The Diseases Of Electrical Machinery

66 THE DISEASES OF potash, or soda solution, to the top of the iron plates. The current flows through this capacity, passing from plate to plate through the liquid. The action of the capacity depends entirely upon the concentration and the quantity of the liquid in the pots. If the solution evaporates in summer, or freezes partly in winter, the value of the capacity is altered, and the motor will no longer start satisfactorily. It may, indeed, refuse to start at all. If, therefore, motors having starting apparatus—that is,motors fittedwith capacities—refuse for no apparent reason to start, in 99 cases out of a 100 it is due to the capacity. Choking coils are naturally not liable to such changes, and are therefore much to be preferred. Auto-transformers are also sometimes employed in starting apparatus. They are unlikely to give trouble. Polyphase Motors. Stator Faults.—Two-phase and three-phase motors require no such auxiliary appa- ratus for starting. They can, however, be troubled with the faults which we have already learnt in connec- tion with generators. If, for example, a three-phase motor heats abnormally, so that parts of the stator winding are hot while the rest is normal, it is at once clear that a short-circuit is present, either in one phase or between two different phases. The motor will then work with small torque but large current, which will be unequally divided amongst the three phases. A polyphase motor with star connection will not start with a break in one phase. Should the break occur