Important Short Questions and Answers: Design of Electrical Machines - Transformers

TRANSFORMERS

1. Define transformation ratio.

                                  It is defined as the ratio of secondary terminal voltage to primary terminal voltage.

 It is denoted by k.

 K = Vs /Vp = Ts / Tp = Ip / Is

2. Name the types of transformer.

Based upon construction, the types are

 Core type and

 shell type transformer

Based on applications, the types are

^Ø                                                          Distribution transformers

^Ø                                                          Power transformers

^Ø                                                          Special transformers

^Ø                                                          Instrument transformers

^Ø                                                          Electronics transformers

Based on the type of connection, the types are

^Ø                                                                       Single phase transformer

^Ø                                                                       Three phase transformer

Based on the frequency range, the types are

^Ø                                                                                  Power frequency transformer

^Ø                                                                                  Audio frequency transformer

^Ø                                                                                  UHF transformers

^Ø                                                                                  Wide band transformers

^Ø                                                                                  Narrow band transformer

^Ø                                                                                  Pulse transformer

Based on the number of windings, the types are

^Ø                                                                                            Auto transformer

^Ø                                                                                            Two winding transformer

3.     Define windows space factor or window area constant.

It is defined as the ratio of the are of copper in the window to the window area.

Kw = Ac / Aw < 1

Ac is the area of copper in m²

Aw is the area of window in m²

4.  Define iron space factor.

It is defined as the ratio of gross core area to the area of the circumscribing circle.

Kis = Agi / Ace < 1

Agi is the gross core area in m²

Ace is the area of circumscribing circle in m²

5.     What is a function of a transformer?

^Ø                                                        It increases or decreases the voltage at same frequency.

^Ø                                                        It transforms energy from one winding to other winding at constant frequency.

^Ø                                                        It is used in electronic circuits with rectifying units to convert ac to dc.

^Ø                                                        It provides isolation between to electrical circuits.

6.     What is the function of transformer oil?

^Ø                                                                  It provides cooling.

Ø                                                               It acts as insulation.

^Ø                                                                              It protects the paper from dirt and moisture.

7.                 What is the cause of noise in transformer?

^Ø                                                              Mechanical forces developed during working

^Ø                                                              Loosening of stampings in the core

^Ø                                                              Expansion and contraction of oil level

8.                 What are the properties of transformer oil?

^Ø                                                           High dielectric strength

^Ø                                                           High resistivity and density

^Ø                                                           Low viscosity

^Ø                                                           Low impurity

^Ø                                                           Reasonable cost and flash point

9. Difference between core type and shell type transformer.

10. Difference between distribution and power transformer.

11.      Mention the important characteristics desirable in transformer oil.

^Ø                                                                    Electric strength

^Ø                                                                    Resistance to emulsion

^Ø                                                                    Viscosity

^Ø                                                                    Purity

^Ø                                                                    Flash point

^Ø                                                                    Sludge formation

12.      Why is transformer oil used as cooling medium?

When transformer oil is used as coolant, the heat dissipation by convection is 10 times more than the convection due to air specific heat dissipation by convection due to air = 8 W/ m² – C.

Specific heat dissipation by convection due to oil = 80 to 100 W/ m² – C.

13. Mention the factors to be considered for selecting the cooling method of a transformer.

The choice of cooling method depends on KVA rating of transformer, size, application and the site condition where it has to be installed.

14.      List the different methods of cooling of transformer.

^Ø                                                                    Air natural

^Ø                                                                    Air blast

^Ø                                                                    Oil natural

^Ø                                                                    Oil natural – air forced

^Ø                                                                    Oil natural water forced

^Ø                                                                    Forced circulation of oil

^Ø                                                                    Oil forced – air natural

^Ø                                                                    Oil forced – air forced

^Ø                                                                    Oil forced – water forced

15. Give an expression for the heating time constant of transformer.

Where G is weight, h is specific heat, λ is the specific heat dissipation.

16. Why cooling tubes are are provided?

Cooling tubes are provided to increase the heat dissipating area of the tank.

