Motor Notes

Wanganui Chronicle, Volume LXXXII, Issue 19346, 27 June 1925, Page 9

Motor Notes

THE CAR’S GENERATOR AND STARTER.

KEEPING THEM IN TUNE. HOW TO AVOID SERIOUS TROUBLE The construction of a car lighting dynamo is of so simple a nature and at the same time so substantial and durable that it should, if anything, require less attention to maintain than a magneto, which is comparatively a much more complicated arrangement of mechanical and electricdl parts. The armature of a dynamo is a completely self-contained unit. The commutator, given proper working conditions, is not subject to appreciable wear; the slight amount of possible wear, in fact, is, or should be, taken by the’carbon brushes which press lightly and evenly on the commutator. Assuming the latter to be perfectly true and smooth, and the spring pressure on the brushes correct, the maximum amount of wear of the carbon brushes should not exceed 3-32nds of an inch in 5000 miles’ running. If it be found that the wear is greater than this, it would be well to investigate the cause. Attending to Commutator and Brushes. The commutator may not be so smooth as it should be, or the brushes may have too much spring pressure behind them and be pressing too strongly on the commutator. At the same time, it must be remembered that insufficient pressure between brushes and commutator may cause trouble. There should be no sparking visible between the brushes and commutator when the dynamo is generating its full output of current. Sparking creates just the trouble it is important to avoid; as the sparks pit or corrode the surface of the commutator all over, as is readily seen by examining it with a magnifying glass. One simple way of testing whe-

ther the contact between the brushes and commutator is sufficient is to note whether the output of current shown on the armature increases appreciably when the brushes are pressed by the fingers rather more firmly against tho commutator than the springs press them. Examine for Carbon Dust. Where there is a minimum wear of the brushes there will be practically no carbon dust formed. The presence of this fine dust is a potential source of trouble, as, by collecting near the terminals, it tends to cause leakage ot current, if not causing an actual short circuit. To ensure efficient working of the dynamo, the armature should at all times be clean, and it is a good plan lightly to brush round about the armature connections —that is, where the wires are soldered to the bars or segments—and also brush round about the field oils and terminals, blowing away any remaining dust by means of a tyre pump, certain amount of iron dust or fillings will find its way inside the machine, attracted therein by the powerful magnetic field. It is preferable to brush this dust away rather than let it accumulate. Loose or Broken Connections. The security of the connections inside the dynamo is a matter of great importance., as cithel a broken or loose connection will prevent the dynamo working properly. As a rule, manufacturers give special attention to the matter of providing reliable terminals and connections, and it is unusual for trouble to arise. A loose connection, however, may be suspected if the ammeter indications are erratic. Should the dynamo not generate any current (or only a small current at low speeds, which falls away to zero at higher speeds), probably the shunt circuit is broken. Usuallv a fuse in the switch-

box protects the shunt field coils from burning out in the event of the machine being run with tho battery disconnected; therefore, the conditions of the shunt fuse should be the first detail investigated. If it proves to be intact, the trouble is likely to be caused by a broken or disconnected shunt wire in the dynamo. Care of the Belt Drive. A dynamo may not work properly for reasons which cannot be attributed to the machine itself. There must necessarily be an efficient drive to the 1 armature, such as a mechanical direct drive by gearing or coupling. More usually a belt drive is used. The belt must have sufficient grip to enable the dynamo to generate full output. Although the armature runs on ball bearings and there is very little friction when current is not being generated, there is great resistance offered to its rotation but demand becomes greater. This resistance is due to the magnetic drag between the armature and field magnets, and increases greatly as the output demand becomes greater. This fact should always be grasped in tho case of a belt drive, as the tension may only be effective up to a certain point until it just balances the resistance; then it slips. At the same time, it is just as unadvisable, for mechanical reasons, to run the belt tighter than necessary. Only experience can teach the degree of tension to which to adjust the belt. Avoid Over-Oiling. Ball bearings do not run any easier for being oiled, but, as it is important that the bearings should not become rusty or pitted, as they would be by being run dry, it is necessary to give them a very few drops of oil from Lime to time, such us at intervals of 100 G miles running. The idea that a ball bearing is like a plain bearing in the matter of lubrication, and, therefore, requires frequent and is quite erroneous, and will lead to trouble. It is often found, on taking a defective dynamo apart, that oil has leaked into the interior, and, by collecting carbon dust and filings has caused leakage of current and injury to the windings. Oil also works under terminals and contact surfaces, causing bad connections. Oil getting on the commutator will soon cause it to get into bad condition. Improving a Rough Commutator. As a rule it should suffice merely to clean the commutator by holding a piece of soft rag, very slightly moist ened with kerosene, against it whilst running, moving the rag with the first finger from end to end of the commutator. Owing to tho small amount of carbon picked up by the rag acting as a fine abrasive, a clean glossy surface should result, which is all that is required. It may, however, be tho ease that the commutator is rough from the causes already described. First, it should be stated that the commutator may be in such a condition that it would have to be touched up in the lathe to get the surface right, but if the condition be not so bad, it can easily be got right by a glass-paper “lap,” consisting of a small block of wood as wide as the commutator accurately trimmed out to the correct curvature, within which recess is placed a piece of the finest glasspaper. The lap is held against the rotating commutator until a good surface is obtained, although it must not be expected that it will remove deep pitting caused by sparking. Care of a Starter. A dynamo and a starter are practically the same in construction. The only electrical difference is that the field magnet coils and armature coils of a starter are, so to speak, much more “massive,” as they have to carry, momentarily, several hundred amperes, instead of wire being used, the coils are made of copper tape for this reason. There is no control of third brush on a starter, and, electrically, it is a rather simpler machine than .a dynamo; it has also an important advantage over a dynamo, in so much that it only has to run for a few seconds at a time, and is practically always out of action.

There is no wear or tear, and the commutator and brushes should last indefinitely. It is usual on many starters to fit oil-less brushings for the armature shaft, so that even the question of lubrication has not to be considered. Of course, during the few seconds the starter is in action it is subjected to a most severe mechanical strain, and its reliability depends on ample strength being provided in the design. The Bendix drive is the part that requires a little attention occasionally to ensure that the pinion does not “stick” instead of freely moving along the screw shaft. The presence of grit or gummy oil is likely to cause “sticking” of the pinion. As a matter of fact, it is bettor not to use oil at all on the Bondi x assembly, but to give it an occasional wipe over with a rag or brush moistened with kerosene.

The advice with regard to keeping the interior of a dynamo clean applies to a starter, and it should be seen that the brushes always press firmly on the commutator, as, if there be not the smallest possible electrical resistance here, the starter will not be able to develop its full power. The starter switch should, similarly, always be in good order, so as to offer very little resistance, and the same applies to the cable connection to the starter.

The starter torque depends on a maximum rush of current, and this is, in turn, dependent on the electrical resistance of the whole circuit being of the lowest possible value.