Transformer at a glance
Transformer is a static piece of equipment used either for raising or lowering the voltage of an ac supply with the corresponding decrease or increase in current.It essentially consists of two windings primary ans secondary,wound on a common laminated magnetic core.Winding connected to AC source is called primary winding and the one connected to the load is secondary winding.
The loss in a transformer basically takes place in the form of power,so depending on power the losses are of two types-:
- Iron or core losses
- Copper losses
These losses appear in the form of heat and produce an increase in temperature and a drop in efficiency.
Core or iron losses
These consist of hysteresis and eddy current losses and occur in transformer core due to alternating flux.
These can be determined by open circuit test,
Hysteresis loss= k f (B)^1.6 watts/m^3
Eddy current loss= k (f B t)^2
Both hysteresis and eddy current losses depend upon maximum flux density B in the core and supply frequency ‘f’ .
Iron or core loss = Hysteresis loss + eddy current loss
Iron loss is a constant loss.
These losses occur in both the primary and secondary windings due to their ohmic resistance. These can be determined by short circuit test,
Total copper losses= ( I^2)R
Copper losses vary as the square of load current .It may be noted that in a transformer copper losses account for about 90% of the total transformer losses.
Copper loss is a variable loss.
Total losses in transformer
Total losses in transformer=constant losses+variable losses
EFFICIENCY OF A TRANSFORMER
Efficiency of a transformer is defined as the ratio of output power(in watt or kW) to input power (in watt or kW).
efficiency=(output power/input power)
Efficiency can be determined by directly loading the transformer and measuring the input power and the output power.Since the efficiency of the transformer is very high, even 1% error in each watt meter may give ridiculous results.This test , for instance,may give efficiency higher than 100% and it also gives no information about the proportion of various losses.
Since the test is performed with transformer on load, considerable amount of power is wasted.For large transformers,the cost of power alone would be considerable,it is generally difficult to have a device that is capable of absorbing all the output power.
QUESTIONS AND ANSWERS
Q1.What are the type of losses in a transformer?
ans-:there are two types of losses:
1. Copper losses:- when the transformer is loaded, current flows in primary and secondary winding, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.
2. Iron losses or core losses:-The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:
> eddy current los
> Hysteresis loss.
Q2.How are iron losses reduced of a transformer ?
ANS:-Core loss (or iron loss) is a form of energy loss that occurs in electrical transformers and other inductors. The loss is due to a variety of mechanisms related to the fluctuating magnetic field, such as eddy currents and hysteresis.
Eddy currents are usually mitigated by using laminated steel cores that don’t have circular paths for current to flow in.
‘Soft’ magnetic materials with low hysteresis, such as silicon steel, rather than the ‘hard’ magnetic materials used for magnets, are usually used in cores.
Lamination of the core material can reduce eddy current loss, and also making the core of a nonconductive magnetic material like ferrite.
Q3.Why are iron losses constant at all loads in a transformer?
ANS-: Since the induce primary ampere turns and secondary ampere turns always nutralise one another, the flux in the core on load is the same as the flux on no load.
Q4.What is the principle of operation of transformer?
ANS-:Since transformer consist of two windings placed on an iron core so when alternating voltage is applied to one winding it sets up an alternating flux in the core,this flux links the other winding and induces voltage in it and thus load is connected to secondary and current flows and deliver power to load.