电气机器与系统｜ECS790P Electrical Machines and Systems

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Resit Examination Period 2021 – May
ECS790P Electrical Machines and Systems Duration: 2 hours
This is an open-book exam, which should be completed in approximately 2 hours.
You MUST submit your answers within 24 hours from the exam being released.
Follow instructions on download page.
You can refer to textbooks, notes and online materials to facilitate your working, but
normal referencing and plagiarism rules apply and you must cite any sources used.
You must upload a single PDF document containing your solutions. These can be typed
or handwritten, or a mix of the two. Multiple submissions are not permitted, so be sure
that you check your submission before uploading it.
Calculators are permitted in this examination.
Instructions:
This paper contains FOUR questions. Answer the FIRST question. Then answer ANY TWO
of the REMAINING THREE questions. If you answer all the remaining questions, only the
first two answers (up to the specified number) will be marked.
YOU MUST COMPLETE THE EXAM ON YOUR OWN, WITHOUT CONSULTING OTHERS
Examiners:
Dr Kamyar Mehran and Dr Tijana Timotijevic

c Queen Mary University of London, 2021
Page 2 ECS790P (2021)
Question 1 [50 marks]
(a) Which one of the following statements is true for microgrids.
[6 marks]
(i) A microgrid can better integrate renewable energy sources
(ii) A microgrid in islanded mode can provide ancillary services to the main grid like
degraded power quality or shortage.
(iii) The energy management of a microgrid is NOT complex
(iv) The operation of a microgrid is reliable and economical
(b) Which one of the following statements is true in using a nuclear reactor for electricity
generation.
[6 marks]
(i) A nuclear reactor produces damaging and polluting gases
(ii) A nuclear reactors cannot be used in marine vessels
(iii) Storage of radioactive waste is one of the main challenges for a nuclear reactor
(iv) The risk of severe accidents for modern nuclear power stations is zero
(c) Which one of the following statements is true for commercial electro-chemical grid
batteries.
[6 marks]
(i) It reduces peak demand charges
(ii) They increase the rate of electricity for residential customers
(iii) They usually can hold power for more than 24 hours
(iv) The residents cannot use them for storing the extra capacity in vehicular batteries
(d) Which one of the following statements is true when comparing a magnetic circuit with
an electric circuit.
[6 marks]
(i) – Flux always flows in a magnetic circuit.
(ii) – Conductivity in an electric circuit is similar to permeability in a magnetic circuit.
(iii) – A magnetic circuit can be an open circuit or closed circuit.
(iv) – Current density does NOT depend on the conductivity in an electric circuit.
(e) Which one of the following statements can better explain the Farday law.
[6 marks]
(i) – A time-varying flux density (current) induces an Electro-magnetic flux (EMF).
The magnitude of the EMF is proportional to the rate of change of flux density
and its direction is in opposition to the change.
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ECS790P (2021) Page 3
(ii) – A conductor moved at a variable velocity through a magnetic field has an EMF
induced in it.
(iii) – A current-carrying conductor has a magnetic field around it.
(iv) – A current-carrying conductor in a magnetic field experiences a force. The force
is NOT proportional to the current
(f) A balanced three-phase system with a line voltage of 240V is supplying a balanced
Y-connected load with 1500W and a leading power factor of PF = 0.9.
(i) – Determine the line current,
(ii) – Determine the per-phase load impedance,
(iii) – A balanced 750W lighting load is added (in parallel) to the system. Sketch the
suitable per-phase circuit and describe how the line current will be changed.
[12 marks]
(g) A two-legged core is shown in Figure 1. The winding on the left leg of the core (N1)
has 600 turns, and the winding on the right (N2) has 200 turns. If the coils are wound
in the directions as shown in the figure, determine the flux produced by currents
i1 = 0.5A and i2 = 1.0A.
Note: Assume μr = 1200 and constant, and the dimension of the core is as shown in
Figure 1.
[8 marks]
Figure 1: A two-legged core in Question 1-part (g)
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Page 4 ECS790P (2021)
Question 2 [25 marks]
A 460V, 60Hz, four-pole, Y-connected induction motor is rated at 25 hp and has a slip of
0.02. The equivalent circuit parameters of this motor are
R1 = 0.35 R2 = 0.4 XM = 30
X1 = 0.925 X2 = 1.5
PF&W = 600W Pmisc = 200W Pcore = 600W
and the power flow diagram of the motor is shown in the Figure 2.
Figure 2: The power flow diagram of the motor in Question 2
(a) Work out and sketch the equivalent circuit of the motor, and find the line current
(armature current) IL of the motor.
[10 marks]
(b) Determine the stator power factor, rotor power factor and rotor frequency.
[6 marks]
(c) Determine the stator copper losses PSCL, the air-gap power PAG, and the electrical
power converted Pconv for this motor.
[6 marks]
(d) Determine the induced torque τind of the motor.
[3 marks]
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ECS790P (2021) Page 5
Question 3 [25 marks]
A 50-hp, 250-V, 1200 r/min DC shunt motor with compensating windings has an armature
resistance (including the brushes, compensating windings, and interpoles) of 0.06 . Its
field circuit has a total resistance Radj + RF of 50 , which produces a no-load speed of
1200 r/min. There are 1200 turns per pole on the shunt field winding.
(a) Sketch the circuit diagram of this motor.
[3 marks]
(b) Determine the speed of this motor when its input current is 100A.
[6 marks]
(c) Determine the speed of this motor when its input current is 200A.
[4 marks]
(d) Determine the speed of this motor when its input current is 300A.
[4 marks]
(e) Plot the torque-speed characteristic of this motor.
[8 marks]
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Page 6 ECS790P (2021)
Question 4 [25 marks]
A 13.8-kV, 50-MVA, 60-Hz, four-pole Y-connected synchronous generator has a syn chronous reactance of 2.5 , an armature resistance of 0.2 , and a power factor of 0.9
(lagging). At 60 Hz, its friction and windage losses are 1 MW, and its core losses are 1.5
MW. The field circuit has a dc voltage of 120 V, and the maximum IF is 10A. The current of
the field circuit is adjustable over the range from 0 to 10A. The open-circuit characteristics
(OCC) of this generator is shown in the Figure 3.
Figure 3: open-circuit characteristics of the synchronous generator in Question 4. The horizen tal axis represents ”Field Current (A)”, and the vertical axis represents ”Open-circuit voltage
(kV)”.
(a) Determine the field current required to make the terminal voltage Vt (or line voltage
VL) equal to 13.8 kV when the generator is running at no load
[2 marks]
(b) Determine the internal generated voltage EA of this machine at the rated conditions
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ECS790P (2021) Page 7
[8 marks]
(c) Determine the phase voltage of this generator at the rated conditions
[3 marks]
(d) Determine the field current required to make the terminal voltage Vt equal to 13.8 kV
when the generator is running at the rated conditions
[3 marks]
(e) Suppose that this generator is running at the rated conditions, and then the load is
removed without changing the field current. Determine the terminal voltage of the
generator.
[3 marks]
(f) Determine the steady-state power and torque that the generator’s prime mover must
be capable of supplying to handle the rated conditions.
[6 marks]
End of questions
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电气机器与系统｜ECS790P Electrical Machines and Systems最先出现在KJESSAY历史案例。