CHAPTER s Synchronous Machines an alternator\'s voltage drop sharply when it is
ID: 2268721 • Letter: C
Question
CHAPTER s Synchronous Machines an alternator's voltage drop sharply when it is loaded down s an alternator's voltage rise when it is loaded down with a he phasor diagrams and magnetic field relationships why does 's 2, with a lagging load? leading load? sy Sketch the phasor dia 54nous generator operating at (a) unity power factor, (b) lagging r factor, (c) leading power factor. ENplain how the synchronous impedance and armature resistance can be determined in a synchronous generator. What is the di generator? 5-6. at is the speed regulation of a synchronous motor? Wh When would a synchronous motor be used even though its constant-speed characteristic is not needed? Why can't a synchronous motor start by itself? What techniques are available to start a synchronous motor? What are amortisseur windings? Why is the torque produced by them nidirectional at starting, while the torque produced by the main field winding alternates direction? Explain, using phasor diagrams, what happens to a synchronous motor as 5-8. L9. 10 12. field current is varied. Derive a synchronous motor V curve from the phasor diagram. ynchronous motor's field circuit in more danger of overheating when it is operating at a leading or at a lagging power factor? Explain, using phasor diagrams. A synchronous motor is operating at a fixed real load, and its field current is increased. If the armature current falls, was the motor initially operating at a lagging or a leading power factor? 5-13. Is a s 5-14. 5-15. Why must a 60-Hz generator be derated if it is to be operated at 50 Hz? How much derating must be done? Would you expect a 400-Hz generator to be larger or smaller than a 60-Hz generator of the same power and voltage rating? Why? Why is overheating such a serious matter for a synchronous machine? What are short-time ratings? Why are they important in regular generator operation 5-16.Explanation / Answer
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5-2. the voltage of the alternator drop sharply when it is loaded with a lagging load due to the effect of armature reaction and reactance voltage drop in the alternator.
5-3.when an alternator is loaded with leading loads , then terminal voltages will increase sharply, this is because of the armature reaction in the machine
5-6.
the complete behavior of a synchronous machine can be determined with the help of its equivalent circuit. the circuit consist of two elements namely resistance and reactance. resistance can be determined simply by ohms law. the calculation of reactance of a synchronous generator is the combination of armature reactance and the equivalent value of reactance due to armature reaction. the reactance of the synchronous generator is called as synchronous reactance. to calculate synchronous reactance you need open and short circuit characteristics
5-15
this can be done in either change in rotor poles or the change in the prime mover speed. when the frequency is reduced to 50Hz then its rated terminal voltage is reduced to 5/6th of the rated terminal voltage, and in the same way the power rating of the machine aslo reduces be a factor of 5/6
5-16
the higher the frequency the less flux is required. lesser flux means lesser core size. so a 400Hz generator is smaller than a 60Hz generator
5-9
The stator carries windings connected to an AC supply to produce a rotating magnetic field. At synchronous speed the rotor poles lock to the rotating magnetic field. ... Because of the constant magnetic field in the rotor these cannot use induction windings for starting
the complete behavior of a synchronous machine can be determined with the help of its equivalent circuit. the circuit consist of two elements namely resistance and reactance. resistance can be determined simply by ohms law. the calculation of reactance of a synchronous generator is the combination of armature reactance and the equivalent value of reactance due to armature reaction. the reactance of the synchronous generator is called as synchronous reactance. to calculate synchronous reactance you need open and short circuit characteristics