17. Give the expression for magnetizing current.

The magnetizing current is given by

18. Write the expression for temperature rise in plain walled tanks.

where P_i = iron loss  ; P_c = copper loss ;  S_t = Heat dissipating surface of the tank

19. Why plain walled tanks are not used for large output transformers?

The plain walled tanks are not used for large output transformers as they are not sufficient to dissipate losses. This is because volume and hence losses increase as cube of linear dimensions while the dissipating surface increases as the square of linear dimensions. Thus an increase in rating results in an increase in loss to be dissipated per unit area giving a higher temperature rise.

20. How is leakage reactance of winding estimated?

It is estimated by primarily estimating the distribution of leakage flux and the resulting flux leakages of the primary and the secondary windings. The distribution of the leakage flux depends upon the geometrical configuration of the coils and the neighboring iron masses and also on the permeability of the iron.

21. Define stacking factor and give its typical value.

Its typical value is 0.9.

22. Why stepped cores are used in transformers?

When stepped cores are used, the diameters of the circumscribing circle is minimum for a given area of the core, which helps in reducing the length of mean turn of the winding with consequent reduction in both cost of copper and copper loss.

23. What is the range of flux densities used in the design of a transformer?

When hot rolled silicon steel is used,

^Ø                                      B_m = 1.1 to 1.4 Wb / m² for distribution transformer

^Ø                                                = 1.2 to 1.5 Wb / m² for power transformer When cold rolled silicon steel is used,

^Ø                              B_m = 1.5 Wb / m² for up to 132 KV transformer

^Ø                                        = 1.6 Wb / m² for 132 KV to 275 KV transformer

^Ø                                        = 1.7 Wb / m² for 275KV to 400 KV transformer

24.            Name the factors to be considered to choose the type of winding for a core type transformer.

^Ø                                  Current density

^Ø                                  Short circuit current

^Ø                                  Surge voltage

^Ø                                  Impedance

^Ø                                  Temperature rise

^Ø                                  Transport facilities

25.            Give typical values of core area factor for various types of transformers.

Core area factor ( K_c ) for various transformers:

^Ø                                  Square core K_c = 0.45

^Ø                                  Cruciform core K_c = 0.56

^Ø                                  Three stepped core K_c = 0.6

^Ø                                  Four stepped core K_c = 0.62

26.            List the assumptions made for calculation of leakage flux and leakage reactance.

^Ø                                      The primary and secondary windings have an equal axial length

^Ø                                      The flux paths are parallel to the windings along the axial height

^Ø                                      Primary winding mmf is equal to secondary winding mmf

^Ø                                      Half of the leakage flux in the duct links with each winding

^Ø                                      The length of the mean turn of the windings are equal

^Ø                                      The reluctance of flux path through yoke is negligible

27.            Define copper space factor.

For a transformer, it is the ratio of conductor area and window area.

28.            Name the various types of cross section used for core type transformer.

^Ø                                      Square

^Ø                                      Rectangle

^Ø                                      Cruciform and

^Ø                                      Multi stepped cores

29.            What is window space factor?

The window space factor is defined as the ratio of copper area in window to total window

Window space factor     = area. Copper area in window / Total Window area

30. How the area of window is calculated?

Are of the window (Aw) = Height of window (H_w) * Width of window (W_w).

31. Why are the cores of large transformers built up of circular cross-section?

The excessive leakage fluxes produced during short circuit and over loads develop mechanical stresses in the coils. These forces are radial in circular coils and there is no tendency for the coil to change its shape. But in rectangular coils, these forces are perpendicular and tend to deform the coil.

32. Give the expression for window width that gives the maximum output.

The width of the window for maximum output is

W_w  = D - d = 0.7 d.

