Electrical Machines VIVA Questions :-
1.What is the power factor of a transformer at no load?
At no load, the power factor of a transformer is very low and lagging, whereas the power factor on load is nearly equal to the power factor of the load, which it is carrying.
2.What is the normal phase difference between the voltage and the no-load current in a transformer?
The no-load current in a transformer normally lags behind the voltage by about 80°– 85°.
3.What are the essential parts of a transformer?
The essential parts of a transformer are as follows:
i.Magnetic circuit consisting of laminated iron core and clamping structure
iii.Tank filled with insulating oil
iv.HT terminals with bushings
v.LT terminals with bushings
vi. Conservator tank
4.What is the name of the winding to which supply is given?
5.What is the name of the winding from which the supply is taken for load connections?
6.Which material is used for the core of a transformer and why?
Laminations of specially alloyed silicon steel (silicon content 4–5 per cent) are used due toits high electrical resistance, high permeability,non-ageing characteristics and minimum ironloss.
7.What is the use of iron core in a transformer?
The iron core is used in a transformer to provide continuous easy magnetic path of low reluctance.
8.How is magnetic leakage reduced?
Magnetic leakage is reduced to a minimum by sectionalizing and interleaving the primary and secondary windings.
9.Why are iron cores in transformers made laminated?
Iron cores are made laminated to reduce eddy current loss.
10.What determines the thickness of the lamination or stamping?
Frequency determines the thickness of the lamination or stamping
11.Why are the laminations insulated from each other?
The laminations are insulated from each other by insulating varnish or thin paper to break the path of eddy currents and thus reduce eddy current loss.
12.What is stacking factor? What is its approximate value?
Stacking factor is the ratio of iron content in the laminated varnished core by volume. Its value is about 90 per cent, that is, 10 per cent volume is occupied by varnish and air.
13.What is called grain-oriented laminations?
Grain-oriented laminations are cold rolled laminations specially annealed to orient the iron crystals, that is, the grains in a uniform way in the direction of rolling to get very high permeability and low hysteresis loss.
14.What is the permissible maximum flux density in transformer core?
15.What is the phase relationship between the primary and secondary voltages of a transformer?
The primary and secondary voltages of a transformer are 180° out of phase.
16.What is turn ratio of a transformer?
The ratio of the number of turns in the primary to the number of turns in the secondarywindings is called the turn ratio or the ratio of transformation of the transformer, which isindicated by a constant.
17.What is voltage ratio of a transformer?
Voltage ratio is the ratio of the voltage between the line terminals of one winding to that between the line terminals of another winding at no load.
18.What current flows in the transformer primary when its secondary is open?
When the secondary is open, there is no current in the secondary of the transformer. The
primary takes a small current I0) from the source called no-load current which has a magnetizing
component (I0 sin?0) producing the magnetic flux and a working component (I0 cos?0) supplying
real power for iron losses.
19.What is the formula for calculating no-load current?
20.Why is the frequency not changed during transformation of electrical energy in a transformer?
As the same flux having a definite frequency is responsible for the production of emf on both the primary and secondary windings, there is no question of change in the frequency.
21. What is the emf equation of a transformer?
Erms = 4.44 Fm fN V
where Fm = maximum value of the flux linkage with both the windings
f = frequency of the
supply,N = number of turns.
22.What are the two basic types of transformers?
The two basic types of transformers are:
i.The isolation type in which the two windings are physically isolated and electrically
insulated from each other.
ii.The auto transformer type in which one coil is used for both the windings.
23.What are the types of transformers according to the arrangement of iron cores?
There are three types: (i) core type, (ii) shell type and (iii) distributed core or Berry type.
24.What magnetic circuit is formed in Berry-type constructions and why?
Distributed magnetic circuits are formed in Berry-type construction because of its distributed cores.
25.What is called limb of a transformer?
The vertical portion of the iron core where the windings are placed is called limb of a transformer.
26.Why are LT windings placed near the core?
LT windings are placed near the core to reduce the total dielectric strength of the insulating materials provided on the winding.
27.What are the types of windings according to the construction?
Sandwich type and cylindrical type.
28.What is the difference between cylindrical-type and sandwich-type winding?
In cylindrical-type winding, the length of the coils is equal to the length of the core limb.
The primary and secondary windings are placed one over the other, placing low-voltage winding
nearer the core, whereas in sandwich-type winding HT and LT windings are placed lengthwise one above and other like a sandwich.
29.What are the types of transformers ?
(i) Step up transformer (ii) Step-down transformer.
30 .hat do you mean by step-up and step-down transformers?
When a transformer converts low voltage to high voltage it is called a step-up transformer,and when the transformer converts high voltage to low voltage it is called a step-down transformer.
31.What is an ideal transformer?
A transformer having an overall efficiency of 100 per cent is called an ideal transformer.
32.What do you mean by power transformer?
Transformers that are used on transmission lines for the transmission and distribution of
relatively large quantities of energy are called power transformers.
33.What do you mean by distribution transformers?
When transformers are used for distributing the energy from transmission lines as well as
net-works for local consumption and the secondaries are directly connected to the consumer’s
load, they are called distribution transformers.
34.What do you mean by lighting transformer?
A transformer used to supply a distribution circuit having no motors connected to it is
called lighting transformer.
35.How does a transformer contribute towards the widespread popularity of AC system over DC?
High voltage of AC system can be obtained using a transformer for transmission of electrical power.Using a transformer, AC voltage can be increased or decreased without any power loss.
36.The required thickness of lamination in a transformer decreases when
The applied voltage increases
37.Oil in transformers is used to –
Cool the windings
38.The windings of a transformer are divided into several coils because
It is difficult to wind as one coil
39.The size and construction of bushings in a transformer depend upon the
Size of tank and voltage supplied.
40.What is the resistance of the field winding of a d.c. shunt generator kept low?
If the field resistance of a d.c. generator is more than particular value ( critical resistance),
The generator will fail to build up the voltage. For this reason, the field resistance of a d.c. shunt
generator is kept low.
41.What do you understand by external characteristics of a d.c. generator?
The graph between the terminal voltage and load current is known as external
characteristics of a d.c. generator, provided speed and field current remain constant.
42.What will happen if the d.c. machine is operated below rated speed?
This will result in overheating due to two reasons; first, more field current has to be maintained in order to produce the rated voltage. Second, decrease in fanning action due to dsecrease in speed.
43.What is the most important precaution in any experiment with d.c. shunt motor?
Before switching on d.c. supply, a sufficient resistance should be put in series with the
armature of the d.c. shunt motor.
44.What range of speed can you get with the field control method of speed control of d.c. shunt motor?
Speed higher than rated speed can be obtained by using this method.
45.What range of speed can you get with the armature control method of speed control of d.c. shunt motor?
Speed lower than the rated speed can be obtained by the armature control method.
46.Does the direction of rotation of d.c. shunt motor would get reversed if the armature current and field current both are reversed?
47.If the rated speed of a d.c. shunt motor is1440 r.p.m, which method of speed control would you suggested to obtain a speed of 1500 r.p.m?
Field control method of speed control is suggested.
48.What will happen if the d.c. shunt motor opened accidentally running on no-load has its shunt field winding?
- The field will be reduced to only to the value of residual flux. The speed will be very high.
- The parts of motor may even fly apart.
49.What is the most essential condition for the voltage build up for a d.c. shunt generator?
There should be a residual magnetism in the poles of the DC shunt generator.
50. A d.c. shunt motor is found suitable to drive fans because they require
Small torque at start up and large torque at high speeds