Latest Transformer Interview Questions For Freshers Part – 5
Q: What is a Transformer?
A: A transformer is a static device which can transfer power from one circuit to another at same frequency.
Q: How Does a Transformer Work?
A: Transformer consists of two coils.If one coil is connected with ac voltage source then it will produce alternating flux in the core. Most of the flux is linked with second coil hence mutually induced emf will produce in the second coil as per faraday’s law of electromagnetic induction.
Q: Can DC be applied to transformers ?
No
i) Transformer works on Faraday’s law of Electromagnetic Induction for which current in coil must change. If DC is applied current will not change and transformer will not work.
ii) Practically winding resistance is very small. For DC ,inductive reactance is zero and frequency is zero. Therefore impedance is low. Thus winding draws more current which may damage the winding.
Q: What are the various types of Transformers?
Based on supply
1.Single phase transformer
2.Three phase transformer
Based on winding
1.Auto transformer (single winding)
2.Two winding transformer
3. Three winding transformer
4. Six winding transformer
Based on construction
1. Core Type transformer
2. Shell Type transformer
Based on the service
1. Distribution transformer
2. Power transformer
Based on measurement
1. Current transformer
2. potential transformer
Based on cooling
1. Dry type Transformer
2. Oil immersed type transformer
Based on function
1. Step up transformer
2.Step down transformer
3.Isolation transformer
Q:What is single and three phase transformer?
The transformer which works on single phase is called single phase transformer.
The transformer which works on three phase is called three phase transformer.
Q: What is Auto-transformer?
In these transformer only one winding is used as primary and secondary. Also, primary and secondary are conductively coupled.
Q:What is Two winding transformer?
Two separate windings one as primary and other as secondary are used. Both windings are magnetically coupled.
Q: What is Three winding transformer?
Three windings are used each work as primary and secondary. Also primary and secondary windings are conductively couple. They are three phase auto transformer
Q:What is Six winding transformer?
Three windings in primary and three windings in secondary are used, They are three phase transformers.They are connected in either star-star, delta-delta, star delta or delta star.
Q: What are the advantages of shell type transformer over core type transformers?
In shell type transformers the coils are well supported on the all sides and so they can withstand higher mechanical stresses developed during short circuit conditions. Also the leakage reactance will be less in shell type transformers.
Q: Where is core type and shell type construction suitable for a transformer?
Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. Where as, Shell type transformers are popular in Low voltage applications like transformers used in electronic circuits and power electronic converters etc
Q: What are power transformer?
Power transformer is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It is big in size as compared to distribution transformer, it used in generating station and Transmission substation. It is generally rated above 200MVA.
Q: What are distribution transformer?
The distribution transformer is used for the distribution of electrical energy at low voltage as less than 33KV in industrial purpose and 440v-220v in domestic purpose. It works at low efficiency at 60-70%, small size, easy in installation, having low magnetic losses & it is not always fully loaded.
Q: Why the efficiency of distribution transformer is 60 to 70 % and not 100%?
Distribution transformer is designed for maximum efficiency at 60% to 70% load as it normally doesn’t operate at full load all the time. Its load depends on distribution demand. Whereas power transformer is designed for maximum efficiency at 100% load as it always runs at 100% load being near to generating station.
Q: What are the salient features of distribution transformer?
1. The distribution transformer will have low iron loss and higher value of copper loss
2. The capacity of transformers will be up to 500 KVA
3. The transformers will have plain walled tanks or provided with cooling tubes or radiators.
4. The leakage reactance and regulation will be low.
Q: What are Current transformer?
They are protective devices used to measure very high value of current in power system. The Current Transformer ( C.T. ), is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary
Q: What are Potential transformer?
They are protective devices used to measure very high value of voltage in power system.
Q: What are dry type transformer ?
In this type of transformer, air is used as coolant.The heat is taken to walls of tank and dissipated to the surrounding air.
Q: What is oil immersed type transformer?
In this type of transformer, oil is used as coolant. Entire assembly including core and windings is kept immersed in oil. The developed heat is transferred to the walls of tank through oil. Finally the heat is transferred to the surrounding air from tank wall by radiation.
