Motoring Notes

Hutt News, Volume 11, Issue 10, 11 August 1937, Page 5

Motoring Notes

B—.8 —.

(By E. C. MeKinnon, M.1.E.U.. Chief Engineer to Exide Batteries.) Periodically one conies across a less ifottunate inem'ber of tho motoring fraternity who has had to park his car because of burnt-out bulbs. It would be quite erroneous to assume that this is necessarily caused by a defective battery, because this latter, often maligned, accessory is rarely the root of the trouble. The immediate cause of the lamp bulbs burning out must, obviously be that an excessive voltage has accidentally been applied to them. Fortunately the ris<k of, this happening has been much reduced in recent years ,owing to the widespread adoption by car manufacturers of the so-called "controlled voltage" tyipe of generator. In -this type of machine the voltage is controlled by means of a regulator, quite independently of the speed of the car and of the current load on the generator. In no circumstances can any excessive voltage rise take plaice, provided that the regulator is working correctly. Should the lanjp bulbs burn out on a car fitted' with this type of equipment, it may be assumed with confidence that "the generator and not the battery is at fault. In the ordinary type of car dynamo, however, voltage control is secured by third brush excitation in conjunction with the battery coupled across the generator terminals. The speed of an automobile varies so much and so continuously in practice that with this type of machine voltag# control would be impossible without the buffering effect whiii'h the battery provides. The voltage of a third brush generator shows a tendency to rise when the current load upon it is reduced. If the current load' suddenly fell to zero the voltage of the generator would build upat once towards open circuit value and the lamps would be burnt out.

A typical electric lighting icii'cuit, using a third brush type of generator has the battery coupled across the terminals of a generator in parallel with the lamps. So long as this arrangement is preserved, the 'battery acts as an efficient buffer and the generator voltage is held down to limits within which the lamps are protected. If, however, a definite break, or even an intermittent break, occurs at any point in the circuit between the generator and the battery (either in the wiring or in the battery itself), then the condition arises which leads to burnt-out bulbs. Thus, if a fuse or some special so-called master switch be inserted: between the generator and the battery, this lessens the stability of the system and increases the risk of damaged lamps. Even when, in the case of, say, a .12volt (G-ccll) battery the plates of one cell are touching, causing the cell to be shorted, the cell simply acts as so much resistance inserted in series with the other five cells. The voltage of the remaining five will then suffice to control the generator voltage. Under these circumstances, if we assume the battery to be charging, the voltage across the generator terminals may be anything between say, 1U to 13A- volts. From this we can conclude that so long as a voltage is maintained, even within wide limits, by a battery coupled across the - terminals of a third brush dynamo feeding the lamps on a motor car, the risk of burning out the lamp bul'bs is non-existent. The following suggestions may help to trace the .cause for bulbs blowing on cars fitted with third brush equipment. On cars fitted with controlled voltage equipment, it may be assumed that the generator is at fault and the best plan will then lie to disconnect the generator from the 'battery, replace the lamp bulb and carry on with the lights supplied direct from the .battery until the generator can be repaired. Obviously the battery will only carry on by itself for a limited period 'before 'becoming run down, so the lights would have to be used sparingly in the meantime and the generator attended to at the earliest possible moment. After replacing the bulbs, switch the lights "on" from .the battery. The generator should not be run until the cause for the la imp blowing is traced. If the lamps do not light there is a break in the circuit. Look for a 'burntout fuse or broken lead or switch not making contact. I,t' the lamps bum dimly, seek out a bad contact at a switch or in the battery (see below). If the lamps burn brightly as soon as they are replaced without having done anything else, do not be content that all is well. There may be an intermittent niake-and-break. Try and reproduce this liy vibration, but still without running the engine. Work gradually 'back to the battery and check the following points:— "Make sure that the bolted connection on the 'battery terminal posts arc making good contact and are properly tightened up. Even if they are, a hartl insulating scale sometimes forms on the positive terminal post, and electrical conductivity can only be regained by scraping off this liai'il coat. There remains only the possibility of an internal break in the battery, either in the inter-cell or in a terminal post, or in one of the joints between an iutercell connector or in a terminal post. A fault of 'this kind could be detected by means of a cell testing voltmeter, taking care not to apply so much pressure with the voltmeter spike that sufficient contact is 'temporarily made to obtain a normal voltage reading. To rectify some of these possible defects in a battery is, of course, outside the scope of the ordinary motorist's repair equipment. The local Exide service station is Brunette and Co., Queen's Eoatl.

Small Boy (who lias been Kcoldci by his mother): I say, dad, you and me would have had a fine time doing as we ked if you hadn't married mother! .