Introduction: Design of Linear AutoTransformer

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This is an innovative design of three-phase linear autotransformer with variable voltage. The design of three phase autotransformer by changing the conventional design we can reduce the costing, size, and inrush current as well as increased in efficiency.

Linear Autotransformer As compared to laminated E-I types lamination because of the lack of a residual gap in the magnetic path, toroidal transformers take higher inrush current, Toroidal core transformer needs higher labor cost of winding, costly equipment. In toroidal core, it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil.

In this prototype work by using I lamination we are going to design linear autotransformer to improve the cooling, efficiency, to reduce labor cost and also to reduce inrush current.

Step 1: Transformer Core Assembly

1) Calculate volt ampere (VA) (Vs*Is)

2) Calculate Turns/volts (Te) From the given table below

3) Calculate flux Φm = 1/ 4.44*f*Te

4) Consider maximum flux density Bm= 1wb/mm2

5) Calculate net area of core Ai= Φm/Bm

6) Gross area of core Agi= Ai/0.9 (where 0.9 is stacking factor ) (Note:- Stacking factor gives an approximate number to how much of core is effective when calculating flux)

7) Width of central limb 8) Wd= (Agi)1/2

More detailed calculation visit

Transformer Core

Cold Rolled Grain Oriented Electrical Steel (CRGO) and Non Cold Rolled Non Grain Oriented Electrical Steel CRNGO. Grain oriented Electrical Steel CRGO is undoubtedly the most important soft magnetic material in use today. Whether in small transformer, distribution transformer or in large transformer & generator, grain oriented electrical steel CRGO is a must for the production of energy saving electrical machines. Grain oriented Electrical Steels are iron-silicon alloys that provide low core loss and high permeability needed for more efficient and economical electrical transformers.

Core Material: 0.5mm thick CRNO (Cold Rolled Non-Grain Oriented ) Grade of 50C530, coating C6A. and Watt losses of 4.09 watt/kg.

Step 2: ​Design of Primary Winding

1) Calculate current in primary winding

Ip= VA/Vp*η


VA =Volt ampere

η = Efficiency and its value is to be considered inbetween 80 to 96%)

2) Calculate area of primary winding conductor

Ap= Ip/δp mm2


δp – Current density of primary winding conductor in A/mm2 value is generally taken as 2.3 A/mm2

3) From the area diameter of bare conductor is calculated as

d = (4*Ap)/π mm

1) Diameter
of insulated conductor is calculated from given table (di)

2) New Area of primary conductor used

Ap= (π/4)*d2

3) Number of primary turns

Tp= Primary voltage* Turns/volts

Tp= Vp*Te

4) Window space required

= (Tp*Ap) mm2


After calculation to design transformer which is cost effective, durable, and as per standard specification it is very challenging work. for the best optimum design the material selection plays the important role.

Following three basic materials required to design transformer

• Magnetic Material (Core Material)

• Electrical Material ( Winding Material)

• Dielectric Material (Insulation Material)

Step 4: ​Working of Linear Autotransformer

We named this transformer as linear autotransformer as its work in linear fashion. Working of the transformer is same as a conventional transformer with delta star connection. The primary side of transformers is connected in delta and secondary is connected in star connection. Carbon brushes are mounted on the slider for current collection.

The whole three phase winding of the transformer is divided into three limbs with equal no of turns.


  • Reduce the inrush Current·
  • Reduction in cost of less labor and core material·
  • Because of less leakage flux, it gives Higher efficiency


  • Adjusting AC voltage·
  • Induction Motor Starter·
  • Lab testing power supply