Dear Readers, Welcome to Transformer Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Transformer. These Transformer Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
A: A transformer is a static device which can transfer power from one circuit to another at same frequency.
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.
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.
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
The transformer which works on single phase is called single phase transformer.
The transformer which works on three phase is called three phase transformer.
In these transformer only one winding is used as primary and secondary. Also, primary and secondary are conductively coupled.
Two separate windings one as primary and other as secondary are used. Both windings are magnetically coupled.
Three windings are used each work as primary and secondary. Also primary and secondary windings are conductively couple. They are three phase auto 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.
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.
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
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.
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.
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.
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.
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
They are protective devices used to measure very high value of voltage in power system.
In this type of transformer, air is used as coolant.The heat is taken to walls of tank and dissipated to the surrounding air.
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.
They step up voltage from lower value to higher value.
They step up voltage from higher value to lower value.
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.
(i) To reduce the space effectively.
(ii) To obtain reduced length of mean turn of the windings.
(iii) To reduce I² R loss.
The sections of the core which connect the limbs are called yoke. The yoke is used to provide a closed path for the flux.
In order to minimize eddy current loss.
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.
This is because the core is laminated & on each lamination the insulations are are used.
The ratio of total cross section of iron to the total cross section of core is called stack factor.
Erms = 4.44 f F maxT volt
It is expressed as ratio of the primary and secondary voltages of transformer.
It is expressed as ratio of the primary and secondary turns of transformer.
E1/ E2= T1/T2 = K
K>1 for step up transformer
K<1 for step down transformer
K=1 for isolation transformer
1.There should be no losses
2.The winding should have zero resistance
3.The leakage flux should be zero
4. The permeability of core should be so high that the negligible current is required to establish the flux in it.
No-load current produces flux and supplies iron loss and copper loss on no-load.
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
i) IM (Magnetizing/reactive /wattless) current. It magnetizes core
ii) Iw (Coreloss/ active/wattful ) current. It supplies hysteresis and eddy current loss and negligible I² R loss.
No load curent of transformer is 3 to 5% of rated current in transformer.
Req = R1 +R2' = R1+a² R2'
Xeq = X1 +X2' = X1+a² X2'
Zeq = Z1 +Z2' = Z1+a² Z2'
Req = R2 +R1' = R2+ R1'/a²
Xeq = X2 +X1' = X2+ X1'/a²
Zeq = Z2 + Z1' = Z2+ Z1'/a²
Equivalent impedance is essential to be calculated because the electrical power transformer is an electric power system equipment, for estimating different parameters of electric power system which may be required to calculate total internal impedance of an electric power transformer, viewing from primary side or secondary side as per requirement. This calculation requires equivalent circuit of transformer referred to primary or equivalent circuit of transformer referred to secondary sides respectively.
Voltage regulation in transformers is the difference between the no load voltage and the full load voltage.
This is usually expressed in terms of percentage. For example: A transformer delivers 100 volts at no load and the voltage drops to 95_volts at full load, the regulation would be 5%.
dry type distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage istermed as regulation.
%regulation =E2-V2/E2 *100
%regulation = voltage drop in transformer/ no load voltage of transformer
V2>E2 for leading p.f load
V2<E2 for lagging p.f load
Negative sign indicates zero voltage regulation. It occurs when load is capacitive and power factor is leading.
It occurs when load is inductive and power factor is lagging.
In transformer, we supply alternating current in the primary, this alternating current produces alternating magnetizing flux in the core and as this flux links with secondary winding, there will be induced voltage in secondary, resulting current to flow through the load connected with it. Some of the alternating fluxes of transformer; may also link with other conducting parts like steel core or iron body of transformer etc. As alternating flux links with these parts of transformer, there would be a locally induced emf. Due to these emfs, there would be currents which will circulate locally at that parts of the transformer. These circulating current will not contribute in output of the transformer and dissipated as heat. This type of energy loss is called eddy current loss of transformer.
By using less thickness of laminations
By using less value of Magnetic flux density
The magneto motive force or mmf applied in the transformer core is alternating. For every cycle due to this domain reversal, there will be extra work done. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss of transformer.
By using good magnetic material.
By using less value of Magnetic flux density
Iron losses depend on supply frequency and flux density in the core. For all normal operations, the frequency of flux reversals which is same as supply frequency is constant and the value of flux density more or less remains constant. Hence iron losses remain constant under all load conditions. i-e from no-load to full-load
A:Copper loss of a transformer depends on current and iron loss on voltage . Hence total losses depends on Volt- Ampere and not on the power factor. That is why the rating of transformers are in kVA and not in kW.
Transformer efficiency ?= (output power/input power) x 100
A:The efficiency of the transformer will be in the range of 94% to 99%. Among the available electrical machines the transformer has the highest efficiency
Transformers operate at higher efficiency when compared to other electrical machines.This is due to the absence of mechanical losses which is due to the absence of moving parts
When Iron losses is equal to copper losses.
A:It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. All day efficiency=output in kWh/input in kWh for 24 hrs.
