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1. What is a transformer?

A transformer transfers electrical energy between two circuits. It usually consists of two wire coils wrapped around a core. These coils are called primary and secondary windings. Energy is transferred by mutual induction caused by a changing electromagnetic field. If the coils have different number of turns around the core, the voltage induced in the secondary coil will be different to the first.

History of Transformers:
Transformers are based on the theory of electromagnetic induction, which was discovered by Michael Faraday in 1831. It was not until 1836 that the first device, an induction coil, was invented. William Stanley, who designed the first commercial model, introduced the term "transformer" in 1885.

Types of Transformers:
The two major types of transformer are laminated cores and toroidals.
• Laminated cores are those common cube-shaped transformers, which are used in power adapters. They are stronger and cheaper than toroidals.
Toroidals are smaller and lighter, for the same power rating. They also produce less electrical noise and are more efficient. The secondary winding can be joined in series to double the voltage or joined in parallel for higher current.

Other types of transformers include the variac, audio, and balun.

• Variacs have a movable carbon brush that connects to the winding, providing a large range of voltages.
• Audio transformers are used to amplify signals and drive speakers.
• Baluns are short windings that convert impedances, such as those between a television and an antenna.

2. How does a transformer work?

Alternating current in the primary winding creates an electromagnetic field that induces a current in the secondary winding when the field changes. Small transformers use enameled wire for their windings, while large transformers use insulated copper strips. Transformers can be single winding, center-tap, or multi-tap. Center-taps have a terminal at the middle point of the secondary winding, which has half the voltage of the end terminal. Multi-taps have many terminals along the winding, whose voltages depend on their locations. The purpose of the core is to direct the electromagnetic field through the secondary winding. Silicon steel cores are used for their high magnetic permeability. The insulated laminations work better than solid cores, by confining eddy currents, which reduces their losses.

3 How to test Power Transfomer vector group in the field?

Put the tap position on the nominal taps.  
Connect the 4 core cable with MCB on the HV side and the
leads cable at LV side and make sure that  
the LV side in open circuit. Then loop together between HV
red phase and LV red phase. 
Inject 415V (Switch on the MCB) at HV side and measure the
voltage at LV side for each phase. record results and by
using the test result and following proper scale draw the
vector group of the transformer winding.

4 What are the testing to be done before the installation of 1000 KVA Transformer?

Normally test are done before installation and commissioningof the transformer. The following test are done;
1. Visual Inspection
2. Insulation Resistance Test
3. Ratio Test
4. Vector Group Test
5. Magnetizing Current Test
6. Core Balance Test
7. Winding Resistance Test

5 Why distribution transformer is uesd in Delta/Star formation?is there any significance in doing this?

we need neutral point for single phase only star connection we can get this thats why distribution transformer delta star connected

6 Why the size of transformer decreases if we increases the frequency?

EMF of a transformer increases with frequency. As a result the transformer can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are required to achieve the same impedance.

7 What will happen at secondary side of transformer if i increase voltage at primary side?
While"V1*I1=V2*I2" ,will secondary voltage or current increase?

If you increase the voltage at the primary of the transformer, then definitely at the secondary side voltage increases, but it will cause loading of the winding. so you should not increas voltage at the primary side beyond the rated voltage.

8 What is the transformenr turns ratio ?


N2=number of secondary turns
N1=number of primary turns
V2=secondary voltage
V1=primary voltage
I1=primary current
I2=secondary current

For Impedance use:


9 What are the saftey parameters in transformers?

1.Pressure relieve valve
2. Oil surge relay
3. Buchltze relay
4. Breather
5.Auto fire extinguishing(by sprikler) for big transformers
Different protection Systems also come under Safety devices.

10 What is differential relay in power transformer?

It is a relay that checks for current balance between the primary and the secondary side of a transformer.it is also used in other components of the power system like to protect cables.itis also known as a unit protection as it does not discriminate with any other protection schemes.the ct secondary currents that circulates in the coil of this relay from primary and secondary of the transformer cancells each other when the system is healthy.when the fault occurs in a system the balance is disturbed and the resultant current activate the relay and cause trip.

11 Why transformer is not rotating?

Transformer is a static device which transfers voltage fromone circuit to another circuit without changing thefrequency(changes the voltage level). The working principleis dpends upon mutual induction which is created by thealternating or rotating magnetic field. SO transformers are not rotating type.

12 Which type of material is used to transformer laminated core?

Type of material is transformer laminated core silicon seal,it can reduce hysteresis loss.

13 How can be decreased the size of transformer?

By increasing the frquency, we can reduce the size of transformer (In air craft,size of tranformer are very small as there frequncy is 400Hz)

14 Why the secondary terminal of current transformer should not be lift open circuited?

The MMF developed in the primary side will depend only on load current which is highdue to which there will be high flux  in secondary also hence if we open circuit the secondary the current present in sec will be zero,a very high voltage will be generated which cause hazard.
15 Why dose an electric meter record KVA when there is no load on a 500 KVA transformer?

This is related to the iron losses & copper losses of the
Trafo. Iron losses are constant ie from No load to full
load. Copper losses varies depending on the load current.

16 What are the Steps for Testing of 20MVA Transformer? How many Test we have to do?

1) IR
3) Magnetic CORE balance
5) Ratio test
6) Vector group
8) BDV
12) Visual inspection
13) Tap changer tests
14) Special tests to see the condition of the dielectric

17 What is impedence (Z) of transformer, what indicate in transformer , which perameter indicate in transformer ?

Transformer is always interposed between a primary source of energy and a secondary that makes use of that energy. Hence, the transformer constitutes an impedance to current flow from source to load. The higher that impedance, the more voltage drop will occur for a given load current. If a secondary fault occurs, its effect on the circuit will be lessened to the extent that transformer impedance increases. The circuit designer therefore needs to know the value of transformer impedance.

