Turbine Engine Electric and Starter Generator Starting System

Electric starting systems for gas turbine aircraft are of two general types: direct cranking electrical systems and starter generator systems. Direct cranking electric starting systems are used mostly on small turbine engines, such as Auxiliary Power Units (APUs), and some small turboshaft engines. Many gas turbine aircraft are equipped with starter generator systems. Starter generator starting systems are also similar to direct cranking electrical systems except that after functioning as a starter, they contain a second series of windings that allow it to switch to a generator after the engine has reached a self-sustaining speed. This saves weight and space on the engine.

The starter generator is permanently engaged with the engine shaft through the necessary drive gears, while the direct cranking starter must employ some means of disengaging the starter from the shaft after the engine has started. The starter generator unit is basically a shunt generator with an additional heavy series winding. [Figure 1]

Turbine Engine Electric and Starter Generator Starting System
Figure 1. Typical starter generator

This series winding is electrically connected to produce a strong field and a resulting high torque for starting. Starter generator units are desirable from an economical standpoint, since one unit performs the functions of both starter and generator. Additionally, the total weight of starting system components is reduced and fewer spare parts are required.

The starter generator internal circuit has four field windings: a series field (C field), a shunt field, a compensating field, and an interpole or commutating winding. [Figure 2] During starting, the C field, compensating, and commutating windings are used. The unit is similar to a direct cranking starter since all of the windings used during starting are in series with the source. While acting as a starter, the unit makes no practical use of its shunt field. A source of 24 volts and 1,500 peak amperes is usually required for starting.

Turbine Engine Electric and Starter Generator Starting System
Figure 2. Starter generator internal circuit

When operating as a generator, the shunt, compensating, and commutating windings are used. The C field is used only for starting purposes. The shunt field is connected in the conventional voltage control circuit for the generator. Compensating and commutating or interpole windings provide almost sparkless commutation from no load to full load. Figure 3 illustrates the external circuit of a starter generator with an undercurrent controller. This unit controls the starter generator when it is used as a starter. Its purpose is to assure positive action of the starter and to keep it operating until the engine is rotating fast enough to sustain combustion. The control block of the undercurrent controller contains two relays. One is the motor relay that controls the input to the starter; the other, the undercurrent relay, controls the operation of the motor relay.

The sequence of operation for the starting system is discussed in the following paragraphs. [Figure 3] To start an engine equipped with an undercurrent relay, it is first necessary to close the engine master switch. This completes the circuit from the aircraft’s bus to the start switch, to the fuel valves, and to the throttle relay. Energizing the throttle relay starts the fuel pumps, and completing the fuel valve circuit gives the necessary fuel pressure for starting the engine. As the battery and 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; the battery cutout relay disconnects the battery. Opening the battery circuit is necessary because the heavy drain of the starter motor would damage the battery. Closing the motor relay allows a very high current to flow to the motor. Since this current flows through the coil of the undercurrent relay, it closes. 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 is allowed to return to its normal off position, and all units continue to operate.

Turbine Engine Electric and Starter Generator Starting System
Figure 3. Starter generator circuit

As the motor builds up speed, the current draw of the motor begins to decrease. As it decreases to less than 200 amps, the undercurrent relay opens. This action breaks the circuit from the positive bus to the coils of the motor, ignition, and battery cutout relays. The de-energizing of these relay coils halts the start operation.

After these procedures are completed, the engine should be operating efficiently and ignition should be self-sustaining. If, however, the engine fails to reach sufficient speed to halt the starter operation, the stop switch may be used to break the circuit from the positive bus to the main contacts of the undercurrent relay.

Troubleshooting a Starter Generator Starting System

The procedures listed in Figure 4 are typical of those used to repair malfunctions in a starter generator starting system similar to the system described in this section. These procedures are presented as a guide only. The appropriate manufacturer’s instructions and approved maintenance directives should always be consulted for the aircraft involved.

Starter Generator Starting System Troubleshooting Procedures
Probable Cause Isolation Procedure Remedy
Engine does not rotate during start attempt
Low supply voltage to the starter Check voltage of the battery or external power source Adjust voltage of the external power source or charge batteries
Power switch is defective Check switch for continuity Replace switch
Ignition switch in throttle quadrant Check switch for continuity Replace switch
Start-lockout relay is defective Check position of generator control switch Place switch in OFF position
Battery series relay is defective With start circuit energized, check for 48 volts DC across series relay coil Replace relay if no voltage is present
Starter relay is defective With start circuit energized, check for 48 volts DC across starter relay coil Replace relay if no voltage is present
Defective starter With start circuit energized, check for proper voltage at the starter Replace the starter if voltage is present
Start lock-in relay defective With start circuit energized, check for 28 volts DC across the relay coil Replace relay if voltage is not present
Starter drive shaft in component drive gearbox is sheared Listen for sounds of starter rotation during an attempted start. If the starter rotates but the engine does not, the drive shaft is sheared Replace the engine
Engine starts but does not accelerate to idle
Insufficient starter voltage Check starter terminal voltage Use larger capacity ground power unit or charge batteries
Engine fails to start when throttle is placed in idle
Defective ignition system Turn on system and listen for spark-igniter operation Clean or replace spark igniters, or replace exciters or leads to igniters

Figure 4. Starter generator starting system troubleshooting procedures