3 Phase
Power Is More Efficient Than Single Phase
Three phase electricity powers large industrial loads more
efficiently than single-phase electricity. When
single-phase electricity is needed, It is available
between any two phases of a three-phase system, or in
some systems , between one of the phases and ground.
By the use of three conductors a 3 phase system can
provide 173% more power than the two conductors of a
single-phase system. Three-phase power allows heavy
duty industrial equipment to operate more smoothly and
efficiently. 3 phase power can be transmitted over
long distances with smaller conductor size.
Also read
about 3
phase isolation transformers here. For an
excellent source for these all transformer types check
out TEMCo 3
phase transformers. Or check with Isolation
Transformer Sales for 3
phase isolation transformers. These two
companies manufacture some of the most recognized high
quality 3 phase transformers available today.
In a three-phase transformer, there is a three-legged iron
core as shown below. Each leg has a respective primary
and secondary winding.
The three primary windings (P1, P2, P3) will be connected
at the factory to provide the proper sequence
(or correct polarity) required and will be in a
configuration known as Delta. The three
secondary windings (S1, S2, S3) will also be
connected at the factory to provide the proper
sequence (or correct polarity) required.
However, the secondary windings, depending on
our voltage requirements, will be in either
?Delta? or a ?Wye? configuration.
3
Phase Transformer Delta and Wye Connections
In a 3 phase transformer, there is a three-legged iron core
as shown below. Each leg has a respective
primary and secondary winding.
3
Phase Transformer Winding Combination
As can be seen, the three-phase transformer actually has 6
windings (or coils) 3 primary and 3 secondary.
These 6 windings will be pre-connected at the
factory in one of two configurations:
Configuration 1. Three primary Windings in Delta and Three
Secondary Windings in Wye
Note: These are the designations which are marked on the
leads or terminal boards provided for customer
connections and they will be located in the
transformer wiring compartment.
In both single and 3 phase transformers, the
high voltage terminals are designated with an
h and the low voltage with an X
Configuration 2. Three Primary Windings in Delta and Three
Secondary Windings in Delta
Note: These are the designations which are marked on the
leads or terminal boards provided for the
customer connections and they will be located in
the transforming wiring compartment.
In both single and three-phase transformers, the
high voltage terminals are designated with an
H and the low voltage with an X.
3
Phase Transformer Voltage in Delta and Wye Connections
Different brands of 3 phase
transformers handle the windings in different manners.
All Federal Pacific 3 phase transformers have their primary
windings pre-connected in a Delta configuration.
Therefore, when connected to a three-phase
source, each primary winding will have the same
voltage across it.
For Example: 480V 3 Phase Source
If the secondary windings are also connected Delta then
they have equal voltages across each winding. Of
course, this voltage will be either higher or
lower than the primary depending upon the
turns ratio.
480V Primary Source with 240V Secondary Output @ 2/1 Turns
Ratio (Delta-Delta)
Note: it is important to note that three-phase
transformers with Delta-connected primaries when
connected to a 30, 4-wire supply system do not
utilize the 4th wire or neutral.
Wye: If the secondary is not connected in Delta it will be
pre-connected at the factory as a Wye secondary.
All Wye connections provide two voltages due to
the common point or neutral connection. A
typical rating would be 208/120V. The 208Y
indicates the voltage between phases of the
secondary windings.
For Example:
The 120 volt portion indicates the voltage from each phase
to the common point or neutral
For Example:
This Phase-to-Neutral voltage in a Wye is always equal to
the Phase-to-Phase voltage divided by
For Example:
Therefore a 3 phase transformer with its secondary
connected in a Wye configuration for 208Y/120
volts will have the available: Common
Three-Phased Transformer Voltage Combinations
Special Three Phase Delta Connected Transformers
There are certain situations where only a very small
portion of a building loads require 120V
single-phase . A special
power transformer is
available and you should be familiar with it.
The 240 Volt 30 Delta Connected Secondary With 120 Volt 10
Lighting Tap
As you can see there is no point in a Delta at which an
equal potential to all three lines and the
grounded neutral can be made. This is a
disadvantage of a Delta compared to a Wye
secondary connection
This Delta secondary connection has only one
winding (S3) with a neutral conductor. The
mid-point of winding S3 is tapped which gives
the XI and X3 to neutral a voltage reading of
120 volts. In a 3-phase system, winding S3
is the workhorse; it has to carry all the 120V
lighting and appliance loads plus one-third of
all the 3 phase loads. (The 120V loads must not
exceed 5% of the nameplate KVA, and the total of
the nameplate KVA must be derated by 30%).
Winding S1 and S2 cannot carry any 120 volt
loads as there is no neutral connection to these windings. Windings S1 and S2 can only carry
one-third of the three-phase loads each, and the
240 volt single-phase loads.
*Caution: A240 volt Delta connected
transformer with a 120 volt neutral tap creates
a condition called high leg As indicated
in the above diagram, the voltage between Phase
B (X2) and the neutral tap will be 208 volts;
therefore, no 120 volt single-phase loads can be
connected between X2 and the neutral tap.
Single Phase Transformers Connected to Form Three Phase
Bank
Normally , when 3 phase is required, a single enclosure
with three primary and three secondary windings
wound on a common core is all that is required.
However three single-phase transformers with the
same rating can be connected to form a
three-phase bank. Since each single-phase
transformer has a primary and a secondary
winding, then 3 single-phase transformers will
have the required 3 primary and 3 secondary
windings and can be connected in the field
either Delta-Delta or Delta-Wye to achieve the
required 3 phased transformer bank, as shown
below.
