The Lycoming T53 turboshaft engine is started using a starter generator. The starter generator is a shunt generator. When the T53 is started, the starter generator is used as a DC motor to get the engine turning. During this process, the manual for the engine specifies that the shunt field is disconnected from the armature.
Why does this need to be done? Why can't current simply be fed to the armature and field in parallel to get the motor turning? I understand that a shunt generator with insufficient residual magnetism in the stator might need to be started with the shunt field disconnected, but it seems to me that this problem doesn't apply when starting the shunt generator as a motor.
When the starter generator is put back into generator mode, the field is reconnected to the armature. The best reason for this that I can come up with is that the starter generator is wound so that it can be operated as a series wound motor during the start to take advantage of high starting torque, before operating as a shunt generator in which no external power is required to maintain field excitation, which is a nice feature to have for an aircraft engine.