Download Eddy Current Losses

SARVAJANIK COLLEGE OF ENGINEERNG
AND TECHNOLOGY , SURAT
2nd YEAR
ELECTRICAL ENGINEERING DEPARTMENT
EVENING SHIFT
DC Machine & Transformers
Rangat Mehta
Unnati Natali
Nishant Shah
Kumar Pawar
Yash Lakdawala
130420109518
130420109523
130420109525
130420109538
130420109558
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
O/C Test : Open Circuit Test
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
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.
O/C Test : Open Circuit Test
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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
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.
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.
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.
Polarity Tests
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
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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 = V1 ~ V2
If it reads V1 + V2 , the polarity marking of one of the winding must be
interchanged.
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.
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.
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, T1 and T2 are identical transformers. Secondaries of them are
connected in voltage opposition, i.e. EEF and EGH. 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 V1 is 2I0, where I0 is equal to no load current of each transformer. Thus input
power measured by wattmeter W1 is equal to iron losses of both transformers.
i.e. iron loss per transformer Pi = W1/2.
Sumpner’s Test

Now, a small voltage V2 is injected into secondary with the help of a low voltage
transformer. The voltage V2 is adjusted so that, the rated current I2 flows through the
secondary. In this case, both primaries and secondaries carry rated current. Thus
short circuit test is simulated and wattmeter W2 shows total full load copper losses
of both transformers.
i.e. copper loss per transformer PCu = W2/2.
From above test results, the full load efficiency of each transformer can be given
as:-
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.
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.
Auto Transformers
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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.
Auto Transformers
Auto Transformers
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.
o
On long rural power distribution lines, special autotransformers with automatic tapchanging 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.
o
A special form of autotransformer called a zig-zag is used to provide grounding on threephase 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).
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.
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.
Thanking You !