MASCOT September - October 2004

How does it work? No 10 - The Charging System - Part 2a - The Charging Regulator - Current regulation

from the Tool Chest

In Part 1 we saw that the generator output voltage can reach a very high value as the speed increases, therefore some form of control is necessary to make it suitable for charging the battery.

Two main systems are used - "Current Regulation" and "Voltage Regulation", the former on most 1930s cars and the latter increasingly on post-war vehicles up to the introduction of the alternator, in which control is done electronically. From the late 1950s a combined voltage and current regulator was used on most cars. This overcame the disadvantages inherent in controlling only the current or only the voltage, both of which usually resulted in overcharging of the battery, thus greatly increasing battery life and reducing the propensity to acid corrosion at the battery terminals.

Current Regulation. This is achieved by the use of the Third Brush Dynamo, see Fig 1. Regulation is based on distortion of the magnetic field created by the field coils, by the magnetic field that builds up in the armature as the dynamo speed increases, a process known as the armature reaction. The third (narrow) brush is mounted on the left of the main negative brush and is connected to the field coils, becoming the negative terminal of the field system. The other end of the field is connected to the dynamo positive terminal through a field fuse, thus tapping off the dynamo output to supply current for the field coils.

At low speeds the armature current will be small and the magnetic lines of force (flux) will take an almost direct path from the north field pole to the south field pole - see Fig 2a. The current carrying conductors in the armature under this condition will be mainly those between the positive dynamo brush and the third brush. As the dynamo speed increases, the field flux will be distorted in the direction of rotation, the effect being to weaken the flux between the positive and third brushes - see Fig 2b. This reduces the voltage tapped off for the field coils, with a corresponding fall in the dynamo output voltage.

Thus the increase in voltage normally associated with increased armature speed is offset by the weakening of the field, which reduces the voltage and hence the charging current. At low speeds the effect of the armature reaction is less than at increased speeds, so that when the car is first started the charging rate will increase with speed until a certain rpm has been reached. The rate then peaks until the point when the armature reaction is greater than the effect of increasing speed, with the result that further increases in speed result in a decrease in the charging rate -see fig 3.

The peak value of the charging current can be varied by moving the third brush. You can see from Fig 1 that the closer the third brush is moved to the dynamo negative brush, the more armature coils will be tapped into for the field current and vice versa, so it is moved in the direction of rotation to increase the current and against the direction of rotation to decrease it.
Further control of the charging rate can be achieved mechanically by inserting a series resistance into or out of the field circuit. This usually takes two forms manual or automatic.

Manual 1 - by a Summer/Winter (or Low/High) Charging Switch, where an external resistance is connected in series with the field circuit to reduce the charge rate for summer use, and vice-versa.

Manual 2 - via the lighting switch, when resistance(s) are shorted out of the field circuit when the lights are switched on, thereby increasing the charge rate to meet the additional load - see the wiring diagram at Fig 4.

Automatic - by a Thermostat, comprising a resistance in series with the field circuit and a set of contacts operated by a bi-metal strip - see Fig 5. This is mounted inside the dynamo so that it is affected by the internal temperature. When the dynamo is cold the contacts are closed, which shorts out the resistance, giving a higher charging rate than when hot and the contacts open, which brings the resistance into operation and reduces the charging rate. With this type of control the contacts should be cleaned occasionally, as if they become dirty or pitted, the charging rate will be reduced.

In Part 2b we will look at methods of Voltage Regulation.