Q: What are step up transformers?
They step up voltage from lower value to higher value.
Q: What are step down transformers?
They step up voltage from higher value to lower value.
Q: What are isolation transformers?
In this type of transformer the primary and secondary are same. When it is required to isolate the primary and secondary circuits these type of transformers are used. It protects the circuits connected in secondary side when major fault occurs in primary side.
Q: Why stepped cores are used?
(i) To reduce the space effectively.
(ii) To obtain reduced length of mean turn of the windings.
(iii) To reduce I² R loss.
Q:. What is yoke section of transformers?
The sections of the core which connect the limbs are called yoke. The yoke is used to provide a closed path for the flux.
Q:. What is the purpose of laminating the core in a transformer?
In order to minimize eddy current loss.
Q: Explain on the material used for core construction?
A:The core is constructed by sheet steel laminations assembled to provide a continuous magnetic path with minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities.
The eddy current loss is minimized by laminating the core, the laminations being used from each other by light coat of coreplate varnish or by oxide layer on the surface. The thickness of lamination varies from 0.35mm for a frequency of 50Hz and 0.5mm for a frequency of 25Hz.
Q: Why the cross-section of iron is less than total cross section area of core?
This is because the core is laminated & on each lamination the insulations are are used.
Q:What is Stack factor?
The ratio of total cross section of iron to the total cross section of core is called stack factor.
Q: What is the EMF equation of transformer?
Erms = 4.44 f Φ maxT volt
Q:What is voltage transformation ratio or turn ratio?
It is expressed as ratio of the primary and secondary voltages of transformer.
or
It is expressed as ratio of the primary and secondary turns of transformer.
E1/ E2= T1/T2 = K
Q:What is transformation ratio for step up, step down and isolation transformer?
K>1 for step up transformer
KE2 for leading p.f load
V2It should be free from moisture
–> It should have high dielectric strength
–> It should have thermally stability and higher thermal conductivity
–>It should be contaminated by temperature rise.
Q: Why transformer oil is used as a cooling medium?
When transformer oil is used as a coolant the heat dissipation by convection is 10 times more than the convection due to air. Hence transformer oil is used as a cooling medium.
Q: What is conservator?
A conservator is a small cylindrical drum fitted just above the transformer main tank. It is used to allow the expansion and contraction of oil without contact with surrounding atmosphere. When conservator is fitted in a transformer, the tank is fully filled with oil and the conservator is half filled with oil.
Q: What is buchholz relay?
It protects the transformer from their internal faults like earth faults, winding short circuit, short circuit between phases, Puncture of bushing etc..
Q: Where is buchholz relay located?
It is located between transformer tank and conservator.
Q.In buchholz relay how the top and bottom mercury switches are connected?
Top mercury switch is connected for alarm and bottom switch for trip.
Q: What is the purpose of providing Taps in transformer and where these are provided?
In order to attain the required voltage, taps are provided, normally at high voltages side(low current).
Q: In transformers, why the low voltage winding is placed near the core?
The winding & Core are both made of metals and so an insulation have to be placed in between them, the thickness of insulation depends on the voltage rating of the winding. In order to reduce the insulation requirement the low voltage winding place near the core.
Q: Why the tapings are placed on HV side?
1.A fine voltage regulation is possible with hV side as it has large number of turns.
2.LV side carries large current
3. HV side is placed outside so its feasible
4. In case of step down transformer at light loads the HV side voltage increases, it is required to decrease this voltage by adjusting tappings on HV . With large no. of turns the flux and flux density decreases. Hence core losses decreases results in higher efficiency.
Q:List some methods of cooling of transformers.
A:Air natural, Air blast, Oil Natural, Oil natural air forced, Oil natural water forced, Oil forced, Oil forced air natural, Oil forced air natural, Oil forced water forced.
Q:What are the factors to be considered for choosing the method of cooling?
The choice of cooling method depends on KVA rating of transformer, size, application and the site conditions where it will be installed.