Where D = distance between adjacent limbs d = width occupied by iron

33. Give the expression for KVA rating of a single and three phase transformer.

Rating of a single phase & three phase transformer in KVA is given as

Q = 2.22 f B_m δ K_w A_w A_i * 10^-3

Where f = frequency, Hz

B_m = maximum flux density, Wb/m²

δ = current density, A/mm²

K_w = Window space factor

A_w = Window area, m²

A_i = Net core area, m²

34.      Mention different types of low voltage windings.

^Ø                                                  Cylindrical windings

^Ø                                                  Helical winding

35.      What is the range of efficiency of a transformer?

The efficiency will be in the range of 94% to 99%.

36.      In transformers, why the low voltage winding is placed near the core?

The winding & core are both made of metals and so insulation has 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 is placed near the core.

37. What are the disadvantages of stepped cores?

With large number of steps a large number of different sizes of laminations have to be used. This results in higher labor charges for shearing and assembling different types of laminations.

38.      What is the objective behind using sheet steel stampings in the construction of electrical machines?

The stampings are used to reduce the eddy current losses. The stampings are insulated by a thin coating of varnish, hence when the stampings are stacked to form a core, the resistance for the eddy current is very high.

39.      What type of steel is commonly used for the core of transformer?

The hot rolled and cold rolled silicon steel with 3 to 5%silicon are used for the laminations of the core of transformers. The hot rolled silicon steel allows a maximum flux density of 1.45 Wb/m² and the cold rolled silicon steel permits a maximum flux density of 1.8 Wb/m².

40.      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:

^Ø                                          To supply small additional load at a different voltage

^Ø                                          To give supply to phase compensating devices such as capacitors which work at different voltage

Ø                                       To limit short circuit current

^Ø                                          To indicate voltage in high voltage testing transformer

41.      How the tertiary winding is connected? Why?

The tertiary winding is normally connected in delta. When the tertiary is connected in delta, the unbalance in the phase voltage during unsymmetrical faults in primary or secondary is compensated by the circulating currents flowing in the closed delta.

42.        What are the salient features of distribution transformer?

^Ø                                            The distribution transformers will have low iron loss and higher value of copper loss.

^Ø                                            The capacity of transformers will be up to 500 KVA

^Ø                                            The transformers will have plain walled tanks are provided with cooling tubes or radiators

^Ø                                            The leakage reactance and regulation will be low.

42.      What types of forces acts on the coils of a transformer in the event of a short circuit on a transformer?

During short circuit conditions the radial forces will be acting on the coil, which is due to short circuit currents.

43.      What is the range of current densities used in the design of transformer winding?

The choice of current density depends on the allowable temperature rise, copper loss and method of cooling. The range of current density for various types of transformers is given below:

^Ø                                                                    δ = 1.1 to 2.2 A/mm²  - For distribution transformers

^Ø                                                                    δ = 1.1 to 2.2 A/mm²  - For small power transformers with self oil cooling

^Ø                                                                    δ = 2.2 to 3.2 A/mm²  - For large power transformers with self oil cooling

^ δ = 5.4 to 6.2 A/mm² - For large power transformers with forced circulation of oil

44. How the heat dissipates in a transformer?

The heat dissipation in a transformer occurs by conduction, convection and Radiation.

45. How the leakage reactance of a transformer is reduced?

In transformers the leakage reactance is reduced by interleaving the high voltage, and low voltage winding.

46. How the magnetic curves are used for calculating the no-load current of a transformer?

The B –H curve can be used to find the mmf per metre for the flux densities in yoke and core. The loss curve can be used to estimate the iron loss per Kg for the flux densities in yoke and core.

47. 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.

48. Why silica gel is used in breather?

The silica gel is used to absorb the moisture when the air is drawn from atmosphere into the transformer.

49.      What are the merits and demerits of using water for forced cooling of transformers?

The advantage in forced water cooling is that large amount of heat can be removed quickly from the transformer.

The disadvantage in forced water cooling is that the water may leak into oil and the oil may be contaminated.

50.      In mines applications transformers with oil cooling should not be used, why?

The oil used for transformer cooling is inflammable. Hence leakage of cooling oil may create fore accidents in mines. Therefore oil cooled transformers are not used in mines.