Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 255 winding has a larger wire diameter and shorter length, and the start winding has thinner wire diameter and longer length. Since the run winding has higher inductive resistance, the run winding current lags behind the start winding current. Hence, out-of-phase cur- rents and out-of-phase magnetic polarities are created. The two out-of-phase currents make it possible to generate a rotating magnetic field. It allows the rotor to start turning without the aid of an external force. This is called self-start. Note the next remaining figures in this section illustrate the automatic starting of a two- pole motor and describe a full 360° rotor rotation. Each winding requires two poles, which means there is a total of four poles. However, only the run winding poles are used to describe the number of poles a motor contains. The start winding poles are required to produce the rotating magnetic field, so there is an equal number of run and start winding poles. In Figure 13-6, the leading start winding current flows from L2 to the first start winding pole, making it a south pole. Current continues onto the second pole, which becomes a north pole. The start winding poles induce currents in the rotor, so the rotor poles are opposite in polarity to the starting poles. The start winding current starts to decrease as the run winding current begins to increase. The rotor has not yet moved but has established magnetic poles. Figure 13-7 shows the progression of a sine wave in time. It demonstrates how the run winding reaches its peak current and magnetic field. The start winding current decreases toward zero. The rotor’s poles are attracted to the run winding poles, and so the rotor moves. The strong attraction allows the rotor to complete 180° of travel, Figure 13-8. The second half cycle of the sine begins in Figure 13-9. The start winding poles change polarities, causing the rotor polarities to change. As run winding current increases, the rotor is attracted to the run winding poles. The run winding current then peaks and provides the rotor with enough inertia to complete 180° rotation back to the original starting point. See Figure 13-10. Thus, the rotor has completed 360° of rotation. After the rotor develops enough inertia to maintain rotation, the start winding can be removed. 1 1 S N + – L1 S N L2 Centrifugal switch Start winding (blue lines) reaches peak current and maximum magnetic pole strength Run winding (red lines) is a zero current and no magnetic field First Half Cycle of Sine Wave Goodheart-Willcox Publisher Figure 13-6. The sine wave cycle begins with start winding current leading the run winding current. Magnetic poles are established by the start winding.