How current lags the voltage in inductor?

In a perfect inductor (one with no series internal resistance), the current lags the voltage by 90 degrees. If the inductor has series internal resistance, then the current will lag the voltage by less than 90 degrees - the more the resistance in series with the inductor, the smaller the angle. The tangent of the angle can be found from the ratio of the inductive reactance of the inductor to the DC resistance of the inductor.

That is, Tan (phase angle) = (2 x pi x frequency (Hz) x inductance (H)) divided by resistance (ohms)

eg, a 1 henry, 100 ohm inductor on 60Hz would give:

(2 x pi x 60 x 1) / 100 = 3.77; tan-1(3.77) gives 75 degrees lag of current behind voltage.

The cosine of this angle gives the 'power factor' for the inductor - that is, the amount of useful energy dissipated in the inductor. Cos 75 is about 0.25 - so 25% of the energy actually does useful work (heat) - the rest of the energy (75%) is returned to the supply mains when the inductor discharges its magnetic field.