MASCOT November - December 2004

How Does It Work? No 10 - The Charging System Part 2b - The Charging Regulator - Voltage Regulation

from the Tool Chest

In Part 2a we looked at Current Regulation, which gives a constant current at a given speed regardless of the state of charge of the battery. This, of course, often results in over-charging, which can reduce the life of the battery. It also causes acid corrosion of the battery terminals and other surrounding items, due to the gases given off. Ideally when the battery is low it needs a high charge rate, reducing as the battery voltage builds up, and this is achieved by regulating the voltage output from the generator. So how does it work?

The regulator works by governing the amount of current fed back to the field coils - you can see how it functions from the basic layout in Fig 1. A lead from the field connects to the main output lead of the regulator through a pair of contacts and a resistance. Opening of the contacts is controlled by the dynamo output - when this is low the contacts are closed and the field receives a large current, giving a high charge rate, but when the output reaches a pre-determined level, the contacts are opened and the connection is via the resistance, thus lowering the charge rate. This causes the contacts to close again, so they vibrate, the rate of vibration governing the current applied to the field circuit.

An early regulator is illustrated at Fig 2. A "shunt" winding of many coils of fine wire is mounted on the "L" shaped regulator frame and connected across, or in parallel with, the generator output terminals. One of the pair of contacts is fixed, and is mounted on a strip secured to the frame. It is insulated from the frame and connected to the field coil. The moving contact is mounted on the regulator armature and is held against the fixed contact by a leaf spring. It is connected via the armature to the dynamo output terminal and the battery. When the generator is working, a current flows through the shunt winding, the core of which becomes an electro-magnet. The strength of the magnet is determined by the generator output voltage. As the output rises, a point is reached where the magnet is strong enough to overcome the pressure of the leaf spring and the armature is pulled down, opening the contacts which then vibrate, thus controlling the output voltage as in Fig 1. By adjusting the strength of the leaf spring the output voltage can be set at the desired level. This is nominally 16 volts for a 12 volt system. For cars operating in cold conditions this can be increased to between 16.1 and 16.7 volts, and for hot climates reduced to between 15.3 and 15.9 volts. This type of regulator, whilst limiting the voltage, does not limit the current, so if there is a short circuit, or if the battery is faulty or fully discharged, the generator can be overloaded. To reduce this risk, a winding of a few turns of thick wire wound around the core was connected in series with the generator and battery - see Fig 3. A large current flowing through this winding, opens the contacts earlier, thus reducing the output and preventing an overload condition.

To further reduce the potential for overloading the generator, the Voltage-Current Regulator was introduced - see Fig. 4. This has two regulators mounted side by side, the voltage regulator with its shunt wound coil, and the current regulator with a series wound coil. The two sets of contacts are connected in series with the field circuit so that if either contact opens, the field current, and hence the dynamo output, are reduced. The current regulator contacts are normally set to limit the current to around 25 amps. This is sufficient to recharge a low battery in a reasonably short time whilst minimising the problems of excessive gassing and subsequent corrosion in the battery area, particularly around the battery terminals. Some corrosion may still occur, and the best way to prevent this is to keep the terminals clean and well greased. Badly corroded cable terminals are best cleaned by holding them over a suitable receptacle and gently pouring boiling water on them. Thoroughly grease them immediately afterwards. The battery posts will probably also have a layer of crusty corrosion and should be scraped dean before coating with grease and refitting the cables. The regulator(s) and cut-out are usually mounted together to form a Control Box. A typical Voltage Control Box is at Fig 5 and a Voltage-Current Control Box at Fig 6.