The power factor is defined as the cosine of the angle between the current and voltage waveforms in an electrical circuit. It represents the efficiency of power transfer in the circuit.

When the current and voltage are 90 degrees out of phase, it means that the current waveform reaches its peak value at a different time than the voltage waveform. This situation occurs in circuits with inductive or capacitive loads.

In an inductive load, the current lags behind the voltage waveform by 90 degrees. This is because inductors store energy in a magnetic field, which takes time to build up and collapse. As a result, the current waveform reaches its peak value after the voltage waveform.

In a capacitive load, the current leads the voltage waveform by 90 degrees. This is because capacitors store energy in an electric field, which takes time to charge and discharge. As a result, the current waveform reaches its peak value before the voltage waveform.

In both cases, the angle between the current and voltage waveforms is 90 degrees, which means the power factor is zero. This is because the power factor is the cosine of the angle, and the cosine of 90 degrees is zero.

A power factor of zero indicates that the circuit is not utilizing the transferred power effectively. Reactive power, which is the power oscillating back and forth between the source and the load, is present in the circuit. This reactive power does not contribute to useful work and can cause inefficiencies in the system.

To improve the power factor, power factor correction techniques can be employed. These techniques involve the use of capacitors or inductors to offset the reactive power and bring the power factor closer to unity (1). A power factor of 1 indicates that the current and voltage waveforms are in phase, resulting in efficient power transfer.

In summary, when the current and voltage waveforms are 90 degrees out of phase, the power factor is zero. This signifies a poor power factor and indicates the presence of reactive power in the circuit.