It's my first project using arduino. Simply trying to rotate three identical stepper motors, using three 293d h-bridges.
Although it does work properly (meaning the motors do rotate as they should), after the second or third rotation, they start to make a whirring electrical noise -when not moving.
I have attached a schematic of my circuit. The power supply is 5v dc 16A (old computer psu).
You can see exactly what I am experiencing on this video I made (try to listen in closely).
Update: I checked by touching the h-bridges at the same time the noise starts, and I can feel them getting really hot.
Best Answer
What you're hearing is the effect of actually stepping the current. A sharp change in current produces a sharp change in the forces acting on the motor structure, which produces a certain amount of acoustical output. It's not quite the same as hitting it with a hammer (it's a step, not a pulse) but the idea is the same.
If you listen closely, you'll realize that the pitch of the sound you hear is determined by the stepping rate of your driver. If you vary the step rate, the pitch will change as well. The effect is not "pure", since the excitation (a step) produces lots of harmonics.
If you replace your steppers with a pair of sine wave drivers, 90 degrees out of phase, you can get rotation which is essentially silent.