Function of Each Part of D.C. Generator

Function of Each Part of D.C. Generator

In the preceding article, a simple loop generator has been considered in detail to understand the basic principle underlying the construction and working of a practical generator illustrated in fig. which mainly consists of the following parts:

  i. Yoke or magnetic frame

  ii. Pole cores and pole shoes

  iii. Field windings

  iv. Brushes and brush holders

  v. End covers

  vi. Armature core

  vii. Armature winding

  viii. Commutator

  ix. Shaft

i. Yoke or magnetic frame:  The outer most frames is known as yoke. It made of high permeability (magnetic) material possessing sufficient mechanical strength. Cast iron is used in small machines and cast steel or rolled steel is employed in large machines.

The yoke serves two fold as:

A) It protects the entire machine from dust and dirt and also provides mechanical support for the                  magnetic poles,

b) it acts as the return path for the magnetic flux.

ii. Pole cores and pole shoes: The cores and poles: The field magnet consists of pole core and poles shoe. In modern design, the complete pole cores and pole shoes are made of thin (about 0.5 mm thick ) laminations of annealed steel which are pressed together and riveted and are secured to the yoke by means of screws bolted in to steel rod as shown in fig.

The main function of the pole core is to establish the required magnetic flux. The field winding is placed on the pole core. When (field) current passes through this field winging the core becomes as electromagnet and establishes the magnetic flux. The flux can be varied by varying the current through field winding.

The pole shoe serves two flod as :

1. It distributes the magnetic flux uniformly in the air gap and reduces the reluctance of the magnetic path due to its larger cross-section.
2. It supports the field winding (also called as exciting coils) as show in fig.

(iii) Field winding: the field winding  consists of enamel coated copper/aluminium wire and are former-wound to fit over the pole core as show in fig. (a)

When current is passed through winding coils, they electro-magnetise the pole core which produce the necessary magnetic flux. All the field coils are connected in such a way that the adjacent poles are made of opposite polarities.

(iv) Brushes and Brush holders: The function of the brush is to collect current from rotating commutator and deliver it to the external stationary load circuit. They are usually made of high grade carbon and are housed in holders. The brush holders are secured to the front end housing with clamps. The brushes are held under pressure over the commutator, the pressure being provided by a tension adjusting spring. A flexible copper pigtail mounted at the top of the brush conveys current from the brushes.

Advantages:

  1. Facilitates the collection of current from rotating commutator to stationary terminals.

  2. carbon brushes minimizes the sparking.

  3. carbon brushes are cheap.

Disadvantages:

  1.   Wear and tear takes place and necessitates replacement of new one.

  2.   Requires maintenance.

  3.   Terminal voltage reduces due to BCD (brush contact drop).

(v) End covers: The end covers are usually made of cast iron or cast steel. Its main function is to protect the inner parts i.e., armature, commutator etc. from dust and other foreign particles also provides the protection to the workers. One end cover holds the brush assembly.

(vi) Armature core: Armature core is cylindrical in shape and is built up of high permeability silicon steel stampings or laminations of 0.5mm (approximately) thick. Each stamping being separated from the neighboring one by varnish. The purpose of using laminations is to reduce eddy current loss whereas the high permeability silicon steel is used to minimize the hysteresis loss.

Slots are provided on the outer periphery of the core to house the winding in proper position and the key way is provided on the inner diameter as show in fig. to fix it on the shaft. AIR holes or ventilating ducts are provided, which permits the axial flow of air for cooling purpose.

All the laminations are immersed in varnish and after drying up they are pressed together by means of hydraulic press or by other means to form an armature as show in fig. insulating paper is placed in the slots first and then winding coil sides are placed in the slots first and then winding coil sides are placed in the slots to form a closed loop and may be either in lap or wave fashion. When the shaft is driven the armature and armature winding also rotates in between the magnetic poles.

The armature core serves the following purposes:

• a)      It houses the armature conductors in slots.
• b)      It provides a path of low reluctance to the magnetic flux.

The armature is mounted on the shaft so that when it is rotated, the conductor housed in it cut the magnetic flux.

(vii) Armature winding: The armature windings are usually former-wound. These are first wound in the form of rectangular coils and are then pulled into the proper shape. The coils are insulated from each other. The conductors are placed in the armature slots which are lined with insulating materal (like paper). This slot insulation is folded over above the armature conductors placed in the slots (or a fresh insulating paper will be placed over the conductors) and is secured in place by wooden or fibre wedges.

EMF can be induced in a stationary conductor (statically induced e.m.f.) or moving conductor i.e., in the armature winding (dynamically induced e.m.f.) according to faradays laws of EMI. A.C generator works on former principle and D.C. Generator works on later principle.

Usually copper wires coated with enamel is used for winding purpose. Aluminim is the next best material for winding. Of course the use of Aluminium reduces the cost of the machine.

(viii) Commutator: The Commutator is of cylindrical in shape and is made up of wedge-shaped high conductivity hard drawn copper segments. These segments are insulated from each other by a thin layer of mica. Each commutator segments. These segments are insulated from each other by a thin layer of mica. Each commutator segment is connected to armature conductors by means of a copper riser (or lug). The number of segments are equal to the number of armature coils.

The commutator is an important part of a d.c. generator and it serves the following purposes:

• a)      It facilitates the collection of current from armature conductors.
• b)      It converts the alternating current induced in the armature conductors in to unidirectional current in the external load circuit.

It is fixed on the main shaft on one of the sides of armature and rotates with armature winding. The armature winding terminals are connected to the commutator risers.

(ix) Shaft: It is usually made of cast iron or cast steel and is supported between two bearings. The armature and commutator are housed on the shaft. It is provided with a key to hold the armature firmly (not to slip) in proper position. Its main function is to rotate.