ECE Electrical Engineering 2 Marks PDF
ECE Electrical Engineering 2 Marks PDF
ECE Electrical Engineering 2 Marks PDF
UNIT -I
DC MACHINES
1. State the basic parts of a DC machine.
Stationary Parts: Frame, Main pole, field coils, interpoles, interpole winding
Rotating Parts: Armature core, Armature winding, Commutator, Shaft.
2. Name the various parts of a DC machine that control the magnetic circuit
Poles, Air-gap, Armature core, Yoke.
E = (NZ / 60)(P/A) V
Where,
P= number of poles
Z= Total number of conductors
A= number of parallel paths
= flux per pole
N= speed in rpm
10. What is the function of a no-voltage release coil provided in a dc motor starter?
As long as the supply voltage is on healthy condition the current through the
NVR coil produce enough magnetic force of attraction and retain the starter handle in the
ON position against spring force. When the supply voltage fails or becomes lower than a
prescribed value the electromagnet may not have enough force and the handle will come
back to OFF position due to spring force automatically. Thus a no-voltage or under
voltage protections given to the motor.
14. Why is the emf not zero when the field current is reduced to zero in a dc
generator?
Even after the field current/magnetizing force is reduced to zero the machine is
left out with some flux as residue. Emf due to this residual flux is available when field
current is zero.
17. What are the conditions to be fulfilled for a dc shunt generator to build up emf?
The generator should have residual flux
The field winding should be connected in such a manner that the flux set up
by the field
winding should be in the same direction as that of residual flux
The field circuit resistance should be less than critical field resistance
Load circuit resistance should be above its critical load resistance
22. Why starters are used for DC motors? Or Why a starter is necessary for a DC
motor?
Starters are used in DC motors to limit the starting current within about 2 to 3
times the rated current by adding resistance in series with the armature circuit. Apart
from starting resistances starters are invariably fitted with protective devices such as No-
voltage protection.
It acts as a protecting cover for the whole machine and provides mechanical
support for the machine.
It carries the magnetic flux produced by the poles. The flux per pole divides at
the yoke so that; the yoke carries only half the flux produced by each pole.
Under loaded condition, the shunt field resistance should be less than the critical
field resistance.
29. Why a dc shunt motor is also called a constant flux motor or constant speed
motor?
In shunt motor, flux produced by field winding is directly proportional to the
field current i.e. ( Ish). Here, the input voltage is constant and so the flux is also
constant. Therefore, DC shunt motor is also called a constant flux motor or constant
speed motor.
Unit-II
TRANSFORMERS
5. Does the transformer draw any current when secondary is open? Why?
Yes, it (primary) will draw the current from the main supply in order to
magnetize the core and to supply iron and copper losses on no load. There will not be
any current in the secondary since secondary is open.
7. Full load copper loss in a transformer is 1600 watts. What will be the loss at half
load?
If x is the ratio of actual load to full load then copper loss = x2(full load copper
loss).Here Wc = (0.5)2 x 1600 = 400 watts
11. A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary
winding. Calculate the number of turns on its primary.
We know V1 / V2 = k = N2 / N1
Substituting 400/1100 = 100/N1
N1 = 100/400 x 1100
= 275 turns.
12. What are the functions of no-load current in a transformer?
No-load current produces flux and supplies iron loss and copper loss on no-load.
18. What is the purpose of conducting open circuit and short circuit tests in
transformers?
Open circuit Test : (i) To find out the equivalent circuit parameters R0 & X0 or no load
resistance and reactance
ii) To find out the Iron loss of the transformer
Short circuit Test : (i) To find out the equivalent circuit parameters R01 & X01 or
resistance and reactance of the transformer referred to primary or secondary
ii) To find out the copper loss of the transformer
By using these two tests we can find out the efficiency and regulation of the transformer.
Unit-III
INDUCTION MOTORS
1. State the condition for maximum torque of a three phase induction motor.
What is the maximum torque equal to?
When the slip is maximum, the torque of a three phase IM is maximum.
The maximum torque is equal to sm E22 R2
R22 + sm2 X22
4. What are the purposes that could be served by external resistors connected
in the rotor circuit of phase would induction motor.
To limit the starting current
To increase the starting torque
To control the speed.
