CHAPTER 2 DC GENERATOR Electrical Machines

Introduction A DC generator converts mechanical energy into electrical energy (DC). The working of a DC generator is based on the principle that, when a conductor cuts a magnetic field an e.m.f. is induced in the conductor. A dynamically induced e.m.f. will be produced in the conductor when it will cut the magnetic flux as per the laws of electromagnetic induction Electrical Machines

Introduction If the circuit is closed this induced e.m.f. will cause a current to flow through circuit. Basically a D.C. machine is an alternating current machine. The e.m.f. generated in the conductor is alternating in nature. With the help of the commutator segments this alternating e.m.f. is converted into a direct one. Electrical Machines

Construction of DC Machine Electrical Machines

Construction of a DC Machine Pole Armature Yoke Commutator Brushes Interpole Electrical Machines

Working Principle of a DC Generator Electrical Machines

Working Principle of a DC Generator Electrical Machines

Armature Winding There are two types of windings Lap winding The armature windings are divided into no. of sections equal to the no of poles Wave winding A = 2 Used in high voltage low current output. Requires 2 brushes Electrical Machines

Lap Winding Electrical Machines

Wave Winding Electrical Machines

Coil Terminologies Coil Coil span Commutator pitch Pole pitch Back pitch Front pitch Commutator pitch Resultant pitch Full pitch Fractional pitch Electrical Machines

EMF Equation Let, Ø= flux per pole in weber Z = Total number of conductor P = Number of poles A = Number of parallel paths N =armature speed in rpm Eg = emf generated in any on of the parallel path Electrical Machines

EMF Equation Flux cut by 1 conductor in 1 revolution = P * φ 60 sec = P φ N /60 Avg emf generated in 1 conductor = PφN/60 Number of conductors in each parallel path = Z /A Eg = PφNZ/60A Electrical Machines

Armature Reaction Electrical Machines

Armature Reaction Magnetic neutral axis: It is perpendicular to the lines of force between the two opposite adjacent poles. Leading pole Tip (LPT): It is the end of the pole which first comes in contact with the armature. Electrical Machines

Armature Reaction Trailing pole tip: It is the end of the pole which comes in contact later with the armature. Electrical Machines

Effect of Armature Reaction Efficiency of the machine is decreased Sparks are produced at the brushes. There is demagnetising effect on. the main poles. e.m.f induced is less Self-excited generators some times fail to build up e.m.f. Electrical Machines

Remedies of Armature Reaction Brushes must be shifted to the new position of the MNA Field winding must be provided with extra turns Reluctance is increased by making the slots on the tips The laminated cores of the shoe are staggered Electrical Machines

Remedies of Armature Reaction In big machines the compensating winding at pole shoes are used. Electrical Machines

Demagnetising and Cross Magnetising Ampere Turns Let us consider a DC machine having P poles with Z number of armature conductors. Let us assume that the armature is having A parallel paths. Electrical Machines

Commutation in DC Generator The change in direction of current takes place when the conductors are along the brush axis . During this reverse process brushes short circuit that coil and undergone commutation. Due to this sparking is produced and the brushes will be damaged and also causes voltage dropping. Electrical Machines

Methods to Improve Commutation Use of high resistance brush Brush shifting Interpoles Compensating winding Electrical Machines

Classification of DC Generator Separately excited DC machine Self excited DC machine Shunt wound DC generator Series wound DC generator Compound wound Electrical Machines

Separately excited DC machine In this type of machine, the field of the machine is excited by an independent external source. This source can be a battery, rectifier circuit or a D.C. generator Electrical Machines

Separately excited DC machine V be the field excitation voltage Rf be the field resistance. If be the field current Eg be the generated e.m.f. in the armature Ra be the armature resistance Ia be the armature current IL be the load current VL be the load voltage Electrical Machines

Series Wound DC Generator In this type of machine the field winding is connected in series with the armature winding of a DC generator Electrical Machines

Series Wound DC Generator In this type of machine, since the field winding carry the full armature current, the field winding has less number of turns of wire of large cross sectional area. From the machine circuit shown, Electrical Machines

Shunt Wound DC Generator In this type of machine, the field winding is connected in parallel with the armature winding. Electrical Machines

Shunt Wound DC Generator Electrical Machines

Compound Wound DC Generator In this type of DC machine the poles are having two separate windings. That is for a compound wound DC machine there are two field windings, namely, the shunt field winding and the series field winding. Electrical Machines

Compound Wound DC Generator Depending upon how these two windings are connected with the armature of the DC machine, they are classified as, Long shunt compound DC machine. Short shunt compound DC machine. Electrical Machines

Compound Wound DC Generator Short shunt Long shunt Electrical Machines

Magnetisation Characteristics This characteristic is called the magnetic characteristic or the Open Circuit Characteristic (O.C.C.) of a separately excited D.C. generator. Electrical Machines

Characteristic of a Separately Excited DC Generator Without Armature reaction With Armature reaction With Armature reaction and IR drop Electrical Machines

Characteristic of a DC Series Generator Open Circuit Characteristic Internal characteristic The external characteristic Electrical Machines

Critical Load Resistance Line OA represent the resistance of the external load circuit. Line OB, the critical load resistance Electrical Machines

Voltage Build-up in a DC Shunt Generator The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator. Electrical Machines

Characteristic of a DC Shunt Generator Internal characteristic External characteristic Electrical Machines

Critical Field Resistance Line OA represent the field resistance Line OB is called the critical field resistance. Electrical Machines

Critical Load Resistance Line A represent the external load resistance Line C is called the critical load resistance. Electrical Machines

Characteristic of a Compound DC Generator The external characteristic of a compound generator Electrical Machines

Application of DC Generator DC shunt generator is used for such applications where constant voltage is required. DC series generators are used as Boosters and also as a supply for arc lamps. Differential Compound generators are used to supply dc welding machines. Level compound generators are used to supply power for offices, hostels and Lodges etc. Electrical Machines

Application of DC Generator Over compound generators are used to compensate the voltage drop in Feeders. Separately excited generator are used as a supply for DC Motors ,it is also used for such applications where a wide range of voltage is required for the testing purposes. Electrical Machines