Electrical Machines Lab Viva Questions and Answers

Question No. 01: Which material is used for the core of a transformer and why?
Answer: Laminations of specially alloyed silicon steel (silicon content 4–5 per cent) are used due to its high electrical resistance, high permeability, non-ageing characteristics and minimum iron loss.

Question No. 02: What is stacking factor? What is its approximate value?
Answer: 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.

Question No. 03: What is the emf equation of a transformer?
Answer: 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.

Question No. 04: Why are LT windings placed near the core?
Answer: LT windings are placed near the core to reduce the total dielectric strength of the insulating materials provided on the winding.

Question No. 05: What do you mean by power transformer?
Answer: Transformers that are used on transmission lines for the transmission and distribution of relatively large quantities of energy are called power transformers.

Question No. 06: The windings of a transformer are divided into several coils because?
Answer: It is difficult to wind as one coil

Question No. 07: What will happen if the d.c. machine is operated below rated speed?
Answer: 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 decrease in speed.

Question No. 08: What will happen if the d.c. shunt motor opened accidentally running on no-load has its shunt field winding?
Answer:
  1. The field will be reduced to only to the value of residual flux.
  2. The speed will be very high.
  3. The parts of motor may even fly apart.

Question No. 09: A d.c. shunt motor is found suitable to drive fans because they require?
Answer: Small torque at start up and large torque at high speeds

Question No. 10: What is the power factor of a transformer at no load?
Answer: 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.

Question No. 11: What are the essential parts of a transformer?
Answer: The essential parts of a transformer are as follows:
  1. Magnetic circuit consisting of laminated iron core and clamping structure
  2. Primary winding
  3. Tank filled with insulating oil
  4. HT terminals with bushings
  5. LT terminals with bushings
  6. Conservator tank
  7. Breather
  8. Vent pipe
  9. Thermometer

Question No. 12: How is magnetic leakage reduced?
Answer: Magnetic leakage is reduced to a minimum by sectionalizing and interleaving the primary and secondary windings.

Question No. 13: What is the permissible maximum flux density in transformer core?
Answer: 1.6–1.8 Wb/m2

Question No. 14: What are the two basic types of transformers?
Answer: The two basic types of transformers are:
  1. The isolation type in which the two windings are physically isolated and electrically insulated from each other.
  2. The auto transformer type in which one coil is used for both the windings.

Question No. 15: What are the types of windings according to the construction?
Answer: According to the construction, the types of windings are:
  1. Sandwich type and
  2. Cylindrical type

Question No. 16: What is an ideal transformer?
Answer: A transformer having an overall efficiency of 100 per cent is called an ideal transformer.

Question No. 17: Oil in transformers is used to?
Answer: To cool the windings

Question No. 18: What do you understand by external characteristics of a d.c. generator?
Answer: 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.

Question No. 19: If the rated speed of a d.c. shunt motor is 1440 r.p.m, which method of speed control would you suggested to obtain a speed of 1500 r.p.m?
Answer: Field control method of speed control is suggested.

Question No. 20: What is the normal phase difference between the voltage and the no-load current in a transformer?
Answer: The no-load current in a transformer normally lags behind the voltage by about 80°– 85°.

Question No. 21: What is the use of iron core in a transformer?
Answer: The iron core is used in a transformer to provide continuous easy magnetic path of low reluctance.

Question No. 22: What is called grain-oriented laminations?
Answer: 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.

Question No. 23: Why is the frequency not changed during transformation of electrical energy in a transformer?
Answer: 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.

Question No. 24: What are the types of transformers?
Answer: Types of transformers are:
  1. Step up transformer
  2. Step-down transformer.

Question No. 25: How does a transformer contribute towards the widespread popularity of AC system over DC?
Answer: 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.

Question No. 26: What is the most important precaution in any experiment with d.c. shunt motor?
Answer: Before switching on d.c. supply, a sufficient resistance should be put in series with the armature of the d.c. shunt motor.

Question No. 27: Does the direction of rotation of d.c. shunt motor would get reversed if the armature current and field current both are reversed?
Answer: No.

Question No. 28: What is the difference between cylindrical-type and sandwich-type winding?
Answer: 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.

Question No. 29: What is the name of the winding to which supply is given?
Answer: The name of the winding is Primary winding.

Question No. 30: What is the name of the winding from which the supply is taken for load connections?
Answer: Secondary winding.

Question No. 31: Why are iron cores in transformers made laminated?
Answer: Iron cores are made laminated to reduce eddy current loss.

Question No. 32: What determines the thickness of the lamination or stamping?
Answer: Frequency determines the thickness of the lamination or stamping.

Question No. 33: Why are the laminations insulated from each other?
Answer: 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.

Question No. 34: What is the phase relationship between the primary and secondary voltages of a transformer?
Answer: The primary and secondary voltages of a transformer are 180° out of phase.

Question No. 35: What is turn ratio of a transformer?
Answer: The ratio of the number of turns in the primary to the number of turns in the secondary-windings is called the turn ratio or the ratio of transformation of the transformer, which is indicated by a constant.

Question No. 36: What is voltage ratio of a transformer?
Answer: 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.

Question No. 37: What are the types of transformers according to the arrangement of iron cores?
Answer: There are three types:
  1. Core type,
  2. Shell type and
  3. Distributed core or Berry type.

Question No. 38: What magnetic circuit is formed in Berry-type constructions and why?
Answer: Distributed magnetic circuits are formed in Berry-type construction because of its distributed cores.

Question No. 39: What is called limb of a transformer?
Answer: The vertical portion of the iron core where the windings are placed is called limb of a transformer.

Question No. 40: What do you mean by step-up and step-down transformers?
Answer: 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.

Question No. 41: What do you mean by distribution transformers?
Answer: When transformers are used for distributing the energy from transmission lines as well as networks for local consumption and the secondaries are directly connected to the consumer’s load, they are called distribution transformers.

Question No. 42: What do you mean by lighting transformer?
Answer: A transformer used to supply a distribution circuit having no motors connected to it is called lighting transformer.

Question No. 43: When the required thickness of lamination in a transformer decreases?
Answer: When the applied voltage increases

Question No. 44: The size and construction of bushings in a transformer depend upon the?
Answer: Size of tank and voltage supplied.

Question No. 45: What is the resistance of the field winding of a D.C. shunt generator kept low?
Answer: 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.

Question No. 46: What range of speed can you get with the field control method of speed control of d.c. shunt motor?
Answer: Speed higher than rated speed can be obtained by using this method.

Question No. 47: What range of speed can you get with the armature control method of speed control of d.c. shunt motor?
Answer: Speed lower than the rated speed can be obtained by the armature control method.

Question No. 48: What is the most essential condition for the voltage build up for a d.c. shunt generator?
Answer: There should be a residual magnetism in the poles of the DC shunt generator.
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