C.K. PITHAWALA COLLEGE OF ENGINEERING & TECHNOLOGY ACTIVE LEARNING ASSIGNMENT

Presented By :- GROUP :- 8 YEAR :- 2nd YEAR – 3rd SEM SUBJECT :- DC Machine & transformer TOPIC :- Basics of DC Machines

Construction of DC machine

Fig. shows the construction of DC generator Fig. shows the construction of DC generator. The construction of a DC generator is same as that of DC motor. Important parts of a Generator: Yoke Field winding Poles Armature Commutator, brushes and gear Bearings

Yoke Yoke is also called as frame. It provides protection to the rotating and other parts of the machine from moisture, Yoke is an iron body which provides path for the flux. This is essential to complete the magnetic circuit. It provides a mechanical support for the poles.

Poles, Pole shoe and pole core A pole of a generator is an electromagnet. The field winding is wound over the poles. Poles introduce the magnetic flux when the Field winding is excited. Pole shoe is an extended part of a pole. Due to its typical shape , it enlarges the Area of the pole .

Field Winding The coils wound around the pole cores are called field coils. The field coils are connected in series to form the field winding. The field winding is also called as exciting winding. The material used for the field conductor is copper.

Armature core Armature core is a cylindrical drum mounted on the shaft. It is provided with a large number of slots All over its periphery. All these slots are parallel to the shaft axis. Armature conductors are placed in these slots. Armature core provides a low reluctance path to the flux produced by the field winding.

Commutator A commutator is a cylindrical drum mounted on the shaft along with the armature core. It is made of a large number of wedge-shaped Segments of hard-drawn copper. The armature winding is tapped at various Points and these tappings are succesively Connected to various segments of the Commutator.

Working Principle of DC Machine

A DC generator operates on the principle of dynamically induced emf in a conductor. The dynamic induction of emf can be explained with the help of Faraday’s laws of electromagnetic induction. The relative motion is produced by either moving the magnetic flux or by moving the conductor. The practical DC generator operates on the principle of dynamic induction. To implement the principle , the parts of dc generator are as follows: An electromagnet to produce the magnetic flux. A movable conductor placed in this field. A mechanism to rotate the conductor. Hence, in a practical dc generator a stationary winding is called field winding Is used to produce flux.

Types of DC Generators. (Methods of Excitation) A dc generator consists of two winding namely the field winding which produces the magnetic flux and the rotating armature winding. The current can be supplied to the field winding of a generator in two different way. The way in which field current of a generator I supplied will decide the type of dc generator. The two different ways of supplying the field current are as follows : To connect the field winding to an external source such as battery. To use the emf induced in the armature winding of the generator to supply its own field current.

Generators Separately excited generators Self excited generators Shunt Generators Series generators Compound Generators Cumulative compound generators Differential compound generators Long shunt cumulative generator Short shunt Long shunt differential generator Short shunt

Armature Windings Pole Pitch : It is defined as the periphery of the armature divided by the number of poles of the generator or distance between two adjacent poles. Conductor : We can define the conductor as the length of wire lying in the magnetic field and in which an e.m.f. is induced.

Coil span or coil pitch : Coil span or coil pitch Is defined as the distance measured in terms of armature slots between two sides of couple. Commutator pitch : The commutator pitch is defined as the distance between the segments to which the two ends of a coils are connected

Types of Armature Winding Armature winding Lap winding Wave winding

Lap winding For lap winding the number of parallel paths is exactly equal to the number of poles P. Number of parallel paths(A) = Number of poles(p) The lap winding is useful foe law voltage high current machines

Wave Winding This type of winding will crate only two parallel paths irrespective of the number of poles. Number of poles(A) =2 Wave winding is useful for high voltage law current machines.

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