called single-phasing). However, this condition is very bad for the motor. It will pull higher current on the other two phases and can quickly result in motor failure. Single-phasing of a three-phase motor is the most common failure of a three-phase motor. Changing Rotation Direction of rotation is determined by direction of the rotating zero, where no polarity is generated. A three-phase motor is easily reversed by changing Three-phase alternating current induction motors offer high starting torque and high running torque without the need for a start winding, capacitors, or relays. Only the three “hot” wires (L 1 , L 2 , and L 3 ) are needed for proper operation, but the ground (green) wire is included for safety. The green wire is connected to the motor frame to provide an escape for electrons if the motor becomes shorted to ground (grounded). 25.7.1 Method of Operation Three-phase motors have three pairs of stator poles (windings), one pair for each supply wire. Each winding should have identical resistance. As shown in Figure 25‑46, each pair of windings has a north pole and a south pole directly opposite each other. The stationary poles are equally spaced around a circle, exactly 60° apart (6 × 60 = 360°). In this way, three-phase motors have “one pole per phase.” With three-phase alternating current, the three power-supply wires take turns changing polarity from north to south to zero. See Figure 25‑47. Changing of polarity in the supply wires produces a strong rotating magnetic field. The alternating zero does not produce polarity in the stator poles, which permits the other two poles to produce the rotating push-pull effect on the rotor. Because the zero pole is constantly rotating around the stator poles, the rotor chases the rotating magnetic fields. The alternation of north, south, and zero in a three-phase motor produces a strong starting torque and a strong running torque. If the motor is running and one wire is disconnected from the circuit or one phase is lost, the motor may continue to run as a single-phase motor (a condition Goodheart-Willcox Publisher Figure 25‑45. Colors used by manufacturers to code wires used for making external connections to multi- speed motors. Coding for Multi-Speed Motors Tap Color Common High Medium Low Capacitor White Black Yellow Red Brown (2 wires) N N N S S S #1 First winding (2 poles) Second winding (2 poles) Third winding (2 poles) #2 #3 1 2 3 1 2 3 Goodheart-Willcox Publisher Figure 25‑46. The three windings, or stators, of a three- phase motor each have a north pole and a south pole. The stators are equally spaced around a circle. 510 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.