quarter-phase
Differing in phase by a quarter of a cycle; two-phase.
quarter-phase: 90 degrees out of sync
Quarter-phase refers to an electrical condition where two alternating currents or voltages are displaced by exactly 90 degrees of phase angle, or one quarter of a complete 360-degree cycle. In a 60 Hz power system, this means one signal lags or leads the other by roughly 4.17 milliseconds. This is the defining characteristic of two-phase power, a system that predates three-phase distribution but remains in use for specific industrial applications.
The most common application is the two-phase induction motor, which uses two separate windings on the stator, each fed with currents 90 degrees apart. This creates a rotating magnetic field that drives the rotor without requiring a separate starting mechanism. The phase shift is typically created by a capacitor in the auxiliary winding circuit, which stores and releases energy in such a way that current through that winding leads the main winding current by approximately 90 degrees. These motors are robust and self-starting, making them useful in applications like air compressors, pumps, and woodworking machinery.
Why quarter-phase versus three-phase?
Three-phase systems with three windings 120 degrees apart dominate industrial power distribution because they deliver more balanced, constant power and are more efficient at large scales. However, two-phase systems require only two power conductors plus a neutral or ground return, making them simpler to wire in older installations or retrofit into existing equipment. The quarter-phase relationship is also mechanically simpler to achieve with a capacitor than maintaining precise three-phase generation and distribution infrastructure.
A practical limitation of quarter-phase motors is that they typically run at lower power ratings than equivalent three-phase machines and can draw higher starting currents. The capacitor itself ages over time and can fail, causing the phase relationship to drift and the motor to lose starting torque or run inefficiently. This is why capacitor condition is a critical maintenance item in any facility relying on quarter-phase equipment.