C. The frame is the name given to the stationary part of the machine to which ате fixed the main & commutating poles & by means, of which the machine is bolted to its bedplate. The ring-shaped portion is called the yoke. The frame of modern machine is made of cast iron or steel. The rotating part is called the armature. The armature is a cylindrical body, rotating in the space between the magnetic poles & comprising: d/ a slotted armature core; e/ a winding inserted in the armature slot; f/ the commutator & g/ the brush gear.
D. The armature core is assembled of steel sheet laminations.
To reduce eddy current losses the sheets are insulated with a thin layer of varnish. The armature core is pressed from both sides by clamping device either specially fixed onto the shaft or tightened by bolts. To improve the cooling of machines they have a special ventilator fan mounted on the shaft.
E. Armature winding. All modern dc machines employ the drum-wound armature. The winding is laid entirely on the outside of the armature core periphery.-The winding is built up in sections wound on special moulds & then laid in the armature core slots.
F. The commutator. The ends of all armature coils are joined to the commutator. A simple, cylindrical commutator it is. It consists of a number of small wedge-shaped segments of copper, which • when assembled together form a cylinder. The copper bars are separated from each other by sheet mica & are insulated from their supporting ring§ by means of mica cones & gaskets.
G7 Brush gear. To collect the' current of a rotating commutator or to lead the current to it a brush-gear is used which consists of: a) brushes, b) brush-holders, c) brush studs or brush-holder arms, d/ brush-rocker, e/ the current-collecting busbars. The brushes employed in modern machines are solely composition’of carbon & graphite or carbon & copper, the latter used for low-voltage commutator DC machines. The brush is inserted into the brush-holder where a spring presses it against the commutator. The brush-holder is mounted on
r \
a stud w;hich maintains the brush in a definite position in relation to the commutator. The current from the brush is led to the stud by means of a special flexible lead. Each brush stud usually carries two or more brushes working in. parallel. All brushes of the same polarity are interconnected by busbars to the machine terminals.
