SG 1 SG 2 GCU GCU F1 igen1 ac Bus 1 vHVAC1 SATRU1 F3 igen2 SHVB ac Bus 2 vHVAC2 SWIPS SATRU2 SACL Essential Bus iATRU2 iATRU1 SEMA1 WIPS ATRU1 SEMA2 Other ac Loads iCRU1 dc Bus 1 vHVDC1 CRU SDCL idcEMA F2 idcECS1 vHVDC2 dc Bus 2 CRU SECS1 ATRU2 CIU SECS2 CIU idcECS2 Other dc Loads PMM PMM EMA1 EMA2 ECS1 Other dc Loads ECS2 PMM PMM Figure 3. A possible More Electric Aircraft power system. systems, represented in this diagram by two electromechanical actuators (EMA1 and EMA2), which are driven by permanent magnet motor drives. The most significant loads found on the dc buses are the environmental control systems, which maintain the temperature and pressure of the passenger cabin of civilian aircraft. Generation of electrical Power variable-speed shaft in the gas turbine; this gearbox can be expensive to purchase and maintain. This constantspeed shaft drives a generator to give a constantfrequency electrical supply, typically at 400 Hz. The voltage from the generator can then be controlled using an exciter and a simple control loop, giving the generation system shown in Figure 4(a). Figure 4(b) shows an alternative technique for generating a constant frequency supply with the generator connected directly to the engine shaft. The output of the generator will then have a variable frequency related to the speed of the turbine. This electrical frequency will change during the phases of flight of the aircraft as a response to changing demands for the gas turbine speed. This variable-frequency supply can then be processed by a suitably designed ac/ac power converter and filter to produce a fixed frequency and voltage supply for the aircraft's electrical Electrical systems offer far more options for reconfigurability as well as for advanced prognostics and diagnostics. The increase in the requirement for electrical energy on future More Electric Aircraft increases the demands on the design of the generation systems. For the More Electric Aircraft to be competitive with existing systems in terms of weight and reliability, the industry has seen the start of a number of fundamental developments in the generation of electricity, including the generator technology as well as the choice of electrical system voltage and frequency. Today, most civilian aircraft use a complex mechanical gearbox to create a constant-speed shaft from a IEEE Electrific ation Magazine / d ec em be r 2 0 1 4 9