Interview questions for Transformer

1. What Are Step Down Transformers?

They step up voltage from higher value to lower value.

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.

2. What is voltage transformer?

Voltage transformer is sometimes called as potential transformer which is used to measure the voltage of the line. Basically it is a step down transformer which reduces the line voltage to minimum voltage (safe voltage) which a voltage meter can measure. It also isolates the high voltage line and metering circuit to safe guard the metering circuit during a fault condition.

Voltage transformers are usually connected between line and ground in which it is protected with the fuses.

3. What is current transformer?

Current transformer is an instrument transformer used to measure the current flowing through the windings. Normally primary winding will be single wire and secondary will be many turns so the secondary measuring current will be less.

The current transformers can be used for metering purpose or for protection purpose. The output of the CT will be normally 5 or 10 amps. So according to the turn’s ratio will be arranged. Figure shows the arrangement of CT where the primary conductor carrying current induces the magnetic flux in secondary turns to produce the respective turns.

4. What Is The Purpose Of Laminating The Core In A Transformer?

In order to minimize eddy current loss.

5. What is step-up transformer?

Step-up transformer is a transformer whose secondary voltage is higher than the primary voltage. To do this the secondary coil tunes are wounded high compared to the primary coil turns. This transformer has main application in power plants where the generation voltage has to be stepped up to transmit for long distance.

6. 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.

7. What Are Potential Transformer?

They are protective devices used to measure very high value of voltage in power system.

8. What step-down transformer?

Step down transformer is a voltage down transformer where the secondary voltage is less compared to primary voltage. This is achieved by providing lower turns of coil in secondary compared to primary. The main application of this type of transformer is in distribution side where as the transmitted high voltage has to be stepped down to distribute it for households purposes.

9. Explain On The Material Used For Core Construction?

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.

10. What is power transformer?

Power transformer is a transformer used to transmit the power from the generating station to the destination. These are big in size and used for large power applications. These are typically used between the two ends of transmission lines to step-up the voltage at the generating end of transmission line and to step down the voltage at the receiving end of the transmission side.

11. What is a distribution transformer?

Distribution transformer is basically a step down transformer which decreases the transmission high voltage in to low voltage. As the name specifies it distributes the power for various house and industrial purposes with a reduced end user voltage. So the load fluctuations are very high on these transformers. These are normally operated less power compared to power transformers. These transformers are also provides isolation between primary and secondary to limit the secondary fault current. The power should be passed through the distribution transformer before supplying it to the end user.

12. Advantages of iron core transformer?

• The magnetic reluctance of iron is less so that it allows more flux through it.
• Efficiency is high compared to air core
• Heat losses are less.
• It is used for low frequencies (<10KHz).

13. What is isolation transformer?

Isolation transformer is a static machine used to isolate the two circuits and provides a magnetic coupling between two circuits. It avoids the coupling losses and provides better safety to the both the circuits from the fault in other circuit. As the name specifies it provides only isolation, so the primary and secondary windings turns are same to ensure that there is no change in signal quantities like voltage and current.

14. What is transformer tap changer?

A tap changer is a device used to change the primary or secondary voltage of the transformer by changing the turn’s ratio without remodeling the transformer. The primary function of the transformer is it removes or connects some portion of the winding either load side or source side. The importance of tap changer is maintaining the load side voltage constant irrespective of load variations. There are two types of tap changer one is off-load changer and on-load tap changer.

15. What is off-loading tap changer?

Off-load tap changer works on the principle “Break before Make” which breaks the current flow and makes the new tap connection. So this tap changer works when there is no current flow in the transformer windings i.e. it cannot work even when the transformer is energized because it can break the magnetizing current. It is nowadays called as De-energizing tap changer instead of No-load or Off-load tap changer.

16. What is on-load tap changer?

On-load tap changer is device which operates and changes the tap connection even when the transformer is supplying the load. This works on the principle of “Make before Break” which makes the new circuit before breaking the old tap circuit. It is very important invention that it can keep the transformer load voltage irrespective of load varying during operation itself.

17. What is the difference between isolating and shielding transformer?

Isolating transformer: This transformer is used to provide isolation between primary and secondary to protect the winding from the faults.
Shielding transformer: In high power applications the isolation transformers are provided with shielding plates between primary and secondary to reduce the high frequency noise in the transformer. When high frequency surge pulse is introduced in system, the capacitor near primary winding creates a short circuit path and diverts this pulse to the ground.