EEM507 ELECTRICAL TECHNOLOGY
SYLLABUS
No. of hr. 4
Unit -1 D.C.Motor
Armature reactions, Methods to eliminate undesirable effects of armature reaction.
Commutation, Resistance commutation, reactance voltage, starting of DC motor, three
point and four point starters, speed control of DC motors..
Unit - 2 Transformers
Types, difference between distribution and power transformers, Three-phase
connections, Parallel operation, and concepts of transformer harmonics, Brief idea
about cooling methods.
Unit - 3 Induction motors
No-load and Block rotor test, Starting methods, methods of Speed control, Crawling,
Double cage induction motor, Applications.
Unit -4 Fractional horsepower (FHP) motors
Single phase induction motor. Shaded pole motor, Reluctance motor, Hysteresis
motor operation and Applications.
Unit -5 Synchronous Machines
Types, constructional details, EMF equation, Synchronous reactance, voltage
regulation - synchronous impedance methods. Power angle characteristic, Power
factor improvement methods. V-curve, Applications.
Books referred
(1) M.G. Say
(2) H. Cotton
(3) S.K.Pillai
(4) D.K. Chaturvedi
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Alternating current Machines
Electrical Machines
Electrical Drives
Electrical Machines Lab Manual
QUESTION BANK
EEM 507 ELECTRICAL TECHNOLOGY
Unit -1 D.C.Motor
Armature reactions, Methods to eliminate undesirable effects of armature reaction.
Commutation, Resistance commutation, reactance voltage, starting of DC motor, three
point and four point starters, speed control of DC motors..
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1. What is the need of the starter in case of large rotating machines.
2. Explain the construction and working principle of three point and four point starters.
Also state in brief the no volt and over load release.
3. Draw speed torque and slip torque characteristics of the DC shunt, series and
compound motors.
4. Explain which type of motor is best suited for the traction purposes and why.
5. Describe the various methods of speed control of d.c. motors.
6. Compare the different methods of speed control of DC shunt motor.
7. A d.c. shunt motor is running at 1500 rpm at rated load torque. Discuss what would
happen to the motor operation if the following changes are made :
1- armature or Field terminals are reversed.
2- Supply wires are reversed.
8. What will happen if :
a. the armature is rewound with the fewer no. of turns of thick wire.
b. Some of field turns are short circuited.
9. Why in the starting of d.c. motors the field current should be kept maximum.
10. What is the effect of excitation, speed and load on the losses of a d.c. machine.
11. What is meant by constant power drive and constant torque drive.
12. Explain with neat sketch, the armature reaction in DC shunt motor. Mention
disadvantages of it.
13. Explain the methods to overcome the armature reaction.
14. What do you mean commutation in DC machines.
15. Why sparking takes place when load on simple DC machine increases.
Numerical
1. Two DC motors used for traction purposes run at a speed of 600 rpm and 700 rpm.
When taking a current of 60 amperes from a supply mains of 400 volts each motor has
an effective resistance of 0.25 ohm. Calculate the speed and potential difference across
each machine when both the motors are connected in series and are mechanically
coupled and drawing a current of 60 amperes from 400 volts mains.
2. A 2-pole series motor runs at 600 rpm on full load at 500 volts when taking a current
of 10 amperes. If the armature has 500 conductors and the flux per pole is expressed
by  0.03. I1/2 wbs, where I is the motor current. Calculate the speed at which the
motor will run if the supply voltage and torque are both halved. Ignore iron and
windage losses.
3. It is desired to reduce the speed of a 450 V 20 hp shunt motor by 25 % by the insertion
of resistance in the armature circuit. The field current is constant at 2 A and the
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armature resistance is 0.25 ohm. Calculate the necessary resistance if the torque is to
remain constant and the efficiency 85%.
4. A 230 V d.c. motor takes no-load current of 2 A and runs at a speed of 1200 rpm. If the
full load current is 40 A. Find (a) the speed on full load (b) percentage speed
regulation. Assume that the flux remains constant. Resistance of armature is 0.25 ohm.
5. A 250 V 20 h.p. (British) shunt motor has a maximum efficiency of 88% and a speed
of 700 rpm when delivering 80 % of its rated output. The resistance of its shunt field is
100 ohm. Determine the efficiency and speed when the motor draws a current of 78 A
from the mains.
6. A 3kW series motor runs normally at 800 rpm on a 240 V supply, taking 16 A, the
field coils are all connected in series. Estimate the speed and the current taken by the
motor, if the coils are connected two in parallel groups of two in series. The load
torque increases as the square of the speed. Assume that the flux is directly
proportional to the current and ignore losses.
