What's the easy way to measure a DC hobby motor's inductance?

When using simulations(SPICE), I need to model a DC motor or an electromagnet, relay ect. as an RL in series.

I can measure the R resistance by an ohmmeter. But I couldn't find an easy way to find out L value. Should I use a sinusoidal AC signal and measure the impedance from the phasor equations for that? But then I would need other equipment.

Any ideas?

Answer

If you have a square wave generator and an oscilloscope then you can use the L/R time constant method. Hook the generator up to the coil in series with a resistor, and put the scope across the resistor. Adjust the resistor value and square wave frequency to get an exponential waveform like this:-

Now measure the time it takes for the voltage to drop to 37% of its peak value. This is the L/R time constant, T = L/R. R is the total resistance in the circuit (resistance of the coil + your resistor). You know T and R, so put those values into the formula and rearrange it to get L = T*R.