Core of Transformer and Design of Transformer Core

Purpose of Transformer Core

An electrical power transformer has a primary, a secondary and sometimes a tertiary winding. Performance of a transformer depends mainly on flux linkage between those windings. Efficient linkage needs one low-reluctance magnetic path common to all windings. That path is the core of a transformer. The three main types of transformer cores are:

1. Core Type Transformers
2. Shell Type Transformers
3. Air Core Transformers

Influence of Diameter of Transformer Core

Let the diameter of the transformer core be ′D′.
Then the cross-sectional area of the core is

Now, voltage per turn,

B_m is the maximum flux density of the core.

E is proportional to D².
So voltage per turn rises as the core diameter rises.
If the voltage across the winding is V.
Then V = eN, where N is the number of turns.
If V is constant, e is inversely proportional to N, so D² is inversely proportional to N. A larger core diameter therefore means fewer turns in the transformer winding. Fewer turns cut the height of the core legs, but the larger diameter still increases the overall diameter of the magnetic core of transformer. The extra steel weight raises core losses in transformer. The larger core also increases the mean winding diameter. Even so, fewer turns still cut copper loss in the transformer.

A larger core diameter raises core loss and cuts copper loss. A smaller diameter cuts steel weight and core loss but raises copper loss in transformer because more turns are needed. Core diameter is therefore chosen to balance core loss and copper loss.

Material for Transformer Core

The main problem with the transformer core is its hysteresis and eddy current losses. Hysteresis loss depends mainly on the core material. A small amount of silicon alloyed with low-carbon steel gives a core steel with low hysteresis loss and high permeability. Rising power ratings then required still lower core loss, so a further process called cold rolling is used. Cold rolling aligns the grain of the ferromagnetic steel with the rolling direction.

After silicon alloying and cold rolling, manufacturers call the sheet CRGOS (Cold Rolled Grain Oriented Silicon Steel), the usual power-transformer core steel.

CRGO steel has low specific iron loss, but it still has drawbacks. Loss rises if flux flows off the grain direction. Bending and cutting also hurt performance. Both faces of the sheet are therefore coated with an insulating oxide layer.

Optimum Design of Cross-Section of Transformer Core

CRGO steel saturates near 2.0 T; at about 1.9 T it is already close to saturation. Cores are therefore designed so they do not saturate in normal service. The Voltage of a transformer depends on its total magnetizing flux, which is flux density times core cross-sectional area. Changing the cross-sectional area therefore sets the working flux density.

The ideal core cross-section is a circle. A true circle would need every lamination cut to a different size, which is not economical. In practice, packets of equal-size laminations are stacked in steps so the outline approximates a circle, as in the figure below.

Oil ducts cool the core because hotspot temperature can rise too far, and their number depends on core diameter and the core material. Steel clamp plates are also needed on both sides of the core to clamp the laminations. The laminated packets, oil ducts and clamping plates should all lie inside the optimum core circle.
Net sectional area is taken from the packet dimensions. An allowance is made for space lost between laminations (the stacking factor). For 0.28 mm steel sheet with an insulation coating that factor is about 0.96. Area is also deducted for oil ducts. The ratio of net core area to the gross area inside the imaginary peripheral circle is the utilization factor of the transformer core. More steps raise the utilization factor but also raise manufacturing cost. A typical step count is between 6 (smaller diameter) and 15 (larger diameter).

Manufacturing of Transformer Core

When a transformer core is built, the main factors taken into consideration are:

1. Higher reliability.
2. Reduction in the iron loss in transformer and magnetizing current.
3. Lowering material cost and labor cost.
4. abatement of noise levels.

Quality checks are needed at every manufacturing step. The steel sheet must be tested for its specific core-loss or iron-loss values. Laminations should be inspected visually; rusty or bent sheets should be rejected. To cut transformer noise, laminations should be clamped tightly. Punch holes should be avoided where possible to limit cross-flux iron loss. Air gaps at limb-to-yoke joints should be kept as small as possible so magnetizing current has a smooth iron path.

Corner Jointing of Limbs with Yokes

Core losses in transformer happen mainly due to:

1. Magnetic flux flow along the direction of the grain orientation,
2. Magnetic flux flow perpendicular to the direction of the grain orientation, this is also known as cross grain iron losses. The cross grain loss mainly occurs in the zones of corner jointing of limbs with yokes and it can be controlled to some extent by applying special corner jointing techniques. There are normally two types of joints used in a transformer core,
   1. Interleaved joints
   2. Mitre joints

Interleaved Joints in Transformer Core

An interleaved joint is the simplest core joint. The figure shows it. The flux leaves and enters the joint perpendicular to the grain, so cross-grain loss is high. The joint is still used on small-rating transformers because it is cheap to make.

Mitred Joints in Transformer Core

Here the laminations are cut at 45^o. Limb and yoke edges meet face to face at the mitred joints. Flux then enters and leaves along the grain, so cross-grain loss is low. Manufacturing cost is higher. The joint is used on types of transformers where cutting loss is the main aim in designing of a transformer core.