Question

how in pure inductive circuit voltage lead in current by 90 degree?

Answer 1

The voltage in an inductor is L(dI/dt). If your current is a sine wave, the voltage will be its derivative, which is cosine. They are 90 degrees apart. You can conceptually see this by realizing the current in an inductor cannot be changed instantly because time is required to build a magnetic field. As you apply power to an inductor, the current is initially 0 while the voltage is at its peak. As the magnetic field builds the impedance to current goes down as the amount of current goes up. When the full amount of current available to the inductor is flowing, the voltage is zero (ideally). This is why no real power is lost, the current and voltage are not occurring in phase. In reality there is resistance in the windings and in the source, that is where the voltage drops occur when the current is flowing. Power losses also occur in the eddy currents produced in the core, which create heat losses.