Why is the shunt field of a DC starter-generator disconnected when used as a starter?

Question

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.

Answer 1

There is probably a second field winding in series with the armature.

In starter mode with the shunt field disconnected it acts as a series would motor providing high starting torque. This is a the most common arrangement for automotive starters.

In generator mode, power is supplied to the shunt field so the it acts as a compound wound generator. The shunt field is arranged to be dominant so that it acts similarly to most DC generators.

Answer 2

The T53 turbo shaft engine like most others except turboshaft engines that utilize a DC powered shunt start generator utilizes a compound wound combination series/shunt wound starter generator. In starting mode battery or APU power is routed through the series field (in series with the armature) to create the high torque necessary to turn the N1 gearbox/compressor. Also during the starting process the shunt field of the starter generator is disconnected inside the voltage regulator so it doesn’t act as a brake while utilizing the series field for starting. Once the engine reaches self sustaining rpm battery/APU current to the series field is disconnected by de-energizing the start relay(s) by turning the start switch off or letting the switch go. Now the starter generator can be utilized as a generator driven by the engine N1 gear train by exiciting the shunt field the output being controlled by a voltage regulator and a reverse current relay RCR. Once the voltage is of sufficient value the voltage coil in the RCR closes the contact which now puts the generator current in series with the RCR current coil and in series with the aircraft electrical load helping the voltage coil to keep the RCR main contact closed and the current goes off and feeds the aircraft electrical system busses.