TM 1-1520-240-102-11-1SECTION XI. ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEMS2-11-1. Electrical Power Supply System.Alternating current (AC) is the primary source of powerto operate the electrical and electronic equipment. ThreeAC generators, two driven by the aft transmission andone driven by the APU, produce 115/200-volt 3-phase400-Hz power. The system develops 28-volt DC throughtwo transformers rectifiers (RECT) one each in the for-ward section of the left and right fuselage pods. DC isalso supplied by a 24-volt nickel-cadmium battery.Both 115/200-volt 3-phase AC and 28-volt DC can besupplied by operating the APU or by connecting an ACexternal power source to the external power receptacles(fig. 2-11-1). If the APU is running or AC external poweris connected, DC power is supplied by the helicoptertransformer rectifiers (RECT). If only DC external poweris supplied, AC power is not available on the helicopter.Circuits are protected by circuit breakers (fig. FO-2 andFO-5). The electrical load is divided between the two ACgenerators (fig. FO-3 and FO-6). Should one generatorfail, the other will automatically take over the entire load.When APU is running, its single generator powers theentire load.Figure 2-11-1. External Power Receptacles2-11-2. AC System.The AC system supplies 115/200-volt three-phase400-Hz power from No. 1 AC generator to No. 1 three-phase AC bus and from No. 2 AC generator to the No. 2three-phase AC bus (fig. FO-3 and FO-6). The AC equip-ment is powered by these buses. Some of the equipmentis operated by 115-volt single-phased AC and someequipment by 26-volt AC power supplied through thetransformers.The AC system is protected from overvoltage, undervol-tage, and underfrequency conditions by generator con-trol units. The generators will be disconnected from theAC buses any time the RRPM drops below 82 to 85percent for more than 3 to 7 seconds. The AC powerdistribution system has four power sources, a contactor,control circuit, an AC power transfer circuit, and two ACbuses.The No. 1 and No. 2 generator power sources are twomain generators driven directly by the aft transmission.The APU generators is driven directly by the APU. Theexternal power source is an AC power supply connectedto the helicopter.No. 1 and No. 2 generators feed their respective buses.If No. 1 and No. 2 generator fails (or are shut down), thefailed generator is isolated from its bus and the operatinggenerator feeds both buses. When No. 1 or No. 2 or bothgenerators are operating, APU generator and externalpower are blocked from the AC buses.When the APU generator is operating and the main gen-erators are shut down (or rotors turning below about84%) or switched off, the APU generator feeds both bu-ses. When the APU generator is operating, external pow-er is blocked from the AC buses. When external poweris applied to the helicopter (GEN APU, GEN 1, and GEN2 are OFF), the external power source feeds both buses.The Generator Control Unit (GCU) also provides genera-tor feeder fault protection. If a fault occurs between thefeeder and the airframe, the GCU will disable the genera-tors. This prevents structural damage to the airframewhen a ground fault occurs.The Permanent Magnet Generator (PMG) section withinthe generator is used to power the main contactors (re-lays) in the distribution system. A pickoff coil within thePMG provides an RPM signal for the rotor tachometerindicators. This tachometer signal is available wheneverthe rotor are turning.2-11-3. Generator Control Switches.The generator control switches are located on the ELECpanel of the overhead switch panel (fig. 2-11-2). Thethree switches are labeled GEN 1, GEN 2, and GENAPU. The switch positions are TEST, OFF RESET, andON.
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