7. A 20 kW, 500V shunt motor has an efficiency of 90 % at full load. The armature
copper loss is 40 % of the full load loss. The field resistance is 250 ohm. Calculate the
resistance values of a 4-section starter suitable for this motor in the following two
cases:
Case 1 : Starting current ≤ 2 IFL
Case 2 : Starting current (Min) = 120 % IFL
Unit - 2 Transformers
Types, difference between distribution and power transformers, Three-phase
connections, Parallel operation, and concepts of transformer harmonics, Brief idea
about cooling methods.
_____________________
1. Differentiate between a core type and shell type transformer. Which type of core
construction is used and why?
2. How is the power Transformer is different from distribution transformer? Where
is each employed and why?
3. Why are the transformer placed in oil filled tank? Why are tanks provided with
tubes / fins.
4. Compare operationally and economically a bank of three single phase
transformers with a single 3-phase unit for use in a power system.
5. What purpose does an open delta connection serve and how much load it could
take?
6. How does the two-phase output obtain from a 3-phase supply.
7. What are the necessary conditions to operate the transformers (1 and 3) in
parallel.
8. Two transformers with unequal turns ratios are connected in parallel. Derive the
expression for the load shared by each transformer.
9. Explain in brief the different connections of three phase transformers.
10. Which type of connection you prefer for the distribution transformer and why?
11. Explain different methods of cooling in case of transformers.
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Numerical
1. Two transformers A and B are connected in parallel to carry a load of 20000
KVA at 0.707 p.f. and 14000 volts. The transformers are rated at 10000 kVA each
and each transformers has a per unit resistance of 0.01 and p.u. reactance of 0.08.
Transformer A has voltage ratio of 33000 / 14000 and that of B is 26700 / 14000.
Find the current, kVA and p.f. of each transformer.
2. An ideal 3-phase step down transformer, connected delta / star delivers power to a
balanced 3-phase load of 120 kVA at 0.8 p.f. The input line voltage is 11 kV and
the turns ratio of the transformer, phase-to–phase is 10. Determine the line
voltage, line currents, phase voltages and phase currents on both primary and
secondary sides.
3. Two transformer each rated 250kVA, 11/2 kV and 50Hz, are connected in open
delta on both the primary and secondary.
(a) Find the load kVA that can be supplied from this transformer connection.
(b) A delta connected three-phase load of 250kVA, 0.8 pf., 2kV is connected
to the LV side of open delta transformer. Determine the Transformer
currents on the 11kV side of this connection.
4. A Scott connected transformer supplied from 11kV, 3-phase, 50 Hz mains.
Secondaries series connected; supply 1000 A at a voltage of 100 √2 to a resistive
load. The phase sequence of the 3-phase supply is ABC.
(a) Calculate the turns-ratio of the teaser transformer,
(b) Calculate the line current of B-phase.
Unit - 3 Induction motors
No-load and Block rotor test, Starting methods, methods of Speed control, Crawling,
Double cage induction motor, Applications.
1. Why the speed of the induction motor is always slightly less than the synchronous
speed.
2. Why the cage bars of the squirrel cage induction motor is skewed.
3. What happens to rotor copper loss at s = ∞.
4. Draw the external characteristic of the induction motor. Also show the region in
which it operate under normal operating conditions.
5. What will be the efficiency of the induction motor under no load and blocked
rotor condition.
6. Describe a method to make an induction motor a two-speed motor.
7. Briefly explain the phenomenon of cogging and crawling.
8. Why are large squirrel-cage induction motors not started on full line voltage?
How are they normally started?
9. Explain the speed control methods for plain induction motor and slip ring
induction motor.
10. Explain how the double cage IM develops high torque at starting and give good
running performance.
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11. Calculate the relative values of starting currents and starting torque of a three
phase squirrel cage Induction Motor. when it is started by
1- D.O.L. starting
2- Auto-transformer starter with 60 % tapping
3- Star delta starte
12. How the characteristic of induction motor is modified if
1- rotor resistance is increased &
2- rotor reactance is increased.
Numerical
1. A 4-pole three phase 50 Hz induction motor has a star connected rotor. The rotor has a
resistance of 0.1 ohm and standstill reactance 2 ohm per phase. The induced emf
between the slip rings is 100 V. If the full load speed is 1460 rpm.
Calculate (a) the slip (b) the emf induced in each phase of the rotor (c) the rotor reactance
per phase (d) the rotor current (e) the power factor. Assume slip rings short circuited.
2. A 3-phase induction motor is fed with a power of 48 kW. If the stator losses are 1.2
kW find
(i) the mechanical power developed if the motor is running at a slip of 3%
(ii) Copper losses in the rotor.
