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BVRIT HYDERABAD College of Engineering for Women Department of Electrical and Electronics Engineering

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BVRIT HYDERABAD College of Engineering for Women Department of Electrical and Electronics Engineering BVRIT HYDERABAD College of Engineering for Women Department of Electrical and Electronics Engineering Hand Out Subject Name: Electrical Machine-III Prepared by (Faculty(s) Name): Babita Gupta, Assistant Professor, EEE Year and Sem, Department: III Year- I Sem, EEE Unit – I: synchronous Machines & characteristics Important Points / Definitions: Synchronous machine is know as doubly excited machine. Synchronous machines can be operated as either generators or motors., The main parts of a synchronous machine are identified: rotor (field); stator (armature); prime mover ; exciter. An alternator which converts mechanical energy from a prime mover to AC electric power at specific voltage and frequency. It is also known as synchronous generator. The alternator work on the principle of faraday’s law of electromagnetic induction. The direction of the induced current can be determined by Flemming's right-hand rule. The armature winding in an alternator either open & closed type. Closed winding forms star connection in an alternator. There are different types of armature winding used in alternator. 1. Single phase and poly phase armature winding. 2.Concentrated winding and distributed winding 3.Full pitch coil winding and short pitch coil winding. 4.Intergral winding and fractional slot winding. There are two types of rotors in an alternator:- 1. Salient pole type. 2. Smooth cylindrical type. The e.m.f equation of an alternator is Eph= 4.44fΦTphKcKd volts Synchronous generator always runs at synchronous speed i.e Ns = 120f/P. Harmonics which are generated in the e.m.f due to slotting is called slot harmonics. Different ways to eliminate the harmonics from generated voltage: 1. Distribution of armature winding 2.Chording 3.Fractional slot winding 4.Skewing 5.Large length of air gap. Effect of armature flux on the main flux affecting it’s value & the distribution is called armature reaction. The effect of armature reaction depends on the power factor i.e the phase relationship between the terminal voltage and armature current. Questions 1. Define synchronous machine & its caharacteristics. 2. Distinguish between salient pole & non- salient type rotor 3. Explain construction & working principle of alternator 4. Derive emf equation of three phase of alternator 5. Define harmonics 6. What are the different ways to eliminate the harmonics in generated emf. 7. What is slot harmonics 8. What is expression for pitch factor 9. What is the expression for distribution factor 10. What is the effect of harmonics in generated emf of an alternator. 11. Which type of alternator is used in hydroelectric power station? 12. Which type of alternator is used in steam turbines? Fill in the blanks / choose the Best: 1. When a turbine drives an alternator it is termed as turboalternator 2. A two pole alternator running at 1500 rpm will generate emf at 25 Hz. 3. A generator has two different windings, armature,field winding. 4. In a fractional-pitch winding the coil span is less than1800 electrical degrees. 5. In a synchronous generator, the armature is always a stationary member, and the field moving member. 6. High capacity generators are usually Hydrogen cooled. 7. The efficiency of a synchronous motor is more than that of an induction motor of same size and output rating. 8. The cost of a synchronous motor is High in comparsion to that of an induction motor of same size and output rating 9. Alternator operates on the principle of Electromagnetic induction 10. In synchronous machine stator frame is made of cast iron or welded steel plates 11. The slip rings employed in a three phase synchronous machine are insulated for very low voltage 12. The stator core of a synchronous machine is built up of silicon steel laminations. Unit – II: Regulation of synchronous generator Important Points / Definitions: The voltage regulation is the percentage of voltage difference between no load and full load voltages with respect to its full load voltage. Voltage regulation = Ef - Vt Vt Voltage regulation can be determined by direct method and indirect method. Direct method can be applicable for small rating of an alternator and it can be done by o.c and s.c test. Indirect method can be applicable for large rating of an alternator. Different method for finding voltage regulation of an alternator:- 1. Synchronous impedance method or E.M.F method 2. Ampere-turns method or M.M.F method 3.Zero power factor metod or potier triangle method 4. ASA modified form of M.M.F method Synchronous impedance method is called pessimistic method Ampere-turns method is called optimistic method American standard Association method is superior to ZPF method because it consider drop due to Armature Reaction, Leakage Reactance and also consider Saturation. Emf generated in a synchronous motor is given by: Potier triangle is drawn in between Open circuit characteristics and Zero power factor characteristics. Slip test is used to determine the value of xd and xq Synchronous reactance is the combined reactance due to leakage flux and armature reaction. Questions 1. Define Voltage regulation. 