DC GENERATOR FOR SSC JEE, RRB JEE , BPSC AE AND ENGINEERING COLLEGE

4.1 DC Generator

An electro-mechanical energy conversion device (or electrical machine) that converts mechanical energy or power into DC electrical energy or power is called DC generator.

Fig. 4.1 Block diagram of electro-magnetic energy conversion (Generator action)

Working Principle DC Generator 

The basic principle of a DC generator is electro-magnetic induction i.e.,

“When a conductor cuts across the magnetic field, an emf is induced in it.”

  

Consider Fig. 4.2(a), here, when a conductor is moved vertically upward or downward, the deflection in the galvanometer clearly shows that an emf is induced in the conductor since flux is cut  by the conductor. But, when it is moved horizontally (left or right),there is no deflection in  the galvanometer which shows that no emf is induced in the conductor since flux cut is zero and conductor moves just parallel to the magnetic lines of force.

    In fact, in a generator, a coil is rotated at a constant speed of Z radians per second in a strong magnetic field of constant magnitude as shown in Fig. 4.2(b). An emf is induced in the coil by the phenomenon of dynamically induced emf (e =Blv sin T; e v sin T). The magnitude and direction of induced emf changes periodically depending upon sine of angle T. The wave shape of the induced emf is shown in Fig. 4.2(c), which is AC for internal as well as external load.

   This AC is converted into DC with the help of commutator, as explained in the Articles to follow. Thus, "the working principle of a DC generator is electro-magnetic induction".

4.2 Main Constructional Features

The complete assembly of various parts in a scattered form of a DC machine is shown in Fig. 4.3.The essential parts of a DC machine are described below:                                       .. 

1. Magnetic Frame or Yoke

The outer cylindrical frame to which main poles and inter poles are fixed is called yoke. It also helps to fix the machine on the foundation. It serves two purposes; 

(i) It provides mechanical protection to the inner parts of the machine.

(ii) It provides a low reluctance path for the magnetic flux.

    The yoke is made of cast iron for smaller machines and for larger machines, it is made of cast steel

or fabricated rolled steel since these materials have better magnetic properties as compared to cast iron.

2. Pole Core and Pole Shoes

The pole core and pole shoes are fixed to the magnetic frame or yoke by bolts. They serve the following purposes:

(i) They support the field or exciting coils.

(ii) They spread out the magnetic flux over the armature periphery more uniformly.(

iii) Since pole shoes have larger X-section, the reluctance of magnetic path is reduced.

  Usually, the pole core and pole shoes are made of thin cast steel or wrought iron laminations which are riveted together under hydraulic pressure.

3. Armature Core

It is cylindrical is shape and keyed to the rotating shaft. At the outer periphery slots are cut, which accommodate the armature winding. The armature core  serves the following purposes:

(i) It houses the conductors in the slots.

(ii) It provides an easy path for magnetic flux.

Since armature is a rotating part of the machine, reversal of flux takes place in the core, hence hysteresis losses are produced. To minimise these losses silicon steel material is used for its construction. When it rotates, it cuts the magnetic field and an emf is induced in it. This emf circulates eddy currents which results in eddy current loss in it. To reduce these losses, armature core is laminated, in other words we can say that about 0.3 to 0.5 mm thick stampings are used for its construction. Each lamination or stamping is insulated from the other by varnish layer 

4. Armature Winding

The insulated conductors housed in the armature slots are suitably connected. This is known as armature winding. The armature winding acts as the heart of a DC machine. It is a place where one form of power is converted to the other form i.e., in case of generator, mechanical power is converted into electrical power and in case of motor, electrical power is converted into mechanical power. On the basis of connections, there are two types of armature windings named (i) Lap winding and (ii)Wave winding 

 (i) Lap winding: In this winding, the connections are such that the number of parallel paths is equal to number of poles. Thus, if machine has P poles and Z  armature conductors, then there will be P parallel, paths, each path will have Z/P conductors in series. In this case, the number of brushes is equal to the number parallel paths. Out of which half the brushes are positive and the remaining (half) are negative.

 (ii) Wave winding: In this winding, the connections are such that the numbers of parallel paths are only two irrespective of the number of poles. Thus, if machine has Z armature conductors, there will be only two parallel paths each having Z/2 conductors in series. In this case, the number of brushes is equal to two i.e., number of parallel paths.

5. Commutator

It is an important part of a DC machine and serves the following purposes:

 (i) It connects the rotating armature conductors to the stationary external circuit through brushes.

 (ii) It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit in generator action, whereas, it converts the alternating torque into unidirectional (continuous) torque produced in the armature in motor action.

The commutator is of cylindrical shape and is made up of wedge-shaped hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica. The segments are held together by means of two V-shaped rings that fit into the V-grooves cut into the segments. Each armature coil is connected to the commutator segment through riser. The sectional view of the commutator assembly

6. Brushes

The brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit. They are usually made of high grade carbon because carbon is conducting material and at the same time in powdered form provides lubricating effect on the commutator surface. The brushes are held in particular position around the commutator by brush holders and rocker.

7. Brush Rocker

It holds the spindles of the brush holders. It is fitted on to the stationary frame of the machine with nut and bolts. By adjusting its position, the position of the brushes over the commutator can be adjusted to minimise the sparking at the brushes.

8. End Housings

End housings are attached to the ends of the main frame and support bearings. The front housing supports the bearing and the brush assemblies whereas the rear housing usually supports the bearing only.