sizing calculator, we get the following specifications for our motors:

- Velocity: 45.47 rpm
- Torque on wheel: 2.2 N.m
- Power: 12.9 W

- Velocity: 45.47 rpm
- Torque on wheel: 4.4 N.m
- Power: 25.8 W

After some research, I chose a ZD Motors 42mm motor (reference Z42BLDPN2440-30S with a planetary gearbox having a ratio of 58.22).

- The motor can reach a speed of 52 rpm.
- Maximum torque is 6 N.m

The motor power is 40W. But considering 52 rpm = 5.45 rad/s, we can deduce that the maximum output power of the gearbox will be 5.45 rad/s x 6 N.m = 32.7 W.

All that's left to do now is to check that the motor is actually suitable!

- Documentation of the Z42BLDPN2440-30S motor
- Complete motor series documentation
## See also

- Brushless Hall sensor frequency calculator
- Commutation sequence for BLDC motors
- Inside a Maxon EC45 BLDC motor
- How to size a motor?
- Online motor sizing calculator

Last update : 09/20/2023