2. At what power factor Zero Voltage regulation occurs 3. What is the use of Damper Windings? 4. Why ASA method is superior to ZPF method? 5. Define Slip test? 6. What is the relation between Xd &Xq for non salient pole machine 7. Which method is called pessimistic method? 8. Which method is called optimistic method? 9. What is meant by Quadrature and Direct axis reactance 10. What are the advantages of Slip Test? 11. Why do you conduct Slip-test 12. Salient pole Alternators are used in which type of power plants 13. What is meant by two axis reactance theory Fill in the blanks / choose the Best: 1. Synchronous impedance method is called pessimistic method . 2. Ampere-turns method is called optimistic method 3. Slip test is used to determine the value of xd and xq 4. Slip test is used for non-salient pole machine. 5. Salient pole machine has non-uniform air gap. 6. Salient pole alternators are used in Hydro-electric plants. 7. Zero power factor method of an alternator is used to find its voltage regulation 8. During slip test for maximum current, the armature mmf aligns along d-axis. 9. The frequency of voltage generated in an alternator depends on no. of poles 10. In synchronous impedance method the value of synchronous method is more than normal value 11. Synchronous reactance is the combined reactance due to leakage flux and armature reaction. 12. The leakage reactance of a three phase alternator is determined by performing open-circuit and zero power factor test. Unit – III: Parallel operation of synchronous generator Important Points / Definitions: The process of connecting an alternator in parallel with another alternator or with common bus- bars is called the synchronization. Bus bars are copper rods which operate at constant voltage & frequency. The synchronous machine which is to be synchronized is called incoming machine. Synchronization can be done by lamp method in case of single phase motor. A synchroscope is a special device used for synchronizing the machine more accurately. An alternator synchronised to the infinite bus delivers lagging power to the bus when the field is overexcited. When two alternators are operating in proper synchronism,the synchronism power will be zero. synchonizing current circulating in the local circuit of alternator operating in parallel which brings the alternator in synchronism once they are out of it. Necessary condition to be satisfied for synchronising an incoming alternator to an already operating alternator should have: 1.Same voltage magnitude. 2.Same frequency. 2. Same phase sequence. Questions 1. Define Synchronism. 2. Define infinite busbar. 3. What conditions must be fulfilled before an alternator can be connected to an infinite bus.? 4. Why parallel operation of alternators is necessary? 5. What are the advantages of connecting alternators in parallel. 6. What are the various methods of synchronization of alternators. 7. Define significance of transient and subtransient reactances in an alternator? 8. Explain the effect of change in excitation on the parallel operation of two alternators? 9. Define synchronizing torque and synchronizing power? 10. Define load sharing between two alternators. 11. Define synchonizing current? Fill in the blanks / choose the Best: 1. For parallel operation , the frequencies of two machines should be same 2. When two alternators are operating in proper synchronism,the synchronism power will be zero. 3. An alternator synchronised to the infinite bus delivers lagging power to the bus when the field is overexcited. 4. The process of connecting an alternator in parallel with another alternator or with common bus-bars is called the synchronization. 5. Two- bright and one dark lamp method is used for synchronising of three phase alternator. 6. For successful parallel operation of two alternators should have high reactance in comparsion to resistance. 7. Transient response analysis is done for stable systems. 8. Synchronizing torque will come into operation whenever there is difference in the magnitude of voltages. 9. A stationary alternator should not be connected to the live bus-bar it will get short circuited. 10. If the prime mover of an alternator supplying load to an infinite bus suddenly shut down then it will continue to run as synchronous motor in the same direction. Unit – IV: Synchronous Motors Important Points / Definitions: Electrical motor in general is an electro-mechanical device
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