*92101150615*
9210-115 JUNE 2015
Level 6 Graduate Diploma in Engineering
Electrical machines and drives
Monday 22 June 2015
09:30 – 12:30
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, drawing
instruments
No additional data is attached
General instructions
• This paper consists of nine questions.
• Answer five questions only.
• A non-programmable electronic calculator may be used but candidates must show
sufficient steps to justify their answers.
• Drawings should be clear, in good proportion and in pencil. Do not use red ink.
• All questions carry equal marks. The maximum marks for each section within a
question are shown.
1st PROOF
DATE
COMMENTS
PASSED
SIGNED .......................................
© The City and Guilds of London Institute 2015
ST00044364 H1 PO4500106983 10187138
1
a)
b)
Explain the procedure adapted for smooth synchronisation of a synchronous
generator to an infinite bus.
Two synchronous generators GA and GB are supplying a load of 200 kW with
a power factor of 0.85 lagging.
(4 marks)
Generator GA: 480 V, 200 kW, 3 phase, 2 pole, 50 Hz
Generator GB: 480 V, 150 kW, 3 phase, 4 pole, 50 Hz
The generator GA is connected to a prime mover with a no-load speed of 3040 rpm
and a full-load speed of 2975 rpm. The generator GB is connected to a prime
mover with a no-load speed of 1500 rpm and a full-load speed of 1485 rpm.
i)
Determine the governor droops of generator GA and GB.
ii) Calculate the operating frequency of the system.
iii) Determine the power supplied by each generator.
iv) What will be the system frequency and power delivered by each generator
if the governor set point of generator GB is increased by 0.5 Hz in no load.
2
a)
b)
c)
d)
3
a)
b)
c)
Explain the armature reaction of a synchronous machine.
Sketch the vector diagram for the terminal voltage of a salient pole
synchronous machine.
Derive expressions for the active power and reactive power developed in a
salient pole synchronous machine with negligible armature resistance.
The direct axis reactance xd and quadrature axis reactance xq of a salient-pole
synchronous generator are 0.95 and 0.7 p.u. respectively. The armature resistance
is negligible. It is connected to an infinite bus through a link with reactance of 0.2 p.u.
The excitation voltage is 1.3 per unit and infinite bus voltage is maintained at 1 p.u.
Calculate the active power and reactive power supplied to the infinite bus when
power angle is 25°.
Explain what is meant by slip of an induction motor and express the rotor speed
in terms of slip and synchronous speed.
Sketch the approximate equivalent circuit for a three phase induction machine.
State the assumptions made.
Results of no-load, locked rotor and DC test for a 5.5 kW, 205 V, 60 Hz, three phase,
star connected, class A induction motor are given below.
(2 marks)
(4 marks)
(4 marks)
(6 marks)
(2 marks)
(4 marks)
(8 marks)
(6 marks)
(4 marks)
(4 marks)
No load test at 60 Hz
Applied voltage = 205 V
Line current
= 8.0 A
Power input
= 410 W
Locked rotor test at 15 Hz
Applied voltage = 25 V
Line current
= 28 A
Power input
= 910 W
Per phase stator resistance is 0.24 Ω.
For class A machines,
Assume the ratio x1 / x2 as 0.5 : 0.5.
Determine the parameters of the approximate equivalent circuit.
2
(12 marks)
4
a)
b)
c)
5
Explain the differences between the motoring, generating and braking modes
of operation of three phase induction machines.
Derive the torque versus speed curve for a three phase induction motor.
i)
Explain the speed control of an induction motor by varying frequency.
ii) A 3 phase, 400 V, 50 Hz induction motor is connected to a constant torque
load and runs at 1460 rpm. The motor driver is operating in constant air-gap
flux mode. What will be the speed of the motor if the frequency is reduced
to 30 Hz?
a)
i)
ii)
b)
i)
ii)
6
7
a)
b)
c)
a)
b)
c)
8
a)
b)
Explain the voltage buildup process in a shunt DC generator.
