Not to mention the noise that results from driving a DC coil with an AC or PWM (variable pulse width) signal... Also there is the issue of the back EMF developed every time the coil is energized/de-energized. When one powers up a coil the coil makes a magnetic field, storing the electrical energy as a magnetic field. When the power is removed/de-energized the magnetic field is converted back to electrical energy or ElectroMotive Force (EMF)
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