CAR ALTERNATOR DEVELOPMENT 1964

To meet the requirements of increasing electrical loading on the modern car, and to cater for city traffic density conditions by providing useful output even when the engine is idling, recent generating system developments have been directed towards the multi-pole three-phase alternator. This can be designed to meet both these requirements and at the same time be reduced In size and weight by comparison with the more conventional dynamo widely employed up to the present time. Hitherto, however, the problem of output rectification to direct current for battery charging had prevented much progress being made in this direction, so far as the private car was concerned; although alternators had in fact been used on certain passenger service vehicles for several years, in conjunction with copper-oxide or selenium rectifiers. These, while being devices of a semiconductor nature, are necessarily large and heavy, with limitations in operating temperature, and are subject to changing characteristics with age. Consequently, while such rectifiers could usually be accommodated and fairly adequately cooled on passenger service vehicles, their use on the private car was quite impracticable. Wlth the advent of silicon diodes, this state of affairs changes completely. By virtue of being so small and light in weight, six diodes can readily be accommodated in the end cover of the alternator to give built-in full-wave rectification of the three-phase output. Moreover, they can be cooled by the ventilatlng air stream provided for the alternator. Thus it is due to the development of the silicon diode that it has become possible to consider the use of an alternator on the private car as a practical proposition, and future generating system developments will undoubtedly be based on this type of machine.

ALTERNATOR OUTPUT CONTROLS

The only form of control of alternator output required is one which will maintain the terminal voltage at a substantially constant predetermined value, that is, a voltage regulator. Hitherto, a vibrating-contact electromagnetic device connected in the field circuit had been employed for this purpose, while a later development was the use of a transistor to interrupt the field current, the vibrating contacts being connected in the base circuit, so breaking only a small current (Fig. 2)

In each of these instances, the voltage reference ls provided by an armature tensioning spring. A further development takes advantage of the availability of Lucas silicon semiconductor devices to provide an electronic control in which all moving partsare eliminated; thus the control has increased reliability, since there are no moving parts to wear or to require adjustment, and this results also in greater stability in output control. In addition, the electronic control unit is reduced in size and weight.

The circuit of this control is shown in Fig. 3, and it will be seen that it contains a Zener diode and two transistors TR1 and TR2. In effect, the action is similar to that of the electromagnetic regulator in that the current in the alternator field winding is varied to maintain the generated output voltage within close limits, but switching is achieved by the transistors instead of vibrating contacts, while a Zener diode and potentiometer provide the voltage reference in place of the voltage coil and tension spring system. It is not proposed in this article to give a detailed description of how this control operates but, briefly, at rest or very low speeds the held circuit is completed through TR2 which is held conducting by virtue of the connection through R1. As the alternator rotor is driven at increasing speed by the engine, the rising voltage generated in the stator winding is applied to the potential divider consisting of R3, R2 and R4, and according to the position of the tapping point on R2, a proportion of this potential is applied to the Zener diode. When the value of this potential reaches the Zener diode breakdown voltage (corresponding to a known output terminal voltage) the diode conducts, and current flows in the base circuit of TRl . The latter becomes conducting, lowering the current in the base circuit of TR2 and, as a result, so also the alternator field excitation. Consequently, the alternator output voltage will tend to fall, and this in turn will reduce thebase current in TRI, allowing increased field current to flow in TR2. By this means, the field current is continuously varied to keep the output voltage substantially constant at the value determined by the setting of R2. Basically, this is the principle of operation of the Lucas Model 4TR Control, but there are certain desirable additions which are beyond the scope of this explanation.

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