3. An induction motor has an efficiency of 0.9 when delivering an output of 37 kW. At
this load the stator Cu loss and rotor Cu loss each equals to the stator iron loss. The
mechanical losses are one third of the no load loss. Calculate the slip.
4. A 4-pole 50 Hz slip ring induction motor has a rotor resistance of 0.24 ohm per phase
and runs at 1455 rpm. Calculate the external resistance /phase that must be added to
lower the speed to 1200 rpm torque being constant.
5. A 4-pole 50 Hz 7.46 kW motor has at rated voltage and frequency starting torque of
160 per cent and a maximum torque of 200 per cent of full load torque.
Determine (i) full load speed (ii) speed at maximum torque.
6. A 10 kW 3-phase 50 Hz 4-pole induction motor has a full load slip of 0.03. Mechanical
and stray load losses at full load are 3.5 % of output power compute
1- Power delivered by stator to rotor.
2- Electromechanical torque at full load.
3- Rotor ohmic losses at full load.
7. No load and short circuit test of 3 phase, 20 h.p., 400 V 6-pole induction motor gave
the following test result (line values)
no load 400 V
11 A
p.f.=0.2
short circuit 100 V
25 A
p.f.=0.4
Rotor copper loss at standstill is half the total copper loss. From the diagram find (a)
line current slip efficiency and p.f. at full load. (b) maximum torque.
Unit -4 Fractional horsepower (FHP) motors
Single phase induction motor. Shaded pole motor, Reluctance motor, Hysteresis
motor operation and Applications.
_________________
1. Explain why a single phase single winding induction motor produces no starting
torque.
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2. What design features incorporated in a split phase induction motor to make it self
starting.
3. Why is the starting torque relatively small in single phase induction motors.
4. What are advantages of capacitor run over a capacitor start motor.
5. Why is a shaded pole motor is recommended over a resistance split phase motor
for same application.
6. Which type of motor would you use in the following applications and why:
Desk fan, sewing machine, food mixer, dishwasher, washing machine, portable
drill.
7. Why can single-phase IM be speed controlled by voltage whereas three phase
IM’s are not suitable for this type of control.
8. Give reasons for the low efficiency of
(a) hysteresis motors
(b) reluctance motors.
9. What are DC servo motors.
10. State various applications of stepper motors.
Numerical
1. A 220V, 50 Hz, 4-pole, single phase induction motor has the following circuit
model parameters:
R1m=3.6 ohm, (x1m+x2) = 15.6 ohm, r2 = 6.8 ohm, x=96 ohm
The rotational losses of the motor are estimated to be 75 W. At a motor speed of
940 rpm, determine the line current, the power factor, the shaft power, and the
efficiency.
Unit -5 Synchronous Machines
Types, constructional details, EMF equation, Synchronous reactance, voltage
regulation - synchronous impedance methods. Power angle characteristic, Power
factor improvement methods. V-curve, Applications.
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1. What are various advantages to keep armature stationary in case of synchronous
machine.
2. Which type of synchronous machines is used in hydro generators and in steam
turbine generators?
3. Develop the general phasor diagram for cylindrical rotor alternator.
4. Explain why the short circuit characteristic of alternator is a straight line but open
circuit characteristic is a curve.
5. Define regulation of an alternator. Why the terminal voltages changes when the
load is applied to it.
6. How the synchronous machine power factor can be changed from lagging to
leading. Why it is necessary.
7. Why the air gap of the rotating machine is kept very small.
8. What are the functions of pole shoes in the rotating machines?
9. What are the iron losses and how they can be reduced?
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10. Show that iron losses as well as copper losses both depend on the supply
frequency.
11. Explain why alternators are rated in kVA. What is the necessity of mentioning the
p.f. at their name plates.
12. A 3300-volt 3-phase star connected 50Hz alternator has 12 poles. The stator of the
alternators has 144 slots each with 5 conductors per slot. Calculate the necessary
flux per pole to give 3300 volts on no load. Assume the winding concentric and
take the value of form factor as 1.11 and breadth factor as 0.96.
13. Show that a synchronous motor has no net starting torque.
14. Describe the various methods of starting the synchronous motor.
15. Find the expression for power in terms of load angle for a salient pole
synchronous motor working at a lagging p.f. Armature resistance may be
neglected.
16. What is the function of damper winding in case of
1- Alternator 2- Motor.
17. Draw the phasor diagram of a synchronous motor at - lagging leading and unity
power factor.
18. A synchronous motor is operating at a certain load. Explain how you will find out
in the laboratory whether it is operating at a leading or lagging p.f.
19. A salient pole synchronous motor is connected to infinite bus. If the field current
is reduced to zero will it stop or continue running? If the latter what will be its
speed.
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