Single Phase Induction Motors.—The general utility of single phase motors, particularly the smaller sizes, is constantly being enlarged by the growing practice of central stations generating polyphase current, of supplying their lighting service through single phase distribution, and permitting the use of single phase motors of moderate capacity on the lighting circuit.
Fig. 1,779.—External resistance or slip ring induction motor connections. The squirrel cage armature winding is not short circuited by copper end rings, but connected in Y grouping and the three free ends connected to three slip rings, leads going from the brushes to three external resistances, arranged as triplex rheostat having three arms rigidly connected as shown, so that the three resistances may be varied simultaneously and in equal amounts.
Fig. 1,800.—Allis-Chalmers phase wound external resistance type or slip ring armature construction. The winding is for three phases and the terminals are brought out to three slip rings. The front bracket is slightly modified to make room for these rings on the inside. For starting duty sufficient resistance is supplied to reduce the starting current taken by the motor to 1¼ times the normal full load current. In the running position the resistance is all cut out of the circuit. For speed regulation sufficient resistance is supplied to reduce the speed 50% on normal full load torque.
Figs. 1,801 to 1,828.—Disassembled view of Western Electric three phase external resistance or slip ring mill type induction motor. It is adapted to severe working conditions, such as are met with in steel mills, crane and hoist service, etc. Designed for 220 or 440 volt, 25 cycle circuits. The frame is divided horizontally into an upper and a lower steel casting, both of which are bolted together at the corners by four heavy bolts. The lower casting is provided with four feet for bolting the motor to its foundation. The end of the upper frame which covers the slip rings is equipped with malleable iron covers held in place by lock bolts. The field and armature are of the usual construction. One end of the armature winding is protected against mechanical injury by the slip rings which are of heavy construction and of practically the same diameter as the armature, and the other end by a detachable flange of the same diameter as the outside of the winding. The slip rings are mounted on the same spider as the armature, so that the shaft can be removed without disturbing any of the connections. The brushes are equipped with riveted pigtails, and held in brass brush boxes machined to gauge. Heavy coiled clock springs are used to maintain an even pressure of the brushes on the slip rings. The armature leads are brought out through holes in the upper half of the frame, and the field leads are brought through a block, which fits in an opening in the upper edge of the lower half.
The simplicity of single phase systems in comparison with polyphase systems, makes them more desirable for small alternating current plants.
The disadvantage of single phase motors is that they are not self-starting.