Q: How the heat dissipates in a transformer?
The heat dissipation of a transformer occurs by convection, conduction and radiation.
Q: Why cooling tubes are provided?
Cooling tubes are provided to increase the heat dissipating area of the tank.
Q: How the heat dissipation is improved by providing the cooling tubes?
The cooling tubes will improve the circulation of oil. The circulation of oil is due to effective pressure heads produced by columns of oil in tubes. The improvement in cooling is accounted by taking the specific heat dissipation due to convection as 35% more than that without tubes.
Q: How the leakage reactance of the transformer is reduced?
In transformers the leakage reactance is reduced by interleaving the high voltage and low voltage winding.
Q: How does change in frequency affect the operation of a given transformer?
With a change in frequency, iron and copper loss, regulation, efficiency & heating varies so the operation of transformer is highly affected.
Q: What is the Difference Between “Insulating”, “Isolating”, and “Shielded Winding” Transformers?
Insulating and Isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded winding transformer, on the other hand, is designed with a metallic shield between the primary and secondary windings, providing a safety factor by grounding, thus preventing accidental contact between windings under faulty conditions.
All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, which are a type whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.
Q: Can Transformers be Operated at Voltages other than Nameplate Voltages?
In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its name- plate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly.
For example, if a 480 volt primary trans¬former with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the trans¬former were originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.
Q: What are the applications of step-up & step-down transformer?
A:Step-up transformers are used in generating stations. Normally the generated voltage will be either 11kV. This voltage (11kV) is stepped up to 110kV or 220kV or 400Kv and transmitted through transmission lines (simply called as sending end voltage). Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV are stepped down to 3phase 400V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations are called power transformers.
Q.What is the vector group of distribution transformer?
Dy11
Q.What is the humming of transformer?
Humming is a sound, which is produced due to the vibration of the cores in the transformer. The vibrations are produced due to the change in polarity of an alternating current or voltage and by the loose of lamination of the core. Both can be minimised by tightening the core of the transformer.
Q: Can 60 Hz transformers be operated at 50 Hz?
The transformers rated below 1 KVA can be used on 50 Hz service. Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate on a 60 Hz service.
Q:Is it Possible to Change Three Phase to Two Phase or Vice-Versa with Standard Transformers?
Yes. This is a very practical application for standard single phase off-the-shelf transformers. Some typical voltage combinations are as follows: 480 volts three phase to 240 volts two phase, or 240 volts three phase to 480 volts two phase, or 240 volts three phase to 240 volts two phase. Please refer to us for an exact schematic.
Q:List four applications of a transformer.
It can raise or lower the voltage or current in an AC circuit.
It can act as an impedance transferring device by increasing or decreasing the value of a capacitor, inductor or resistance in an AC circuit.
It can isolate two circuits electrically.
It can be used to prevent DC from passing from one circuit to another
Q:What is the cause of noise in transformer?
The cause of noise in the transformer is mainly because of magnetostriction effect and also loosening of stampings and mechanical forces produced during working.
Q:What is tertiary winding?
Some three phase transformers may have a third winding called tertiary winding apart from primary and secondary. It is also called auxiliary winding or stabilizing winding. The tertiary winding is provided in a transformer for any one of the following reasons, i) to supply small additional load at a different voltage ii) to give supply to phase compensating devices such as capacitors which work at different voltages. iii) to limit short circuit current. iv) to indicate voltage in high voltage testing transformer
Q:The area of the yoke in a transformer is taken 15 to 20% larger than that of the core. Why?
By keeping yoke area 15-20% higher ,the flux density in the yoke is reduced, resulting into reduction in iron losses for yoke. The reduced core area results working flux density and needed to increase the number of turns.
Q: Why we need isolation transformers in power circuit ? why cant we use breakers to isolate?
In Isolation transformer is used, if there is change in parameter values of any side, the other side parameters will remain unchanged.
Q:Why do transformers hum?
Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.
The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet. These extensions are proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level.
Q:Why not reduce the noise in the core by reducing the amount of flux?
Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration.