The Diseases Of Electrical Machinery

40 THE DISEASES OF namely, for cranes, hoists, lifts, etc. Now consider what will happen if the series winding is wrongly con- nected. "The current will now flow in opposing direc- tions in the two coils. The result is that the series coil, instead of strengthening the magnetic field, will weaken it. With a weakened field, however, the speed will increase, and this may lead to a catastrophe. When starting compound motors for the first time, one must for this reason be more careful than with ordinary shunt motors. Faults in Starting and Regulating Resistances. We will now pass on to those faults which may occur in the accessories of the machine, namely, in the starters and the regulating resistances. There are naturally not many such faults, and when they do occur they are, owing to their great accessibility, easy to find out and to repair. The most usual fault is a break in one spiral or element of the resistance. Then no more current will flow through the apparatus. The result is that a dynamo will not give any voltage, and that a motor will not run : the former because the ex- citing current cannot flow, and the latter because the path of the armature current has disappeared. In both cases it is possible, in spite of this, to run the machines as long as the particular coil is near the end (i.e. the “off”) point of the resistance. For then in the case of a dynamo we may certainly, and in the case of a motor we may usually, move the switch over without danger to the next “ point ” in the circuit, or