# Choosing motor to buy after calculations

I am building a two wheeled robot which will have:

• max speed of 10km/h and acceleration of 0.5m/s²
• can climb 12° slopes
• weighs at most 30 kg
• wheel diameter is 50cm

By doing the calculations myself, and checking with many online automatic calculator I find that in the "worst" possible conditions I would need :

• Around 9.5Nm of torque
• Around 100Rpm for speed Which results in approximately a 100W motor.

When I look for suitable motors online I come across a lot of sites recommending different motors depending on robot weight.

An example would be.

Where this kind of motor is recommended for up to 150lbs robots and it is a 30-50W motor (I believe they use 4 of them).

Is the power I'm looking for in my case too "overkill"?

Seeing as this is an indoor robot but which will need to be able to go up slopes to reach different areas, do I need to find a motor that corresponds exactly to my numbers or do I need to get something even more powerful since efficiency is never 100%?

Since you don't know what assumptions they had in mind when they estimated the motor was good for a robot of a specific weight, I would not trust that. You have established your own requirements and calculations, which is awesome. Trust and build on that. Use the provided datasheets for the motors. The datasheets from Shayang Ye (the manufacturer of the gearmotor you linked) tend to require a bit extra conversion and work to use, but all the info you need is there.

The more operating conditions you can predict, the better you will be able to select the best motor the first time. Your 9.5*Nm*100*RPM is probably an estimate of the power to the environment and so you will need to estimate the power losses from the wheel-environment contact and the gearbox and other friction to arrive at the motor power you need. Losses of 30%-50% is what I would use for high level calculations like this. With a clean operating environment and high quality components losses might be as low as 10% but you are starting with low quality gearmotors so...

Electric motors operating at their maximums generate a lot of heat and can usually be operated at that maximum for a short amount of time.

High quality DC motor manufacturers usually show an estimate of what performance to expect if you want to guarantee no overheating. You can use their catalogs (Faulhaber, Maxon Motor are examples) to get an idea of how this will affect you. Look up motors of similar power or size to what you want and look at the graphs they provide. Expect that your low quality motor will heat a bit faster.

Your robot is big enough that you may find a larger motor to be useful. Search for brushless wheelchair motors for some more options a bit bigger but with similar cost/quality.

From my experience, you should try to find motor that is 1.5 to 2 times the calculated power. This is to account for additional losses mainly due to the friction and inefficiency in the power transmission mechanisms.

In some designs, even after you have factored in all the losses and you have found the right motor, there are other criteria such as space constraint and suitability of the power source that should be taken into account.