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Tuesday, 10 December 2019

Armature Construction

Armature forms an important part revolving within the magnetic field. It consists of the following parts built up over the shaft

a. Armature core
b. Armature winding
c. Commutator
d. Brushes and brush holder

Armature Core :

The primary function of an armature core is to provide magnetic path of low reluctance for the flux which have the north pole of the main field to enter south pole. Necessarily, therefore, it should be made of magnetic material.

Armature core is made up of stampings punched from annealed onto the shaft and mounted over the spider, axial slots are formed in which conductors are wound. The thickness of lamination varies from 0.5 mm to 1 mm depending upon the frequency of magnetic reversal. The stampings are insulated on both sides before stacking onto the shaft. For small machines they are locked at both ends with the help of lock washer and nut and for larger machines it is mounted on the spider. The stampings are held in position with the help of the key way. Usually these laminations contain perforated air ducts to permit axial flow of air through the armature for purpose of cooling. When the diameter of the armature increases beyond one metre, it would be difficult to handle the cut-out stampings for assembly as they tend to distort. In such cases, they are cut into a number of segments which form part of the ring. Usually four six or eight segmental laminations are required to form a complete set. Two keyways are notched in each segment and are dovetailed to make the lamination self locking in position. Fig. (a) shows the different types of lamination stampings of the armature core while Fig. (b) shows the assembly of the stampings on the shaft. On assembly the slots formed are indicated.

The shaft is made of mild steel, and its dimensions depends upon the various components to be mounted on it. The length of the shaft depends on the length of the core which has a direct bearing on the effective length of the conductor for generation of emf. A key way is formed on one side for locking the pulley. The shaft is supported by two bearings either bush or ball bearing, one on each side to enable the shaft to rotate freely within the stator.

Armature Winding:

The simplest type of winding consists of a series of coils wound in the slots of an armature and are connected in succession to a commutator. There are mainly two types of winding namely Lap Winding and Wave Winding. Here we will limit our blog post to the procedure of winding only. The exact details of the windings will be dealt at a later stage. The slots are first insulated with proper insulating material to prevent the conductors from touching the iron core causing ground fault. The insulation so inserted into the slot should protrude approximately 3 mm outside the slot ends and 6 mm above the slot. This will prevent the winding touching the core. The windings are inserted and the lead connections are soldered to the commutator bars. Wedges made out of fibre, wood etc., are inserted between the winding and top portions of the core slot so that the wires do not fly out of armature slot while rotating at full speed. After the armature is wound, leads are soldered to commutator. It has to be made moisture proof besides the coils should be vibrations proof. It is therefore varnished with either baking varnish or air drying varnish. The length of the conductor inside the slot is the effective length for calculating the induced emf.

Armature windings are generally of two types ,

a. Lap winding
b. Wave winding

In the lap winding the beginning and end of the coils are connected to adjacent commutator segment bars. A wave winding is one in which the beginning and end leads of a coil are connected to a definite number of poles. In wave winding, the number of parallel paths for current is equal to two irrespective of the number of poles. But in the lap winding, the number of parallel path for current is equal to the number of poles. Therefore, the number of parallel paths (A) for

a. Wave winding is A = 2
b. Lap winding is A = P

Commutator :

Commutator is an extension of armature to which the terminal connections of the armature winding is soldered. It is a device which automatically reverses the direction of the current to achieve a single direction in the external circuit. It is cylindrical in shape and consists of a number of copper segments of high conductivity insulated from each other as well as from the shaft. Each segment is insulated from the other with the help of mica and is connected to armature by means of lugs. The thickness of mica varies from 25 to 40 mils. The segments are tapered from top to bottom with a V-cut at the bottom on both sides. The back end of the segment has a raised platform with slots for soldering leads of armature winding.
This arrangement prevents adjacent copper bars from touching each other. Onto the iron shell with V ring at the bottom and threaded at the top several layers of mica insulation is used, The insulated segments are then assembled on the other mica ring to form a cylinder all round. The front V ring is then slipped into position and tightened by a nut which can be screwed to the inner threaded portion of the iron shell. On assembly the commutator should be tight and all segments aligned. It is then tested for segmental insulation.

The basic function of a brush is to collect current with the least possible voltage drop from the rotating coil to a stationary position. The type of brush used depends upon the speed of rotation of the. commutator and the voltage involved in it. 

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