The output power in practical real transformer is slightly less than input power since there are some power losses occurs within the transformer. Different types of transformer losses are as follows:

Core or Iron losses

Copper loss

Stray Loss

Dielectric loss
Core losses occur in the core of transformer while copper loss occur in the primary and secondary windings of transformer. Core losses are constant in transformer while copper loss varies with the square of load current. The reason for being fixed core losses is due to the fact that these losses depend on the magnetic properties of the material used in the construction of core. Now, let discuss the detail of transformer losses.
COPPER LOSS:
In transformer, primary and secondary windings have some resistance. When current flows through the copper coils (Primary and secondary windings) of transformer, windings resistance consume some of the power and gets heated up. The resistive heating losses occur in the primary and secondary windings of transformer is known as copper loss. These are called copper loss because the coil generally used is copper. Since copper losses are resistive and proportional to the square of load current, they are also known as load losses or losses. The copper loss formula can be expressed as:
Pc=
Assuming R to be remains constant. Hence copper loss varies as the square of load current. This means that if load current (load) is doubled, copper loss will become four times.
How can we reduce Copper Losses in Transformer?
Since Copper loss Pc is the power loss equal to the product of resistance R in ohm and square of the current (I) in amps Pc= .
If we reduce any of the term on the R.H.S, we can reduce copper loss in transformers.
Copper wire windings are made thick in order to reduce resistance.(Since resistance is inversely proportional to the area of cross section of wire).
Voltage is stepped up so that current can be reduced. In this way, the product of is reduced. This technique is generally used in large distance transmission.
IRON OR CORE LOSSES:
Hysteresis loss and eddy current loss occur within the core of transformer is known as Iron or Core losses of transformer.
Eddy Current Loss:
When we applied AC voltage at the primary (winding) side of transformer, current flows in the primary winding produces alternating magnetic flux. This alternating magnetic flux also links with the core of transformer. Since core of the transformer is conducting material, emf is induced in it according to the faraday law of electromagnetic induction. The induced emf causes the flow of current in the core of transformer. Circulating currents produced in the magnetic core does not contribute in the output of transformer and dissipated in the form of heat. This type of loss is known as transformer eddy current loss.
These losses occurs when the conductor (magnetic core of transformer) experiences changing magnetic field. If the mass of the core is highly conductive, eddy current losses are quite high since the resistance of material is very low in that case.
Here the question arises:
How can we reduce Eddy Current Loss in Transformer?
Eddy currents produced in the core of transformer are not desirable and need to be reduced. In order to reduce eddy current losses, we generally increase the resistance of path where produce eddy currents circulates. The resistance(R) of the wire is given as:
Where
ρ= Resistivity of material.
L = length of wire.
A= Cross sectional area of the wire.
From the above equation, we can say that reducing the crosssectional area(A) increases the resistance(R) of material. Now, reducing of cross sectional area is done by using core of thin laminated sheets which are insulated from each other by thin layer of varnish. Laminations are made to be thin and done in such a manner so that the crosssectional area for the flow of current is reduced. Once the cross sectional area is reduced, resistance of the material increases.
Eddy currents circulates within these thin lamination sheets. Large resistance between the lamination sheet limits the currents within the sheets. In this way, eddy current losses are minimized to a great extent. Moreover, the sum of all the individual eddy currents flowing in the thin laminations is very less as compared to the solid mass of the core (which has a much higher crosssectional area.)
One more thing to be noted that the use of laminations is not required if the core of transformer is made up of highly resistive material like ferrite. Since the resistivity of the core material is itself quite high which will reduce transformer eddy current loss.
Stray loss:
Stray losses occur due to the presence of leakage flux in the transformers. Leakage flux links with the metallic structural parts (i.e. tank ) and produce eddy current losses in them. Since they take place all over the transformer instead of one definite place, hence the name “stray” is given to them.
Dielectric loss:
Dielectric losses occur in the insulating material (in the oil of transformer) or in the solid insulations. If the oil gets deteriorated or solid insulation is damage, efficiency of the transformer become affected.
Hysteresis loss:
Magnetic core of the transformer is made up of ferromagnetic materials. These materials are very sensitive to be magnetized (i.e when magnetic field passes through it, they behave like a magnet).
Since every magnetic material has some domains in their structure. In normal condition, these magnetic domains(particles) move in arbitrary directions. When the core is subjected to alternating magnetic field, randomly moving domains began to align themselves in parallel with the magnetic lines of forces(flux). When applied magnetic source is removed, maximum number of domains goes back to their arbitrary state, but some of them remains in their new position. Due to these magnetic domains, the substance is still slightly magnetized. In order to neutralize this magnetism, we have to apply some amount of magnetomotive force(H=NI) in the opposite direction known as coercive force (see the below fig). That extra force applied is nothing but hysteresis loss.
Since the applied magnetic field in the transformer is alternating, there are reversal of cycles. Hence the domains present in the core material will change their directions after every half cycle. Due to the frequent changings of domain positions, there will be extra work done after every cycle. For this reason, there will be continuous consumption of electrical energy which is known as hysteresis power loss. The below diagram is the BH hysteresis loop curve↗.
How can we reduce hysteresis loss in transformer?
Although we cannot prevent hysteresis losses in transformer but we can minimize these power losses to a great extent. Hysteresis losses occur in the core of transformer is directly proportional to the area of the hysteresis loop of the material. From the above figure, we can analyze that area cover under the BH loop is the power dissipated in the form of heat which is known as hysteresis loss.
Hysteresis loss can be reduced if this loop becomes narrow. Hence these losses are reduced by using material having less hysteresis loop area. Silicon and steel used for the manufacturing of transformer core have very less hysteresis loop area. In this way, we can minimize these losses by using a material having less hysteresis loop area.
Conclusion:
From the above discussion, we conclude that both hysteresis and eddy current losses depend on the magnetic properties of the core material used in the construction. Copper losses are current dependent and varies as the square of load current. Moreover, the percentage of dielectric and stray losses are very low as compared to the iron and copper losses so they can be neglected. Hence, we can write transformer losses in equation form as:
Input Power= Output Power – transformer losses.
Primary Power = Secondary Power hysteresis loss eddy current losscopper loss.