The AC Advantage
Here’s where AC excitation is usually more efficient for solenoid valves, from a power consumption standpoint. Typically, an end-user will apply a voltage to the coil, for example, via a mechanical or solid-state switch. But it’s the current passing through the coil (multiplied by the number of coil turns) that actually creates the magnetic field. This current is equal to the voltage divided by the coil impedance. For an AC sine wave (remember Circuits 101?), this impedance is calculated as R + j*2*pi*f*L, where R is the coil resistance, L is its inductance, f is the AC frequency, and j is a mathematical operator that results in a 90-degree phase shift. As the solenoid valve opens, the air gap quickly narrows (the core accelerates as the magnetic circuit becomes more efficient). That makes the coil’s inductance, L, (and thus the impedance) increase dramatically. As a result, the current decreases after an initial inrush or spike. So using AC excitation achieves the desired result — to open the valve against pressure, then cut down on power (Figure2).