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Using scope to mend model train motor

Posted: 20 Jan 2016 ?? ?Print Version ?Bookmark and Share

Keywords:oscilloscope? trigger? motors? commutators? ammeter?

The averaging function of digital oscilloscopes is well-appreciated. This can remove random noise from a repetitive signal without sacrificing bandwidth, so long as you can trigger in spite of the noise. For very noisy signals, a clean external trigger source is needed and must be synchronised to the signal on display. Trying to average on a noisy trigger is futile.

I occasionally use an external trigger and averaging for a somewhat unusual task: measuring hobbyist motor current. Small motors such as those used in small-scale model train locomotives have tiny brushes and commutators. The brushes bounce and arc when the motor spins, resulting in a lot of "hash" in the current. Driving the motor with a half-wave rectified sinusoid results in higher torque at slow speed, and it provides a clean trigger from the AC power.

These photos show the usefulness of averaging. The blue trace is the current at 100mA/div, measured as the voltage dropped across a 0.5 resistor. The yellow trace is the applied voltage at 4V peak. We see the current peaking at 600mA, way too high for this motor. No wonder it's been running a bit hot.

The measurement tells me it's time to open up the locomotive and inspect the motor. This reveals a dirty commutator. The brushes have worn and clogged the slots in the commutator segments, shorting them together.

Taking the motor apart and cleaning out the "crud" restores it to good health. As shown below, the current is back to its normal peak of 300mA.

Now, an ammeter would also have indicated higher-than-normal average current. The point is to demonstrate that this noisy signal, and others like it, would be impossible to display on a scope without the averaging function. The spikes from the bouncing brushes were so strong that the best you could do is guess where the average fell, and the frequency was so low that the scope's bandwidth limit of 20MHz did not make any difference. I wish I had taken a photo of the non-averaged spikes for comparison, but it seemed rather pointless at the time.

Note: Glen Chenier, the author, passed away on January 5, 2016. This article originally appeared on Scope Junction.

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