Transformer Winding

Transformer which works on the principle of electromagnetic induction is an electrical device which helps in the transfer of electric energy between the circuits. The main parts of the transformer are cores and windings. There are two types of windings in a transformer: primary and secondary winding.

A primary winding produces the magnetic flux when connected to the electrical source. While the transformer magnetic core helps in linking the magnetic flux of the primary winding with the secondary winding to create a closed magnetic circuit.

Here, the tutorial gives you the basic transformer winding considerations, tips and tricks while winding the coil and how to find the right core for the transformer.

Transformer Winding Considerations

Before winding the coils in a transformer, many points have to be considered like:

  • Type of coils being used,
  • Whether the coils are used for high radio frequencies,
  • Turns of the coil,
  • Type and length of wire used for winding the coils,
  • Build up of transformer windings,
  • Temperature, Stray capacitance, Leakage Inductance.
  • Screening or shielding,
  • Method of transformer winding- Balanced winding, Toroidal winding, High Current Winding.

Types of coils 

Coils used for the applications of electronics & communication and electrical engineering can range from a few to several turns.

  • Coils with few turns

Applications: Circuits handling radio frequencies, tuning circuits, intermediate frequency transformers, chokes and inductors.

  • Coils with large number of turns

Applications: Low frequency chokes, inductors, audio transformers, power transformers etc.

High frequency and Iron core coils are the most common types of coils.

1. High Frequency Coils

Coils used at very high frequency have the following characteristics:

  • They will be having very few turns of wire.
  • The wire used will be thick.
  • Coil will be self-supporting.
  • Usually they are wound over bakelite, plastic, paper formers.

2. Iron-core coils

  • These coils are wound with solid Cu wire insulated with enamel.
  • Layer winding and random winding is most commonly used.

     Random Winding   

                                                                     Random Winding                      

Layer Winding                                                                                                                                        

                                                                             Layer Winding

Type of wire

Inorder to perform winding of coils, an insulated wire is commonly used.

Build up of windings

Build is an important dimension while winding the coils. This generally indicates the thickness of the winding denoted by b.

Build up of windings

                                                                              Build up of Windings

Wire length calculations

                 calculating mean length of turn

Consider a transformer with two windings having thickness denoted by W1 and W2. Let m1 and m2 denote their mean length.

Length of the wire= MxN, here M denotes the mean length and N denotes the number of turns.

m1, mean length of primary winding = circumference of the circle + (sum of the sides of the bobbin).

m1= 2(E+D+4B) +6.28 W1/2.

m2, mean length of secondary winding= circumference of the circle+ (sum of sides of the bobbin)

m2= 2(E+D+4B) + 6.28 W1/2.

Voltage drop

Voltage drop in windings can be calculated by Ohms law,

Vd =IxR.

To compensate the voltage drop, one must increase the number of turns of the secondary winding.

Copper Loss

Copper loss is defined as the power lost while the windings get heated up.

Copper loss in a transformer= Power lost in primary windings + power lost in secondary windings

W= I12R1+I22R2+.....+In2Rn.

Temperature, Stray Capacitance & Leakage Inductance

Temperature consideration is really important in a transformer because the heat developed inside, can cause more power losses. Usually, the transformers are designed to work at temperatures not more than 105 degrees Celsius.

Stray capacitances are capacitances that exist between the windings, between the turns and between the core and the windings. The presence of stray capacitances can cause shunt paths that will help in modify the transformer performance. Usually, these capacitances are considered at high frequencies only. While at low frequencies, stray capacitance is a problem. This can be reduced by dividing the windings.

Leakage Inductance

The flux linking the windings via the core can cause leakage inductance. To minimize the leakage inductance, the windings are placed one over the other. But this can reduce the inductance to a limit only. The effect of leakage inductance is not a great concern at low frequencies, but it is not true at all cases. While, effect of leakage inductances should be considered at high frequencies.

        Bifilar Winding

                                                                                   Bifilar Winding

Reduction: Bifilar Winding and splitting the windings with intermingling the primary and secondary winding.

minimizing leakage inductance

                                                                             Intermingling Primary and Secondary Windings

Screening/Shielding

Due to many reasons placing an electrostatic shield/ screen and connecting it to the circuit ground between primary and secondary windings is really advantageous.

Reasons why screening is good:

1. It prevents high frequency interference.

2. It also reduces capacitance between the windings.

Two types of screening

  • Thin copper foil wrapped around the winding.
  • Single layer winding

Method of Winding

  • Balanced Windings

Balanced windings are really advantageous while considering the resistance, number of turns and capacitance factors. A simple balanced winding diagram is shown in the figure below.

  • Toroidal Windings

Toroidal word is used generally to describe the shape of the toroid. Usually if the windings occupy only a short length of the available 360 degree sweep of the core, toroidal windings cannot be used. Normally toroidal winding is used to occupy the entire length or periphery of the core. Winding on the toroid core is shown in the figure below.

Windings on a toroid core

                                                     Windings on a Toroid Core

Advantages of toroidal winding

  • It has a small external field.
  • Leakage inductance is less.
  • Effect of external interfering fields tends to cancel in the windings.

In a toroidal winding, every layer of the winding has a different length and different number of turns. Here, the length of the first layer is the length of the inside circumference. While, the winding length of the second layer is less than the first and the third is shorter than the second and so on.

  • High current winding

For large currents, it is often difficult to do the winding of the thick wire. Usually, in such cases, it is used to wind two coils of thinner wire.

Tips and Tricks to find the right core for the transformer

Before making a transformer, we have to choose the right core. Some tips and tricks while finding the transformer core are:

  • Find the purpose of the circuit. This will give the idea what type of material has to be used in the core.

Wounding of the coils

Coils are generally wound over bobbins or formers. The bobbins are made in such a size that it can fit the limb of the core. Bobbins can be of two types: Plastic moulded bobbins and bakelite bobbins. Plastic moulded bobbins are used of many sizes. For large sizes, bakelite bobbins are available.

Tips and Tricks: Winding the coil

Tip1: While winding the coil, it should be firmly anchored. This can be done by holding the piece of tape around the wire and winding over the tape……

Tip2: Start the winding from the correct face of the transformer.

Tip3: After the starting turn is anchored correctly, then only proceed with the winding.

Tip4: Now cut the inter-layer paper strips of correct width before starting the winding.

Tip5: While the winding process is done, in case of a layered wound coil, the turns must be laid neatly side by side.

Tip6: On reaching the last part of the layer, a strip of interleaving paper is wound on the first turn of the next layer. This procedure must be repeated until reaching the end of the final layer.

Coil Winder

Coil winder is basically a machine used for wrapping the coil. When the winding progresses, the shape of the coil will be becoming circular and the winding will be inserted to the core.

Assembly

At last, the laminations will be inserted to the coil. This process is done by placing the coil on the bench side and using the hands to control the insertion of the laminations.

Clamping

Clamping means to make the core tightly packed and let the whole assembly of the transformer to be mounted on a frame or chassis.

Mounting and Ventilation

Mounting of the transformer needs to be done carefully, since a transformer is the most heat generating component. So it must be mounted in such a way that adequate ventilation must be provided inorder to keep it cool. Depending upon the size and type of the transformer, methods of mounting will be varying. Always the top of the transformer must be clear, inorder to keep the transformer cool. The second thing to note is that, the transformers must be placed away from the components that are affected by stray magnetic fields.

Last is the testing and measurement step. Here the ohmmeter tests, inductance measurement, leakage inductance, turns ratio measurement, number of turns and the insulation is being checked out and detected.

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