1. Rangat Mehta 130420109518 Unnati Natali130420109523 Nishant Shah130420109525 Kumar Pawar130420109538 Yash Lakdawala130420109558 SARVAJANIK COLLEGE OF ENGINEERNG AND TECHNOLOGY, SURAT 2 nd YEAR ELECTRICAL ENGINEERING DEPARTMENT EVENING SHIFT DC Machine & Transformers

2. Topics Covered :-  Testing – O/C & S/C Tests.  Polarity Tests.  Sumpner’s Test.  Separation of hysteresis and eddy current losses.  Auto Transformers  Construction  Principle  Applications  Comparison with two winding transformer

3. O/C Test : Open Circuit Test  The secondary of the transformer is left open-circuited. A wattmeter is connected to the primary. An ammeter is connected in series with the primary winding. A voltmeter is optional since the applied voltage is the same as the voltmeter reading. Rated voltage is applied at primary.  If the applied voltage is normal voltage then normal flux will be set up. Since iron loss is a function of applied voltage, normal iron loss will occur. Hence the iron loss is maximum at rated voltage. This maximum iron loss is measured using the wattmeter.

4. O/C Test : Open Circuit Test  Since the impedance of the series winding of the transformer is very small compared to that of the excitation branch, all of the input voltage is dropped across the excitation branch. Thus the wattmeter measures only the iron loss. This test only measures the combined iron losses consisting of the hysteresis loss and the eddy current loss. Although the hysteresis loss is less than the eddy current loss, it is not negligible, & hence it cannot be neglected. The two losses can be separated by driving the transformer from a variable frequency source since the hysteresis loss varies linearly with supply frequency and the eddy current loss varies with the square.  Since the secondary of the transformer is open, the primary draws only no-load current, which will have some copper loss. This no-load current is very small and because the copper loss in the primary is proportional to the square of this current, it is negligible. There is no copper loss in the secondary because there is no secondary current.  Thus, the open-circuit test is used to “DETERMINE THE IRON LOSSES” of the transformer

5. O/C Test : Open Circuit Test These values are referred to the side (usually LV) from which the test is conducted and could easily be referred to the other side if desired by the inverse square of transformer ratio. The transformer ratio if not known can be obtained by connecting a voltmeter on HV side.

6. S/C Test : Short Circuit Test  The test is conducted on the high-voltage (HV) side of the transformer where the low-voltage (LV) side or the secondary is short circuited. The supply voltage required to circulate rated current through the transformer is usually very small and is of the order of a few percent of the nominal voltage and this 5% voltage is applied across primary. The core losses are very small because applied voltage is only a few percentage of the nominal voltage and hence can be neglected. Thus the wattmeter reading measures only the full load copper loss.

7. S/C Test : Short Circuit Test These values are referred to the (HV) side from which the test is conducted. If desired, the values could be easily referred to the other side. It is observed that the Short-Circuit test has given us the equivalent resistance & reactance of the transformer, it has not yet yield any information for separating these into respective primary & secondary values.

8. Polarity Tests  Similar polarity ends of the two windings of a transformer are those ends that acquire simultaneously positive or negative polarity of emf’s induced in them. These are indicated by the dot convention.  In determining the relative polarity of the two-windings of a transformer the two windings are connected in series across the voltmeter, while one end of the winding is excited from a suitable voltage source. If the polarities of the windings are marked as that shown in the diagram, the voltmeter should read V = V 1 ~ V 2  If it reads V 1 + V 2, the polarity marking of one of the winding must be interchanged.

9. Polarity Tests  The above method of polarity testing may not be convenient in field testing of a transformer. Alternatively the polarity testing can be easily carried out by a DC battery, switch and DC voltmeter.  As the switch on the primary side is closed, the primary current increases and so do the flux linkages of both the windings including emf’s in them. The positive polarity of this include emf in the primary is at the end to which the battery positive is connected (as per lenz’s law).  The end of secondary which acquires positive polarity is the similar polarity end. The reverse happens when the switch is opened i.e. the similar polarity end of the secondary is that end which acquires negative potential.

10. Sumpner’s Test  Sumpner's test or back to back test on transformer is another method for determining transformer efficiency, voltage regulation and heating under loaded conditions. Short circuit and open circuit tests on transformer can give us parameters of equivalent circuit of transformer, but they can not help us in finding the heating information. Unlike O.C. and S.C. tests, actual loading is simulated in Sumpner's test. Thus the Sumpner's test give more accurate results of regulation and efficiency than O.C. and S.C. tests.

