The Diseases Of Electrical Machinery

20 THE DISEASES OF P namely, at the place where they enter into the slots, or close to where they are joined to the commutator lugs. It is therefore advisable not to carry the armature wire too stiff or too straight from the slot to the commutator, but to give it a slight bend, through the elasticity of which bend a break due to undue strain is averted. While in the case of dynamos this break usually shows itself by the machine refusing to give its voltage (that is, refusing to excite itself), in the case of motors (which are, so to speak, separately excited) we have a slight increase in the speed, and also very heavy sparking often running right round the commutator. When the motor has come to rest, the insulation between those segments between which the defective coil is connected will be seen to be badly burnt. There is, perhaps, no fault of electric machines which can be so easily diagnosed as the above. If, in the case of a dynamo, we suspect this particular fault, we can obtain the same effect by running the machine without load, but with separate excitation of the magnets, as can always be done where there is a battery of accumulators. The machine, though driven as a dynamo, will then spark in exactly the same way it would do if running as a motor. The cause of the breaking of the armature wires in a motor is often due to an incorrect proportioning of the starter. When motors are to start without load, the starter must not be designed for starting at full load, for otherwise, in consequence of the low resistance of the starter, the