As you can see, the current through the coil is 90° out of phase – and delayed. Why? The periodically alternating current flow creates a magnetic field in the coil that also alternates periodically. This alternating magnetic field in turn results in an induction voltage, which (according to Lenz’s rule) is opposite to its cause – namely the applied supply voltage. With sinusoidal current, the slope or rate of change is maximum at the zero crossing. The self-induction voltage of the coil therefore also must have its maximum at these points. Conversely, at maximum or minimum of the current curve, the rate of change of the current has the value 0. The value of the self-induction voltage is therefore also 0.
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