Download EE3003-ModelPaper-2013

INSTITUTE OF ENGINEERS, SRI LANKA
COLLEGE OF ENGINEERING
ADVANCED DIPLOMA STAGE-I
EE3003 – ELECTRICAL MACHINES
ANSWER ANY 05 QUESTIONS
TIME: 03 HOURS
Question 1
(a) Explain what is meant by armature reaction of a DC machine. What are the undesirable
effects of armature reaction and discuss the methods that can be used to minimize its
effects.
(b) A separately excited 230V, 90kW dc generator has a field winding with 1000 turns per pole.
To generate rated voltage at rated speed, the field current requirement is 4A on no-load and
5.3A on full load. Calculate the number of turns/pole of a series winding to be fitted in order
to convert the machine to a self-excited generator, level-compounded to give an armature
voltage of 230V. Explain briefly how a field divert resistor would be designed to cater for the
situation when the requiring number of turns is not an integer. If the resistance per turn of a
series field winding is 0.01Ω, calculate the value of filed divert resistor required.
Question 2
(a) Draw a 3-point starter arrangement connected to a DC shunt motor.
Explain what would happen if,
i) The field winding becomes open-circuited when the motor running at no-load
ii) The starter handle is moved rapidly from OFF position to ON position
(b) Draw the following output characteristics for a DC shunt motor
i) Torque – Armature current
ii) Speed – Armature current
(c) A DC shunt motor is fed by a 500V supply, motor rotates at 1500rev/min at its rated load and
the armature current is 50A. The resistances of the armature and field windings are 0.5Ω and
200Ω. The mechanical loss torque is 5Nm. Estimate the following,
i) Electromagnetic torque
ii) Mechanical output
iii) Efficiency
Question 3
(a) What is an electric transformer?
(b) Explain why the power factor of a transformer is poor at no-load and normally improves as
the load is increased.
(c) The no-load test results obtained on a 50kVA transformer are as follows,
Open circuit test:
Primary voltage
= 3.3kV
Secondary voltage
= 400V
Input Power
= 430W
Input Current
= 15A
Short circuit test:
Primary voltage
Primary Current
Power Input
= 124V
= 15.3A
= 525W
i) Calculate the efficiencies at full-load and half full-load at a power factor of 0.7
ii) Voltage regulation at power factor of 0.7 lagging and 0.7 leading at full-load
iii) Secondary terminal voltages at both lagging and leading power factor at full-load
Question 4
(a) Compare and contrast a three phase core-type transformer with a similar rated three phase
bank type transformer.
(b) Draw the vector diagrams and winding connections for high voltage and low voltage
windings of Yy6 and Dy11 three phase transformers.
(c) A Dz three phase transformer has 400V between lines in the low voltage side. What will be
three line voltages if one of the half sections of a low voltage phase is reconnected with
reverse polarity?
Question 5
(a) Draw the typical Toque/Slip and Torque/Speed characteristics of a three phase induction
motor separately.
Indicate all the operating modes on the same plot.
(b) A three phase 4-pole, 400V, 50Hz squirrel cage induction motor has the following equivalent
circuit parameters referred to the stator side, per phase in Ω.
r1= 0.2
r2= 0.50
x1= 1.2
x2= 1.0
xm= 44
Total frictional and windage losses may be assumed to be at 1120W.
For an operation at slip 5% calculate the followings by using an approximate equivalent
circuit
i) Rotor speed
ii) Stator current
iii) Shaft torque
State if any assumptions made.
Question 6
(a) List the common types of single phase induction motors and explain the operation of one of
them
(b) State the principle of operation of a three phase induction motor and describe how a self
starting torque is produced
(c) Compare and contrast the following three phase induction motor starting methods with a
Direct-On-Line (DOL) starter and derive equations for starting current and torque in terms of
IDOL and TDOL
i) Star-Delta starter
ii) Auto-transformer starter
Question 7
(a) Briefly explain how the airgap flux of a three phase induction motor is retained constant in
practice, during speed control by frequency/voltage control method.
Give reasons why voltage is not applied in excess of rated voltage.
(b) A 4pole, three phase, 50Hz, 1470rev/min cage rotor induction motor is used in wide range
adjustable speed drive requiring a maximum speed of 8200rev/min.
Give a sketch of its operating chart indicating the limits of speed. What is the range of
operating frequency?
Take the pull-out speed as 1350rev/min at 50Hz.
Question 8
(a) Draw a typical operating characteristic for a cylindrical rotor synchronous generator.
Indicate all the limits applicable for stable operation of a generator without exceeding its
safe thermal limits.
(b) State the advantages of parallel running synchronous generators. State all the important
things to consider when running synchronous generators in parallel.
(c) A 150MW synchronous generator (G1) operates in parallel with a 250MW synchronous
generator (G2) to meet the demand. The governor droops of G1 and G2 are 3% and 5%
respectively.
i) Calculate the contributions from each generator when the combination supplies a
total load of 320MW at 50Hz. What are no-load frequencies of each generator?
State if any assumptions made.