How to calculate torque of the motor?

Electrical Engineering Asked by Tushar Dubey on December 27, 2020

I have bought a motor from amazon. But the problem is no data is provided with it.

Now I want to calculate the torque of the motor. I have measured the current and voltage of the motor.
At 8.57V it consumes 0.05Amps of current. Now I want to calculate its torque.
Size of motor- 1cm dia and 3 cm length 200gms and cylindrical in shape.

The method I used is taking Electrical power= Mechanical power
So the electrical power for the motor is $ 8.57Vcdot 0.05A=0.4285 W$

Mechanical power is $Fcdot v= 0.4285[W]= Fcdot 10[m/s]$

$F= 0.04285 N$

Now torque $T=Fcdot r$

$T=0.04285cdot 0.01$

$T=0.004285 Nm $

So it this calculation correct or not.
Also what does T=0.04285Nm signifies in practical terms for example- it can lift 0.04285N weight for 1 meter vertically.

Also how to calculate how much weight it can carry horizintally.

One Answer

With a 200 g motor, 400 mW power consumption sounds awfully low. It sounds like you are running the motor at no load.

If you want to measure or estimate the torque into a load by the mechanical power = electrical power method, then you need to put a load on the motor. Suitable loads include a fan, another motor/generator driving resistors for a load, or a friction or electromagnetic brake.

Assuming 100% efficiency would give you an upper bound on mechanical power output. Given the motor size, it's likely to be much less than that.

Once loaded with a suitable mechanical load, you'll need to measure or estimate the motor speed, as the output power in watts is torque (Nm) times angular speed (radians/s). Conversion between rpm or rev/s to rads/s is fairly straightforward.

Once you have measured voltage V, current I and motor speed ω when loaded, and estimated your expected efficiency η, your torque at that load and current will be

torque = ηVI/ω

If you can actually measure the torque, by mounting the load on a balance, or by winding a weight up round a drum, then a more interesting calculation is to calculate the efficiency.

Correct answer by Neil_UK on December 27, 2020

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