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.