TRANSFORMER

TRANSFORMER

The transformer is an electrical device, which can convert electrical

energy from one coil to another coil. The transformer is working on

the principle mutual induction. The transformer basically consists of

two coils, namely, primary and secondary . These coils are wound on a iron core. The alternating voltage, which is to be transferred,is applied in the primary coil as input. This produces a changingmagnetic flux in the iron core, which produces an alternating emf in the secondary coil. The induced emf’s in the coils are directly proportional to the respective number of turns of the coil.

TRANSFORMER LOSSES

In practice, the output power is always lesser than the input power

and hence, the efficiency of the transformer is always less than 100%.

This implies that some amount of energy is lost in the form of heat.

This energy loss can be considered as copper losses, eddy current

losses, hysteresis and flux leakage losses.

Copper Losses

Whenever a current I flows through a resistance R, an amount of

power equal to I2× R × t watt is converted into heat. This can arise

in both copper coils and iron core. The copper coil has resistance.

If current flows through this coil, electrical energy equal to I2Rt is

converted into heat. To reduce this loss, the current cannot be reduced because the normal operation of the transformer will be affected. Instead, the resistance of the coil must be minimized by using wire of low resistivity. Therefore, thicker wire should be used as transformer coil. The optimum thickness will be decided by comparing the cost,space and saving of power. Copper is the best coil material available to day and hence, it is commonly used.

Eddy Current Losses

The iron core consists of concentric layers of iron, each acts as a

circuited single turn coil. Whenever the magnetic field changes, an

emf will be induced in the core. The current produced by the induced

emf in the core is called eddy current, which will give rise to I2Rt

heat losses. These eddy currents can be eliminated by making the

iron core in the form of thin sheet of metal, and each sheet is insulated

from its neighbor by a thin layer of paper. This type of core is known

as laminated core. The core is usually made up of stelloy, an alloy

of steel. Some design employ high resistance ceramics as core material.

Hysteresis Losses

The transformer core is a magnetic material. The core is magnetized

twice in each cycle of the alternating voltage. When the direction of

AC changes, the magnetization is also gets reversed. During this

reversal, some energy is lost due to the molecular friction and the

energy appears as heat. The loss of energy by molecular friction is

called hysteresis loss. This can be reduced in practice by choosing

a suitable magnetic material, such as mu-metal, which has low

hysteresis loss. Mu-metal is a ferromagnetic alloy containing 78% nickel, 17% iron, and 5% copper. It has high permeability.

Flux Leakage

All the magnetic flux linked with the primary is not linked with the

secondary coil. This is said to be flux leakage, which results in loss

of energy. This can be minimized by using good core design like

shell type of core.