In general, gas turbine aircraft use one of two types of electric starting systems: direct cranking electrical systems or starter generators. Direct cranking electric starting systems are mainly used on small turbine engines, such as auxiliary power units, and some small turboshaft engines. Adversely, starter generator systems are found on gas turbine engines. The two systems are similar, except that after functioning as a starter, starter generators feature a second series of windings that allow it to operate as a generator once the engine has reached the appropriate speed. This saves weight and space within the engine.
The starter generator is engaged with the engine shaft via the necessary drive gears, while the direct cranking starter must employ some means of disengaging the starter from the shaft after it has started. A starter generator is essentially a shunt generator with an added heavy series winding. The series winding is electrically connected to produce a strong field and high torque for starting. Starter generators also offer economic benefits, as they are effectively two units in one. Furthermore, as fewer spare parts are required, the weight of starting system components is reduced.
The internal circuitry of the starter generator features four field windings: a series field, shunt field, compensating field, and commutating winding or interpole. During starting, the series field, compensating field, and commutating windings are used. It is similar to a direct cranking starter as all of the windings used during starting are in series with the source. When acting as a starter, the unit doesn’t use the shunt field. To start, a source of 24 volts and 1,500 peak amperes is needed.
While working as a generator, the shunt, compensating, and commutating windings are used. The series field is only needed for starting. The shunt field is connected in the conventional voltage control circuit for the generator. The compensating and commutating or interpole windings provide virtually sparkless commutation from no load to full load. Now let’s take a look at the sequence of operation for the starting system.
To start an engine with an undercurrent relay, the engine master switch must first be closed. This completes the circuit from the aircraft’s bus to the start switch, fuel valves, and throttle relay. Energizing of the throttle relay starts the fuel pumps, and completing the fuel valve circuit provides the necessary fuel pressure to start the engine. As the battery starts and the start switch is turned on, three relays close: the motor relay, ignition relay, and battery cutout relay.
The motor relay closes the circuit from the power source to the starter motor, the ignition relay closes the circuit to the ignition units, and the battery cutout relay disconnects the battery. Opening the battery circuit is required, as the heavy drain of the starter motor would damage the battery. Closing the motor relay allows a high current to the flow through the motor. As this current flows through the coil of the undercurrent relay, it causes it to close. Closing the undercurrent relay completes a circuit from the positive bus to the motor relay coil, ignition relay coil, and battery cutout relay coil. The start switch then returns to its normal position with all other units continuing to operate.
As the motor speed increases, its current draw decreases. When it decreases to less than 200 amps, the undercurrent relay opens, breaking the circuit from the positive bus to the motor, ignition, and battery cut out relays. The de-energizing of these relay coils ends the start operation, at which point the engine should be operating efficiently and ignition should be self-sustaining.
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