Inductors and RL Charging

Inductors in series and parallel, and RL charging time. An inductor's impedance increases in response to current changes; in other words, an inductor drops or supplies voltage to oppose current changes. (Therefore, the change in voltage across an inductor precedes the change in current through the inductor; this phase difference is 90°.) Initially, when an inductor is fully de-energized, it behaves like an open circuit (infinite impedance): when a constant voltage is suddenly applied, the increasing current generates a magnetic field in the inductor, which induces a voltage in the opposite direction, thereby opposing the change in current; at first, all the electrical energy goes toward generating this magnetic field. Over time the inductor's magnetic field builds up, and its impedance decreases: the voltage induced across it decreases, and the current through it increases. Eventually, when the inductor is fully energized, it behaves like a short circuit (no impedance): no voltage is induced across it, and current flows constantly through it, maintaining its consistent magnetic field. With the inductor fully energized, when the applied voltage is suddenly removed, the opposite occurs: the inductor opposes the changing current through it by collapsing the magnetic field to induce a voltage in the opposite direction, in order to maintain current in original direction. (A diode can be placed in reverse across the inductor to prevent this reverse voltage from being applied across the rest of the circuit.) Over time, as the inductor de-energizes, the voltage and current supplied by the inductor decrease until it is fully de-energized. The greater the inductance, the more electrical energy it can store: the longer it takes to energize, and the more voltage it can induce for longer. (In this way, a larger inductor, when energized, could induce a burst of voltage that exceeds that of the voltage source; however, the current supplied by the inductor can never exceed the current to which it was energized.) As described above, for DC (direct current), as the applied voltage remains constant, once the inductor is fully energized, its impedance is effectively zero, and it allows current to flow constantly through it. In contrast, for AC (alternating current), if the frequency and inductance are great enough, the inductor is constantly energizing and de-energizing – there isn't enough time for the inductor to fully energize before the applied voltage reverses itself. Therefore, the inductor's impedance remains high, and it prevents the AC current from flowing through it. The greater the frequency and/or inductance, the less the inductor can energize during that time, the higher its impedance remains, and the less current it allows to flow through it. Thus, the lower the frequency and/or inductance, the more the inductor approaches a short circuit, whereas the greater the frequency and/or inductance, the more the inductor approaches an open circuit.