can you explain how it works? interesting.
Well engines like that are typically used for moving heavy loads, with the loads being relative to the engine's physical size as a rule of thumb. For such applications you need a strong current. The voltage isnt critical, should be high enough tho as well, so the engine can provide a constant high torque when its revolutions are increased.
A stepper on the inside normally looks like this:
In the middle you can see a bearing. Attached to that is a gear what gives the name to the engine. The gear gives the engine the ability to do 360 steps at precisely 1 degree. So you can use it for placing a load accurately. Around the gear are electromagnets, which when they are charged with a current, start to form a magnetic fields, which moves the magnetic gear. Those coils operate in harmony, so depending on what goal you want to reach, physically opposing coils are charged and the things starts turning.
The coils basically work like a sponge. Meaning, if you'ld attach the stepper directly to a current, it would suck all the current it can get, effectively dieing in flames or killing your houses electric. To prevent that you need to regulate the amount of current it gets. Most steppers operate within a range of .5 - 6 Amperes. If you for example had an stepper that needs 2A, you attach a comparator circuit between the current supply and the stepper. That comparator, compares the current current to a set value ( 2 Ampere for example ) and if set value is reached, it shuts down the power supply for another set value. After that it reactivates again, the stepper sucks current until it has reached 2 Amperes again and is shut off again.
Those rising and falling edges normally happen at a frequency around 30 kHz, what are approximately 29 µs, so 30 millionth of a second. Since, everytime the stepper starts again, has to overcome the inertia of its rotor dictated by earths gravity, it produces a high torque to start, so a punch wave. This punch or vibration is accelerated depending on the used materials eigenfrequency.
Since all solid materials are a grid of atoms, you can punch that grid on the one side and have a wave travelling to the other side. Depending on the material that wave will arrive at the other side differently. Either it will arrive with a high force or it will arrive with a high frequency, so very often. You can project that with a sine curve, where you can see the amplitude as density and the occurences of the spikes as frequency. The eigenfrequency is the point of every material at which its grid structure breaks under the vibration. That happens for example to bridges, who are hit by wind. If you hit the eigenfrequency, the vibrations in the material start to build up and emphasize each other, meaning they dont leave the material no more. If that gets too strong at some point you reach the so called resonance catastrophy when the material breaks.
Of course that doesnt happen to the stepper, within the stepoper the vibrations are transmitted and emphasized according to the material used. In the video you can see the guy placing the engine on certain ground (wood and plastic are good carriers and bad dampeners) to emphasize the vibrations and so the sound. So if you basically hit the right frequency for the electric pulses, you can play every song you like on that piano
