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Wednesday, 11 December 2019

DC Generator Parts and their Functions

The essential parts of a dc generator are :

a. Magnetic frame (yoke)
b. Pole core and field coils
c. Armature
d. End shield and bearings

The constructional details are shown in Figure. The functions of each parts of d.c. generator is dealt in the succeeding paragraphs.

a. Magnetic Frame or Yoke :

For smaller machines the yoke is made of cast iron but for larger capacity machines, it is made of fabricated rolled steel or steel casting. It serves two fold functions; one it acts as a magnetic path and the other to hold the field magnets in position. It is accommodated with two end shields housed with bearings and supports the armature at the centre and also protects the Machine from dust, dirt, moisture, etc. in case of smaller machines the yoke is made in one piece but for larger machines they are split into two halves horizontally. The surface where they meet are carefully machined and the two halves are bolted together by which the difficulty in removing larger armature without damage is eliminated. The frame is with foot rest as far apart as possible is casted to give maximum stability and right support. Tapped holes are provided at the inner side of the frame to accommodate field poles.

b. Pole core and field coils

Pole Core and Pole Shoe:

There exists different forms and shapes for pole core but all serve the same  purpose of concentrating the magnetic field. The most commonly used pole core is shown in Fig. These are made of MS Plates for smaller machine. Present day design use laminated soft sheet steel  punched to suitable shape and size. These laminations are then stacked and riveted to form pole core. Two tapped holes of 1.5 Φ and 30 mm deep are provided on top for fixing the poles inside the frame by means of screws. The field coils are slipped onto the poles before fixing it inside the frame. The pole face serve the purpose of spreading the magnetic flux uniformly in the air gap. The pole face is so curved that it gives a uniform air gap between the armature and the pole face.

Poles are of two types :

a. Main pole
b. Inter pole

Main poles have a wider area of pole face and is intended to spread the main flux uniformly in the air gap. Inter pole provides flux in the air gap between the main pole for

a. neutralising the armature reaction of the field.
b. inducing an e.m.f. in the coil undergoing commutator in opposition to the reaction voltage.

These may be solid or laminated and are made out of wrought iron or mild steel. Figure shows the interpole with its coil. These are fastened to the yoke by means of screws between two main poles and the interpole coils are connected in series to armature. The length of air gap below the inter pole is usually kept about 100% greater than that of the main pole air gap.

Field Coils :

Field coils are used to produce magnetic field. Usually these coils are wound with conductors on a former to suit the pole and the machine capacity. The cross section of the coil is generally rectangular and the number of turns and gauge depend on the type of connection and the capacity of the machine. In small generators where high quality of insulation and reliability is required, the coils are wound on paper stick or tube of paper or core tube either manually or by automatic winding machines.

Each layer is insulated by paper. In the case of cotton inter-woven coils special machine which winds the coils in layers also inter-weaves cotton yearn between the layers separating the layers are used. This type of coil is used where atmospheric conditions are severe. Coils are also wound on bobbins made of insulating material like bakelite or fibre. Where high current capacity coils are required rectangular copper strips are used instead of round wire and are wound in the form of a spiral on the flat side. The layers are insulated with strips of asbestos. The coils after winding is complete are taped with cotton tape and impregnated in varnish before use. In the case of compound machines both the shunt and series windings are done on the same pole. In the case of interpole, the windings are single layered. These windings are insulated and inserted in the interpole before fixing into the yoke.

Interpoles are narrow poles fixed exactly midway between the main poles. It is also known as commutating poles or compoles. The polarity of the interpole is the same as that of the main pole for the generator whereas for motor, the polarity is the same as that of the main pole behind it. Interpoles are fitted to improve commutation. It is of rectangular in section with brass flanges on top and bottom. Coils are well insulated before use.

c. Armature:

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. Shaft and Bearing:

Shaft plays an important role of housing armature core and commutator besides keeping them at the centre of the stator, free to rotate with the support of the bearings. The diameter and length of the shaft vary depending on the machine capacity and design. The shaft is generally made of mild steel with or without case hardening. To keep the armature core in position locking arrangements like lock nut, rivet, cross pin or V washers are used. For small machines the shaft is supported by the end plates through the gun metal bush but for medium and larger machines ball bearings are used. The ball bearings are housed in the end plates with a dust proof covers on the outer side of it. Provision is made for oiling through oil holes for smaller machines and grease cup for larger machines. The size of the bearing varies depending on the shaft diameter and the load.

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