How to find out the DC motor(Small motor - 5 to 12v motor) torque using current/voltage.
Formula:
τ = (I * V * E *60) / (rpm * 2π)
I current
V Voltage
E Efficiency
RPM speed
We can measure the current, Voltage and Speed with the help of Arduino sensors.
unfortunately, we are unable to measure the efficiency of the motor which is an important factor here.
How to manage this and find out torque?
Note: I am ready to change motor type like DC burseless or Stepper or any other small motor
Your equation τ = (I * V * E *60) / (rpm * 2π) is just a restatement of P=τw with efficiency thrown in. E is the ratio of power out to power in, with power in being VI. So you would need measure the power out, that is work done / time, by getting the motor to do some work that you can measure like lifting a weight through a distance.
ARDUINOAGENT003:
How to manage this and find out torque?
I think that will be difficult with a small DC motor because the power is likely to be very small compared to potential friction losses in the measuring apparatus.
In theory, lifting something (as suggest in Reply #1) is simple, but small motors usually run much too fast to make that practical without a reduction gearbox, which will have a lot of friction.
Maybe you could mount the motor on a gimbal and use some sort of propeller to create the load. The load will cause the motor to rotate in the opposite direction on the gimbal and then measure the force needed to prevent it rotating. Different propellers could be used for tests at different motor speeds.
I suspect most DIY apparatus could be useful for comparing motors, rather than getting absolute measurements.
...R
The torque/current curve of a motor is easy to determine as the motor doesn't need to turn while measuring it,
its independent of speed. Its also a straight line.
In brushed motors the available torque is reduced by the brush-friction torque, so the straight line is offset
from the origin - solid friction is pretty constant and speed-independent.
Brushless motors only have air and bearing friction to worry about which are both speed-dependent but
much smaller than brush friction at moderate speeds.
The motor datasheet will have an efficiency curve, but all DC motor curves look the same, efficiency is mainly
determined by winding resistance and brush-friction (though air drag gets significant for small motors
and faster motors). Both of these are simple to model by adapting the ideal motor equations.
T = kI - T0 - where T0 is brush friction torque (Nm), k is motor constant
w = (V-IR) / k - where R is winding resistance, w is angular velocity (rad/s)
Thus power out = (kI-T0)(V-IR)/k, power in = VI,
power out = power in - IIR - w*T0
IIR is power lost to winding's resistance as heat, the other term is power lost to friction, and can
be neglected for a brushless motor (although at high speeds a more complex friction term would come
into play)
Note that k, the motor constant has units of Nm/A or of V/(rad/s), which are actually equivalent (nice
exercise to prove that BTW)
T0 for a brushed motor is easy to determine, its k*I0 where I0 is the no-load current (again assuming
air drag is not a big issue - ie measure I0 at a slow speed).