DC Generator Made By : Kaushal V Sangdot(130990109017)

Introduction Generator is a type of machine which convert mechanical energy into electrcal energy. It is rotated by a prime mover to produce electricity.

Principle of operation of a DC generator A DC generator operates on the principle of dynamically induced emf in a conductor. The dynamic induction into the conductor with the help of Farraday`s Law of electromagnetic induction. If the flux linkage with a conductor changes due to the relative motion b/w the magnetic field & conductor then the emf is induced into the conductor.

DC Generator Construction The main parts of a DC machine are Yoke Poles, pole shoes Field Coils Armature Commutator Brushes & bearings

YOKE Gives mechanical support for poles Protects whole machine as a protecting cover Provides path for the circulation of magnetic flux Small generators – cast iron Large machines – cast steel

Pole cores & Pole shoes Field magnets has two parts Pole cores Pole shoes: spread out the flux in the air gap & reduce the reluctance Support the exciting coils Pole cores: solid piece made of cast iron & cast steel Pole shoes are laminated to the pole face by screws.

Field Winding Function : To carry current due to which pole core behaves as an ELECTROMAGNET, producing necessary flux. It helps in producing magnetic flux. Field winding is divided into various coils called field coils. These are connected in series with each other and wound in such a direction that an alternated ‘N’ and ‘S’ poles are created.

Armature More loops of wire = higher rectified voltage In practical, loops are generally placed in slots of an iron core The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature. The windings of armatures are connected in different ways depending on the requirements of the machine.

Commutator To facilitate collection of current from the armature conductors. & it rectified. i.e. converts the alternating current induced in the armature conductors in unidirectional current in the external load circuit. It is of cylindrical and made up of copper.

Brushes & bearings Brushes : To collect current from commutator Made-up by carbon or graphite with in rectangular shape. Bearing is used for friction less smooth operation of DC Machine

Differential Compound Methods of Excitation Excitation Separate Excitation Self Excitation Series Excitation Shunt Excitation Compound Excitation Cumulative Compound Differential Compound Short Shunt Long Shunt

Separate Excitation It consists of several hundreds of turns of fine wires and is connected to separate or external D.C. source. External D.C. voltage source has no relation with the armature voltage.

Methods of Excitation Self Excitation Field winding is excited by its own armature . Based on the connection of field winding there are three sub division of self excitation are (a) Series Excitation, (b) Shunt Excitation and (c) Compound Excitation are shown in figure.

Methods of Excitation Self Excitation (Cont.) Cumulative Compound Differential Compound Short Shunt Long Shunt

E.M.F. Equation of a D.C. Generator Let φ= flux/pole in Wb Z = total number of armature conductors P = number of poles A = number of parallel paths = 2 for wave winding = P ... for lap winding N = speed of armature in r.p.m. Eg = e.m.f. of the generator = e.m.f./parallel path Flux cut by one conductor in one revolution of the armature, dφ = Pφ wb Time taken to complete one revolution, dt = 60/N second

E.M.F. Equation of a D.C. Generator e.m.f generated/conductor e.m.f. of generator, Eg = e.m.f. per parallel path = (e.m.f/conductor) *No. of conductors in series per parallel path where A = 2 for-wave winding = P for lap winding Where K=PZ/A Eg α φn where n=N/60= r.p.s

Differential Compound Types of Generator DC Generator Permanent Magnet Separate Excited Self Excited Series Wound Shunt Wound Compound Wound Cumulative Compound Differential Compound Short Shunt Long Shunt

Types of Generator Permanent Magnet Generator When the flux in the magnetic circuit is established by the help of permanent magnets then it is known as Permanent magnet dc generator. This type of dc generators generates very low power. So, they are rarely found in industrial applications. They are normally used in small applications like dynamos in motor cycles.

Types of Generator Separately Excited Generator These are the generators whose field magnets are energized by some external dc source such as battery . Ia = Armature current IL = Load current V = Terminal voltage Eg = Generated emf Voltage drop in the armature = Ia × Ra (Ra is the armature resistance) Let, Ia = IL = I (say) Then, voltage across the load, V = Eg - IRa Power generated, Pg = Eg×I Power delivered to the external load, PL = V×I.

Types of Generator Self Excited Generator These are the generators whose field magnets are energized by the electric current supplied by themselves. In these type of machines field coils are internally connected with the armature. Due to residual magnetism some flux is always present in the poles. When the armature is rotated some emf is induced. Hence some induced electric current is produced.  As the pole flux strengthened, it will produce more armature emf, which cause further increase of electric current through the field. This increased field electric current further raises armature emf and this cumulative phenomenon continues until the excitation reaches to the rated value.

Types of Generator Series Wound Generator In these type of generators, the field windings are connected in series with armature conductors whole electric current flows through the field coils as well as the load. As series field winding carries full load electric current it is designed with relatively few turns of thick wire. The electrical resistance of series field winding is therefore very low (nearly 0.5Ω ). Rsc = Series winding resistance, Isc = Current flowing through the series field Ia = Isc = IL=I (say) Voltage across the load V = Eg -I(Ra+Rsc), Power generated Pg = Eg×I Power delivered to the load PL = V×I

Types of Generator Shunt Wound Generator In these type of DC generators the field windings are connected in parallel with armature conductors In shunt wound generators the voltage in the field winding is same as the voltage across the terminal. Rsh = Shunt winding resistance Ish = Current flowing through the shunt field The effective power across the load will be maximum when IL will be maximum. So, it is required to keep shunt field electric current as small as possible. For this purpose the resistance of the shunt field winding generally kept high (100 Ω) and large no of turns are used for the desired emf. Ia=Ish + IL Shunt field current, Ish = V/Rsh Voltage across the load, V = Eg-Ia Ra Power generated, Pg= Eg×Ia Power delivered to the load, PL = V×IL

Types of Generator Compound Wound Generator In a compound-wound generator, there are two sets of field windings on each pole—one is in series and the other in parallel with the armature. This combination of windings is called compound wound DC generator. Compound wound generators have both series field winding and shunt field winding. One winding is placed in series with the armature and the other is placed in parallel with the armature. This type of DC generators may be of two types- short shunt compound wound generator and long shunt compound wound generator.

Types of Generator Compound Wound Generator (Short Shunt) The generators in which only shunt field winding is in parallel with the armature winding as shown in figure. Series field current, Isc = IL Shunt field current, Ish = (V+Isc Rsc)/Rsh Armature current, Ia = Ish + IL Voltage across the load, V = Eg - Ia Ra - Isc Rsc Power generated, Pg = Eg×Ia Power delivered to the load, PL=V×IL

Types of Generator Compound Wound Generator (Long Shunt) The generators in which shunt field winding is in parallel with both series field and armature winding as shown in figure. Shunt field current, Ish=V/Rsh Armature current Ia= series field current Isc= IL+Ish Voltage across the load V=Eg-IaRa-Isc Rsc =Eg-Ia (Ra+Rsc) [∴Ia=Ics] Power generated, Pg= Eg×Ia Power delivered to the load, PL=V×IL

Types of Generator Compound Wound Generator (Cumulative & Differential Compound) In a compound wound generator, the shunt field is stronger than the series field. When the series field assists the shunt field, generator is said to be commutatively compound wound. On the other hand if series field opposes the shunt field, the generator is said to be differentially compound wound.

THANK YOU