Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 265 Recall each phase is 120° apart. Figure 13-22 shows the rotating magnetic field of a two-pole, three-phase stator for one ac cycle. Note that each phase requires two poles. The squirrel cage rotor is used as with single-phase motors to fol- low the rotating magnetic field. The rotor is being rotated clockwise. To change the rotation direc- tion, any two phases can be swapped. By stan- dard, phase one and three are normally swapped. The ability to easily change direction in three-phase also creates a problem when the phases are accidentally switched before reach- ing the motor. A phase sequence and rotation tester is available to test three-phase power sta- tus. See Figure 13-23. Tester leads are connected to three-phase wires that are connected to the motor terminals. The tester displays the rota- tion based on the sequence. Three-phase supply must be labeled L1, L2, and L3, and motor termi- nals labeled T1, T2, and T3. However, the power source could be mislabeled or the identification not legible. Another problem with three-phase motors is single phasing. This occurs when one phase is not supplying power. The motor is then operating with only two hot legs or a single phase. This causes the motor to burn out. Also, a line imbalance of 2% or greater can cause an increase in current through one phase and overheat the motor winding. To ensure all three phases are applied to the motor at the same time, a three-pole single-throw switch, or a three-pole contactor, is required. A A A A A A B B C C B B C C B B C C S N N N S S A A B B C C S N A A B B C C N S A A B B C C N S Goodheart-Willcox Publisher Figure 13-22. A three-phase motor’s rotating magnetic field. Goodheart-Willcox Publisher Figure 13-23. Phase sequence and rotation tester.