# How to decide the motor rating and torque required ? Answered

I want to make manual  wheelchair to be electrically driven.
For that I have assumed wheelchair weight to be=20 kg

Patient's weight to be=100kg ( max )

Motor  with battery = 30 kg (approx)
Thus total weight to be = 150 kg (approx)

what kind of DC motor along with its rating should be used ?

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## Comments

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You can physics your way through this problem with some basic formulas and some empirical tests. Do you have a "minimum" acceleration and speed requirement? and a minimum slope you must be able to climb with it? You can get an idea for the Force needed to get up the hill with this formula:

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F = M*A + 9.8M*sin(θ) + μ*9.8M*cos(θ)

1) A is the minimum acceleration (ms^-2)up a slope you need. Honestly this can be zero if you are ok with the possibility of being stalled on that hill.

2) Accounting for the hill, we know that the force working against you is 9.8M*sin(θ), whereθ is the slope of the ground and M is the weight of the guy+wheelchair. (the 9.8 is the strength of gravity) This makes sense because on flat ground, sin(0) = 0, so gravity doesn't matter. But if you go vertical, then gravity is 100% against you!

3) μM*cos(θ) is the force needed to overcome friction. μ is the coefficient of friction, it is just a fudge number, you need to measure it. Notice I used cos(θ) instead of sin again, because the friction is the biggest on a flat hill. On a vertical hill, there is nothing "pushing" you against the surface, no friction.

suppose

M = 200Kg (a heavy person + wheelchair)

A = 0 (because you don't care about accelerating while the steepest hill)

μ = 0.001 (typical for rolling resistance according to wikipedia)

θ = 5 degrees

then it's easy to punch in the numbers and see what F is. (171 Newtons)

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Knowing F, you can easily figure out the Torque your motor needs. Just take that F figure and multiply it to the radius of the wheel. That gives you Nm. So at a given minimum speed up that hill, (which translates to a minimum rotational frequency) you can find out how beefy your motor needs to be. Online calculators make this trivial. It's easy mathematically, too. Just be sure to stick with SI units until the end, or make sure to use the correct conversion factors.

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If you want to get straight to knowing the minimum power-rated nessesary for the motor, you need only know the minimum speed (|v|) of the wheelchair up that incline and the force you need to meet in order to get up it. ( P = F*|V|).

When trying to decide what spec to apply to your project it is always good to look at what others have already done by looking at the specs for similar products.

A quick glance through Google shows me that typically:

Battery 22Ah

Motors 250 - 300 watts. This for both motors if one on each wheel.

MAx weight 136 Kg

Range 27 Km

Simple really just look at what others are doijng,.