Windings
Electrical Parts of a transformer comprise mainly of the primary and secondary windings.
The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions.
Copper and Aluminum are the primary materials used as conductors in power transformer windings.
These conductors may be of circular or rectangular cross-section, depending on the current and voltage ratings of the machine and are insulated using enamel, paper, or cotton.
Desirable properties of a conducting material for use in transformer ( or any Electrical Machine ) are:-
- High Conductivity
- Low temperature coefficient of resistance
- High Thermal Conductivity
- Low coefficient of thermal expansion
- High ductility
- High tensile strength
- Not too heavy
- Low corrosion by chemicals
The two materials mostly commonly used for windings of transformers are copper and aluminum. Both have their relative merits and demerits.
Electrical conductivity of copper is higher than that of aluminum so that it offers less resistance.
Per Kg cost of aluminum is less than copper, but because of higher resistivity, the amount of material to be used to allow same amount of power loss (I^2 . R) in the winding is more if aluminum is used.
Mechanical strength of copper is also higher than aluminum that makes copper to be more popular except for small sized ones where cost saving is one major concern.
High voltage coils are mostly made of conductors of circular cross-section so that escalation of electric field at sharp edges of the conductor can be avoided.
Low voltages coils, because of its high current rating, will generally need a number of conductors (strands) to be connected in parallel. In order to reduce the size of such stranded coil, low voltages coils are generally made of rectangular cross-section conductors.
Types of Winding
Shell type transformers use sandwich type winding with both HV and LV coils split up in a number of sections, with HV coils placed between two LV coils.
core type transformers can have the following types of windings:
- Disc winding
- Helical winding
- Double helical winding
- Multi-layer helical winding
- Cross over winding
The types of windings to be used depends on:
- Current rating
- Voltage rating
- Short circuit strength
- Allowable temperature rise
- Surge current rating
Placement of Windings
There are two types of windings generally employed for Transformers. The concentric coils are used for core type of Transformers. Interleaved or Sandwiched windings are used for Shell Type Transformers.
Sandwiched windings are preferred to reduce magnetic leakage flux thereby increasing coefficient of coupling between the windings. In Sandwiched windings the Low Voltage winding is usually placed near to the core and High Voltage winding outside the Low Voltage winding. Insulation is provided between two windings and between core and winding.
Advantages of placing LV winding nearer to the core:
- The amount of insulation required for transformer between core and winding reduced.
- It also reduces the amount of Copper (Cu) required for windings.
- If HV windings is outer to low voltage, then HV is accessible to provide tapings for voltage control purpose.
The directions of flux in HV and LV are of Opposing as per Lenz Law because induced emf must oppose the cause of induction.
- If HV and LV winding are placed on same limb then they should carry 'I' in opposite directions to satisfy Lenz's law.
- If HV and LV are placed on separate limbs, then they should carry 'I' in same direction to satisfy Lenz's law.
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Magnetic Parts
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