road and workshop
Page 42
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by Handyman
Auto-electrics for the mechanic (6)
WHERE a 24V system has four 6V batteries coupled in series, and interconnected by bridging pieces — ie lengths of starter cable and the appropriate terminals — these short bridge pieces come in for some rough handling in removal and refitting. So it is to be expected that repairs or replacements will be needed from time to time, and this is where I have seen mistakes made by mechanics faced with a rotted or failed bridge piece.
All too often, they have had little enough time to spare and have seized upon the handiest short length of battery cable to make the new bridge, but without taking any special care to establish that it is heavy enough for the starting currents; in other words, no check was made to see that the cable had the number of strands and thickness of section required. Consequently, while the lighting section worked as it should, the thinner cable proved unequal to the starting load, could suffer overheating during a cold start or fail at some inopportune moment on the road.
Earth-return circuits A similar trouble can arise with earth-return circuits, where the battery earth is taken to the chassis. This may be a section of starter cable and spade terminal, or it Can be a braided strap, designed for the purpose. As a rule this strap is heavily tinned against corrosion, and is very flexible, but as the strap ages and stiffens with acid corrosion, it can begin to break up strand by strand, ending up one day unable to handle the starter current.
Often missed also, is the other end of the lead where bolted to earth on the chassis frame. This connection is not always in view; it may be hidden below the batteries, and can become loose or heavily rusted, as it is invariably a steel bolt and nut.
It is wise to bring arty partly concealed connections like this up into full view, but make sure that the bolt used is in the chassis proper or fastened to an equally good point for earth, as bolting to light frames or the battery carrier sheet metal casing does not always provide The best earth.
A further cause of starting trouble can arise from a poor link or connection between engine and chassis to complete the battery starter circuit. Earth straps or leads are usually fitted between engine and chassis to ensure a good circuit, as it is not enough to rely upon the rods and wires, etc, that often provide the only other connection between engine and chassis. Sometimes during engine change or repair, these earths are removed and not refitted, yet this is a most important part of an earth-return system. I knew a driver who regularly wedged his steel jack-bar between chassis and engine mounting before pressing the starter. He said it was the only way to guarantee a start from cold, and he was quite right, simply because the linking earth strap was rotted away.
Next in line with the battery we should look at the first source of electrical supply, the dynamo or generator, and here there have been some changes over the years. Up to about the mid-thirties the standard vehicle dynamo was equipped with three brushes, and at that time was accepted as self-regulating. However, it had limitations and was duly replaced by what we know as the plain shunt wound dc generator. This unit is controlled by an external regulator and has served well, only now being replaced by an ac alternator for both psv and goods use. The ac alternator must, of course, be provided with a means of rectifying the current to dc before it can be used to charge the battery.
However, it is necessary at this time and probably for some time ahead, to understand how the two-brush dc dynamo can start a flow of current, and for this we should look at the simplest form of electrical generator. In order to start a flow of electricity, we must in the first instance ha a small, established magnetic field, and tl can be arranged in the form of the poles o permanent magnet, or the small remaini field between north and south poles of t field of a dynamo. When a conductor moN through the field, a voltage is induced whi depends upon the length of the conduct+ its speed and the strength of the field. II loop of wire is revolved between the tl poles about an axis at right angles to I field between the two poles, we can doul the voltage obtainable from only one wire.
By revolving the loop within the field at of the poles, we can indicate ciirrent flom a circuit is arranged down one leg of 1 loop through a sensitive instrument a back to the other leg. By making ma turns so that the loop becomes a coil obtain a much higher voltage. Whether have a dynamo (dc) or alternator (. depends upon whether we connect the et of the coil to two separate slip rings so tl each collector is permanently connected one coil end, or whether we connect the c ends to a two-segment commutator a collect via brushes so that each brush permanently connected to a coil side of polarity. With the separate slip rings collect alternating current, and with two-part commutator we will have dir current. How the two-brush shunt dyna differs from the simple magnet exercise, be dealt with next week.