phase (alternating current relationship). The angular relationship between voltage and current in an electrical circuit. Voltage and current are in phase when they both pass through zero, moving in the same direction at the same time.
If changes in current occur before changes in the voltage, the current is leading the voltage. But if changes in the current occur after changes in the voltage, the current is lagging the voltage, or the voltage is leading the current.
phase angle (alternating current electricity). The number of electrical degrees between the time an AC voltage passes through zero, going in a positive direction, and the time the current passes through zero going in the same direction. If it were possible for a circuit to be purely inductive, having neither capacitance nor resistance, the voltage would lead the current by 90°. This circuit would have a phase angle of 90°.
phased array antenna (radio antenna). A complex antenna which consists of a number of elements. A beam of energy is formed by the superimposition of the signals radiating from the elements. The direction of the beam can be changed by varying the relative phase of the signals applied to each of the elements.
phase inverter (electronic circuit). An electronic circuit that inverts the phase of a signal. Phase inverters change the phase of a signal by 180° to make it usable for the input of one side of a push-pull amplifier.
phase lock (electronic circuit). A method of adjusting the phase of one electronic oscillator, so it follows that of another oscillator. The phase of the two signals is compared, and a resultant signal is produced. This resultant signal is used to adjust one oscillator to bring its output exactly into phase with the other.
phase modulation (PM). A method of modulating a sine-wave carrier, so its phase is changed by an amount proportional to the instantaneous value of the modulating voltage.
phase-reversal protection. A circuit or device used in a three-phase alternating current system that interrupts the power if reversal of the phase rotation should occur.
phase-rotation indicator. An indicator, consisting of two lamps, used to indicate the direction of phase rotation of a three-phase alternating current generator. One lamp is bright and the other dim if the phase rotation is A-B-C. If the phase rotation is A-C-B, the condition of the lamps reverses. The first lamp will be dim and the second will be bright.
phase sequence, or phase rotation. The sequence with which the output phases of a three-phase generator are connected to the load. Reversing the phase sequence of a generator from A-B-C to A-C-B prevents the generator being synchronized with the others on the bus.