I got a problem I have not found the solution to. Got a motor that runs at 1000 rpm, when I hit with the strobe light at strobe frequency of 1000 rpm's the motor appears to stop which is normal. It also stops at 2nd harmonic and 3rd and 4th and so on. But when I start slowing down the strobe under 1000 rpm I also find other frequency that also stop the motor motion. What are these called, I mean below the fundamental.

 

I got a problem I have not found the solution to. Got a motor that runs at 1000 rpm, when I hit with the strobe light at strobe frequency of 1000 rpm's the motor appears to stop which is normal. It also stops at 2nd harmonic and 3rd and 4th and so on. But when I start slowing down the strobe under 1000 rpm I also find other frequency that also stop the motor motion. What are these called, I mean below the fundamental.

Sub-harmonic, the undertone series..

From Wikipedia
Subharmonic frequencies are frequencies below the fundamental frequency of an oscillator in a ratio of 1/n, with n a positive integer number. For example, if the fundamental frequency of an oscillator is 440 Hz, sub-harmonics include 220 Hz (1/2) and 110 Hz (1/4). Thus, they are a mirror image of the harmonic series, the undertone series.

 

Consider a motor rotating at 1,000 rpm, with the strobe set at 16.66 strobes/s will show the rotating part to be ‘still’ – this will also be true at 8.33 strobes/s (where the motor has rotated 2 revolutions between illuminations by the strobe).

But if the rotating part is illuminated at 33.33 strobes/s then every other strobe illumination will show the motor out of position by 180 degrees – indicating that the fundamental rotation speed is 1,000 rpm.

 

Consider a motor rotating at 1,000 rpm, with the strobe set at 16.66 strobes/s will show the rotating part to be ‘still’ – this will also be true at 8.33 strobes/s (where the motor has rotated 2 revolutions between illuminations by the strobe).

But if the rotating part is illuminated at 33.33 strobes/s then every other strobe illumination will show the motor out of position by 180 degrees – indicating that the fundamental rotation speed is 1,000 rpm.

An interesting effect can be had if you colour the blades different colours / patterns.
try black lines on a white background, at 45 degrees offset.

different speeds will show up as different patterns,

 

Wow, thanks all. That gave me alot of examples to play with. Was having hard time explaining what I was seeing in terms of when the thing would appear motionless. Thanks again. I can get it now.

 

It's like the old cowboy movies with the Waggon Wheels, going backwards in and out of sync with the TV, cos cinema use 24Fps, and TV uses 25Fps...

 

Except in North American and a few other places where TV was 30 frames per second.