Hardware Introduction of Transformer

Hardware of a simplest transformer consist of

 i. Copper Conductor Coil
ii. Soft magnetic material core

Other hardware components for higher rating transformers are oil tank, cooling fins, reserved tank, cooking fans, breathers, protection circuits and relays.

Core:

On basis of construction of transformer core, it is divided into two types. One is core type transformer and other is shell type transformer. The core in its simplest form is rectangular soft iron material and the primary and secondary windings are wound simply on both sides of rectangle. Each side of rectangle is called as limb of transformer. It is shown in figure below. Which shows the primary and secondary on opposite sides of rectangle.

                Shell type transformer has three legged core i.e. has three limbs. In this transformer the primary and secondary windings are wound one on the top of other on the central limb. In this type of transformer the low voltage windings are place first and high voltage is placed above it. This is because it insulates the high voltage from the core and eliminates the need of larger insulation. Another advantage of this arrangement is that it reduces the leakage flux in the core which could be a serious problem when two windings are placed on a distance.

shall type transformer
Figure 1: Shall type (two windowed) transformer core with windings

Each type of transformer core is made up of soft magnetic material which can be magnetize and demagnetize easily without loss of power in it. And core is laminated very thin strips stacked together instead of single solid core.

The reason of using soft material (ferromagnetic material) is that as it is operating on Alternating current so with each of half cycle starting, the atoms inside core starts to align them in one direction and in opposite direction in next opposite half of AC. So there must be such material which is not very hard to magnetize and demagnetize. For this purpose, soft iron or silicon steel is used.

Laminated strips (each strip is laminated from each other) as used to limit the eddy currents in the core which is due to self-induction effect in core and causes the loss of power.

It will not be easy to wind on the central limb of transformer as can be seen in figure above so instead of making two windowed strip, it is divided into two parts and placed inside the windings and joined together to give the same effect as single two windowed (shell type) core.

For a shell type transformer, the strips (also known as stampings) can be divided into two by different arrangements. Some of them are shown in following figures:First arrangement is known as E and I. there are stampings of the shape of alphabet E and others are I. windings are done on some insulating material on one to another and then placed on central limb of E strip. And the whole winding is filled with E and on the other side I stampings are placed and it acts like shell type core. (Figure 1)

E AND I Stamping of core
Figure 2: E and I stamping of core (silicon steel)

 

double E stamping
Figure 3: Double E Stamping of core

 

 

 

 

 

 

 

 

 

The size of central limb is greater than the other two limbs on the ends and the size of two windows are decided by net windings of transformer (Figure 1). For window size following formula is used:

Window space = (number of turns x area of wire) / space factor

Where,

Space factor = (area of copper in window)/ area of window

And it must be less than 1.

       In other arrangements, the two windowed strip can be divided into two at diagonals or from the center (figure 3) and arranged with winding in same fashion as disused above. But for small transformer design, the most commonly used stamping is E and I strips.

cold rolled grained oriented steel laminated core stamping
Figure 4: Cold rolled Grained Oriented (CRGO) Steel laminated core stamping

The three phase transformer core type winding is two windowed (same as shall type of single phase transformer) but the difference is that all the three limbs or pillars are of same size and window size is enough for housing of winding on both adjacent limbs leaving the space between them. On each limb the high voltage winding is placed above the low voltage winding for each phase.

   In three phase transformer primary and secondary winding can be joined together in either star or delta combinations.

Hysteresis loop of iron core
Figure 5: Hysteresis loop of iron core

The material for the core is decided on the basics of hysteresis loop or the graph between magneto-motive force (the cause) and the magnetic flux (the effect). When the AC is applied to the coil of transformer the domains of atoms inside the material stats to align them in one direction (direction of half cycle of AC) with next negative half cycle it aligns in opposite direction. So aligning of all the domains of atoms inside the material is called as magnetization of material(Figure 6). For aligning in opposite direction it must come to neutral state (called as demagnetization of material).

In magnetizing and demagnetization energy is lost. The loss of energy in magnetizing and demagnetizing the core is called as hysteresis loss. (Figure 5) The area under the magnetization curve is energy loss. So, the material should have thin magnetization curve.

Alligning of domain when AC applied
Figure 6 (a) Figure 6 (b) Figure 6: Domains of Atoms inside the magnetic material (a) random- without applying external force (b) aligned in one direction

 With the magnetic field of coil emf is generated in coil causing the current to flow in core (called as eddy currents) also results in energy loss. To reduce these losses area must be minimum. For this purpose instead of solid core laminated strips are used.

  1. Which can be magnetize and demagnetize easily (soft material)
  2. Very low current permeability.

Other Core Materials:

The above discussed transformer core stamping are made up of ferromagnetic materials like soft iron or silicon steel. For high voltage/power rating transformers CRGO (cold rolled grained oriented) silicon steel stamping are used. The thickness of these stamping could be 0.27 mm hence results in smaller eddy currents. CRNO (cold rolled non-grained oriented) silicon steel stamping are also used but it could only be 0.50 mm thick hence greater losses. CRNO stamping are used in cheap high voltage transformers.

   For transformer operating at very high frequencies, ferrite core is used. It is made up of iron oxide with zinc, nickel and manganese resulting in material which has very high magnetic permeability and very low electrical permeability. So, for transformers to operate at very high frequencies, these core are used which allows very fast magnetic change and prevent eddy currents in core due to low current permeability. These type of transformers are used in RF circuits or switch mode power supplies.

Winding:

The other main part of a transformer is its coils. For a simplest transformer action there are two coils one act as primary and other is secondary. For best performance transformer and least losses in winding, copper is used. Aluminum wires are also used as it has very low density as compared to copper but its resistance is much higher than copper. So, low price and small rating transformers are wound with aluminum enameled wires.

Wires for making coils of transformer are in different sizes. Two main standards of wire are followed around the world. One is AWG (American Wire Gauge) standard and second is British Standard Wire gauge (SWG) or imperial wire gauge. In these tables, wires are arranged on basis of diameters. With increase gauge size the diameter goes smaller hence maximum current it can handle also reduces.The copper wire used in windings is enameled wire to make insulated each turn from one another. The standard tables for wire gauge are also available listing the diameter of wire with enamel on it.

For Power (high rating ) transformers instead of wires, copper strips are used to handle high power. These strips are also need insulation from each other and from surroundings. So, very high insulation is placed on the copper strips and between the layers of winding. Sometimes for high current handling, parallel paths of currents are made to divide the current.

Asad Ullah

I am MSc Electrical Scholar under a fellowship program. Working with Modeling and Simulation software related to my field are my activities in leisure.

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