Single Phase Transformer

PROCEDURE:

   The transformer was examined, and given a special attention to the construction, rated voltage, kVA, frequency, etc. The rated currents for each side were calculated. Then the

1. Terminal identification test

2. Polarity test

3. Open circuit test

4. Short circuit test

                           are made on the transformer as follow the instructions which are given in the handout sheet.

DISCUSSION                                                                 

Transformer is an electrical device consisting of one coil of wire placed in close aproximity to one or more other coils, used to couple two or more alternating-current (AC) circuits together by employing the induction between the coils. It has two windings, which are connected through a magnetic core.

The coil connected to the power source is called the primary coil, and the other coil is known as secondary. A transformer in which the secondary voltage is higher than the primary is called a step-up transformer. If the secondary voltage is less than the primary, the device is known as a step-down transformer. The product of current and voltage is constant in each set of coils, so that in a step-up transformer, the voltage increase in the secondary is accompanied with a corresponding decrease in the current.

Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current. Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used for long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually 230V.

Power Transformers

Power Transformers are large devices which are used in electric power systems, and small units in electronic devices. Industrial and residential power transformers that operate at the line frequency, may be single phase or three-phase, and are designed to handle high voltages and currents.

Power transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 percent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings. The dissipation of even 0.5 percent of the power transmitted in a large transformer generates large amounts of heat, which requires special cooling provisions. Typical power transformers are installed in sealed containers that have oil or another substance circulating through the coils to transfer the heat to external radiatorlike surfaces, where it can be discharged to the surrounding atmosphere.

 

Transformer in Electronics

In electronic equipment, transformers with capacities in the order of 1 kw are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment. Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1- to 100-W power level are used principally as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment. These are known as audio transformers and they use only a small fraction of their power rating to deliver program material in the audible ranges, with minimum distortion. The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level.

At power levels of 1 milliwatt or less, transformers are primarily used to couple ultrahigh-frequency (UHF), very-high frequency (VHF), radio-frequency (RF), and intermediate-frequency (IF) signals, and to increase their voltage. These high-frequency transformers usually operate in a tuned or resonant circuit, in which tuning is used to remove unwanted electrical noise at frequencies outside the desired transmission range.

 

Parallel Operation of Transformers

The following conditions must exist for transformers to operate satisfactorily in parallel:

·   Connection diagrams must be identical. Paralleling transformers with different connection diagrams is similar to short circuiting their secondary windings.

·   Voltage ratios must be the same. If voltage ratios are not the same, circulating currents will flow in the secondary with no or little load and the division of load will be improper.

·   Percent impedance, including primary and secondary leads to each transformer, should be nearly equal. If the impedance's are equal and the turns ratios are identical, the paralleled transformers will divide the load currents (properly) in proportion to their kVA ratings. If the percent impedance's are different, the transformer with the lower percent impedance will take more than its proper share of the load.

Example: Operation with unequal turns ratios:

Two transformers with similar characteristics (both rated 7,200-240 Volts, 3% Z, equal X/R ratios but with different kVA ratings) are to be operated in parallel to serve a 75 kVA load. One unit is rated 25 kVA and the other is rated 50 kVA.

By mistake, the 25 kVA unit has its primary tap set 5% low (6840 V tap) which would give an open-circuit voltage of 252.6 V instead of the desired 240 V. 

When paralleled and energized from a 7,200 V primary, the circulating current is 117 percent of rated current for the 25 kVA unit before any load current is drawn. The open-circuit voltage from the combination is 244.2 V.

Solution:

Difference between O-C voltages = 12.6 V (= 5.25%)
Loop impedance (25 kVA base) = 4.50%
Circulating current (use per-unit values) = 1.17 per unit
(by Ohm's Law) = 117%
Impedance voltage divider ratio = 1.5%/4.5% = 1/3
Terminal voltage rise above 240 V = (1/3) x (12.6 V) = 4.2 V
"No load" terminal voltage = 240 V + 4.2 V = 244.2 Volts

Errors in the Experiment

There may be several reasons which cause errors in the experiment. 

·   Resistance of the connecting wires.

·  Reading errors in volt meter and ammeter.

·  Errors in measuring instruments as they are not ideal.

Also during the experiment as we expect to get some readings at the rated voltage of the transformer it was not possible. The maximum voltage we could take by the supply was 220 V as the voltage is dropped due to other connected equipments which take high currents. Therefore the calculations are done using 220V. Also in the experiment measuring instruments had to be selected with appropriate scales to minimize the possible errors.

REFERENCES

·  Encarta Encyclopedia Deluxe version 2002

·   Electrical Machinery        -    Chalres Kingsley

A.E. Fitzgerald

Stephen D.Umans

·   Electric Machines           -      I J Nagarath & D P Kothari

                             -      Eight reprint 1993

·   www.citycollegiate.com

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