Gas Turbine Handbook : Principles and Practices

88 Gas Turbine Handbook: Principles and Practices
these accessory gearboxes are located under the gas generator, it is a
hostile (temperature) environment for an electric motor.
Once the gas generator has reached self-sustaining speed, the
motor is de-energized and mechanically disengaged through a clutch
mechanism. In some applications a clutch mechanism is not included
and the motor is simply de-energized. In applications where the
clutch mechanism is not provided, the gas generator must carry the
motor load throughout its operation. This imposes additional wear on
the motor.
Direct Current
Where AC power is not available, such as black start applications,
direct current (DC) motors may be used. The source of power
for the DC motor is a battery bank of sufficient capacity to carry the
cranking and starting loads of the gas generator. DC starting motors
are more commonly used with small gas turbines and aeroderivativetype
gas turbines (where stating torque is relatively low). As in the
AC starter motor application, the DC starter motor may be configured
with a clutch mechanism to dis-engage the motor from the gas generator.
Another approach is to convert the DC motor (electrically) into a
electric generator to charge the battery system. This is a convenient
arrangement where large battery packs are also used to provide direct
power for other systems (controls, motorized valve operators, etc.).
Battery powered DC motor starters are predominately used in small,
self-contained, gas turbines under 500 brake horsepower (BHP).
Electric motors require explosion proof housings and connectors
and must be rated for the area classification in which they are
installed. Typically this is Class I, Division 2, Group D. See Appendix
C-4 for more details on area classifications.
Pneumatics Motors
Pneumatic starter motors may be either impulse-turbine or vane
pump type. These motors utilize air or gas as the driving (motive)
force, and are coupled to the turbine accessory drive gear with an
overriding clutch. The overriding clutch mechanism disengages when
the drive torque reverses (that is when the gas turbine self-accelerates
faster than the starter) and the air supply is shutoff. The housing
of this type starter must be sufficiently robust to sustain the high
gas generator speeds in the event the mechanism fails to disengage.