4.1 transformer part 2

DJJ2022: Electrical Technology[Transformer]PN NURUL NADIAH BT SHAHROM

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r.m.s value of e.m.f induced in primary is E1 = 4.44 N1 f m voltsr.m.s value of e.m.f induced in secondary isE2 = 4.44 N2 f m volts

EMF EQUATION


Example 4

A 250 kVA, 1100 V / 400 V, 50 Hz single-phase transformer has 80 turns on a secondary. Calculate : the approximate values of the primary and secondary currentsthe approximate number of primary turnsthe maximum values of flux


Example 5

An ideal 25 kVA transformer has 500 turns on the primary winding and 40 turns on the secondary winding. The primary is connected to 3000 V, 50 Hz supply. Calculate:primary and secondary currents on full-loadsecondary e.m.f. andthe maximum core flux


Losses in transformer


Core losses2 jenis kehilangan:Histerisis yang disebabkan oleh ulangan pemagnetan atom yang terbentuk di dalam bahan bermagnet di dalam teras(the heating of the core as a result of the internal molecular structure reversals which occur as the magnetic flux alternates. The loss is proportional to the area of the hysterisis loop and thus low loss nickel iron alloys are used for the core since their hysteresis loop have small areas)Arus pusar yang berputar mengalir di dalam perlapisan-perlapisan teras(the heating of the core due to e.m.f. s being induced not only in the transformer windings but also in the core. These induced e.m.f.s set up circulating currents call ed eddy currents. Owing to the low resistance of the core, eddy currents can be quite considerable and can cause a large power loss and excessive heating of the core)


Efficiency

=Output Power Input power


Example 6

The primary and secondary windings of a 500 kVA transformer have resistances of 0.42 and 0.0019 respectively. The primary and secondary voltages are 11 000 V and 400 V respectively and the core loss is 2.9 kW, assuming the power factor of the load to be 0.8. Calculate the efficiency on:full loadhalf load


Example 6(i)


Example 6 (ii)


Open Circuit Test and Short Circuit Test

These two tests enable the efficiency and the voltage regulation to be a calculated without actually loading the transformer and with an accuracy far higher than is possible by direct measurement of input and output powers and voltages. Also, the power required to carry out these tests is very small compared with the full-load output of transformer.


Open Circuit Test

The transformer is connected as in figure to a supply at the rated voltage and frequency, namely the voltage and the frequency given on the nameplate.The ratio of the voltmeter readings, V1 / V2, gives the ratio of the number of turns. Ammeter A gives the no-load current, and its reading is a check on the magnetic quality of the ferromagnetic core and joints. The primary current on no load is usually less than 5 per cent of the full-load current, so that the I2R loss on no load is less than 1/400 of the primary I2R loss on full load and is therefore negligible compared with the core loss. Hence the wattmeter reading can be taken as the core loss of the transformer.


Short Circuit Test

The secondary is short-circuited through a suitable ammeter A2, as shown in Fig.8.8 and the low voltage is applied to the primary circuit. This voltage should, if possible, be adjusted to circulate full-load current in the primary and secondary circuits. Assuming this to be the case, the I2R loss in the windings is the same as that on full load. On the other hand, the core loss is negligibly small, since the applied voltage and therefore the flux are only about one-twentieth to one-thirtieth of the rated voltage and flux, and the core loss is approximately proportional to the square of the flux. Hence the power registered on wattmeter W can be taken as the I2R loss in the windings.


Auto Transformer

An auto-transformer is a transformer having a part of its winding common to the primary and secondary circuits. In figure below winding AB has a tapping at C, the load being connected across CB and the supply voltage applied across AB.

I1 and I2 = primary and secondary currents respectivelyN1 = no. of turns between A and BN2 = no. of turns between B and Cn = ratio of the smaller voltage to the larger voltageNeglecting the losses, the leakage reactance and the magnetizing current, in the figure:


Auto Transformer

The nearer the ratio of transformation is to unity, the greater is the economy of conductor material. Also, for the same current density in the windings and the same peak values of the flux and of the flux density, the I2R loss in the auto transformer is lower and the efficiency higher than in the two winding transformer.Auto transformer are mainly used for interconnecting systems that are operating at roughly the same voltage and starting cage-type induction motors. Should an auto transformer be used to supply a low voltage system from a high voltage system, it is essential to earth the common connection, for example, B in Fig 8.9 otherwise there is a risk of serious shock. In general, however, an auto transformer should not be used for interconnecting high voltage and low voltage systems.


Advantages & Disadvantages of Auto Transformer

The advantages of auto transformers over double wound transformers included:a) Saving cost since less copper is needed.

b) less volume, hence less weight.

c) higher efficiency, resulting from lower I2R losses

d) continuously variable output voltage is achievable if a sliding contact is used.

e) smaller percentage voltage regulation.

4.1 transformer part 2

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