By:Sheikh Touqir Nisar Registration No.2008-RUET-ELECT-10

1. Presentation Electrical Machines Sourceshttp://www.wikipedia.orghttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://www.phys.unsw.edu.au/~jw/HSCmotors.html By:Sheikh Touqir Nisar Registration No.2008-RUET-ELECT-10 http://www.touqirnisar.edu.tr/~erdas/vcs159.htm

2. Introduction • DC Generators are the machines that convert mechanical energy into DC Electrical energy. • These machines derive their names from the fact that they convert mechanical energy into DC voltage and current. • There is no appreciable difference between DC Motors & DC Generators Except for the direction of Power flow. • All Generators are driven by a source of Mechanical power which is usually called Prime mover(i-e Diesel Engine, Steam Turbine or even an Electric Motor) • DC Generators are quite rare in modern Power Systems. Even DC Power Systems such as those in automobiles now use AC Generators plus rectifiers to produce AC Power.

3. Working Principle of DC Generators The Energy Conversion is based on the principle of production of Dynamically (or motionaly) Induced e.m.f. According to Faraday’s Law of Electromagnetic Induction, “When a conductor moves in the magnetic field, it cuts magnetic flux due to which an e.m.f.is induced which is called Dynamically induced e.m.f “. Its direction is given by the Fleming Left Hand Rule as shown in the above figure.

4. Construction If we talk about the construction of DC Generators then, • From the Mechanical point of view, there are two major parts of DC Generators that are: • Stator • Rotor • And From Electrical Point of view, we have also two major parts of DC Generators that are : • Field • Armature

5. Construction (continued…) Mechanical point of view • Stator Stator of that part of DC Generators that is Stationary. The purpose of stator is to provide the Magnetic Field. 2. Rotor The purpose of the rotor is to provide the rotating element in a DC machine .In a DC generator, the rotor is the component that is rotated by an external force

6. Construction (continued…) Electrical point of view • Armature It is that part of DC Generators, We get the output.In a DC generator, the armature is rotated by a prime mover This rotation induces a voltage And current flow in the armature 2. Field The purpose of the field in a DC machine is to provide a magnetic field for producing voltage .The field in a DC machine is produced by either a permanent magnet or an electromagnet. Normally, electromagnets are used because they have an increased magnetic strength which can easily be varied using external devices.

7. Construction (continued…)

8. Types of DC Generators The dynamic behavior of DC Generators is mainly determined by the type of the connection between the excitation winding and the armature winding. On this basis, there are two types of DC Generators that are as follows. • Separately excited DC Machines In these type of DC Generators, excitation and armature windings are electrically disconnected but Magnetically coupled (i-e Their Field Magnates are energized form an independent source of electric current. • Self excited DC Machines In these type of DC Generators, excitation winding is energized by the current produced by the generators themselves.

9. Types of DC Generators (Continued…) Self Excited DC Generators There are three types of self excited DC machines named according to the manner which their excitation windings are connected to their armature windings. • Shunt wound DC Generators Excitation and armature winding are connected in parallel (i.e. fed by the same source)

10. Types of DC Generators (Continued…) 2. Series wound DC Generators When the field winding of a DC generator is connected in series with the armature, the generator is called a series-wound generator.The excitation current in a series-wound generator is the same as the current the generator delivers to the load.

11. Types of DC Generators (Continued…) 3. Compound wound DC Generators These are the combination of shunt wound and series wound DC Machines and can be either “Long Shunt” or “Short Shunt”. The two windings of the compounded generator are made such that their magnetic fields will either aid or oppose one another.

12. DC Generators Family

13. Theory of Operation of DC Generator There are three conditions necessary to induce a voltage into a conductor. 1. A magnetic field 2. A conductor 3. Motion between the magnetic field & conductor. A DC generator provides these three conditions to produce a DC voltage output. The magnetic field may be supplied by either a permanent magnet or an electromagnet.

