Automatic Shunt

Under nearly all conditions of practice it is desirable to have the generator automatically perform some switching function when it is operated. As an example, when the generator is connected so that its armature is in series in a telephone line, it is quite obvious that the presence of the resistance and the impedance of the armature winding would be objectionable if left in the circuit through which the voice currents had to pa . For this reason, what is termed an automatic shunt is employed on generators designed for series work; this shunt is so arranged that it will automatically shunt or short-circuit the armature winding when it is at rest and also break this shunt when the generator is operated, so as to allow the current to pa to line. IMG:384091999325875926_fig074_t.gif.png:Illustration_ Fig. 74. Generator Shunt Switch Fig. 74. Generator Shunt Switch View full size illustration. A simple and much-used arrangement for this purpose is shown in Fig. 74, where 1 is the armature; 2 is a wire leading from the frame of the generator and forming one terminal of the generator circuit; and 3 is a wire forming the other terminal of the generator circuit, this wire being attached to the spring 4 , which rests against the center pin of the armature so as to make contact with the opposite end of the armature winding to that which is connected with the frame. The circuit through the armature may be traced from the terminal wire 2 through the frame; thence through the bearings to the armature 1 and through the pin to the right-hand side of the armature winding. Continuing the circuit through the winding itself, it pa es to the center pin projecting from the left-hand end of the armature shaft; thence to the spring 4 which rests against this pin; and thence to the terminal wire 3 . Normally, this path is shunted by what is practically a short circuit, which may be traced from the terminal 2 through the frame of the generator to the crank shaft 5 ; thence to the upper end of the spring 4 and out by the terminal wire 3 . This is the condition which ordinarily exists and which results in the removal of the resistance and the impedance on the armature winding from any circuit in which the generator is placed, as long as the generator is not operated. An arrangement is provided, however, whereby the crank shaft 5 will be withdrawn automatically from engaging with the upper end of the spring 4 , thus breaking the shunt around the armature circuit, whenever the generator crank is turned. In order to accomplish this the crank shaft 5 is capable of partial rotation and of slight longitudinal movement within the hub of the large gear wheel. A spring 7 usually pre es the crank shaft toward the left and into engagement with the spring 4 . A pin 8 carried by the crank shaft, rests in a V-shaped notch in the end of the hub 6 and as a result, when the crank is turned the pin rides on the surface of this notch before the large gear wheel starts to turn, and thus moves the crank shaft 5 to the right and breaks the contact between it and the spring 4 . Thus, as long as the generator is being operated, its armature is connected in the circuit of the line, but as soon as it becomes idle the armature is automatically short-circuited. Such devices as this are termed automatic shunts . In still other cases it is desirable to have the generator circuit normally open so that it will not affect in any way the electrical characteristics of the line while the line is being used for talking. In this case the arrangement is made so that the generator will automatically be placed in proper circuit relation with the line when it is operated. IMG:384091999325875926_fig075_t.gif.png:Illustration_ Fig. 75. Generator Cut-in Switch Fig. 75. Generator Cut-in Switch View full size illustration. A common arrangement for doing this is shown in Fig. 75, wherein the spring 1 normally rests against the contact pin of the armature and forms one terminal of the armature circuit. The spring 2 is adapted to form the other terminal of the armature circuit but it is normally insulated from everything. The circuit of the generator is, therefore, open between the spring 2 and the shaft 3 , but as soon as the generator is operated the crank shaft is bodily moved to the left by means of the V -shaped notch in the driving collar 4 and is thus made to engage the spring 2 . The circuit of the generator is then completed from the spring 1 through the armature pin to the armature winding; thence to the frame of the machine and through shaft 3 to the spring 2 . Such devices as this are largely used in connection with so-called "bridging" telephones in which the generators and bells are adapted to be connected in multiple acro the line. A better arrangement for accomplishing the automatic switching on the part of the generator is to make no use of the crank shaft as a part of the conducting path as is the case in both Figs. 74 and 75, but to make the crank shaft, by its longitudinal movement, impart the nece ary motion to a switch spring which, in turn, is made to engage or disengage a corresponding contact spring. An arrangement of this kind that is in common use is shown in Fig. 76. This needs no further explanation than to say that the crank shaft is provided on its end with an insulating stud 1 , against which a switching spring 2 bears. This spring normally rests against another switch spring 3 , but when the generator crank shaft moves to the right upon the turning of the crank, the spring 2 disengages spring 3 and engages spring 4 , thus completing the circuit of the generator armature. It is seen that this operation accomplishes the breaking of one circuit and the making of another, a function that will be referred to later on in this work. IMG:384091999325875926_fig076_t.gif.png:Illustration_ Fig. 76. Generator Cut-in Switch Fig. 76. Generator Cut-in Switch View full size illustration.