Figure 6.35 Extract from informatio

Figure 6.35 Extract from information supplied by Lucas Automotive Ltd. relating to the Plus Pac alternator
The power curve is used to calculate the type of from information regarding the mounting and drive
drive belt needed to transmit the power or torque to belt fitting for a typical alternator.
the alternator. As an aside, the power curve and the The drive ratio between the crank pulley and alter-
current curve can be used together to calculate the nator pulley is very important. A typical ratio is about
efficiency of the alternator. At any particular speed 2.5 : 1. In simple terms, the alternator should be driven
when producing maximum output for that speed, the as fast as possible at idle speed, but must not exceed
efficiency of any machine is calculated from: the maximum rated speed of the alternator at max-
imum engine speed. The ideal ratio can therefore be
Efficiency  Power out/Power in
calculated as follows:
In this case, the efficiency at 8000 rev/min is
Maximum ratio 5 max alternator speed/
(Power out  14 V  70 A  980 W max engine speed.
980 W/2300 W  0.43 or about 43% For example:
Efficiency at 2/3 rated output 15 000 rev/min / 6000 rev/min  2.5 : 1.
(Power out  14 V  47 A  653 W) During the design stage the alternator will often have
653/1100  0.59 or about 59% to be placed in a position determined by the space
available in the engine compartment. However, where
These figures help to illustrate how much power is
possible the following points should be considered:
lost in the generation process. The inefficiency is
mainly due to iron losses, copper losses, windage  Adequate cooling.
(air friction) and mechanical friction. The energy is  Suitable protection from contamination.
lost as heat.  Access for adjustment and servicing.
 Minimal vibration if possible.
6.6.4 Mechanical and external  Recommended belt tension.
considerations
6.7 New developments in
Most light vehicle alternators are mounted in similar
ways. This usually involves a pivoted mounting on charging systems
the side of the engine with an adjuster on the top or
bottom to set drive belt tension. It is now common
6.7.1 General developments
practice to use ‘multi-V’ belts driving directly from
the engine crankshaft pulley. This type of belt will Alternators are being produced capable of ever
transmit greater torque and can be worked on smaller greater outputs in order to supply the constantly
diameter pulleys or with tighter corners than the increasing demands placed on them by manufactur-
more traditional ‘V’ belt. Figure 6.35 is an extract ers. The main problem to solve is that of producing