A Textbook on Steam Engineering Volume 2 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 edition. Excerpt: ... 25. terminal T and vice versa. If the needle shows a deflection in both positions, but seems to vibrate or tremble, the armature or commutator is probably grounded. If in either case the deflection does not amount to more than about the total E. M. F. of the machine, the ground is not serious, but if the deflection is much more than this, the windings should be examined separately, the ground located, and, if possible, removed. 95. To locate the ground, if thought to be in the field coils, each coil should be disconnected from its neighbor (with the machine shut down, of course) and "tested out" by connecting one terminal of another dynamo (or of a "live" circuit) to the frame of the machine, care being taken to make a good contact with some bright surface, such as the end of the shaft or a bolt head, and the other to a terminal of the coil to be tested, through a voltmeter, as represented in Fig. 26. Here C and C, represent the terminals of a "live" circuit, which should have a difference of potential between them about equal to the E. M. F. of the machine when it is in operation, but not greater than the capacity of the voltmeter will allow of measuring. Tand 7 represent the terminals of the dynamo, as before, and / and./, the terminals of the field coils, which have been disconnected from each other and from the dynamo terminals. One terminal C of the circuit is connected to the frame of the machine; the other terminal Ct of the circuit is connected Fl-. through the voltmeter V to the terminal /, of the field coil. If that coil is grounded, the voltmeter will show a deflection about equal to the E. M. F. of the circuit C C but if the insulation is intact, it will show little or no deflection. The...