Page:Popular Science Monthly Volume 91.djvu/325

 Popular Science Monthly

��been overcome and the field reaches its maximum strength. Such a relay will be slow in operating but may be~ made quick in releasing by choosing such a design that the releasing requirement will be high compared with the operating requirement.

If the winding and operating require- ments are so chosen that the final value of the magnetizing force which the relay receives is much greater than the releasing requirement, then the relay will be slow releasing, as the magnetizing force will not decrease enough to allow the release of the armature, when the circuit is opened, until the effect of the short circuited winding has been overcome. Such a relay may be made quick in operating by choosing such a design that the releasing requirement will be low compared to the operating require- ment.

In neither of the above cases is the rapidity of the movement of the armature itself greatly lessened, the greater delay occurring between the time of closing or opening the circuit and the beginning of the armature movement. The above con- struction is sometimes used for making a relay that will not readily respond to alternating current.

A second method is to use an external inductance or a non-inductive resistance which come under the classification of circuit design rather than the design of the relay itself, except where the non-inductive shunt is wound on the relay merely as a matter of convenience. Either of these means is used to cause the current through the winding to rise or fall more slowly than it would if no outside means was used to affect this time interval. The non-induc- tive shunt slows down the time of release but has practically no effect upon the time of operation, while the external inductance slows down the time of operation but has practically no effect on the time of release. Both effects may be accomplished by the use of the two in combination. As in the case of the first method the movement of the armature itself is not actually retarded. The action of such an arrangement is to increase the time between the closing and opening of the circuit and the beginning of the armature movement.

A third method is to make the moving parts of the relay heavy so that it will be slow in responding to changes in the magnetizing force. If the operating cur- rent is just great enough to pull up the armature, the relay will be slow in operat-

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ing. To make such a relay slow in releas- ing, the restoring force, whether gravity or a spring, must be as small as possible and still cause the armature to fall back. Contrary to methods I and 2, with this construction the actual movement of the armature is retarded. Such relays are used extensively on alternating current, as their heavy moving parts prevent the opening of the relay contacts during the reversals of the current.

The circuit conditions in each case determine which of the above methods should be applied, although the first and second methods are the ones most com- monly used. In some cases two of these methods are used on the same relay to meet certain peculiar circuit con- ditions. — F. H. Tillotson.

��Reversing Rheostat for Controlling a Small Motor

IT is often desired to reverse the direction of rotation of direct current motors and at the same time adjust the speed to suit the new condition of operation. A service- able controller may be made as shown in the sketch. The sets of contacts A, B, and C, should be of brass or copper, and mount- ed on a slate slab 12 in. square. The re- sistance coils R should be fixed to the back of the slate board. These coils, made of

���LINE 6RUSHES FIELD

A reversing switch in connection with a rheostat for controlling a small motor

German silver, should have sufficient re- sistance to give the proper speed control without over-heating.

Along the edge of the slate six binding posts are arranged and connections made, according to the diagram, on the reverse side of the panel. The switch arm D may be made from wood. The brass spring contacts E and F are connected so that

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