ocd048 university of bolton rak academic centre … · out torque) that the motor can deliver. (4...
TRANSCRIPT
OCD048
UNIVERSITY OF BOLTON
RAK ACADEMIC CENTRE
BENG(HONS) ELECTRICAL AND ELECTRONIC
ENGINEERING
SEMESTER TWO EXAMINATION 2017/2018
ELECTRICAL MACHINES AND POWER
ELECTRONIC DRIVES
MODULE NO: EEE6001
Date: Thursday 31 May 2018 Time: 10:00 – 12:00
INSTRUCTIONS TO CANDIDATES: There are 5 questions. Answer any 4 questions. All questions carry equal marks. Marks for parts of questions are shown
in brackets.
Page 2 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
Q1
(a) The Open Circuit and Short Circuit test data are given below for a single
phase, 5 kVA, 200V/400V and 50Hz transformer.
Open Circuit test from Low Voltage side: 200V 1.25A 150W
Short Circuit test from High Voltage side: 20V 12.5A 175W
(i) Draw the equivalent circuit of the transformer referred to both Low
Voltage and High Voltage sides
(6 marks)
(ii) Calculate the efficiency of the transformer at 75% loading with load power
factor = 0.7. Find the load or kVA at which the transformer is to be
operated for maximum efficiency and also calculate the value of
maximum efficiency.
(4 marks)
(b) Instrument transformers are used in AC system for the measurement of large
voltage and current, Justify the statement. If a potential transformer 110K/110
V and a current transformer of 60/5 A are used to measure voltage and
current of a transmission line, and voltmeter indicates 100 V and ammeter
indicates 2 A, what is the voltage and current in the transmission line?
(5 marks)
(c)
(i) Compare and contrast the working of two winding transformer and auto
transformer. (5 marks)
(ii) If an ideal autotransformer has a coil with total number of turns NCD = 200
between terminals C & D and It has got one tapping at A such that NAC =
100 and another tapping at B such that NBA = 50, Calculate currents in
various parts of the circuit and show their directions when 400 V supply is
connected across AC and two resistive loads of 60Ω & 40Ω are
connected across BC and DC respectively
Page 3 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
(5 marks)
Total 25 marks
Please turn the page
Q2.
(a) A 230 V dc machine has an emf constant 212.21 V s/ Wb radK ´
and 0.278 aR . The field is separately excited and produces 0.01 Wb per
pole, 36 AaI for all parts of the problem.
(i) Find the speed and the total power developed by the armature if the
machine is operated as a dc motor with rated terminal voltage.
Determine the speed of the motor if an external resistance 1 xR is
added in series with the armature circuit. Also calculate the rotational
losses at the point of operation if the motor is known to be supplying 10
HP to a coupled mechanical load.
(5 marks)
(ii) Find the speed and the total power developed by the armature if the
machine is operated as a dc generator with rated terminal voltage.
(5 marks)
(b) A dc shunt generator has armature resistance Ra = 0.1 Ω, shunt field winding
resistance Rfw = 80 Ω, and Nf = 1200 turns per pole. The rated power, voltage
and speed of armature are 12 kW, 100 V and 1000 rpm respectively. The rated
field current is 1 ampere. The magnetization characteristic at 1000 rpm is shown
in Figure. (From Figure 1: Generated EMF Eg= 98 V at If= 0.94 A, Eg=112V at
If=1.4 A, Eg=113.18V at If=1.45 A, Eg=99.4V at If=0.99 A)
Page 4 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
Figure 1- Magnetization characteristics
Q2 continued over the page...
Q2 continued
(i) Neglect the armature reaction effect. Determine the terminal voltage at full
load. Also calculate the value of the field circuit control resistance (Rfc) required
to make terminal voltage Vt = 100 V at full load condition. (5 marks)
(ii) Consider that armature reaction at full load is equivalent to 0.06 field amperes.
Find the full-load terminal voltage and also the field current required to make
the terminal voltage Vt = 100 V at full load condition (5 marks)
(iii) The dc machine is provided with a series winding, Rsr = 0.01 Ω so that it can
operate as a cumulatively compound dc machine. The short shunt machine is
required to provide a terminal voltage of 100 V at no load as well as at full load.
If the shunt field winding has 1200 turns per pole along with field control
resistance of 20Ω, how many series turns per pole are required to obtain zero
voltage regulation?
(5 marks)
Total 25 marks Q3.
(a) (i) Analyse the principle of operation of three phase induction motors with
necessary diagrams. (5 marks)
(ii) If a 3φ, 460 V, 100 hp, 60 Hz, four-pole induction machine delivers rated
output power at a slip of 0.05, determine the synchronous speed, motor
Page 5 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
speed, frequency of rotor circuit, slip rpm and speed of rotor field relative
to rotor circuit. Also find the rotor induced voltage at the operating speed, if
the stator-to-rotor turns ratio is 1:0.5. (5 marks)
(iii) Single phase induction motors are not self-starting, Justify the statement.
