1.What are the cooling methods to cool a transformer?

  • Air Natural (AN)
  • Air Forced (AF) or Air Blast
  • Oil Natural Air Natural (ONAN)
  • Oil Natural Air Forced (ONAF)
  • Oil Forced Air Forced (OFAF)
  • Oil Natural Water Forced (ONWF)
  • Oil Forced Water Forced (OFWF)

2. How do we specify the rating of a transformer?

Manufacturer designs transformer based on required voltage and current and specified them on the nameplate of the transformer in terms of VA called the rating. It can be also said that the maximum voltage and current that can be safely applied to transformer called the rating.

3. Why an induction motor is called a rotating transformer?

In induction motor, rotor is short circuited, there is no physical contact between stator and rotor. rotor is made to rotate due to induce effect of stator. Which is similar to transformer operating principle where primary and secondary magnetically coupled. That’s why it is called rotating transformer.

4. What is the difference between earth and neutral?

It is the least resistance path and is used as a safety purpose against residual currentsIn an AC circuit which carries current in normal condition, it is the return path, it balances the load
In normal condition, it doesn’t carry any current but in case of insulation failure, it might carry minor currentA neutral wire is always charged
It cannot be turned into neutralIt can be turned into earth
It can come from a neutral line or can be separately executedIt comes from a neutral line
Earth is the surging point of appliancesNeutral is the return path of the electrical current supply, it is also called a reference point

5. Why is earth pin longest and thickest?

The earth pin is long so that the earth connection is made first. This ensures the safety of the user because if the appliance is defective, the fuse will blow off. The earth pin is made thicker so that even by mistake it cannot be inserted into the hole for the live or neutral connection of the socket.

6. What are the disadvantages of low power factor?

The undesirable effect of operating a low load at a low power factor is due to the large current required for a low power factor. The important disadvantages of low power factor are

  • Higher current is required by the equipment,  due to which the economic cost of the equipment is increased.
  • At low power factor, the current is high which gives rise to high copper losses in the system and therefore the efficiency of the system is reduced.
  • Higher current produced a large voltage drop in the apparatus. This results in the poor voltage regulation.

Since both the capital and running costs are increased, the operation of the system at a low power factor (whether it is lagging or leading) is uneconomical from the supplier’s point of view.

7. What is Hysteresis loss?

Hysteresis loss is caused by the magnetization and demagnetization of the core as current flows in the forward and reverse directions. As the magnetizing force (current) increases, the magnetic flux increases. But when the magnetizing force (current) is decreased, the magnetic flux doesn’t decrease at the same rate, but less gradually. Therefore, when the magnetizing force reaches zero, the flux density still has a positive value. In order for the flux density to reach zero, the magnetizing force must be applied in the negative direction.

8. What is the Form Factor?

The ratio of the root mean square value to the average value of an alternating quantity (current or voltage) is called Form Factor. The average of all the instantaneous values of current and voltage over one complete cycle is known as the average value of the alternating quantities.

Form factor = Ir.m.s / Iav or Er.m.s / Eav

9. Which is more dangerous-shock due to AC or DC?

A.C. is said to be four to five times more dangerous than D.C. For one thing, A.C. causes more severe muscular contractions. The frequency of the AC has a lot to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At this frequency, as little as 25 volts can kill.

10. What is the slip?

Slip is the difference between the synchronous speed of the magnetic field and the shaft rotating speed and would be some number of RPM or frequency. The slip increases with an increasing load, thus providing a greater torque.

11. What is Cogging and Crawling?

Cogging:- This characteristic of I.M. comes into the picture when the motor refuses to start at all. Sometimes it happens because of the low supply voltage. But mainly it is because of Cogging. Stator and rotor slots get aligned such that both face to each other and at this stage reluctance of the magnetic path is minimum and the motor refuses to start.

Crawling:- The phenomenon of rotation of I.M. to 1/7th of sync. speed is known as Crawling.

12. What are the Dummy coils?

The dummy coils are similar to other coils except their ends are cut, short and taped. They do not connect with the commutator bars. The dummy coils are simply to provide mechanical balance for the armature. As they do not connect with commutator bars they do not affect the electrical characteristics of the winding.

13. What are CT and PT?

A current transformer (CT) is a device which is used for the transformation of current at a higher value to a lower value with respect to the earth potential. It is used with the AC instruments for measuring the high value of current.

A voltage transformer or Potential Transformer (PT) is the type of instrument transformer which is used for the transformation of the voltage from a higher value to a lower value.

14. What is Universal Motor?

The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator’s field coils are connected in series with the rotor windings through a commutator.

15. What is the Voltage Regulation?

Voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line.

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