Ques. On what class of circuit are internal resistance motors desirable?
Ans. On circuits devoted to lighting service as well as power service, where a high degree of voltage regulation is essential.
The initial rush of current when a squirrel cage motor is thrown on the line is more or less objectionable and there are central stations which allow only resistance type of induction motor to be used on their lines.
Fig. 1,764.—Western Electric wound armature for internal resistance induction motor. In starting the inductors are short circuited through a resistance which is gradually cut out as the motor comes up to speed.
Figs. 1,765 to 1769.—Western Electric wound armature for external resistance, or slip ring induction motor, showing brush rigging, slip rings and bar winding.
External Resistance or Slip Ring Motors.—In large machines, and those which must run at variable speed, such as is required in the operations of cranes, hoists, dredges, etc., it is advisable that the regulating resistances be placed externally to the motor. Motors having this feature are commercially known as slip ring motors, because connections are made between the external resistances and the armature inductors by means of slip rings.