14. Conditions for the Voltage build up Following are the conditions for the voltage build up: • Presence of “Residual Flux”. • “Field winding” must be properly connected with the “Armature Winding”. • Generator must be rotated at a speed equal to or greater than the “Critical Speed”. • Value of Field Winding Resistance “Rf ” must be greater than the critical value of Resistance. • Armature must be rotated in a proper direction so that the Residual Flux must not be cancelled out by the flux produced by the main winding. • If the Generator is started at loaded conditions then the value of Load Resistance must be much greater than the value of the resistance of shunt winding.

15. Operation of a Single Loop Generator A single conductor, shaped in the form of a loop, is positioned between the magnetic poles as shown in the figure. The magnitude of the induced EMF depends on the field strength and the rate at which the flux lines are cut. The stronger the field or the more flux lines cut for a given period of time, the larger the induced EMF.

16. DC Generator Terminologies Terminal Voltage: Terminal voltage is defined as the voltage that can be measured at the output terminals of the generator. Applied Voltage: voltage is defined as the voltage that is delivered acrossthe load. This voltage should be the same as terminal voltage; however, various circuit faults and losses may reduce the terminal voltage Counter-Electromotive Force (CEMF) In a generator, the conductors cut the magnetic lines of force in the magnetic field. Voltage is induced in the armature conductors. This induced voltage opposes the applied voltage; it counteracts some of the applied voltage, which reduces the current flow through the armature. This induced voltage acts counter to applied voltage; therefore, it is called CEMF

17. DC Generator Terminologies (continued…) Commutation Commutation is the positioning of the DC generator brushes so that the commutator segments Change contact with the brushes at the same time when the armature current changes direction.

18. Theory of Real DC Generator Commutator Action: • The commutator switches the current from one rotor coil to the adjacent coil. • The switching requires the interruption of the coil current. • The sudden interruption of an inductive current generates high voltages . • The high voltage produces flash over and arcing between the commutator segment and thebrush. 5. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce sparking

19. Armature Reaction

20. Theory of Real DC Generator(continued…) Each brush slides along one half of the commutator and then along the other half. The brushes are positioned on opposite sides of the commutator; they will pass from one commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. Every time the ends of the loop reverse polarity, the brushes switch from one commutator segment to the next. This means that one brush is always positive with respect to another The voltage between the brushes fluctuates in amplitude (size or magnitude) between zero and some maximum value, but is always of the same polarity. In this manner, commutation is accomplished in a DC generator.

21. DC Generator Ratings Voltage Current Power Speed Voltage rating of a machine is based on the insulation type and design of the machine. The current rating is based on the size of the conductor and the amount of heat that can be dissipated in the generator. The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors, bearings, and other components of the generator. Speed rating, at the upper limit, is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current (as speed increases, a higher field current is necessary to produce the same voltage).

22. Internal Loses in the DC Generator There are four internal losses that contribute to lower efficiency of a DC generator. • Copper losses Copper loss is the power lost as heat in the windings; it is caused by the flow of current through the coils of the DC armature or DC field & are given by Armature Ia² Ra Field If² Rf • Eddy-current losses As the armature rotates within the field is made of iron, an EMF is induced in the iron, which causes a current to flow. These circulating currents within the iron core are called eddy-currents.To reduce eddy-currents, the armature and field cores should be laminated (layered).

23. Internal Loses in the DC Generator • Hysteresis losses Hysteresis losses occur when the armature rotates in a magnetic field. The magnetic domains of the armature are held in alignment with the field in varying numbers, dependent upon field strength. The magnetic domains rotate, with respect to the particles not held in alignment, by one complete turn during each rotation of the armature. This rotation of magnetic domains in the iron causes friction and heat. The heat produced by this friction is called magnetic hysteresis loss. • Mechanical losses Rotational or mechanical losses can be caused by bearing friction, brush friction on the commutator, or air friction (called windage), which is caused by the air turbulence due to armature rotation. Careful maintenance can be instrumental in keeping bearing friction to a minimum.