Current lags voltage

Inductance, as you recall, stores electrical energy as a magnetic field. When a voltage is placed across a coil, it takes awhile for the current to build up to full value. When ac is placed across a coil, the current lags the voltage in phase.

Pure inductance

Suppose that you place an ac voltage across a low-loss coil, with a frequency high enough so that the inductive reactance, XL, is much larger than the resistance, R. In this situation, the current is one-quarter of a cycle behind the voltage. That is, the current lags the voltage by 90 degrees (Fig. 1).
At very low frequencies, large inductances are normally needed in order for this current lag to be a full 1⁄4 cycle. This is because any coil has some resistance; no wire is a perfect conductor. If some wire were found that had a mathematically zero resistance, and if a coil of any size were wound from this wire, then the current would lag the voltage by 90 degrees in this inductor, no matter what the ac frequency.
When the value of XL is very large compared with the value of R in a circuit—that is, when there is an essentially pure inductance—the vector in the RL plane points straight up along the XL axis. Its angle is 90 degrees from the R axis, which is considered the zero line in the RL plane.

Fig. 1: In a pure inductance, the current lags the voltage by 90 degrees.