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to give the same speed with less current, the advantage is obvious. If, on the other hand, it is geared to take the same current and give a higher speed, it is likewise obvious that the number of turns required to start will certainly not be more and probably will be far less. Therefore, the time required to start will be in versely proportional to the speed or less, and the drain on the battery in ampere-hours will be reduced in the same proportion. It is hoped that the above will make clear that the matter of efficiency is not a minor question, but is vital to the success of the whole matter of electric lighting and starting. We shall have occasion to discuss the same question in connection with lighting generators.
To sum up the question of single versus two-unit sets, the writer (Prof. Bailey) from his experience, would rate them as follows:
Operation. The single-unit set as ordinarily arranged has a slight advantage.
Weight.-For the same cranking speed and the same generator output the two-unit outfit is probably about 20 per cent. lighter. This might not hold for low speed cranking.
Efficiency. The writer is convinced that both as a generator and as a motor the single-unit outfit is at a disadvantage. The efficiency as a generator will average perhaps 5 per cent. lower and as a motor about 2 or 3 per cent. lower. If, as is ordinarily the case, the single-unit set is operated with a double or triple reduction gearing, the efficiency of the gearing will be between 60 and 75 per cent. If the two-unit set is used it is practicable to use a single reduction and obtain the higher efficiency of 90
Generators and Starting Motors.-Essentially there is not much difference in construction between a starting motor and a generator as the principles upon which they operate are practically the same. A machine that is capable of delivering current in one direction when driven by mechanical power will produce mechanical energy if electrical current is passed through the winding in a reverse direction. The construction of typical starting motors and generators may be readily understood if one refers to the illustrations at Fig. 151. That at A is one form of the Gray & Davis governed dynamo, which is of the limited armature speed type. The power is directed to the driving member of a friction clutch which turns the generator armature by means of friction contact with a disc attached to but slidably mounted on the armature shaft. This plate is held in contact by a coil spring. A pair of hinged governor arms are attached to the driven clutch plate, while the other ends are attached to a rotating spider member fastened on the dynamo armature shaft. When the speed increases beyond a given point the governor weights fly out, due to centrifugal force, and reduce the amount of frictional adhesion between the clutch members in proportion as the armature shaft speed augments, until the point is reached where there is no frictional contact between the parts of the clutch and the driving plate is turning at engine speed, while the driven member that imparts motion to the armature is gradually slowing down and permitting the tension of the coil spring to overcome that force produced by rapid rotation, and to bring the discs in contact again for just a sufficient length of time to enable the armature to maintain its rated speed even if the engine is running faster than normal.
A typical starting motor, which is of the Rushmore design, is shown at B. As will be evident, this is practically the same in construction as the generator shown above it, as far as essentials are concerned, except that no governor is provided and the armature shaft is fitted with a small spur pinion designed to engage with the spur gear on the engine flywheel. No mechanical interconnection is necessary between the drive pinion and the electrical starting switch. As soon as the current flows through the armature of the motor it will move that member laterally and automatically engage the pinion of the flywheel gear.
As soon as a starting switch is released, a coil spring will push the starting motor armature back again in the position shown in the illustration, and thus automatically bring the pinion out of mesh with the flywheel gear. In order to obtain a sliding feature this motor armature shaft is mounted on plain bearings instead of ball bearings, which are standard equipment on practically all machines of this nature.
The device outlined at Fig. 151, C, shows the construction followed when the ignition function is combined with a current generator and starting device having the three functions performed by one instrument. The general construction is the same as in the device previously outlined. The drive shaft of the device is adapted to be attached to the engine by direct mechanical means.
When the device is used as a current generator, the armature is driven by the shaft, whereas if the device is used as a motor the armature drives the shaft A through a planetary reduction gearing and roller clutch. Regardless of whether the device is used as a motor or generator, the distributor for ignition purposes is driven in the same direction, and at the proper speed to insure ignition as it is driven directly from shaft A, which turns at crankshaft speed.
An example of a double deck combined instrument in which the generator is carried in the lower portion of the casing and the starting motor at the upper part is clearly shown at Fig. 152, B. The partial section makes clear the arrangement of the reduction gearing and roller clutch. This type has met with favor because it is mounted easily, and also on account of the simple mechanical connection to the engine. While the two units are electrically separate, i.e., each having its own field and armature, it may be considered as one unit mechanically. The double deck instrument shown is designed for application to the side of a gasoline engine connecting by chain or gearing to the pump or magneto drive shaft. It should be noted that this chain or gear is the only connection between the machine and the engine, and that it is used not only for transmitting the engine energy to the generator, but also acts to transmit the power from the starting motor to turn the engine crankshaft when it is desired to start the power plant. It will be apparent that in a combined instrument of this type that it is necessary to have a fairly low gear ratio between the motor and the engine in order to reduce the high speed of the motor armature rotation to a speed low enough to turn over the engine crankshaft.
At the other hand, once the power plant is started the generator armature must turn at a slower speed than that of a starting motor, and if it is run from the pump shaft or magneto drive shaft it will turn fast enough to generate the proper quantity of electricity. The starting motor, however, must be geared down in order that it may exert the starting torque through the high leverage furnished by the reduction gear. The motor occupies the upper position, and carries a pinion P keyed to the end of its armature shaft. This pinion transmits the drive to an intermediate shaft S, which in turn drives the large gear forming the outer casing of an overrunning roller clutch R. The inner or driven member of this clutch is mounted rigidly on the armature shaft of the generator and carries the drive through to an outer chain gear when cranking the engine. As soon as the engine explodes and the speed runs above that represented by the starting motor at the roller clutch the latter comes automatically out of action, thus permitting the generator to obtain its power in the normal way through the chain wheel attached to the dynamo shaft. The motor armature above comes to rest as soon as the starting switch is released. The generator of this device has its output controlled by a combination of armature reactions and a bucking coil, while the battery is protected from discharging back
Ammeter through the generator
Touring Switch by a simple magnetic contact breaker or cutout. The starting mo
Starting Switch tor shown at Fig. 152, A, is a one-function instrument having worm reduction gearing.
The motor generator unit used in the United
Motor-Generator States Lighting system differs from any other form, in that the device is incorporated in the flywheel housing and is driven directly from the motor crankshaft without the interposition of any driving
Storage Battery gearing or chain. This construction is shown at Fig. 153, which shows a side view of Fig. 154.—Diagram Showing Simple Wiring the generator installed of U. S. L. One Unit Starting and Light
ing System. in the flywheel compartment and a face view showing the relation of the fixed and rotating members. A series of fixed pole pieces is attached to a ring bolted to the flywheel case while the rotating pole carrier is driven from the clutch housing and takes the place of the engine flywheel.
When current is passed through the fixed field the rotating armature member will be forced to rotate and turn the crank