DC Motor / Commutator Bar


Using a MTI Microtrak 4 laser triangulation sensor to measure a DC motor’s armature / commutator bar run out.

laser sensor measuring runout

We fitted a DC motor armature (rotor) in a lathe to spin it.

The MTI laser head a DTS 50-20 has 50 mm standoff and a 20 mm measuring range. The laser was placed in a FS-5 magnetic  mount fixture and pointed at the commutator bars. The lathe was spun at 180 RPM and the commutator bar run out was observed on a PC laptop that was connected to the laser sensor. MTI’s MT-4 Basic software was used to observe the run out. Note it’s necessary to use the zoom function to see the bar to bar height for a complete revolution. The MT-4 sensor was set to 500Hz low pass filter, 2000 samples second. This is what the commutator bar run out looks like before zooming. depth strip chartAbout 7.5 seconds of data was taken. The gaps between the bars appears as an upward trending spike and we can see that the commutator bar gaps are about .020 inch (0.63 mm) deep. when we zoom in on the commutator bar’s run out we see it’s about .002 inch (.05mm) p-p run out. If we zoom in closer on an individual commutator bars profile data we see that there is what looks like a choppy signal. This is due to the scratches/ texture of the copper surface and a little noise added by the specular nature of the surface. In a more elaborate program this data could be smoothed. The big upward spikes are the edges of the gap between commutator bars.