How to design single phase transformer

Single Phase Transformer Design

A transformer transfers electric power from one circuit to another circuit without a change in frequency. It contains primary and secondary winding. The primary winding is connected to the main supply and secondary to the required circuit. In our project circuit, we have taken the design of low power (10 KVA) single phase 50 hertz power transformer as per our requirement in the project. 

Construction of core

Generally, the name associated with the construction of a transformer is dependant upon how the primary and secondary windings are wound around the central laminated steel core. The two most common and basic designs of transformer construction are the Closed-core Transformer and the Shell-core Transformer.

In the closed-core type transformer, the primary and secondary windings are wound outside and surround the core ring. In the shell type transformer, the primary and secondary windings pass inside the steel magnetic circuit which forms a shell around the windings as shown below.

In both types of transformer core design, the magnetic flux linking the primary and secondary windings travels entirely within the core with no loss of magnetic flux through air. In the core type transformer construction, one half of each winding is wrapped around each leg of the transformers magnetic circuit. 

The coils are not arranged with the primary winding on one leg and the secondary on the other but instead half of the primary winding and half of the secondary winding are placed one over the other concentrically on each leg in order to increase magnetic coupling allowing practically all of the magnetic lines of force go through both the primary and secondary windings at the same time. However, with this type of transformer construction, a small percentage of the magnetic lines of force flow outside of the core, and this is called “leakage flux”.

Shell type transformer cores overcome this leakage flux as both the primary and secondary windings are wound on the same centre leg which has twice the cross-sectional area of the two outer leg. The advantage here is that the magnetic flux has two closed magnetic paths to flow around external to the coils on both left and right hand sides before returning back to the central coils.

 In both the shell and core type transformer constructions, in order to mount the coil windings, the individual laminations are stamped or punched out from larger steel sheets and formed into strips of thin steel resembling the letters “E”s, “L”s, “U”s and “I”s as shown below.

The core is not designed to have any currents flow through it. It ishowever
 a conducting loop that experiences a changing magnetic field, it will therefore have small currents induced in it - these are called 'eddy current'.

The core is laminated to reduce these to a minimum as they interfere with the efficient transfer of energy from the primary coil to the secondary one.

The eddy currents cause energy to be lost from the transformer as they heat up the core - meaning that electrical energy is being wasted as unwanted heat energy.

Laminated means 'made up of insulated layers of iron 'glued' together' rather than being in a single solid 'lump'. A laminated core has a higher resistance than a non-laminated one with the same number of domains. It therefore does not get such big a currents induced in it

Winding Arrangements

Transformer windings form another important part of a transformer construction, because they are the main current-carrying conductors wound around the laminated sections of the core. In a single-phase two winding transformer, two windings would be present as shown. The one which is connected to the voltage source and creates the magnetic flux called the primary winding, and the second winding called the secondary in which a voltage is induced as a result of mutual induction.

If the secondary output voltage is less than that of the primary input voltage the transformer is known as a “Step-down Transformer”. If the secondary output voltage is greater then the primary input voltage it is called a “Step-up Transformer”.

Calculations

• Secondry Volt Amps(SVA) = Secondry Voltage(Vs) × Secondary current(Is)
• Primary Volt Amps(PVA) = Secondry Volt Amps(SVA) ÷ 0.9 assuming efficiency of transformer  as a 90%
• Primary Current(Ip) = Primary Volt Amps(PVA) ÷ Primary Voltage(Vp)
• Core Area CA = 1.152 ×√ (Secondry Voltage ×Secondry  Current)
• Turns / Volt(TPV) = 42 ÷ Core Area(CA) in cm
• No.of Turns in primary(Np) = Turns / Volt(TPV) × Primary Voltage(Vp)
• No.of Turns In Secondry(Ms) =Turns / Volt(TPV) × Secondry   Voltage (Vs) 



The table below helps you to select the gauge and turns per sq. cm of copper wire by matching them with the selected current rating of the winding appropriately.

SWG-—— (AMP)-—— Turns per Sq.cm.

10———– 16.6———— 8.7

11———– 13.638——- 10.4

12———– 10.961——- 12.8

13———– 8.579——— 16.1

14———– 6.487——— 21.5

15———– 5.254——— 26.8

16———– 4.151——— 35.2

17———– 3.178——— 45.4

18———– 2.335——— 60.8

19———– 1.622——— 87.4

20———– 1.313——— 106

21———– 1.0377——– 137

22———– 0.7945——– 176

23———– 0.5838——— 42

24———– 0.4906——— 286

25———– 0.4054——— 341

26———– 0.3284——— 415

27———– 0.2726——— 504

28———– 0.2219——— 609

29———– 0.1874——— 711

30———– 0.1558——— 881

31———– 0.1364——— 997

32———– 0.1182——— 1137

This Table B enables you to make your own transformer design by comparing the calculated Winding Area with the relevant required Tongue Width and Lamination Type number.

TYPE  ———— TONGUE WIDTH ————WINDOW AREA.

NUMBER.                 IN (CM)                       IN (SQ.CM)

17———–————— 1.27——————————— 1.213

12A——— ——————1.588——————— ——— 1.897

74——— ———————1.748—————————— 2.284

23—————— ————1.905—————————— 2.723

30——— ——— ———–2.000—————— ———–3.000

21——— ——— ———–1.588———— —————–3.329

31— ———— ————–2.223——— ———— —––3.703

10——————————–1.588———————– —–4.439

15— —— ———————2.540——— ———— –—–4.839

33— ————— —— —–2.800—————— —––—5.880

1————— ———— ———2.461———— ——––——6.555

14——————————–2.540———— ——––——6.555

11————————————1.905————— —––——7.259

34—————— ————–1/588—————––———7.259

3———————— ———–3.175———————––—7.562

9———————————2.223—————————–7.865

9A———— ————————2.223————————––7.865

11A——————————–1.905——— ——— ——–9.072

4A———————————–3.335—— ———— –––10.284

2———— —— —————–1.905——————––—–10.891

16————————————3.810———— ——– —–10.891

5———— ————— ———3.810——————––—–12.704

4AX ———— —— ————2.383——————––—–13.039

13———————————–3.175———— ——–– –14.117

75———————————2.540——————––—–15.324

4——— ———— ————2.540————————––15.865

7————————————5.080——————––—–18.969

6————————————3.810——————––——19.356

35A—— —— ————–3.810————— ———––39.316

8——— — ——— ———5.080—————— ——––49.803