Download Sample Question Paper for 9210-115 Graduate Diploma in Electrical Engineering

Sample Question Paper for 9210-115
Graduate Diploma in Electrical Engineering
Electrical machines and drives
Duration: three hours
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, drawing
instruments
General instructions
• This paper consists of nine questions.
• Answer any five questions.
• 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.
1
a)
b)
c)
2
a)
b)
c)
Derive the relationship between mechanical and electrical angles of rotating
electrical machines.
State two advantages of double layer windings over single layer windings.
A six pole machine has an armature with 120 slots and 10 conductors per slot.
The machine is driven by a prime mover at 1000 rpm.
i)
When the machine is working as a dc generator an emf of 1000 V is generated.
Calculate the flux per pole.
Assume the winding to be lap type.
ii) If the machine is working as a three phase star connected induction generator,
calculate the frequency, and the phase and line induced emfs.
Assume a winding factor of 0.96 and all conductors in each phase to be
connected in series.
State the conditions that are to be satisfied for smooth synchronisation of a
generator to an infinite bus.
Describe the three dark lamp method used for synchronisation of an alternator
with an infinite bus.
Three generators A, B, and C are operating in parallel at unity power factor.
The characteristics of the generators are given below.
(4 marks)
(2 marks)
(4 marks)
(10 marks)
(3 marks)
(3 marks)
Generator A: 50 MW, drop in frequency from no load to full load = 2 Hz
Generator B: 50 MW, drop in frequency from no load to full load = 2.5 Hz
Generator C: 50 MW, drop in frequency from no load to full load = 3 Hz
The three generators share a load of 80 MW at a bus frequency of 50 Hz. Under
this condition, generator A and B deliver 20 MW each.
i)
Assuming the generators to be isolated, calculate the no load frequencies
of the three generators. Assume the generators to be isolated.
ii) Determine the load sharing and bus frequency when the total load is
increased to 120 MW at unity power factor.
3
a)
b)
c)
Explain the basic difference between cylindrical and salient rotor synchronous
machines.
Explain the necessity of the two axis theory to analyse salient rotor machines.
i)
Sketch the vector diagram for a salient rotor synchronous machine.
Assume the armature resistance to be negligible.
ii) Show that the power developed per phase in a salient pole synchronous
machine with negligible armature resistance can be expressed through,
VE sin(δ ) V2 sin(2δ ) Xd –Xq
P=
+
Xd Xq
Xd
2
where
δ = torque angle
Xd = direct axis reactance
Xq = quadratic axis reactance
V = terminal voltage
E = excitation
[
d)
e)
(6 marks)
(8 marks)
(1 mark)
(2 marks)
(2 marks)
]
Assume armature resistance to be negligible.
Briefly explain the slip test.
A three phase star connected salient pole synchronous generator is being run at
synchronous speed with its field circuit open. The stator of the machine is supplied
from a three phase balanced supply. A voltmeter which is connected across lines
provides maximum and minimum readings of 2625 V and 2600 V respectively.
The line current varies from 260 A to 340 A.
Calculate the direct axis and quadratic axis synchronous reactance per phase.
Neglect armature resistance.
2
(4 marks)
(5 marks)
(6 marks)
4
a)
b)
c)
i)
Sketch the per phase equivalent circuit of a three phase induction motor,
referred to stator side and indicate all relevant parameters on your diagram.
ii) Sketch the approximate per phase equivalent circuit of a three phase
induction motor. State the assumptions made.
Describe the no load and blocked rotor tests conducted to identify equivalent
circuit parameters.
Test data for a 220 V, 60 Hz three phase star connected class C, induction motor
are given below.
(2 marks)
(2 marks)
(4 marks)
No load test
Applied voltage = 218 V
Line current
= 5.5 A
Power input
= 370 W
Blocked rotor test at 15 Hz
Applied voltage = 27 V
Line current
= 19 A
Power input
= 690 W
Per phase stator resistance is 0.26 Ω.