18 Why we oltc on hv side of tranformer only ?

Normoly in HT side 11kv voltsge and less current will be there But in LT side voltage will be less and current was very high.So if we change tappings in LT side a huge spork will come.to avoid this OLTC has to be fixed in HT side only.

19 How current tranfomer works ?

We know it as a measuring device.But it can act asprotecting device also.

It is basically a step-up transformer.It raises thevoltage thereby reducing the current(as high as 100A or more) in a circuit so that it can be measured using a std 5Aor 1A ammeter.Primary is the conductor containing thecircuit in which the current is to be measured.

20 Why no-load primary current is very small as compared to full load current in transformer (as 1% of full load current?

No load current is only d 3-5%of full load current.if alternating current is applid to d primary then no load current wl flow through only primary circuit since secondary is open circuitd.whn load is connected to the secondary then current wl flow through the secondary aswell..thats y we cn conclude that no load is smaller than full load currents..
21 My requirement is 1VA,50/5A Current Transformer for Metering .If I select 15VA , 50/5A CT.Is there any possibility of Technical Problem ?

No problem at all when we increase the burden . But It should be noted that the current ratio should not be increased, since it leades to sensing problem which inturn hinders the metering values.

22 What happenes if the core of the transformer saturates?

When the core of the transformer become saturated the outputvoltage will remain the same in the average value even if the input increase another issue .. if the core become saturatedthe output voltage will be distortion in shape which lead to harmonics.

23 We wish to test 60 Hz systems from a 50 Hz supply. To acheive frequency conversion, we use a VFD. The VFD powers 40 kVA transformer, which has motor and non-motor loads on it. Will the transformer work?

Since the transformer needs AC suppy , whether it is 50 hz, or 60 hz. so it wll work.

24 What is the procedure to carry out BDV test of a transformer ?

1. first we will take filltered oil in test kit.
2. then maintained 2.5mm gap between electrode and applying rated  40 & 60 Kv for 1 minite .
3. the above test withstood for 1 minite its ok for energiseing the transformer.

25 What is meant by line voltage and phase voltage in a three phase transformer?

The voltage is taking at line to line it is called line voltage Eg in RYB we takes the voltage R&Y, Y&B, B&R this is the line voltage.
And  if we takes the voltage of R&N, Y&N, B&N it is called phace voltage.

26 The voltage per turn of a primary winding of a transformer is the same as the voltage per turn of the secondary winding of the transformer. why?

As the rate of change of flux that links with primary and secondary is same. so induced emf by per unit turn is same for both secondary and primary.

27 Which Transformer is commonly used for power distribution?

Step down transformer which is 11kv/415v.

28 What is the difference between AC voltage and DC voltage?

AC voltage is alternating voltage having frequency of 50Hz.(60 Hz in US)
It has positive peak and negative peak value.DC is direct voltage having 0 frequency and maintains
constant level.

29 Why current Transformer is rated by 100A/5A/5A?

It means 100A for primary, 5A for metering & 5A for protection

30 I have no idea which kind of transformer I should select ?

Don't worry, please click here to see our suggestion,you will know which transformer will meet your requirement.Or contact us now,we'll be glad to get back to you with professional solutions.

31 Can we go to visit your factory ?
Yes, all esteemed client are warmly welcome to visit us ,it'll be great for us to negotiate with esteemed client face to face.

The address of our factory is as below:
Factory address: No.128, Miaozai Zhuang, Jiannan Village, Banghu Town, Baiyun District, Guangzhou City, Guangdong, China
Salesroom address: No.157, Jiefangzhong Road, Guangzhou City, Guangdong, China

32 Toroidal type, EI type, R type and C type transformer,which kind is the best choice for Audio power amplifier?
Toroidal transformer can do the best at min magnetic flux leakage, but EI transaformer is better than toroidal one at the sound listening when put in intermediate frequency density. about magnetic saturation, EI type is better than toroidal type, but toroidal is more efficiency than EI type.
So that, the key to the problem is you must foster strengths and circumvent weaknesses and make sure their respective advantages could be a full play, and this is the most fundamental things to make a transformer.
In the exporting of amplifier, toroidal transformer is still the mainstream, it shows that. you can't take a transformer that is not made out for reference and said it is bad. Some say totoidal transformer is weak at magnetic saturation, why don't you try your best and make it do not easy magnetic saturation? And originally you can do it by technology. If you only want to save cost, of course it would be easy to make it. Similarly, as long as you try your best to make a EI transfomrer, it’s efficiency is also can achieve a very high level.
The quality of the transformer has great influence to the sound, because the transmission of energy is relate to the iron core and coil of the transformer, the transfer rate has great influence to the voice. Like EI type transformer, people usually feel it's intermediate frequency thicker, high frequency is more slender, why? Because it's transmission speed is slow. And toroidal type? The low frequency is fierce, high frequency is in a someway weaker again, why? Because it transmission speed is faster. so if you change the structure of the transformer to a very effective state, you can make the toroidal type and EI type perfectly, so the key is to see how will you do.
But at least we could sure that R type transformer is not easy to make. put it on preamplifiers for low current level power amplifier and CD player power will be bettter, if put it on Level after power amplifier, there will be some serious defects. Because of the structure of the R transformer’s sructure is not so easy to change, and toroidal type and EI type is easier to changing the structure to make the sound more beautiful.using a R type transformer as power amplifier, usually, sound will be very harden and lack of reiki,and low frequency is often no spring and appear more harden.

33 Still I haven't found correct answer to my question ?
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