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How to determine the required torque while choosing a DC motor?

Here is a simple scenario. Think of a 4WD or 2WD vehicle. The max load the vehicle will have is 100kg.
Q1. How do I determine the required torque for the motor?
Q2. Should the Rated Torque(Kg-cm) more than 100 or the Stall Torque more than 100
Q3. And if use 2 Motor rated at 50kg-cm torque can it carry a load of 100kg?

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  • $\begingroup$ You need to specify the speed and acceleration of the vehicle. Then you can use Lagrangians to solve for the required wheel torques. You will probably need to use gears to produce the required torque. $\endgroup$
    – guero64
    Mar 3, 2022 at 13:17

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I believe this is the right way to approach the problem (see this post on ResearchGate):

  1. Need to specify how much you will be accelerating the vehicle. Let's call this $a$. For example, you could decide that you eventually want the vehicle to travel 1 m/s and you want to get to that velocity from a complete stop in 10 s. That means you will need to support an acceleration of 0.1 m/s$^2$.

  2. Once we have $a$, we know that the force required to accelerate the vehicle will be $F = ma$, where $m$ is the vehicle weight.

  3. The torque applied to each wheel will be $T = Fr$, where $r$ is the wheel radius.

  4. You also need to specify the final speed you wish the vehicle to travel at. Let's call it $v_f$.

  5. If $r$ is the wheel radius, then $C = 2\pi r$ is the circumference of the wheel. The vehicle will travel a distance of $C$ per each wheel revolution. If $w$ is the wheel rotation rate, then $v_f = C\omega = 2\pi r\omega$.

  6. Supposing that each wheel uses a gear with ratio $N$, the final rotation rate of the motor at each wheel is $\omega_m = Nv_f/2\pi r$.

  7. Start off assuming $N=1$ (i.e. no gear - direct drive by motor) and look for a motor that can support a repeated peak torque of $T$ and rated output speed of $w_m$.

  8. If $N=1$ doesn't work, try $N=2$, $N=4$, ... up to $N=160$ or so. Look for a motor that can support a repeated peak torque of $T/N$ and rated output speed of $w_m$ and make sure that the gear can support the repeated output torque of $T$. Above that gear ratio (if you really need to go that high) you aren't likely to find many solutions.

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