For class C machines assume x1 and x2I to have a ratio of 0.3 : 0.7.
Stating the assumptions made, calculate the parameters of the approximate
equivalent circuit.
5
a)
b)
c)
d)
6
a)
b)
Compare the squirrel cage and wound rotor induction motors with respect to
construction, efficiency, speed control and maintenance.
Explain what is meant by slip of an induction machine.
Explain the losses in an induction machine.
A 415 V, 40 kW three phase induction motor draws 60 A at 0.85 power factor lagging.
The stator copper losses amount to 2 kW and the rotor copper losses are 700 W.
The friction and windage losses are 600 W. The core losses amount to 1800 W and
the stray losses are negligible.
Calculate
i)
air gap power
ii) output power
iii) efficiency of the motor.
i)
ii)
Sketch torque speed characteristics of DC series motors.
Explain why DC series motors are not recommended to be connected to
loads through belt drives.
iii) State two applications of DC series motors.
A 220 V, DC series motor runs at 500 rpm drawing a current of 60 A. Armature
resistance of the motor is 0.12 Ω and field resistance is 0.08 Ω.
i)
Calculate the torque developed.
ii) Determine the speed of the motor when a diverter resister of 0.16 Ω is
connected in parallel with the field winding.
Assume the torque to remain unchanged and flux to be proportional to the
field current.
3
(12 marks)
(4 marks)
(2 marks)
(4 marks)
(4 marks)
(4 marks)
(2 marks)
(2 marks)
(4 marks)
(2 marks)
(4 marks)
(8 marks)
See next page
7
a)
b)
c)
8
a)
b)
c)
9
a)
b)
c)
d)
Explain how a DC shunt generator is excited.
Draw the equivalent circuit of a short shunt compound DC generator and derive
expressions for armature current, emf generated and power generated in
the machine.
A belt driven 200 kW DC shunt generator is running at 1000 rpm. The generator is
connected to a 250 V bus. When the belt breaks the generator starts to run as a
motor. In motor mode it runs at 890 rpm. Armature resistance of the machine is
0.03 Ω and shunt resistance is 50 Ω. Voltage drop at each brush is assumed as 1.1 V.
Neglecting armature reaction, calculate the current drawn by the machine when
it starts to work as a motor.
Sketch load characteristics of
i)
a compressor
ii) a pump.
i)
State four aspects that should be considered in selecting a motor drive.
ii) List two transducers used in a motor drive.
A separately excited DC motor rated at 5 kW, 230 V is supplied power from a fully
controlled single phase bridge rectifier. The AC power supply is sinusoidal and
rated at 230 V, 50 Hz. The armature resistance of the motor is 0.5 Ω and the motor
constant ‘kφ’ is equal to 2 Nm/A.
Assume continuous conduction and load torque to be constant.
i)
Calculate the average voltage applied to the load when the firing angle is 0°
and 30°.
ii) Assuming that the motor delivers its rated power when the firing angle is 0°,
calculate the full load speed of the motor.
iii) Determine the firing angle required to reduce the motor speed to 900 rpm.
Assume speed to be proportional to average applied armature voltage.
Explain the principle of closed loop control in motor applications.
Describe the speed control of wound rotor induction motors through rotor
resistance control. Your description should contain applications, advantages,
and disadvantages.
Using the per phase approximate equivalent circuit of an induction motor and
assuming small slips r2’ /s >>> r1, x1, x2’, show that
ks
T= ,
r2’
where r2I is the rotor resistance referred to stator side, and T is the torque of
the motor.
A six pole, 50 Hz wound rotor induction motor has a per phase rotor resistance of
0.3 Ω referred to stator side. The motor runs at 960 rpm.
Determine the external resistance that is required to be added to each phase to
change the speed of the motor to 750 rpm.
4
(4 marks)
(4 marks)
(12 marks)
(1 mark)
(1 mark)
(4 marks)
(2 marks)
(2 marks)
(6 marks)
(4 marks)
(4 marks)
(4 marks)
(4 marks)
(8 marks)