30 Questions
What is the conventional method of exciting the field windings of synchronous generators?
The conventional method involves mechanically coupling three machines: a pilot exciter, main exciter, and main 3-phase alternator, with the pilot exciter feeding the field winding of the main exciter, and the main exciter's dc output being given to the main alternator's field winding.
What are the limitations of the conventional excitation method?
The limitations include cooling and maintenance problems associated with slip rings, brushes, and commutators, especially as alternator ratings rise.
What is the purpose of the pilot exciter in the conventional excitation method?
The pilot exciter is a dc shunt generator that feeds the field winding of the main exciter.
What is the main exciter in the conventional excitation method?
The main exciter is a separately-excited dc generator that provides the dc output for the main alternator's field winding.
What is the trend in modern excitation systems?
The trend is to minimize sliding contacts and brushes, leading to the development of static-excitation and brushless-excitation systems.
What is the principle of static excitation?
Static excitation involves drawing the excitation power for the main alternator field from the output terminals of the main 3-phase alternator.
What type of excitation system is used in large turbogenerators due to its many advantages?
Brushless excitation system
What is the purpose of using space and time phasor diagrams in synchronous machines?
To analyze the machine performance
What is the effect of the orientation of the armature mmf wave with respect to the field poles in a synchronous machine?
It is examined in a 2-pole machine
What is the significance of the fir'nB' in the alternator?
It governs the alternator output voltage
What is the advantage of the brushless excitation system in terms of direct current?
It increases the direct current rating
What is the main difference between a brushless excitation system and a traditional brush-gear design?
The absence of sliding contacts and brushes
What is the approximate time-phase angle between Ia and Vt in Fig. 5.35 (b)?
90°
What is the effect of synchronous machine losses on the operating point A in Fig. 5.17 (6)?
Almost negligible
How can the leakage reactance XL be obtained from the diagram in Fig. 5.17 (6)?
By plotting the point A and using the method explained in Art. 5.3.2
What is the assumption made about the alternator in order to obtain its external load characteristics?
The alternator is running at constant speed and with constant excitation
What is the relationship between Ef and Vt at no load, according to Fig. 5.36 (a)?
Ef = Vt = 1.0
What happens to Vt relative to Ef when the load is lagging, according to Fig. 5.36 (b)?
Vt is less than Ef
What is the voltage induced in the armature winding due to the q-axis armature reaction component Faqt?
Eaq = - j K q Fuq = - j C Kq l q = - jX aq 7q
What are Xad and Xaq in a salient pole synchronous machine?
Xd and Xaq are respectively the d-axis and q-axis magnetizing reactances (or armature-reaction reactances)
What is the relationship between Fr, Ef, Ead, and Eaq in a salient pole synchronous machine?
Fr = Ef + Ead + Eaq
How can the armature leakage reactance drop be resolved?
It can be resolved into two components: - j Iqd *aid and - j Id *Xal
What is the voltage equation for each phase of a salient-pole synchronous generator?
E/ = V, + E, - jXadId - jXaqIq
What is the purpose of the phasor diagram in the analysis of salient-pole synchronous generators?
The phasor diagram is used to analyze the relationships between the various voltages and currents in the machine.
What is the sign of the active power P for a synchronous generator and a synchronous motor?
The active power P is positive for a synchronous generator and negative for a synchronous motor.
What is the value of reactive power Qog when the load angle δ is 0°, according to Eq.(5.45)?
Qog = 0.8, which is positive, indicating that the alternator delivers reactive power and operates at a lagging power factor.
What happens to the alternator when the reactive power Qog is negative, according to Eq.(5.45)?
The alternator absorbs reactive power and operates at a leading power factor.
What is the value of reactive power Qim when the load angle δ is 0°, according to Eq.(5.46)?
Qim = -0.8, which is negative, indicating that the motor delivers reactive power to the infinite bus and operates at a leading power factor.
Why is the reactive power Qim plotted above the reference line in Fig. 5.43 when the motor is delivering reactive power?
The reactive power Qim is plotted above the reference line to indicate that the motor is delivering reactive power to the infinite bus.
What happens to the alternator when the load angle δ is 60°, according to Eq.(5.45)?
The alternator neither delivers nor absorbs reactive power and operates at unity power factor.
Learn about the different methods of supplying dc excitation to the field winding of large synchronous machines, including conventional and modern schemes. This quiz covers the basics of synchronous generators and their excitation systems.
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