A 30 kW, 250V DC shunt generator has an armature resistance of 0.12 Ω and
shunt field resistance of 35 Ω. The generated emf is 270 V and the terminal
voltage is 250 V when the generator is supplying a load. Find the output
power and efficiency of the generator.
Draw the equivalent circuit of a DC series generator and derive an expression
for terminal voltage.
A 9 kW, 120 V DC series generator has a field resistance of 0.05 Ω. Determine
the armature resistance. At rated current and terminal voltage, the generator
armature voltage is 135 V.
Explain a starting method for DC motors and why such a starting method is needed.
Derive an expression for torque-speed characteristics of a Shunt DC Motor.
A 7.5 kW, 230 V DC shunt motor has armature resistance of 0.3 Ω. The field
resistance is 160 Ω. The motor draws a line current of 3.9 A on no load at a speed
of 1200 rpm. At full load operation armature current is 40 A.
i)
Determine the armature current and power developed at no load.
ii) Find the full load speed and efficiency of the motor.
Explain magneto-motive force and mmf distribution of electrical machines.
i)
Derive expressions for the power output of a DC series motor.
ii) A 600 V, 110 kW, DC series motor operates at full load at 600 rpm.
The armature resistance is 0.12 Ω and the series field resistance is 0.04 Ω.
Find the rotational losses and full load efficiency when full load output is
110 kW and full load current is 200 A.
Explain the operation of a 12/8 pole (12 stator pole and 8 rotor poles), 4phase,
single stack variable reluctance stepper motor in full step mode.
Sketch circuit diagrams of a 6 pulse dual converter used for four quadrant
operation of high power DC motors.
ii) Explain a firing control scheme for driving a DC motor by a dual converter
given in part i). Assume that the converters are ideal.
The speed of a 7 kW, 230 V, 1200 rpm separately excited DC motor is controlled
by a single phase full converter. The rated armature current is 38 A and armature
resistance is 0.3 Ω. The AC suppy voltage is 260 V. The motor voltage constant
is KaΦ = 0.18 V/rpm. Assume that the inductance of the armature circuit is
sufficient to make motor current continuous and ripple free.
i)
Calculate the motor torque and speed for motoring action with a firing angle
of α = 30° and rated motor current.
ii) If the polarity of the motor field connections is reversed, calculate the firing
angle required to keep the motor current at its rated value and the power
fed back to the supply.
(6 marks)
(6 marks)
(4 marks)
(4 marks)
(5 marks)
(8 marks)
(2 marks)
(5 marks)
(4 marks)
(4 marks)
(6 marks)
(6 marks)
(4 marks)
(4 marks)
(6 marks)
(6 marks)
i)
3
(4 marks)
(4 marks)
(6 marks)
(6 marks)
See next page
9
a)
b)
c)
Explain how the speed of a wound rotor induction motor can be varied by adding
resistance to the rotor circuit.
i)
Sketch circuit diagrams of a three phase full bridge thyristor converter.
ii) Derive an expression for average output voltage of a three phase thyristor
full bridge converter in terms of the ac supply voltage and firing angle for
the controller input.
A three phase full converter is used to control the speed of a 90 kW, 600 V, 1800 rpm
separately excited DC motor. The converter is supplied from a three phase, 480 V,
60 Hz supply. The rated armature current is 165 A. The motor parameters are given
below. The converter and AC supply are considered to be ideal.
Armature resistance
Armature inductance
Motor constant (KaΦ)
i)
ii)
(2 marks)
(2 marks)
(6 marks)
0.08 Ω
6.5 mH
0.33 V/rpm
Find the no load speed at firing angles α = 0° and α = 30°. Assume that the
no load armature current is 10% of the rated current and it is continuous.
Calculate the firing angle to obtain rated speed of 1800 rpm at rated
motor current.
4
(6 marks)
(4 marks)