Fig. 96.
Defects of Armature Windings. Defects in armature windings probably cause one-third the maintenance expenses of electrical equipment of cars. Almost all repair shops have men continually employed in repairing them. The most frequent trouble with armatures is through failure of the insulation of the coils and consequent “grounding.” This term is used in connection with armatures and fields and other electrical apparatus where a direct path exists to ground. As the armature core is electrically connected to the ground through its bearings and the motor casing, a break down of the insulation of the coils in the slots permits the current to pass directly to ground. This shunts the current around the fields and an abnormal current flows because of their weakness. The circuit breaker or fuse is placed in circuit to protect the apparatus in such an emergency, but usually before such devices break the circuit, several of the coils of the armature are burned in such a manner as to make their removal necessary. The coils are so wound on top of one another that in order to replace one coil alone, one-fourth of the coils of the armature must be lifted.
Fig. 97.
With the armature of No. 1 motor grounded the car will not operate and if the resistance points be passed over, the fuse will usually blow. When No. 2 motor is grounded the action of No. 1 motor is not impaired and this latter motor will pull the car until the controller is thrown to the multiple position. But if the motors are thrown in multiple, the path through the ground of No. 2 motor shunts motor No. 1. A study of [Fig. 18] will make this evident.
Fig. 98.
Next to grounding, open circuits are the most serious defects of armatures. These are usually caused by burning in two of the wires in the slot, or where they cross one another in passing to the commutator. Sometimes the connections where the leads are soldered to the commutator become loose.
The effect of an open circuit is shown in [Fig. 96]. The circuit is open at n. The brushes are on segments a and d. By tracing out the winding it will be found that no current flows through the wires marked in heavy lines. Whenever segments c and d are under a brush the coil with the open circuit is bridged by the brush and current flows as in a normal armature. As segment c passes out from under the brush the open circuit interrupts the current in half the armature and a long flaming arc is drawn out.