Fig. 6. Armature Coil.
Armature Coils. Railway motor armatures are to-day universally constructed with form-wound coils, which are wound on a form of proper shape and carefully insulated before being placed in the armature.
The coils of the smaller motors (those up to 40 or 50 horsepower) are usually wound with round wire. The cotton covering of the wire is depended upon for insulation. To strengthen this, however, the coils after being wound are immersed in an insulating compound and then baked in an oven. The whole coil is usually wrapped with insulating tape (See [Fig. 6]). The armatures of larger motors have coils made of copper bars. Mica is often placed between and around the bars for insulation, though oiled linen cloth tape cut bias is also employed, especially in repair work.
Field Coils. Field coils are so constructed that they may be readily removed should they become grounded or short-circuited. Some makers wind them on a brass shell or form which is slipped over the pole piece. In some motors the field coils are composed of copper ribbon, wound bare, with ribbons of insulating material between the turns. Field coils of wire for the smaller motors, if not wound on shells, are wound on forms and before completion are taped in such a manner that they will hold their shape without being enclosed in a spool. The terminals are brought out where they will be of easy access when the field is in place (See [Fig. 7]).
Fig. 7. Field Coil.
Armature Leads. In [Fig. 3] is seen a completed armature in the motor casing of a Westinghouse No. 69 motor. Since the motors are four-pole, the two sides of any one coil occupy slots 90° apart in the armature coil, as indicated in [Fig. 5]. The ends of the coils are connected to commutator bars 180° apart. The relative position of the commutator connections of any armature coil can, of course, be varied so as to bring the brushes in the most convenient position in the motor casing. Brushes are always of carbon, and are placed where they can be easily reached from the opening in the motor casing over the commutator.
Motor Leads. The reversing of the current through the armature, independent of the field current, to secure reversal of direction of rotation of the armature, makes it necessary that four wires enter the motor. The portions of these wires connected permanently to the motor are termed the motor leads because they “lead out” the current. Sometimes an ordinary two-way connector is used in connecting these leads to the wires of the cable, but often a jack-knife connector is employed to facilitate connecting and disconnecting. Considerable difficulty has been experienced by the wearing away of the insulation of the leads where they rest on the motor shell. To avoid this there has recently come into use a lead protected by a spiral metal covering. Brushes. That the motor may operate in either direction equally well, the carbon brushes are placed radially or nearly so. No provision is made for shifting their position relative to the fields. They usually occupy a position equidistant between pole tips. The common types are either ½ or ⅝-inch thick and from 2¼ to 4 inches wide.
Fig. 8. Brush Holder.