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Tuesday, December 4, 2012

CHAPTER 14 QUESTIONS





1.       Show by a sketch the principle of Faraday’s Law of Electromagnetic Induction and explain how it is used to achieve electrical generation.

2.       Explain the difference between a permanent magnet and an electromagnetic field system.

3.       What is the name given to that part of a generator in which voltage is induced?

4.       What is the essential difference between a fixed field and a rotating field generator?  What are the advantages of the latter?

5.       In a fixed field generator, how is the current taken from the rotating conductors?

6.       How is a rotating field generator excited?

7.       What do you understand by ‘brushless excitation’?  What are its advantages over other forms?  Make a sketch diagram of an a.c. generator with brushless excitation.

8.       How is the output voltage of a generator controlled?

9.       What distinguishes a d.c. generator from an a.c.?

10.     Describe briefly the construction of a 3-phase generator.

11.     What are the two main methods of interconnecting the three 3-phase machine windings?

12.     In a star-connected generator feeding a balanced load what current flows in the neutral (star-point) connection?

13.     Where would you expect to find a 3-wire, and where a 4-wire, distribution on a 3-phase system?

14.     What is the effect of including iron inside a coil of wire?

15.     If a single-phase a.c. supply is connected to such a coil, what effect would it have on the magnetic field (a) inside, and (b) outside the coil?

16.     How is a rotating magnetic field produced statically from a 3-phase system?

17.     A 50Hz, 3-phase supply is applied to a ‘2-pole’ a.c. motor.  What is the speed of rotation of the field?

18.     If the motor in Q.17 were ‘4-pole’ and the frequency of the supply were 60Hz, what would then be the speed of rotation of the field?

19.     Can a rotating field be produced from a single-phase supply?  If so, describe one method.


20.     What do you understand by ‘simple harmonic motion’?  Sketch a voltage sine-wave and identify its parts by symbol and name.

21.     What is the ‘amplitude’ of an alternating quantity?  How do you express it in the mathematical form for such a quantity?

22.     In practice are electrical alternating quantities always of pure sine-wave form?  From what sources would you expect to find distortion?

23.     What is the relationship between an rms quantity and its peak value?  Define what is meant by an rms current.

24.     Write down the relationship between the sides of a right-angled triangle.  If the angle at one corner is j, the hypotenuse is ‘H’ and the sides adjacent to and opposite that corner are ‘A’ and ‘O’ respectively, write down the values of cos j, sin j and tan j.

25.     In a right-angled triangle one angle is 40° and the hypotenuse is 15cm.  What are the lengths of (a) the opposite and (b) the adjacent side?

26.     What is a ‘vector’, and how does it differ from a simple measurement?  Sketch a vector system where the voltage is 100V and the current 80A lagging 30° on the voltage.





27.     What is the vector sum A + B?

           
28.     In the figure of Q.27, what is the vector difference A - B?

29.     What is the line voltage of a star-connected 3-phase system whose individual phase voltages are 3.81kV?  What would the line voltage be if these same windings were delta-connected?

30.     If a 3-phase, delta-connected generator supplied a load with balanced line current of 200A in each line, what current flows in each phase-winding of the generator?

31.     What is the greatest voltage experienced under normal conditions between the line conductors of a cable (and therefore the greatest strain on the insulation between them) if line-voltage voltmeter reads 132kV?

32.     What is the phase relationship between current and voltage in an a.c. circuit containing only pure resistance?  If a current of 10A (rms) flows in an a.c. circuit of resistance 4 ohms, at what rate is heat produced, and what unit is used to express it?

33.     What is the effect of inductive reactance on the current in an a.c. circuit?  If that circuit had an inductance of 0.1 henrys and the system frequency were 60Hz, what would be the reactance?  What unit?


34.     What is the effect of capacitive reactance on the current in an a.c. circuit?  If that circuit had a capacitance of 100mF (100 microfarads) and the system frequency were 50Hz, what would be the reactance?  And how would it differ in principle from the inductive reactance of Q.33?

35.     If a single-phase circuit has an inductive reactance of 30 ohms and a resistance of 40 ohms, what is the total impedance?  (State the unit.)  If 250V (rms) a.c. is applied to this circuit, what current would flow?

36.     Three inductive reactances, 12 ohms, 20 ohms and 30 ohms are placed in parallel.  What is the equivalent single reactance?

37.     An inductive reactance of 50 ohms and a capacitive reactance of 25 ohms are placed in parallel.  What is the equivalent single reactance?

38.     A generator of 250V, 60Hz, single-phase feeds two inductive loads of 0.1H and 0.4H and a capacitor of 10mF, all in parallel.  What are the three individual currents, and what is the total current supplied by the generator?

39.     An inductor of 1.013H is placed in series with a capacitor of 10mF and a resistor of 5 ohms.  A voltage is applied overall at 50Hz.  What is the equivalent impendance?  What is noteworthy about it?

40.     Write down three alternative versions of Ohm’s Law for a.c.

41.     What is the impedance of a 50Hz series circuit consisting of a resistance of 40 ohms
and an inductance of 0.2H?

42.     What current does a single-phase, 220V, 60Hz generator supply to a parallel circuit consisting of a resistance of 15 ohms and a capacitance of 100mF?

43.     What do you understand by an ‘impedance triangle’ of a circuit?  How do you use it?

44.     A single-phase circuit has a resistance of 20 ohms and a series capacitive reactance of -15 ohms.  A voltage of 250V is applied.  What total current flows, and by what angle does its vector differ from the voltage vector?  Does it lag or lead?

45.     A single-phase circuit has a resistance of 20 ohms and a series inductive circuit of +15 ohms.  A voltage of 250V is applied.  What current flows and by what angle does its vector differ from the voltage vector?  Does it lag or lead?  What are the individual voltages across the resistive and the inductive elements?  Why do they not add up to 250V?

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