11. Sumpner’s Test  Sumpner's test or back to back test can be employed only when two identical transformers are available. Both transformers are connected to supply such that one transformer is loaded on another. Primaries of the two identical transformers are connected in parallel across a supply. Secondaries are connected in series such that emf's of them are opposite to each other. Another low voltage supply is connected in series with secondaries to get the readings  In above diagram, T 1 and T 2 are identical transformers. Secondaries of them are connected in voltage opposition, i.e. E EF and E GH. Both the emf's cancel each other, as transformers are identical. In this case, as per superposition theorem, no current flows through secondary. And thus the no load test is simulated. The current drawn from V 1 is 2I 0, where I 0 is equal to no load current of each transformer. Thus input power measured by wattmeter W 1 is equal to iron losses of both transformers. i.e. iron loss per transformer Pi = W 1 /2.

12. Sumpner’s Test  Now, a small voltage V 2 is injected into secondary with the help of a low voltage transformer. The voltage V 2 is adjusted so that, the rated current I 2 flows through the secondary. In this case, both primaries and secondaries carry rated current. Thus short circuit test is simulated and wattmeter W 2 shows total full load copper losses of both transformers. i.e. copper loss per transformer P Cu = W 2 /2. From above test results, the full load efficiency of each transformer can be given as:-

13. Separation of hysteresis and eddy current losses Hysteresis Losses  The magnetic core of transformer is made of ′ Cold Rolled Grain Oriented Silicon Steel ′. Steel is very good ferromagnetic material. This kind of materials are very sensitive to be magnetized. That means, whenever magnetic flux would pass through, it will behave like magnet. Ferromagnetic substances have numbers of domains in their structure. Domains are very small regions in the material structure, where all the dipoles are paralleled to same direction. In other words, the domains are like small permanent magnets situated randomly in the structure of substance. These domains are arranged inside the material structure in such a random manner, that net resultant magnetic field of the said material is zero. Whenever external magnetic field or mmf is applied to that substance, these randomly directed domains get arranged themselves in parallel to the axis of applied mmf. After removing this external mmf, maximum numbers of domains again come to random positions, but some of them still remain in their changed position. Because of these unchanged domains, the substance becomes slightly magnetized permanently. This magnetism is called " Spontaneous Magnetism". To neutralize this magnetism, some opposite mmf is required to be applied. The magneto motive force or mmf applied in the transformer core is alternating. For every cycle due to this domain reversal, there will be extra work done. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss of transformer.core of transformermagnetic fluxmagnetic field

14. Separation of hysteresis and eddy current losses Eddy Current Losses  In transformer, we supply alternating current in the primary, this alternating current produces alternating magnetizing flux in the core and as this flux links with secondary winding, there will be induced voltage in secondary, resulting current to flow through the load connected with it. Some of the alternating fluxes of transformer; may also link with other conducting parts like steel core or iron body of transformer etc. As alternating flux links with these parts of transformer, there would be a locally induced emf. Due to these emfs, there would be currents which will circulate locally at that parts of the transformer. These circulating current will not contribute in output of the transformer and dissipated as heat. This type of energy loss is called eddy current loss of transformer. This was a broad and simple explanation of eddy current loss.

15. Auto Transformers  Auto transformers are a special type, since they have no electrical isolation between the primary and secondary windings. A single continuous winding is wound on a laminated iron core, where part of the winding is used as the primary, whilst the other part is used as the secondary.  These transformers can be used to either step-up or step-down the applied voltage, depending on the winding configuration.  In a step-down device, the whole of the winding serves as the primary winding, whilst the lower half of the winding serves as the secondary winding. In this case, there are fewer turns in the secondary than in the primary: so the voltage is stepped-down, but the current is stepped-up. This configuration is typically used to power aircraft instruments where the voltage is stepped down from 115 V 400 Hz to 26 VAC.  The disadvantage of this format is that the full voltage is placed across the load if the coil goes open circuit, since there is no voltage isolation between the two windings.

16. Auto Transformers

17. Applications of Auto-Transformers  Power transmission and distribution o Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application in industry is to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100 V—130 V and 200 V—250 V). The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.powerSuper Grid o On long rural power distribution lines, special autotransformers with automatic tap- changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.voltage regulatorsvoltage drop o A special form of autotransformer called a zig-zag is used to provide grounding on three- phase systems that otherwise have no connection to ground. A zig-zag transformer provides a path for current that is common to all three phases (so-called zero sequence current).zig-zaggroundingzig-zag transformer

18. Auto Transformers Applications of Auto-Transformers  Audio o In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.impedance matching  Railways o In UK railway applications, it is common to power the trains at 25 kV AC. To increase the distance between electricity supply Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire (opposite phase) out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. At frequent (about 10 km) intervals, an autotransformer links the contact wire to rail and to the second (antiphase) supply conductor. This system increases usable transmission distance, reduces induced interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.

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