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Re: [Phys-l] betatron in a buzzer armature?

On 3/21/2011 1:41 AM, Bernard Cleyet wrote:
A poster on PTSOS claims a back EMF is produced by the movement of a buzzer's armature. If so, in a vacuum, it would result in a greater back EMF, because the armature would be dragged less, which would result in the electromagnet consuming less power. I think the changing mag. field will result in an eddy current which will do the opposite, i.e. oppose the motion of the armature. What think you-all?

bc
Here's my take on it.

I visualize a buzzer as a coil wound on an iron u shape, which atrracts an iron armature when current is passed through the coil. I visualize the current path including a contactor attached to the armature, which breaks contact as the armature approaches a stop.
It's at this point that talk turns to back EMF, which will continue to flow some current while a spark issues from the contacts, or through a capacitor across the contacts.
Any current is attractive to the armature, because its material is selected for low remanance. So far, not much to argue about here.
Now the question of what changes if the buzzer is evacuated.
It will spark longer sparks, no doubt. The motion of the armature is less damped
at constant velocity, no doubt. It's easy to suppose that with marginally more energy in the magnetic field, and marginally less air damping, the armature's peak velocity might be found to rise. With a slightly higher armature speed, the rate of increase in magnetic field
might well grow in the vacuum state. And with a greater change in magnetic flux, there might be more eddy current loss. That depends on details of contruction and materials.
Silicon steel cuts eddy current loss, as do build ups of thin laminations or build ups of soft iron wire (one of the attractions of the toroid design, of course.

Now, would there be a net reduction in energy use? Perhaps there would be a little less consumption, if the armature gets from its rest or rebound position to its contact broken displacement slightly sooner.

But you have in mind an updated version of a Shortt syncronome I suppose.
That would take a vacuum can holding a nickel iron bob with four position sensors, coupla flops and a gate to drive it via a solenoid through the center position of the arc, when the amplitude decays after numerous beats.

Brian W