DC Motor

Download files: motor.zip.

This experiment investigates the behavior of a small DC motor, unloaded. The MATLAB instrument control toolkit is used to vary the voltage across the motor and measure the current through the motor.

The first experimental configuration is shown below (click to enlarge):

An Agilent E3631A triple-output power supply provides power to the motor. An Agilent E34401A multimeter measures the current through the motor. The MATLAB script motor2.m is used to control the instruments. The USB output from the instruments must be connected to an USB port on the computer running MATLAB. Note that at the end of the experiemental protocol there is a line of MATLAB code that saves the experimental results (save ommand) to a .mat. This can be loaded later with a load command.

The output for one motor is shown below. DC motors vary somewhat from motor to motor (even the same model).

For low voltages the motor is off. Around 1.2 V the motor turns on at relatively low speed and the current drops. As the voltage is increase the current increases and the motor spins faster. We have observed that the motor does not always turn on at exactly voltage, but that the turn-on voltage varies a few tenths of a volt.

Analysis

The MATLAB script motor2_hand.m was written to evaluate the performance of the motor. It loads a dataset saved by the previous script.

When the motor is on, the current vs. voltage curve has a distinct curvature and is well-represented by a quadratic. In the figure below, the data above 2 volts, shown as circular markers, is fitted by the solid quadratic curve.

A linear approximation is also satisfactory. The inverse slope of the line is the active resistance Ron = 2.8 kΩ and the x-intercept the back emf of the motor Vm = −12.6 V. Then for V > 2 V, we have

i = (V − Vm) / Ron

The resulting fit is shown below

When the motor is off, there is no back emf and the motor behaves like a fixed linear resistor. The calculate resistance is 46 Ω. The motor resistance measured with a multimeter was about 40 Ω. The figure below shows the i-v characteristics when the motor is off.

The transistion region between motor off and motor on is shown below:

Finally we can shown the complete results with the motor off, transistion, and motor on regions:

Maintained by John Loomis, last updated 30 Nov 2010