DC GENERATOR

A generator is a machine which converts the mechanical energy into electrical energy. It works on the principle of Faraday's law of electromagnetic induction. DC Generators produce the direct current.

Ordinary generator working

            The figure shows two pole and one coil generator, two slip rings are connected with the two terminals of the coil. 

Figure 1: The coil is at 90 degree to the field, there will be no cutting of flux so no emf is produced

Figure 2: The coil sides are parallel to the field; hence maximum emf will be induced

Figure 3: The coil is in the same position as in figure 1, but ab side is downwards and cd is upwards, there is no voltage will be induced.

Figure 4: It is similar to figure 2, maximum emf is induced but in the reverse direction.

Figure 5: In this way a cycle of ac voltage is completed. 

Commutation

Commutation in a dc machine is the process of converting alternating current into direct current by the help of commutator

Emf equation of dc generator 

Φ = flux produced by each poles

P = number of poles in a generator  

so total flux produced = Φ x P

Time taken for one revolution = 60/N        where N is the speed of armature conductor in rpm

according to Faraday's law, e = dΦ/dt

e = total flux/time taken = Φ x P / (60/N)

that is emf induced in one conductor is

                                           e = ΦPN/60

Z = total number of conductors

A = number of parallel paths

Number of conductors connected in series = Z/A

total induced emf = emf of one conductor x number of conductors connected in series

                             = ΦPN/60 x Z/A

induced emf of a dc generator e =  ΦPN/60 x Z/A  

notes A = 2 for wave winding, A = P for lap winding

Yoke
     The dc generator is fixed in a foundation, this foundation is known as Yoke/body. It is made either cast iron, cast steel or rolled steel. The two main function of yoke are
1) It support the pole cores and provides mechanical protection to the inner parts of machines
2) It provides a low reluctance path for magnetic flux
Armature
     The rotating part of the dc machine is called Armature. It consists of
1) laminated core
2) Armature core and Armature winding
Armature core provides a) Easy path for magnetic flux
                                       b) Houses conductors in the slots
Armature winding - The arrangement of conductors and the steel wire around the core. their are two types of Armature winding a) lap winding - for low voltage and high current applications
                                            b) wave winding - for high voltage and low current applications
3) Commutator
   It consists of copper bars/segments insulated from each other and from the shaft by a insulator(Mica) pieces. Each commutator segments are connected to the end of armature coils.
Pole Core 
     The pole core and Pole shoes are either cast with frame or bolted to it. The pole core have the following functions
1) Supports the field/Exciting coils
2) It spread out the magnetic flux
Field Coils
      Each pole has one or more field winding's  to produce magnetic field. Most commonly the copper wire is used for construction of field coils.

Brushes
      A dc machine consists of two or more stationary carbon brush set held in box type brush holder. They make contact on the commutator with a certain amount of pressure. The pressure exerted by the brushes on the commutator can be adjusted and is maintained at constant value by means of a spring.
Shaft
   The shaft is used to transfer mechanical power to the machine. It is made by mild steel with a maximum breaking strength