A Textbook on Mining Engineering by International Correspondence Schools

Author:International Correspondence Schools
Language: eng
Format: epub
Publisher: International Text Book Co
Published: 1900-03-25T05:00:00+00:00

CONNECTIONS.

58. Fig, 27 shows the manner in which a shunt motor ■onnected to the terminals -+- and — of the circuit. It ft'will be seen that the ■current through the lehunt field does not

■ pass through the re-(sistance R. which is

[connected in the ar-■mature circuit. This Kis necessary, since to vlceep the field strength (Constant the full difftr-

lence of potential must Kl>e maintained between

■ the terminals of the

■ field coil, which would Btiot tie the case if the 1 rheostat were included ■fn the field circuit, for then the difference of potential would ■be only that existing bolween the brushes -\-B and —B. ■As tin starling the motor this difference of potential is small, ■only a small current would flow through the field coils, which

Would generate such a weak field that an excessive current would be required to furnish the necessary torque for starting the motor.

When connected as shown, however, the field is brought up to its full strength before any current passes through the armature, so this difficulty does not arise.

59. Since in a series

motor the same current flows

irough both armature and

;oils, the starting resist-

niay be placed in any

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