ELECTRICAL STARTER MOTOR
This usually consists of a heavy duty, compound wound, DC motor, which draws its electrical supply from an external source. The motor works in conjunction with a starter control panel, the sequence of events during a start being precisely controlled. To allow the starter motor to overcome the initial inertia of the rotating assembly, the supply to the motor is via a series of resistors, this allows the motor to build up to full speed gradually, reducing the chance of failure within the drive system. The drive from the starter motor to the engine is through suitable reduction gearing and some form of clutch is fitted to disengage the drive when the engine is running.
The start master switch does not just switch the starting system ‘ON'. On some aircraft will prepare the aircraft electrical system for the start operation i.e. starter motors require a very high current for starting which is usually too much for a single Transformer rectifier (TRU), so it will parallel the DC systems. To ensure that a start is not carried out on a single TRU, it will place all the AC power systems onto one generator, so if it fails the start is aborted. It will also ensure that the engine gauging systems are all powered for the start in all conditions.