Bosch Compact Alternator

The Bosch compact alternator is becoming very popular with a number of European manufacturers and others. Figure bellow shows a cut-away picture of this machine. The key points are as follows:

  • 20–70% more power than conventional units.
  • 15–35% better power-to-weight ratio.
  • Maximum speed up to 20 000 rev/min.
  • Twin interior cooling fans.
  • Precision construction for reduced noise.
  • Versions available: 70, 90 and up to 170 A.

Bosch compact alternator Bosch compact alternator

The compact alternator follows the well-known claw pole design. Particular enhancements have been made to the magnetic circuit of the rotor and stator. This was achieved by means of modern ‘field calculation’ programmes. The optimization reduces the iron losses and hence increases efficiency.

A new monolithic circuit regulator is used that reduces the voltage drop across the main power transistor from 1.2V to 0.6V. This allows a greater field current to flow, which again will improve efficiency.

The top speed of an alternator is critical as it determines the pulley ratio between the engine and alternator. The main components affected by increased speed are the ball bearings and the slip rings. The bearings have been replaced with a type that uses a plastic cage instead of the conventional metal type. Higher melting point grease is also used. The slip rings are now mounted outside the two bearings and therefore the diameter is not restricted by the shaft size. Smaller diameter slip rings give a much lower peripheral velocity, and thus greater shaft speed can be tolerated.

Increased output results in increased temperature so a better cooling system was needed. The machine uses twin internal asymmetric fans, which pull air through central slots front and rear, and push it out radially through the drive and slip ring end brackets over the stator winding heads. High vibration is a problem with alternators as with all engine mounted components. Cars with fourvalve engines can produce very high levels of vibration. The alternator is designed to withstand up to 80 g. New designs are thus required for the mounting brackets.