File:The street railway review (1891) (14760142242).jpg

Identifier: streetrailwayrev05amer (find matches)
Title: The street railway review
Year: 1891 (1890s)
Authors: American Street Railway Association Street Railway Accountants' Association of America American Railway, Mechanical, and Electrical Association
Subjects: Street-railroads
Publisher: Chicago : Street Railway Review Pub. Co
Contributing Library: Carnegie Library of Pittsburgh
Digitizing Sponsor: Lyrasis Members and Sloan Foundation

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istanceof one coil will be as small as .02 ohm. For measuringthese small resistances the ordinary Wheatstone bridgeis unsuitable, for the following reasons: It is often of acheap make, equipped with a not over-sensitive galvanom-eter, and often the coils are not calibrated correctly and varying the current to any required number of amperes.The greatest resistance necessary is four ohms. Before the resistance of the field coils is measured thefollowing instructions must be observed: (i) Pull down trolley and throw canopy switch to off position. (2) Remove brushes from each motor. (3) Connect jumper across armature terminals ofeach motor. (4) Prepare terminals of field coils to be tested. (5) Insert adjustable resistance and ammeter acrosscanopy switch. (See Figure i). (6) Throw controller handle on the point connectingthe diverter with the two motors in series (if resistanceof diverter is subsequently proved to be too great, throwcontrolling switch on the point at which the diverter is
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? MILLI-yOLT-METER -CONNECTIONS FOR RESISTANCE TEST. FIG. 2.- -CONNECTION OF VOLTMETER FOR INSULATION TEST. the resistance of the connections due to imperfect con-tacts in plugs vitiates the results. I dare say all streetrailway plants are in possession of a correct portableammeter with a scale reading from o to 150 amperes,and a volt meter with a scale reading from o to 600 volts.It is doubtful, however, if a millivolt meter reading fromsV t° 3 volts can be often found around such plants. Ifthis latter is secured, however, all necessary electricalmeasurements and tests can be correctly accomplishedwith this and the volt and ammeter. I will here only describe the methods of measuringthe conductivity and insulation resistance of motors inthe barn or repair shop. To test the conductivity of a motor connections aremade as in Figure i. An extra resistance is introducedin the circuit. This resistance is made of iron wire coiledand stretched between porcelain insulators fastened ina box. O

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