(5 marks)
(b) Following data is taken from no-load, locked rotor, and DC tests of a 3-phase,
wye connected 40 HP, 60 Hz, 460 V, induction motor with a rated current of
57.8 A. Assume motor is NEMA Design B.
Q3 continued over the page...
Q3 continued
No-load test: Supply frequency = 60 Hz, Line voltage = 460 V, Line current
= 32.7 A and Input power = 4664.4 W
Blocked-rotor test: Frequency = 15 Hz, Line voltage = 36.2 V, Line current
= 58 A and Input power = 2573.4 W
DC Test: Vdc = 12 V, Idc = 59 A.
(i) Determine Total core, friction and windage losses. Find the motor
parameters and draw the approximate equivalent circuit.
(7 marks)
(ii) Calculate the parameters (Vth, Rth, Xth) for the Thevenin equivalent
circuit
(3 marks)
Total 25 marks
Q4.
(a) A 3φ, 5 kVA, 208 V, four-pole, 60 Hz, star-connected synchronous machine has
negligible stator winding resistance and a synchronous reactance of 8 ohms per
phase at rated terminal voltage. The machine is first operated as a generator.
(i) Determine the excitation voltage and the power angle when the machine is
delivering rated kVA at 0.8 PF lagging and draw the phasor diagrams
(4 marks)
Page 6 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
(ii) With the field current as in part 1, the prime mover power is slowly increased.
What are the corresponding values of the stator (or armature) current, power
factor, and reactive power at this maximum power transfer condition?
(2 marks)
(iii)If the machine is operated as a synchronous motor and the field excitation is
adjusted so that the power factor is unity when the machine draws 3 kW from
the supply. Find the excitation voltage and the power angle. Also draw the
phasor diagram for this condition. If the field excitation is held constant and
the shaft load is slowly increased, determine the maximum torque (i.e., pull-
out torque) that the motor can deliver. (4 marks)
Q4 continued over the page…
Q4 continued
(b) Analyse construction, principle of operation and applications of Universal motor.
(5 marks)
(c) A 100 KVA, 3000 V, 50 Hz, 3 phase star connected alternator has effective
armature resistance of 0.2 Ω. The field current of 40A produces short circuit
current of 200 A and an open circuit emf of 1040 V (line value). Calculate the full
load voltage regulation at 0.8 power factor lagging, 0.8 power factor leading and
unity power factor. Also draw appropriate phasor diagrams.
(10 marks)
Total 25 marks
Q5.
(a) A single-phase full converter is used to control the speed of a 5 hp, 110 V,
1200 rpm, separately excited dc motor. The converter is connected to a
single-phase 120 V, 60 Hz supply. The armature resistance and circuit
inductances are Ra = 0.4 Ω, La = 5 mH. The motor voltage constant is KΦ =
0.09 V/rpm.
Page 7 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
(i) Rectifier (or motoring) operation: The dc machine operates as a motor,
runs at 1000 rpm, and carries an armature current of 30 amperes.
Assume that motor current is ripple-free. Determine firing angle α, power
to the motor and supply power factor. (5 marks)
(ii) Inverter operation (regenerating action): The polarity of the motor back
emf Ea is reversed, say by reversing the field excitation. Determine firing
angle to keep the motor current at 30 amperes and power fed back to the
supply when the speed is 1000 rpm.
(5 marks)
Q5 continued over the page…
Q5 continued
(b) A three-phase ac voltage controller is used to start and control the speed of a
3φ, 100 hp, 460 V, four-pole induction motor driving a centrifugal pump. At
full-load output the power factor of the motor is 0.85 and the efficiency is 80
percent. The motor current is sinusoidal. The controller and motor are
connected in delta. Calculate the control range of the firing angle α and also
determine the rms current rating and peak voltage rating of the thyristor.
(5 marks)
(c) With the aid of necessary diagrams , analyse the following operational circuits
(i) DC choppers (5 marks)
(ii) Inverters (5 marks)
Total 25 marks
END OF QUESTIONS
Page 8 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
Please turn the Page for the EQUATION SHEET
Page 9 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
EQUATION SHEET
DC GENERATORS:
Self Excited DC Generator
Shunt generator:
Compound generator:
(+ for cumulative compound generator, - for differential compound generator)
Short Shunt:
Efficiency of machine as generator:
Page 10 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
DC MOTORS:
Self Excited DC motor:
Series motor:
Shunt motor:
Losses:
Efficiency of machine as motor:
Page 11 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
TRANSFORMERS:
Transformer on no-load:
Transformer on Load:
Secondary winding reffered to primary:
Primary winding referred to secondary:
Equivalent circuit of transformer refferd to primary:
Voltage regulation of transformer:
Efficiency of transformer:
Open circuit test:
Page 12 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
Short circuit test:
Efficiency from OC test and SC test:
INDUCTION MOTOR:
Three phase Induction Motor:
Torque Equation:
Losses:
Page 13 of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001
Equivalent circuit reffered to stator:
Thevenin’s Equivalent circuit:
SYNCHRONOUS MACHINES:
Three phase AC voltage controllers
Single phase full converter:
End of Paper