Typically the instrumentation will measure the rms magnitude of the applied voltage
, the short-circuit current
, and the power
. The circuit parameters (referred to the primary) can be found as (2.29)-(2.31).
(2.29)
(2.30)
(2.31)
The equivalent impedance can be referred from one side to the other.
Approximate values of the individual primary and secondary resistances and leakage reactances can be obtained by assuming that
and
when all impedances are referred to the same side.
Note that it is possible to measure
and
directly by a dc resistance measurement on each winding. However, no such simple test exists for
and
.
Open-Circuit Test
The test is used to find the equivalent shunt impedance
.
The test is performed with the secondary open-circuited and rated voltage impressed on the primary. If the transformer is to be used at other than its rated voltage, the test should be done at that voltage.
An exciting current of a few percent of full-load current is obtained.
See Fig. 2.16. Note that
.
Figure 2.13 Equivalent circuit with open-circuited secondary. (a) Complete equivalent circuit.(b) Cantilever equivalent circuit with the exciting branch at the transformer primary.
(2.32)
(2.33)
Typically the instrumentation will measure the rms magnitude of the applied voltage
, the open-circuit current
, and the power
. The circuit parameters (referred to the primary) can be found as (2.34)-(2.36).
(2.34)
(2.35)
(2.36)
The open-circuit test can be used to obtain the core loss for efficiency computations and to check the magnitude of the exciting current.
Note the term “Voltage Regulation” which is to be discussed in Example 2.6.
§2.6 Autotransformers; Multiwinding Transformers
Two-winding Other winding configurations.
§2.6.1 Autotransformers
Autotransformer connection: Fig. 2.14.
Figure 2.14 (a) Two-winding transformer. (b) Connection as an autotransformer.
The windings of the two-winding transformer are electrically isolated whereas those of the autotransformer are connected directly together.
In the transformer connection, winding ab must be provided with extra insulation.
Autotransformer have lower leakage reactances, lower losses, and smaller exciting current and cost less than two-winding transformers when the voltage ration does not differ too greatly from 1:1.
The rated voltages of the transformer can be expressed in terms of those of the two-winding transformer as
(2.37)
(2.38)
The effective turns ratio of the autotransformer is thus
.
The power rating of the autotransformer is equal to
times that of the two winding transformer.
§2.6.2 Multiwinding Transformers
Transformers having three or more windings, known as multiwinding or multicircuit transformers, are often used to interconnect three or more circuits which may have different voltages.