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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