STARTER GENERATORS

Some helicopters are designed to take advantage of the fact that the construction of a DC motor and generator are the same by using a single machine to function in both roles.

Initially, the machine works as an engine starter motor. Once the engine is running, the machine is selected to operate as a generator. Using the same component for both tasks reduces weight.

Starter generators are only practical on relatively small engines, as the energy required to start an engine is related to its size. Small turbine engines may be able to use the battery for a very limited number of start attempts. However, large turbine engines require a greater power source, normally a pneumatic starter powered from the APU or an external air trolley.

A typical starter generator is described: A motor converts electrical energy to mechanical energy while generator converts mechanical energy into energy. This is because when current flows through the coil it induces a magnetic field and when its poles are identical to the permanent magnet at that instant of time which is placed inside the motor, the armature rotates.

This law is invertible. i.e. when a coil is rotated between two permanent magnets of opposite polarity along the axis which is perpendicular to the magnetic lines of force, current is induced in the coil due to the change in magnetic flux associated with the coil.

The starter generator of an aircraft first starts the engine, functioning as a motor. After the engine has been started, it turns into a generator.