Harmony and me, we’re pretty good company.
When I was a technician in the build group of Xidex Data Disc, I used a strobe light to measure the speed of a motor. Turn off the light in the lab and turn on the strobe. If the motor was spinning at 100 rpms and the strobe flashed at 100hz, a mark on the spindle would appear stationary because it was being lit once per revolution. The same is true if the motor is 1000rpms and the strobe is 1000hz.
I would calibrate the speed of the motor with the strobe light. First I would turn the motor on, the dial the frequency of the strobe until the mark on the motor shaft appeared to stop. The frequency of the strobe is displayed, so I would know the speed of the motor. I would then adjust the speed of the motor to obtain the correct speed.
Harmonics speak of frequencies which are in harmony with each other. A simple example would be two frequencies, the first is 60hz and the second 120hz. Each time the first frequency would cycle, the second would cycle twice. If the second frequency is 180hz, it would cycle three times for each of the 60hz cycle. This fact can create problems while adjusting the motor speed as described above. If the motor was spinning at 120rpm and the strobe is set at 60hz, the mark on the shaft would be lit each time it went around twice. The mark would appear to stop in the correct spot but it would only be lit half the time. I had to be careful of harmonics, or I may not calibrate the motor correctly.
I became quite quick at calibrating motor controllers. I would set the strobe light at the desired frequency then turn the potentiometer to change the motor speed to match. One day the motor was spinning a little too fast, so I cranked the potentiometer and waited while the motor slowed down. I was particularly proud of the fact that the mark on the shaft stopped perfectly. I had become so good at calibrating, I could set the speed almost by feel. I turned off the strobe and turned on the lights. Looking at the motor shaft, the mark was STILL STOPPED. I hadn’t set the motor speed correctly, I had STOPPED the motor completely.
I laughed at myself and started over.