Why are transformer cores laminated? 

Concept:

• To reduce the eddy current losses, most  low-frequency power transformers and inductors use laminated cores , made of stacks of thin sheets of silicon steel.
• RF coils are mostly air core types, which can be described as an inductor that does not use a magnetic core made of a ferromagnetic material. The term air-core type refers to coils wound on plastic, ceramic, or other nonmagnetic forms, as well as those that have only air inside the windings.
• Air core coils have lower inductance than ferromagnetic core coils but are often used at high frequencies because they are free from energy losses called core losses that occur in ferromagnetic cores. We, therefore, conclude that the RF coil does not use a laminated core.

Important Points 

Copper loss:

The low-resistance copper cable used for the windings remains resistant and thus leads to heat loss.

Reducing method: By using thick wires with considerably low resistance.

Leakage of flux:

If the core design is not good then the flux produced by the primary coil may not all be connected to the secondary coil.

Reducing method: By considering the shell type core.

Eddy currents loss:

• The varying magnetic field not only induces secondary coil currents but also iron core currents themselves.
• In the iron core, these currents flow in small circles and are termed eddy currents. 

Reducing method: By considering the laminated core.

Hysteresis loss:

This is because of the repeated iron core magnetization and demagnetization induced by the alternating input current. 

Reducing method: By using alloys such as mumetal or silicon steel.