How to find I-V characteristics of motor?
DC motors have a linear I-V relationship, and the specifications give enough information to calculate the I-V slope.

\begin{align}
m &amp;= \frac{12V}{600mA} = 20 \Omega\\
V_{emf} &amp;= 12 - m I
\end{align}

There is technically no way to directly measure \$V_{emf}\$, however it can be deduced by simultaneously measuring the current flowing through the motor along with the rotational speed for various steady loads (you will need at least 2 different loads). A cheap way to apply a load is to use your fingers and vary how hard you hold the shaft (the key is how steady the load is, not what it's actual level is).

There are a few things to note when dealing with real components:


Ammeters have a burden voltage due to how they measure current (they have a known small resistance inside which they measure the voltage across).
Some motors don't like to be stalled for significant periods of time, and may be damaged if stalled for too long.
DC motors age. Depending on the quality of the motor, this may or may not be significant. This is especially a problem with brushed DC motors, which it looks like is what you're using. I've tested DC motors which have lost significant (1/2, or more) performance running them for less than an hour. I've also used DC motors which are many years old and have lost little performance.
Real components have physical friction (either contact friction or air resistance).


Problem 1 is easy to solve: use kelvin sensing and adjust the power supply such that the voltage across only the motor itself is always the same.

Problem 2 is also simple to handle: just don't apply too great a load that the motor stalls.

Problem 3 is unavoidable. The only way to reduce this is to limit the amount of testing you do.

Problem 4 is usually negligible. If you need to characterize this then you will need a way to accurately measure the test torque. I am going to ignore this.

Once you have a few data points, get a line fit of I vs. rotational speed using your favorite line fitting algorithm. From the computed stall speed you can calculate the winding resistance of the motor. Similarly, you can compute the no-load speed from this line fit (point at which I = 0). At this point, \$V_{emf}\$ is the applied motor test voltage. Using the stall point and the no load point you can calculate the linear relationship between \$V_{emf}\$ and I:

\begin{align}
m &amp;= \frac{V_{test}}{I_{stall}}\\
V_{emf} &amp;= V_{test} - m I
\end{align}