Transformer Procurement Course - Transformer Windings

What is a Transformer winding? 

Generally, the windings are coils wound around a ferromagnetic core.

What are the materials used for winding coils? 

Coils material used for the windings depends upon the application as follows:

1- For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enameled magnet wire

2- Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper or other insulating materials

3- High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses

What are the types of Transformer windings? 

• Layer windings (see fig.1)

• Helical windings (see fig.2)

• Disc windings (see fig.3)

• Foil windings (see fig.4)

How to select the transformer winding type? 
The selection will be based on some factors like:

• The required number of turns 

• The required current Value in the winding primary 

fig.1

What is the Layer winding? 
(See fig.1)

The Turns of Layer winding are arranged axially along the winding and wound close to each other without any intermediate space. The Winding may be made as a single or multi-layers winding and it can be used as regulating windings for large transformer.

fig.2

What is the helical winding? 
(See fig.2 )

Helical Winding is made as multi-layer windings with spaces between each turn/layer and other in the winding,

Helical Winding is used for high current transformer although it has a high space Factor which gives a big size transformer but it is mechanically robust and easy to manufacture. 

Handbook of Transformer Design and Applications

fig.3

What is Disc winding? 
(See fig.3)

Disc winding type is build up from a number of Discs connected in series, in each disc; many turns are wound in radial direction.

Disc winding type is used for transformer having large number of turns and relatively small currents and highest voltage windings.

A special type from this winding called the double disc winding is used for highest current transformer (100 KA) as in the furnace transformers and rectifier transformers for industrial applications. 

fig.4

What is the Foil winding? 
(See fig.4)

Foil winding is made from Aluminum or copper sheets to withstand short circuit currents effectively during short duration; therefore it is used in Low voltage distribution transformers and can be used in large transformer for the same reason

What is the regulating winding? 
Regulating winding is not a type from the windings types but only it can be considered as a connection arrangement which be made in these winding types for controlling/regulating the transformer voltage.

In regulating transformer, the regulating turns be arranged in a separate winding from the Layer or Helical types, turns of each regulating step will be connected in series by means of cables outside the winding which provide a connection between each regulating step and the Tap changer.

The regulating winding is preferable to be located at neutral point of a star connected winding where the potential difference between the three phases is small.

What are the types of Tap changers? 
There are two types of tap changers:

1- Tap changer for operation during Load (OLTC) which operates only in energized conditions.

2- Off-circuit tap changer (OCTC) which operates in de- energized conditions.

fig.5

What are the arrangement methods of Tap changer? 
(See fig. 5)

The tap changer and the regulating winding can be arranged in three different methods as follows:

1- Linear: used for small regulating ranges (about 10% of nominal value) and the voltage across the regulating winding will be added to the voltage across the main winding.

2- Plus-Minus: used for large regulating ranges and in this type the tapped winding is connected to the main winding via a separate plus- minus switch which permits the voltage across the tapped winding to be added or subtracted from the voltage across the main winding.

3- Coarse-fine: used when the regulation is done by two windings, one for the coarse step and one for the fine step.