17-09-2012, 04:54 PM
Electrical Machines
1Electrical Machines.pdf (Size: 285.8 KB / Downloads: 136)
Testing of Transformers
The structure of the circuit equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by solving that circuit for any load
conditions. The equivalent circuit parameters are available to the designer of the transformers
from the various expressions that he uses for designing the transformers. But for a user
these are not available most of the times. Also when a transformer is rewound with different
primary and secondary windings the equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily depended upon. From the analysis of
the equivalent circuit one can determine the electrical parameters. But if the temperature
rise of the transformer is required, then test method is the most dependable one. There are
several tests that can be done on the transformer; however a few common ones are discussed
here.
Winding resistance test
This is nothing but the resistance measurement of the windings by applying a small
d.c voltage to the winding and measuring the current through the same. The ratio gives
the winding resistance, more commonly feasible with high voltage windings. For low voltage
windings a resistance-bridge method can be used. From the d.c resistance one can get the
a.c. resistance by applying skin effect corrections.
Polarity Test
This is needed for identifying the primary and secondary phasor polarities. It is
a must for poly phase connections. Both a.c. and d.c methods can be used for detecting
the polarities of the induced emfs. The dot method discussed earlier is used to indicate the
polarities. The transformer is connected to a low voltage a.c. source with the connections
made as shown in the fig. 18(a). A supply voltage Vs is applied to the primary and the
readings of the voltmeters V1, V2 and V3 are noted. V1 : V2 gives the turns ratio. If V3 reads
V1−V2 then assumed dot locations are correct (for the connection shown). The beginning and
end of the primary and secondary may then be marked by A1 −A2 and a1 −a2 respectively.
If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 to a2 in
the secondary. If more secondary terminals are present due to taps taken from the windings
they can be labeled as a3, a4, a5, a6. It is the voltage rising from smaller number towards
larger ones in each winding.
1Electrical Machines.pdf (Size: 285.8 KB / Downloads: 136)
Testing of Transformers
The structure of the circuit equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by solving that circuit for any load
conditions. The equivalent circuit parameters are available to the designer of the transformers
from the various expressions that he uses for designing the transformers. But for a user
these are not available most of the times. Also when a transformer is rewound with different
primary and secondary windings the equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily depended upon. From the analysis of
the equivalent circuit one can determine the electrical parameters. But if the temperature
rise of the transformer is required, then test method is the most dependable one. There are
several tests that can be done on the transformer; however a few common ones are discussed
here.
Winding resistance test
This is nothing but the resistance measurement of the windings by applying a small
d.c voltage to the winding and measuring the current through the same. The ratio gives
the winding resistance, more commonly feasible with high voltage windings. For low voltage
windings a resistance-bridge method can be used. From the d.c resistance one can get the
a.c. resistance by applying skin effect corrections.
Polarity Test
This is needed for identifying the primary and secondary phasor polarities. It is
a must for poly phase connections. Both a.c. and d.c methods can be used for detecting
the polarities of the induced emfs. The dot method discussed earlier is used to indicate the
polarities. The transformer is connected to a low voltage a.c. source with the connections
made as shown in the fig. 18(a). A supply voltage Vs is applied to the primary and the
readings of the voltmeters V1, V2 and V3 are noted. V1 : V2 gives the turns ratio. If V3 reads
V1−V2 then assumed dot locations are correct (for the connection shown). The beginning and
end of the primary and secondary may then be marked by A1 −A2 and a1 −a2 respectively.
If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 to a2 in
the secondary. If more secondary terminals are present due to taps taken from the windings
they can be labeled as a3, a4, a5, a6. It is the voltage rising from smaller number towards
larger ones in each winding.