In [Fig. 97] is shown the result of a short circuit between two coils. The short circuit is at b, c, the two leads coming in contact with each other when they cross. The effect is to short-circuit all of the winding indicated by the heavy lines.
Fig. 99.
Mistakes in Winding Armatures. The armature winder is given very simple rules as to how to wind the armature, but the great number of leads each to be connected to their proper commutator segment sometimes so confuse him that misconnections are made. The effect of getting two leads crossed is shown in [Fig. 98]. The leads to segments b and c from the right are shown interchanged. This short-circuits the coils shown in heavy lines. The abnormal current resulting in these would usually cause them to burn out.
Fig. 99 shows the results of placing all of the top leads, or all of the bottom leads one segment beyond the proper position. This causes the circuit starting from a and traveling counter clockwise around the armature to return on segment m instead of on segment b as is the case in [Fig. 97].
The only result of such connections is to change the direction of rotation of the armature. It may be noticed by comparing the two figures that with the positive brush on segments a the arrows show the currents to be in opposite directions in coils similarly located with reference to the position of the brushes. Some armatures are intended to be wound as in the last case mentioned.
Sparking at the Commutator. As railway motors are made to operate, and usually do operate, almost sparklessly, sparking at the brushes may be taken as a sign that something is radically wrong.
The pressure exerted by the spring in the brush holder may not hold the brush firmly against the commutator.
If brushes are burned or broken so that they do not make good contact on the commutator, they should be renewed or should be sandpapered to fit the commutator.
A dirty commutator will cause sparking.