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Armature

  • The armature consists of core and winding. Iron being the magnetic material is used for armature core. However, iron is also a good conductor of electricity. The rotation of solid iron core in the magnetic field results in eddy currents. The flow of eddy currents in the core leads to wastage of energy and creates the problem of heat dissipation. To reduce the eddy currents the core is made of thin laminations.
  • The armature of D. C. machines (see Fig. 6) is built up of thin laminations of low loss silicon steel. The laminations are usually 0.4 to 0.5 mm thick and are insulated with varnish.
  • The armature laminations, in small machines, are fitted directly on to the shaft and are clamped tightly between the flanges which also act as supports for the armature winding. One end flange rests against a shoulder on the shaft, the laminations are fitted and other end is pressed on the shaft and retained by a key.

The core (except in small size) is divided into number of packets by radial ventilation spacers. The spacers are usually I sections welded to thick steel laminations and arranged to pass centrally down each tooth.

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-        For small machines the laminations are punched in one piece (see Fig. 7). These laminations are built up directly on the shaft. With such an arrangement, it is necessary to provide axial ventilation holes so that air can pass into ventilating ducts.

-        The armature laminations of medium size machines (having more than four poles) are built on a spider. The spider may be fabricated. Laminations up to a diameter of about 100 cm are punched in one piece and are directly keyed on the spider (see Fig. 8).

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-        In case of large machines, the laminations of such thin sections are difficult to handle because they tend to distort and become wavy when assembled together. Hence circular laminations instead of being cut in one piece are cut in a number of suitable sections or segments which form part of a complete ring (see Fig. 9). A complete circular lamination is made up of four or six or even eight segmental laminations. Usually two keyways are notched in each segment and are dove-tailed or wedge shaped to make the laminations
self-locking in position.

  • The armature winding is housed in slots on the surface of the armature. The conductors of each coil are so spaced that when one side of the coil is under a north pole, the opposite is under a south pole .

Fig. 10 shows the arrangement of conductors and insulation in a slot.

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  • In D.C. machines two layer winding with diamond shaped coils is used. The coils are usually former wound. In small machines, the coils are held in position by band of steel wire, wound under tension along the core length. In large machines, it is useful to employ wedges of fibre or wood to hold coils in place in the slots. Wire bands are employed for holding the overhang. The equilizer connections are located under the overhang on the side of the commutator. Fig. 11 shows a typical arrangement for equilizers. The equilizers can be accommodated on the other end of the armature also.