A transformer is a static machine which transfers a.c electrical power from one circuit to another with out any electrical link between them. It essentially consists of two windings the primary and secondary, wound laminated magnetic core. The flux links both the windings and induces e m f according to the Farady’s Laws of Electromagnetic Induction.
- The two windings of an isolation transformer are linked together by the magnetic filed.
All values of a transformer are proportional to its turns ratio.
- The ratio of the primary and secondary voltage determined by the ratio of the number of turns in the primary and secondary windings.
- Step up Transformer:There are more turns on the secondary coil than the primary coil. The induced voltage across the secondary coil is greater than the applied voltage across the primary coil
- Step down Transfoemer:A step down transformer has less turns on the secondary coil that the primary coil. The induced voltage across the secondary coil is less the applied voltage across the primary
Wye and delta connection
- Wye the phase voltage is less the line voltage by a factor of 1.732 .The phase current and line current are the same.
- Delta connection :Te phase voltage is the same as in the line voltage the phase current less than line current by factor of 1.732.
- Harmonics:Are the voltage or current that operate at frequencies that are multiples of the fundamental frequency usually 50 or 60.
- Harmonics reduction :Reduce neutral current in three-phase systems by balancing currents in the phase legs. Reduce triplen harmonics on the neutral by adding filters.
- There are two types of transformers Core type and shell type.
This is the power loss that occurs in the iron part. This loss is due to the alternating frequency of the emf.
Iron losses =Hysteris loss+Eddy current loss=constant loss
This is the loss in the iron core, due to the magnetic reversal of the flux in the core, which results in the form of heat in the core. This loss is directly proportional to the supply frequency.The hysteresis loss can be minimised by using high silicon steel
Eddy current loss:
This power loss is due to the alternating flux linking the corewhich will induced an emf in the core called the eddy emf, due to which a current called the eddy current is being circulated in the core. As there is some resistance in the core with this eddy current circulation converts into heat called the eddy current power loss. Eddy current loss can be reduced by using core of thin lamination.
Copper loss occur in both these windings due to their ohmic resistnaces.Its also called variable resistance
Copper losess Pc=I1(R1)2+I2(R2)2
- The rating of transformer is given in KVA .The total loss in the transformer depends upon Volt Ampere(VA)
- Power Transformer used in generating stations having great efficiency.
- Destribution transformers used for stepping down voltage and kept near consumer’s premises.
- Current transformers for meashuring a.c current
- Potential transformer for meashuring high voltage.
- No winding resistance.
- No leakage flux.
- No iron losses.
Short Circuit test and Open circuit test used for determine the
(i) equivalent circuit of transformer
(ii) voltage regulation of transformer
(iii) efficiency of transformer.
Open circuit (OC) test
open circuit test on transformer is used to determine core losses in transformer and parameters of shunt branch of the equivalent circuit of transformer.
The shunt branch parameters can be determined by performing this test. Since, the core loss and the
magnetizing current depend on applied voltage, this test is performed by applying the rated voltage
to one of the windings keeping the other winding open (generally HV winding is kept open and rated
voltage is applied to LV winding).
Short Circuit Test on Transformer
Suppose the input voltage is reduced to a small fraction of rated
value and secondary terminals are short-circuited. A current will circulate in the secondary winding.
Since a small fraction of rated voltage is applied to the primary winding, the ﬂux in the core and hence
the core loss is very small. Hence, the power input on short circuit is dissipated as heat in the winding.
Effciency of the transformer ratio of output and input.
Eﬃciency of the transformer is deﬁned as=Output power/(Output power+Iron losses+Copper Losses)
- Transformer Engineering: Design and Practice By S.V. Kulkarni, S.A. Khaparde
- Basic Electrical Engineering book M A College Of Engineering Kothamangalam
- Principle of Transformers
- Transformer Basics
- Open and Short Circuit Test on Transformer