1.Direct loading test
2.Open circuit test
3. Short circuit test
4. Sumpner's or back to back test
The high-voltage side is generally kept open because the current in high-voltage winding is less compared to that on low-voltage winding.The LV side has higher current so that maximum no load current can be measured.
The rated current is less on HV side. This will also permit to use ammeter and wattmeter of lower current range.
A:The open circuit on a transformer is conducted at a rated voltage because core loss depends upon the voltage. This open circuit test gives only core loss or iron loss of the transformer.
Efficiency as well as temperature rise of winding.
Loading a transformer is a very difficult thing to test. Say the transformer is rated at 300 MVA, now how to find a 300MVA load to test your transformer? Also the load will waste all of its energy to heat. Sumpner's test allows to fully load a transformer without actually having that load. The energy lost is only equal to the losses in the transformers, nothing which the transformer cooling system cannot handle. In Sumpner's test two transformers are connected back to back means,primaries of two transformers connected in parallel and secondaries side connected in series.
-Non availability of single large transformer to meet the load
-Increased power demand
-To improve reliability
-If many smaller transformer is used one can be used as spare
-Transportation problem for large transformer.
-Equal turn ratio
-percentage impedance should be same
-Equal X/R ratio
-Equal KVA rating
-Equal phase sequence
Connecting transformers with wrong polarity can result in circulating currents or short circuits
Two transformers in Parallel should have the same primary and secondary voltage ratings. Any error in the voltage ratio would cause heavy circulating currents to flow between the transformers. This circulating current will result in a corresponding imbalance in the primary currents, and result in overloading of one transformer. This circulating current will result in increased copper losses.
A difference in the ratio of the reactance value to resistance value of the per unit impedance results in a different phase angle of the currents carried by the two paralleled transformers; one transformer will be working with a higher power factor and the other with a lower power factor than that of the combined output. Hence, the real power will not be proportionally shared by the transformers.
If the phase sequence is in an incorrect manner, in every cycle each pair of phases will get short-circuited.
i) As a booster to compensate the voltage drop for better regulation
ii)As induction motor starters.
iii) In locomotive
iv) As furnace transformer
Require less conductor material
high VA rating
Better voltage regulation
small amount of no load current
The autotransformer cannot provide isolation between HV and LV side. Due to open circuit in the common portion, the voltage on the load side may shoot up to dangerously high voltage causing damage to equipment. This unexpected rise in the voltage on LV side is potentially dangerous. Hence the autotransformer cannot be used as distribution transformer
1. Net core weight: Higher the flux density lesser net weight of core.
2.Saving in copper:Higher the flux density lesser mean turn of windings.
3.Overall economy: Higher the flux density lesser net weight of core and windings. Hence cheaper
4.Net weight: Due to above reasons weight is also reduced.
5. High magnetizing current due to higher flux density
6. High Flux density results in high losses
7. High Flux density results in high losses and hence efficiency decreases
8.Hence high temperature rise and noise
It is the ratio of copper area in the window to the total window area.
A transformer bank consists of three independent single phase transformers with their primary and secondary windings connected either in star or delta.
1. Continuity of supply
2. Less Installation of cost
3. Easy transportation
4.Stand by function
5.Unbalanced load supply
1.Saving in iron material
2. Small size
3. Less transformer oil
5. Higher efficiency
The most common types of transformer connections are,
i. Star-Star (Y-Y)
iii. Star-Delta (Y-?)
iv. Delta-Star (?-Y)
v. Open Delta (V-V)
vi. Scott Connection (T-T)
Distribution transformers are connected in delta-star. This arrangement requires 3 conductors in the high voltage side and 4 in the low voltage side, as well as it provides the star point conductor as a neutral point. This can serve single phase as well as three phase loads.
Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil.
Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases.
Also to avoid sledging of oil i.e. decomposition of oil.
The silica gel is used to absorb the moisture when the air is drawn from the atmosphere in to the transformer.
Dry stage – Deep blue
Saturated stage – Whitish pink
Transformer oil provides , (i)good insulation and (ii)cooling . Nowadays instead of natural mineral oil, synthetic oils known as ASKRELS (trade name ) are used. They are non-inflammable, under an electric arc do not decompose to produce inflammable gases. PYROCOLOR oil possess high dielectric strength.
The following are the desirable properties of transformer oil:
-->It 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.
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.
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.
It protects the transformer from their internal faults like earth faults, winding short circuit, short circuit between phases, Puncture of bushing etc..
It is located between transformer tank and conservator.
Top mercury switch is connected for alarm and bottom switch for trip.
In order to attain the required voltage, taps are provided, normally at high voltages side(low current).
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.
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.
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.
The choice of cooling method depends on KVA rating of transformer, size, application and the site conditions where it will be installed.
The heat dissipation of a transformer occurs by convection, conduction and radiation.
Cooling tubes are provided to increase the heat dissipating area of the tank.
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.
In transformers the leakage reactance is reduced by interleaving the high voltage and low voltage winding.
With a change in frequency, iron and copper loss, regulation, efficiency & heating varies so the operation of transformer is highly affected.
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.
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.