5. How can the reversal of rotation of poly phase induction motor be attained?
The reversal of rotation of poly phase induction motor is obtained by
interchanging any two terminals of the three phase windings when connecting to
the supply.
6. A 3 ph, 50 Hz induction motor runs at 960 rpm on full load. Find the
number of poles and slip speed.
Induction motor always runs nearer to synchronous speed. So, assume
Ns = 1000rpm.
Ns = 120f / p = 1000 p = (120) (50) / 1000 = 4.
Slip speed = Ns N = 1000 960 = 40 rpm.
9. Point out the disadvantages of rotor rheostat control to obtain variable speed
of induction motor.
Large power loss due to the increase in resistance value.
Due to large power loss efficiency is low.
The speed above normal speed can not be obtained.
Bulky and expensive
Wide speed range is not possible. Because it needs large resistance which
will be the cause to increase the losses.
13. Explain the conditions for maximum torque under running condition.
The torque depends on slip at which motor is running. So the only
parameter which controls the torque is slip. So by differentiating the torque
equation with respect to slip the condition for maximum torque can be achieved.
That is slip is equal to the ratio between the rotor resistance and reactance.
R2 = SX2
Motor Stator
Input Rotor Rotor Gross Friction Rotor
Cu & Input Cu Torque &Windage
in Iron Output
Stator P2 Loss Tg losses Pout
losses
P1
17. What are the principles involved in the starting of the rotor to rotate?
Faradays laws of electromagnetic induction.- By this principle an emf
will be induced in the rotor conductors .
Lenzs law By this law the rotor current is produced since the rotor
conductors are short circuited.
. 25. What are the merits and demerits of injecting emf method of speed control?
Advantages:
Wide speed range is possible
Improved power factor.
Demerit:
It can be only applied for slip ring induction motors.
26. What is the name of the starter used for starting the slip ring IM?
Rotor resistance starter is used for the starting of slip ring IM. There is a
provision to change the value of rotor resistance in slip ring IM alone. This type
of starter also improves the starting torque.
UNIT-IV
SYNCHRONOUS AND SPECIAL MACHINES
.
4. List the types of rotors of the synchronous generator. State their features.
There are two types of synchronous generator namely salient type and
cylindrical type rotor.
In salient pole structure, the air gap is non uniform. These poles are
mechanically weak and hence preferred for low speed alternators. Separate damper
winding is provided.
In cylindrical type structure, the air gap is uniform. It is mechanically robust
and hence preferred for high speed turbo alternators. Separate damper winding is not
necessary.
6. Why the field system of an alternator made as a rotor? (Or) Why the
armature is made stationary in alternator?
The field system of an alternator is made rotating to avoid interaction of
mechanical and electrical stresses. Also with rotating field system, it is easier to
collect currents at very high voltages from stationary member. Due to low voltage
on field side, the insulation required is less. The problem of sparking is avoided.
The construction with rotating field is simple and only two slip rings are required
to provide external dc supply
8. What are the reasons for drop in voltage from no load to full load?
The reasons for drop in voltage from no load to full load are,
1. Drop in armature resistance
2. Armature leakage reactance
3. Reaction corresponding to armature reaction.
29. Why is the hysterisis motor free from mechanical and magnetic vibrations?
Due to magnetic retentivity property there is continuous magnetic locking
between stator and rotor which avoids the vibrations.
As there is no rotor teeth and winding there are no mechanical vibrations
31. Define the step angle of a stepper motor and write the expression for the
same.
The angle through which the stepper motor rotates for the train of pulses
is called step angle.
= 360 / mNr
where m denotes the number of phases and Nr denotes the number of rotor teeth.
Unit-V
TRANSMISSION AND DISTRIBUTION
Disadvantages
i) Point to point transmission not possible
ii) Introduction of harmonics
iii) Blocking of reactive power
iv) High cost of terminal equipment
23. What are the types of insulators used in overhead transmission lines
i. Sackle type (230-440 V lines)
ii. Pin type
iii. Suspension type
iv. Strain type
29. Give the reasons of high capacitance in cables than the overhead transmission
lines?
i) High value of permittivity of insulating materials
ii) Distance between the core and the earthed sheath is small
iii) Small distances between the cores (phases) itself