The Diseases Of Electrical Machinery

ELECTRIC AL MACHINERY. 67 while running, it then continues as a single-phase motor so long as the load is not too heavy. A fully loaded three-phase motor will almost always stop if a break occur in one phase. If half-Ioaded, it will con- tinue to run, but the current in the remaining phases will be doubled. A three-phase motor will also often not start if the three phases are wrongly connected, so that, as already mentioned, in the case of the genera- tors, a phase difference of 6o° is caused by changing the beginning and the end of one phase in a star-connected winding. And if it starts, it will start backwards. In faet this is one way of reversing the rotation. I might also mention a wrong connection which could make a motor of no use under certain circum- stances. Although most firms build their three-phase motors with star-connected stator windings, it happens, principally for 11 o-volt motors, that the stator wind- ing is designed as a mesh-connected winding, for the reason that then the same winding, if re-connected with a star-grouping, can be used for 190 volts. If a motor, which should be mesh-connected, is wrongly connected in star, it will start so long as it be not too heavily loaded, but will show a heavy drop in volts ; and with an only average load, drop out of step and stop. Wrong Frequency or Wrong Voltage—An in- duction motor, designed to run at some particular speed on mains that work at a particular voltage with a particular frequency, cannot, in general, be worked . F 2