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Star-Delta Turns Ratio
Likewise, for a delta–star ( Dy ) connected transformer, with a 1:1 turns ratio, the transformer
will provide a 1:√3 step-up line-voltage ratio. Then for a delta-star connected transformer the
turns ratio becomes:
Delta-Star Turns Ratio
Then for the four basic configurations of a three-phase transformer, we can list the transformers
secondary voltages and currents with respect to the primary line voltage, VL and its primary line
current IL as shown in the following table.
Three-phase Transformer Line Voltage and Current
Primary-Secondary
Configuration
Line Voltage
Line Current
Delta – Delta
Delta – Star
Star – Delta
Star – Star
Three Phase Transformer Construction
We have said previously that the three-phase transformer is effectively three interconnected
single phase transformers on a single laminated core and considerable savings in cost, size and
weight can be achieved by combining the three windings onto a single magnetic circuit as
shown.
A three-phase transformer generally has the three magnetic circuits that are interlaced to give a
uniform distribution of the dielectric flux between the high and low voltage windings. The
exception to this rule is a three-phase shell type transformer. In the shell type of construction,
even though the three cores are together, they are non-interlaced.
Three Phase Transformer Construction
The three-limb core-type three-phase transformer is the most common method of three-phase
transformer construction allowing the phases to be magnetically linked. Flux of each limb uses
the other two limbs for its return path with the three magnetic flux’s in the core generated by the
line voltages differing in time-phase by 120 degrees. Thus the flux in the core remains nearly
sinusoidal, producing a sinusoidal secondary supply voltage.
The shell-type five-limb type three-phase transformer construction is heavier and more
expensive to build than the core-type. Five-limb cores are generally used for very large power
transformers as they can be made with reduced height. A shell-type transformers core materials,
electrical windings, steel enclosure and cooling are much the same as for the larger single-phase
types.
Example:- The primary winding of a delta-star ( Dy ) connected 50VA transformer is supplied
with a 100 volt, 50Hz three-phase supply. If the transformer has 500 turns on the primary and
100 turns on the secondary winding, calculate the secondary voltages and currents.
Given Data: transformer rating, 50VA, supply voltage, 100v, primary turns 500, secondary turns,
100.
Scott-T Connection of Transformer:
Transforming 3 Phase to 2 Phase:
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There are two main reasons for the need to transform from three phases to two phases,
1. To give a supply to an existing two phase system from a three phase supply.
2. To supply two phase furnace transformers from a three phase source.
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Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering
that a two-phase system is not 2/3 of a three-phase system. Balanced three-wire, two-phase
circuits have two phase wires, both carrying approximately the same amount of current, with a
neutral wire carrying 1.414 times the currents in the phase wires. The phase-to-neutral voltages
are 90° out of phase with each other.
Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are
electrically 90° out of phase with each other. Two phase 5-wire circuits have four phase wires
plus a neutral; the four phase wires are 90° out of phase with each other.
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The easiest way to transform three-phase voltages into two-phase voltages is with two
conventional single-phase transformers. The first transformer is connected phase-to-neutral on
the primary (three-phase) side and the second transformer is connected between the other two
phases on the primary side.
The secondary windings of the two transformers are then connected to the two-phase circuit.
The phase-to-neutral primary voltage is 90° out of phase with the phase-to-phase primary
voltage, producing a two-phase voltage across the secondary windings. This simple connection,
called the T connection, is shown in Figure
The main advantage of the T connection is that it uses transformers with standard primary and
secondary voltages. The disadvantage of the T connection is that a balanced two-phase load still
produces unbalanced three-phase currents; i.e., the phase currents in the three-phase system
do not have equal magnitudes, their phase angles are not 120° apart, and there is a considerable
amount of neutral current that must be returned to the source.
The Scott Connection of Transformer:
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A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive twophase power from a three-phase source or vice-versa. The Scott connection evenly distributes a
balanced load between the phases of the source.
Scott T Transformers require a three phase power input and provide two equal single phase
outputs called Main and Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The
MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector
current imbalance on the primary side.
MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect
the primary side of the MAIN and Teaser sections.
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The schematic of a typical Scott T Transformer is shown below:
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Scott T Transformer is built with two single phase transformers of equal power rating. The MAIN
and Teaser sections can be enclosed in a floor mount enclosure with MAIN on the bottom and
Teaser on top with a connecting jumper cable. They can also be placed side by side in separate
enclosures.
Assuming the desired voltage is the same on the two and three phase sides, the Scott-T
transformer connection consists of a center-tapped 1:1 ratio main transformer, T1, and an
86.6% (0.5√3) ratio teaser transformer, T2. The center-tapped side of T1 is connected between
two of the phases on the three-phase side. Its center tap then connects to one end of the lower
turn count side of T2, the other end connects to the remaining phase. The other side of the
transformers then connects directly to the two pairs of a two-phase four-wire system.
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The Scott-T transformer connection may be also used in a back to back T to T arrangement for a
three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to
the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to
three-coil secondary transformer. In this arrangement the Neutral tap is part way up on the
secondary teaser transformer . The voltage stability of this T to T arrangement as compared to
the traditional 3 coil primary to three-coil secondary transformer is questioned