The Diseases Of Electrical Machinery

ELECTRIC AL MACHINERY. 69 bars were sweated into or on to rings, it may occur, if the motor is heavily loaded, that the solder at some of the joints will melt, and the bars work themselves loose. The high resistance at the joint may then cause the motor to stop. With later types, the bars are not only sweated but riveted, or, in some cases, screwed on to the end rings, or else by the use of suitably stamped coils all joints are avoided. Some makers wind the rotors with many turns of bare wire, but with one joint only in each phase. In any case, such faults as arise at joints are easily recognised and repaired. Slip-Ring Rotor.—Since the winding of the slip- ring rotor is considerably more complicated, there is more cause for faults. First, a break can occur in one phase, either in the starting resistance or the rotor winding itself. The motor will not then start so well, and will run up only to about half the normal speed. A short-circuit in one phase, or between separate phases, may be recognised from the loud humming noise caused, as well as by the heating of the rotor. It may here be pointed out that all firffls without exception build the slip-ring rotors, not only of their three-phase motors, but also those of their single- phase and two-phase motors, with three slip-rings and a three-phase winding ; and, consequently, a triple starting resistance in the rotor circuit is always used, no matter how many phases the motor may have. We have said above that a three-phase motor will still