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Early Airplanes had very few loads that required electrical power. Batteries supplied a few amps for radio communications and a couple of cockpit lights. Later, alternators were employed on piston engine airplanes to generate more electrical power. As aircraft evolved, increasing amounts of electrical power were required, and power quality standards were introduced to govern the distributed airplane voltage and frequency. Those standards specified either a 28-V dc system or a three-phase system with a regulated voltage at a constant frequency.