Purpose and Operation of the Starter/Generator used in Turbine Powered Aircraft
Most aircraft engines, whether reciprocating or turbine, require assistance during the starting process. This is where the starter/generator (S/G) technology comes in. The S/G combines two primary functions (starting and generating) into a single machine that can save space as well as weight in the engine. In a turbine-powered aircraft, the S/G is required to satisfy two fundamental requirements: to fire and accelerate the engine and to generate electric power to sustain the electrical components. When performing the starter role, the device turns up the engine to a speed that allows the passage of a sufficient amount of air through the engine for fuel to combust/ignite. The S/G then assists the engine in accelerating to self-sustaining speed. Afterward, while being driven by the aircraft’s engine, the S/G switches to a generator and supplies power to onboard loads (Zhang, Huang, Jiang, Geng, & Xu, 2017).
The S/G unit consists of a shunt generator and an extra heavy series winding. This additional series winding is designed and electrically connected to generate a strong field and high torque for cranking the engine. According to Flight Mechanic, (n.d), “the internal circuit of an S/G systems comprises four basic field windings: a series field or a C field, a shunt field, a commutating or inter-pole winding, and a compensating field.” When it is on the starting mode, three of the four windings are used: series field, inter-pole, and compensating windings. During use, the three windings are typically in a series arrangement with the primary source. While in the starting mode, the shunt field has no practical use for the S/G. Usually, the starting process requires a primary source of approximately 24V and a peak current of 1500A.
When the S/G is on the generator mode, the Series field is left out, as the remaining fields swing into action. The Shunt field is connected in the traditional way – the voltage control circuit for the generator. The remaining two windings (compensating and inter-pole) provide a seamless transition from no load to full load. The S/G’s external circuit consists of an “undercurrent controller” that serves to control the S/G when it is being used as a starter. This control function is achieved by ensuring the starter continues to work until the engine attains a predetermined rotating speed that can sustain fuel combustion. The undercurrent controller is made up of the motor relay and the undercurrent relay. While the former regulates the input to the starter, the latter controls the motor relay’s inner workings.
References
Electric Starting Systems and Starter Generator Starting System. (n.d). Retrieved from Flight Mechanic: https://www.flight-mechanic.com/electric-starting-systems-and-starter-generator-starting-system/#:~:text=The%20starter%20generator%20internal%20circuit,and%20commutating%20windings%20are%20used.
Zhang, Z., Huang, J., Jiang, Y., Geng, W., & Xu, Y. (2017). Overview and Analysis of PM Starter/Generator for Aircraft Electrical Power Systems. CES Transactions on Electrical Machines and Systems, Vol.1, NO. 2, pg 117-131.