14.13
New Developments in Charging Systems
14.13.1.
Latest Developments
Alternators are being manufactured capable of producing higher outputs to meet the constantly increasing demands placed on them. The main problem is that of producing high output at lower engine speeds. A solution to this is achieved by adapting a variable drive ratio, but this gives rise to mechanical problems. The current solution is the development of alternators capable of running at much higher maximum speeds so that a greater drive ratio and hence greater speed at lower engine speed can be attained. Radical change in the main design of alternators has not taken place, but through incremental improvements far more efficient machines have been produced.
14.13.2.
Bosch Compact Alternator
The Bosch compact alternator is shown in a cut-way section in Fig. 14.5. The following points are claimed by Bosch for the alternator.
(i) 20-70% more power output than conventional units.
(ii) 15-35% better power output to weight ratio.
(Hi) Maximum running speed up to 20,000 rpm.
(iv) Twin interior cooling fans included.
(v) Precision construction for reduced noise.
(vi) Available in 70 A, 90 A and up to 170 A capacitors.
The compact alternator is based on the claw pole design, with particular enhancements to the magnetic circuit of the rotor and stator. The modern field calculation programmes were used and the iron losses are reduced through optimisation so that the efficiency is increased. A new monolithic circuit regulator is used to reduce the voltage drop across the main power transistor from 1.2 V to 0.6 V. This permitted the flow of a greater field current, which again improves 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 plastic cage type instead of the conventional metal type. Higher melting point grease is also used. The slip rings are mounted outside the two bearings so that 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 causes an increase in temperature, so a better cooling system is necessary. The alternator 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, similar to all engine mounted components. Cars with four valve engines can produce very high levels of vibration. The alternator is designed to withstand vibration up to 80 g. This is achieved through new designs for the mounting brackets.
Although the compact alternator meets the demands of the vehicles at present, but efforts are continuing to further improve the design.
