Figure 44.38 illustrates a typical AC excitation scheme. It shows the shaft-mounted main and pilot exciters together with their brush gear. Permanent-magnet pilot exciters are used to minimize dependency on external power supplies. The pilot exciter provides the excitation power for the automatic voltage regulator (AVR) control equipment. A 660-MW plant has a salient-pole pilot exciter with ratings near 100 kW. The main and pilot exciters are cooled by air. Shaft-mounted fans are used to provide the cooling.