road and workshop
Page 58
If you've noticed an error in this article please click here to report it so we can fix it.
Auto-electrics for the mechanic (8)
FURTHER on the subject of the two-brush dynamo and its external constant voltageregulator, there is another requirement in connection with the needs of the battery, as the constant voltage regulator on its own does not cope too well with the wide variations in the terminal voltage of the battery between a fully charged, and a flat or fully discharged state. Therefore we find a more sophisticated regulator in use, although the earlier type can still be seen.
The later and more efficient unit is known as the compensated voltage regulator, and improved control is effected by adding a number of turns of wire on the electromagnet to carry the battery charging and lead current. These extra turns are wound in such a way as to assist the magnetizing force of the solenoid in opening the contacts. The solenoid then regulates the voltage in proportion to the total amount of current; the advantage here is that since voltage regulation is a function of both the battery voltage and output from the dynamo, there is never enough difference between the battery and dynamo voltage to produce the excessive dynamo voltages which could cause damage. To compensate for both the battery voltages and the additional lighting load when it is switched on, Lucas splits the extra turns on its compensated voltage regulators into two parts.
Changes in temperature
The compensated voltage regulator also caters for another problem, ie changes in temperature, which was something of a problem with the earlier type of regulator. Therefore the mechanic may find what appears at first glance to be an extra-heavy spring with adjusting screw; this is in fact the normal steel tension spring, but fitted behind it is a bi-metal strip. This strip is designed to bend as the temperature rises, and will weaken the effect of the steel armature tension spring, permitting a higher voltage when cold, but cutting back to normal as the regulator comes up to its normal temperature. The advantage here is that a much higher charge rate can be arranged after a cold start.
Usually in the same box and mounted alongside the regulator will be the cut-out, the two units looking rather alike. The duty of the cut-out is to guard against battery current flowing back through the dynamo, when dynamo voltage falls below battery voltage. Again an electro-magnet or solenoid is used, and the winding of the electro-magnet is connected across the dynamo. The contacts operated by this electro-magnet are connected in the series circuit between the battery and dynamo.
With the engine stopped, the tension spring holds the contacts apart and the battery-dynamo circuit is open. As the engine is brought above idling speed, the dynamo begins to energize the cut-out shunt coil enough to attract the electro-magnet, and the series contacts close, remaining closed as long as dynamo voltage exceeds battery voltage. The reason why the cut-out appears to open rather smartly is because the current flowing through the series winding in the charging direction helps the shunt coil to hold the contacts closed, but as the charging current falls the battery current begins to flow in reverse. This cancels out the magnet's effect on the shunt coil, thus the contact spring can open the points quickly and cleanly.
With the recent addition of more lamps and other electrical devices, it has been found that the single compensated voltage regulator had limitations in coping with the changes and the additional load of lamps, signals, gear change, solenoids, etc, and a double unit current voltage regulator will be found on many vehicles. One unit is operated by dynamo voltage and the other by current output, the voltage regulator taking over as the battery is brought up to a. fully charged state. Units of this type are made by Lucas, Bosch and Delco-Remy.
Trouble-shooting
Now that I have given a somewhat broad outline of the dynamo, battery and regulator, it will be as well to look at certain aspects of trouble-shooting, assuming that the battery is in good order.
Whether the vehicle is petrol or diesel there is usually a warning lamp fitted in the dynamo circuit — in the case of petrol engines this is through the ignition switch, and with oil engines a separate switch. Both systems have the lamp wired in parallel with the cut-out contacts, and as the cut-out points close when there is sufficient dynamo output the lamp circuit is shorted out.
Reasons for the warning lamp staying on at a steady brillince can be due to the dynamo voltage being insufficient and hence the battery not being charged for one of several reasons. First and foremost is a broken or slipping belt. The latter can result from a leak of fuel oil on a diesel; on heavy engines with twin belts, one belt on the weak side can cause the other to slip also both belts should have matched tension.
Secondly, the problem could be due to an internal dynamo fault. Thirdly, one of the dynamo connections could be shorting to earth, or fourthly, a simple short to earth of the warning lamp ciruit could cause false operation.
A dimming red warning lamp will indicate faults either on dynamo or regulator, and could mean a broken lead on the dynamo field to control box circuit.