[archiver]

Posted by Raul, Engineer on team #111, Wildstang, from Rolling Meadows & Wheeling HS and Motorola.

Posted on 4/16/99 5:41 AM MST


In Reply to: Re: Drill Motor Inconsistency posted by Jerry Eckert on 4/15/99 5:27 PM MST:



: : Yes, we hooked both drill motors to the same voltage source and used an oscilliscope to count the RPMs. So, we know it is not the speed controllers.

: I'm intrigued... How do you measure the motor RPMs with an oscilloscope?

This is how it was explained to me by Jim Butler, our EE, who did the measurements:

These measurements were made with an oscilloscope and a current-sensing resistor. As conditions required, the period of the brush-induced spikes in voltage (measured at the motor) or current (measured across the resistor)
were measured using the oscilloscope. 'As conditions required' means that sometimes the voltage, and sometimes the current, was the simplest waveform in which to 'see' the spikes.

Spikes are most easy to measure in groups of five and the reason is that there's 'five' spikes per rotation of the armature. It turns out that various conditions, presumably related to the condition of the brushes and commutator, cause the waveform (as least the smooth part of it, which is related to the steady-state flow) to be periodic; I saw quite a bit of variation in this waveform between the six motors and even between direction on each motor, which seems to indicate that certain areas of their brushes/commutators aren't making great contact.