How to Design a Transformer and Parameter Calculations

Admin@AKR Technical

 In this article we will describe the Design and Parameter Calculations of the Transformer. Many types of transformer have using in electricity transferring, now we are design and describe the calculation of a small step down transformer.

How to Design a Transformer and Parameter Calculations

What is doing a Transformer

The transformer transfers power from one coil to another without changing the input and output frequencies. Its core consists of primary and secondary winding. Transformers are designed to connect the primary winding to the main AC power source and the secondary winding to the desired circuit. Transformers are customized in many ways, mainly step-up, step-down and isolation transformers. 

The main advantage is that the secondary winding can be converted to AC or DC and used as needed. The transformer output is modified using some electronic components to make it DC. Step-up and step-down transformers are used in many electronics devices. An inverter is an example of that. Transformer in the inverter is used for battery charging and discharging the battery voltage. But some difference between current transformer and potential transformer

Transformer Parameters 

Technicians using Many parameters for winding a transformer. Here winding wires have two standard using,  AWG and SWG. In this transformer design calculate, the core design of a small power voltage transformer core selection and the bobbin size and its wire thickness (SWG) are explained.

What is the difference between CRGO & CRNO stamping cores?

CRGO and CRNO are electrical grade steels. They are mainly used for stamping and cores of electrical transformers and other electrical equipment. The core is designed by cutting this steel sheet into many models and sizes. Example: E & I core, U & T, F core etc.


CRNOs are cold rolled non-grain oriented steel laminations. The minimum size is 0.50 mm compared to the available thickness. CRGO is available in thicknesses up to 0.27 mm.

Therefore the flux loss in CRNO is four times higher than in CRGO. CRNGO Non-Oriented Fully Processed Steels are iron-silicon alloys with different silicon content. The stamp plan of the sheet has the same magnetic properties in all directions.

Magnetic flux density of CRNO and CRGO

The flux density in cold rolled non-grain oriented (CRNGO) is usually up to 65,000, while in flux density in cold rolled grain oriented steel (CRGO) it is up to 75,000. So it can be said that CRGO lamination is the best for making transformer. Therefore, in the case of high voltage transformers, the CRGO core cross section is preferred.

CRNGO stands for Cold Rolled Non Grain Oriented Steel.

Electrical CRGO stands for Cold Rolled Grain Oriented Steel.

How to design a Transformer

Let us see how to design a step-down transformer using simple formulas. For that we need to determine the output voltage and ampere we need to get in the transformer. 

For example, 230 VAC 50Hz input and 12 Volt 5 Ampere output are selected. Now we have determined our input voltage, output voltage and current.

Bobbin size should be selected for winding the transformer conductor coil. For this, the following formula can be used to describe the voltage and current mentioned earlier. This formula is intended for use with standard transformers only.

VA and Watts must be found before bobbin size can be selected

Secondary VA = Sv x I 

Output values, Sv = 12 V,  I (Amps) = 5A, 

Eff = 80% to 90%, we consider the Efficiency 90% (It may apply only in new core). In old core should be apply 80%. The efficiency of the transformer means the loss of voltage and current in the core and copper. That is why transformers are always referred to as volt ampere (VA).

Secondary Watts = VA + 10% of VA 

The answer of Secondary VA = 12 x 5 = 60 VA

Now we apply the Watts formula, 

Watts = 60 + 6 = 66 Watts

Core Area  (CA)

The formula is  watts ÷ 5.58  

The perpendicular E - core cross sectional area for inches define constantly in '5.58'

CA = sq. root of  66 / 5.58 = 1.456,

Core Area = 1.456" - It can be rounded to 1.5 Inches.

The Transformer Bobbin Size = 1.5 Sq"

Now we can calculate Turns Per Volt (TPV)

The formula is N = 108 ÷ 4.44 x F x Bmax x CA

4.44 is constant value

F means frequency = 50 Hz

Bmax = Maximum of core Magnetic flux density, Normally the core flux density (Bmax) value uses 65000 at 'NO' E and I. 

CA = Area of the core

Turns Per Volt  (TPV) = 100000000 / 4.44 x 50 x 65000 x 1.5 = 4.62

TPV = 4.62 Turns / Volt

That means consider the conductor turns is 4.62 needs for per volt.

Primary winding turns = Pv x Tv = 230 x 4.62 = 1062 Turns

Secondary winding turns = Sv x Tv = 12 x 4.62 = 55.44 Turns. Add 5% extra turns for better performance. 55.44 + 5% = 58.212 = 58 Turns

Primary and Secondary Current Calculation

Primary Current = Secondary watts / Primary Voltage = 66 / 230 = 0.2869 (0.287 Amps)

Secondary current 5 Amps. 

All the parameters for designing a transformer are covered in this article. Now you can compare the current of primary and secondary and select the wire gauge from the chart below.

Select the transformer primary and secondary wire gauge from the SWG current chart. It is only for inch calculation.

How to Design a Transformer and Parameter Calculations

In this case Transformer primary current is 0.287A (287mA), and Secondary current 5 Amps. So Transformer primary wire gauge 29 SWG selected from above table, and Secondary wire gauge 17 SWG. 


If it is below 0.287 Amp from the chart, add 3% to 5% turns. In our case we got 29 SWG because we chose 0.300 Amp from the chart. 1062 rolls should be calculated as 1093 by adding 3% as the current available in the primary is less.

I hope you have learned from this article how to design a transformer and do parameter calculations. Thanks for reading this article to understand Transformer design.

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