Items related to A textbook on steam engineering Volume 5
Synopsis
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 Excerpt: ...BY A I/TERN ATING CURRENT. 63. While direct current is preferable for the operation of electric elevators, in many cases alternating current is the only source of power that is available. Two-phase or three-phase alternating current is generally used for elevator operation. Prior to the introduction of the two-phase and three-phase systems, alternating current was very little used for motive purposes because the single-phase alternating current motor would not start of its own accord under load; on the other hand, two-phase and three-phase motors give a good starting torque and will run up to speed in much the same way as a direct-current motor. An alternatingcurrent induction motor consists of two main parts: the primary, or stator, which is the stationary part, and the secondary, or rotor, which is the revolving part. The primary consists of a laminated body provided around its inner circumference with slots in which the primary coils are placed. These coils are connected together, and the terminals connect to the line when the motor is in operation. The secondary, or rotor, is also a laminated body provided with slots around its circumference in much the same way as a direct-current armature. In many induction motors, each of these slots contains a heavy copper bar, which is connected to a copper ring at each end of the armature, thus forming what is known as a squirrel-cage winding. In other types of machines, especially those that must give a good starting effort and are started and stopped frequently, the armature is provided with a three-phase winding and the three terminals brought out to collector rings mounted on the armature shaft. This is done so that resistance may be inserted in series with the armature windings when the motor is being started...
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