Calculate Rotations per minute using an oscilloscope

This page documents how I used an oscilloscope ( a BK precision 1435) to determine the rpm of motors. I'm not sure that I've got the maths right, so any comments to robotwars101.org would be most welcome.

Firstly I attached a slowish motor to a 4.8v battery and attached the probes of the oscilloscope across the terminals of the motor. My thinking was that the voltage would fluctuate slightly as the motor rotated, and that I would be able to use the 'scope to see the regular fluctuations and thus determine the rpm.

Slow motor

The trace above is on 5 times magnification at the 5ms per division setting.

And here's the same trace at normal magnification.

I estimate that on normal magnification there are six cycles per four divisions, so each cycle is 2/3 of a division, which is in turn 5ms

So .66 x .005 =0.0033

1/.0033 = 303 rotations per second

303 x 60 = 18,000 rpm

Fast motor

This is a Graupner SPEED 300 rated at 29,000 rpm at 6v. I was running it with no load at 4.8v.

The trace above shows the fast motor at 5 times magnification at the 2ms per division setting

And here's the same trace at normal magnification.

I estimate that on normal magnification there are 5 cycles per division

0.2 x 0.002 = 0.0004

1/0.0004 = 2,500 rps

2,500 x 60 = 150,000 rpm

But this motor is rated at 29,000 rpm!

The explanation turns out to be that most motors have more than one winding so you have to find the number of coils and divide the previous number by that.