3
Phase Transformer: Delta-Delta
Utilizing 3 single-phase transformers is normally not done
because it is more expensive than utilizing 1
three-phase transformer. However, there is an
advantage which is called the open Delta or
V-Connection and it functions as follows: A
defective single-phase transformer in a
Delta-Delta 3 phase bank can be disconnected and
removed for repair. Partial service can be
restored using the remaining single-phase
transformer open-Delta until a replacement
transformer is obtained. With two transformers
three-phase is still obtained, but at reduced
power. 57.7 of original power. This makes it a
very practical transformer application for
temporary emergency conditions
Open Delta 57.7%
3
Phase Loads and Single Phase Loads
If the load is 3 phase, then both the supply and the
transformer must be in three-phase. If the load
is single-phase the supply can either be single
or 3 phase but the transformer need only be
single-phase with the primary being connected to
two lines on the three phase circuit. With
single-phase loads, an attempt to use a
transformer with three-phase input and only one
phase connected at the output to convert the
loading on the line to 3 phase is not practical.
3
Phase Transformer Sizing with 3 Phase Loads
1) Determine electrical load
A. Voltage required by load.
B. Amperes or KVA required by load.
C. Frequency in Hz (cycles per second).
D. Verify load is designed to operate on three phase.
All the above information is standard data normally obtained
from equipment nameplates or instruction manuals.
2) Determine supply voltage
A. Voltage of supply (source).
B. Frequency in Hz (cycles per second).
The frequency of the line supply and electrical load must be
the same. A 3 phase transformer is selected which
is designed to operate at this frequency having a
primary (input) equal to the supply voltage and a
secondary (output) equal to the voltage required by
the load.
3) If the load nameplate expresses a rating in KVA, a
transformer can be directly selected from the charts
in the catalog. Choose from the group of transformers
with primary and secondary voltages matching that
which you have just determined.
A. Select a 3 phase transformer with a standard KVA capacity equal to
or greater than that needed to operate the load.
B. Primary taps are available on most models to compensate
for line voltage variations. (Refer to question #2 in
the Transformer Question and Answer Section of Acme's
marketplace.
C. When load ratings are given only in amperes, the following
formulas below may be used to determine proper KVA
size for the required transformer.
(1) To determine three phase KVA when volts and amps are
known:
Three Phase KVA =Volts x Amps x 1.73 /1000
(2) To determine Amperes when KVA and volts are known: Amps =
3 Phase KVA x 1000 /Volts x 1.73
Three Phase Example
Question:
Select a transformer to fulfill the following conditions.
Load is a three phase induction motor, 25 horsepower @
240 volts, 60 Hz and a heater load of 4 kilowatts @
240 volts single phase. The supply voltage is
480Y/277, three phase, 4 wire.
Answer: Compute the KVA required.
28.2 KVA =240 volts x 68 amps x 1.73 /1000
Heater - 4 KVA
A three phase transformer must be selected so that any one
phase is not overloaded. Each phase should have the
additional 4 KVA rating required by the heater even
though the heater will operate on one phase only. So,
the transformer should have a minimum KVA rating of
28.2 - 4 + 4 + 4 or 40.2 KVA.
A 480 delta primary - 240 delta secondary transformer may be
used on a 4 wire, 480Y/277 volt supply. The fourth
wire (neutral) is not Connected to the transformer. To
not overload the transformer, a 45 KVA transformer
should be selected.
NOTE: Any two wires of the 240 volts, 3 phase developed by
the secondary of the transformer may be used to supply
the heater. Any 2 wires of a 3 phase system is single
phase.
Three Phase Transformers Overview
Three phase transformers are used throughout industry to
change values of three phase voltage and current.
Since three phase power is the most common way in
which power is produced, transmitted, an used, an
understanding of how three phase transformer
connections are made is essential. In this section it
will discuss different types of three phase
transformers connections, and present examples of how
values of voltage and current for these connections
are computed.
3
Phase Transformer Construction:
A three phase transformer is constructed by winding three
single phase transformers on a single core. These
transformers are put into an enclosure which is then
filled with dielectric oil. The dielectric oil
performs several functions. Since it is a dielectric,
a nonconductor of electricity, it provides electrical
insulation between the windings and the case. It is
also used to help provide cooling and to prevent the
formation of moisture, which can deteriorate the
winding insulation.
3-Phase Transformer Connections:
There are only 4 possible transformer combinations:
Delta to Delta - use: industrial applications
Delta to Wye - use : most common; commercial and industrial
Wye to Delta - use : high voltage transmissions
Wye to Wye - use : rare, don't use causes harmonics and
balancing problems.
3
phase transformers are connected in delta or
wye configurations. A wye-delta
transformer has its primary winding connected in a wye
and its secondary winding connected in a
delta (see figure 1-1). A delta-wye
transformer has its primary winding
connected in delta and its
secondary winding connected in a wye
(see figure 1-2).
Delta Connections:
A delta system is a good short-distance distribution system.
It is used for neighborhood and small commercial loads
close to the supplying substation. Only one voltage is
available between any two wires in a delta system. The
delta system can be illustrated by a simple triangle.
A wire from each point of the triangle would represent
a three-phase, three-wire delta system. The voltage
would be the same between any two wires (see figure
1-3).
Wye Connections:
In a wye system the voltage between any two wires will always
give the same amount of voltage on a three phase
system. However, the voltage between any one of the
phase conductors (X1, X2, X3) and the neutral (X0)
will be less than the power conductors. For example,
if the voltage between the power conductors of any two
phases of a three wire system is 208v, then the
voltage from any phase conductor to ground will be
120v. This is due to the square root of three phase
power. In a wye system, the voltage between any two
power conductors will always be 1.732 (which is the
square root of 3) times the voltage between the
neutral and any one of the power phase conductors. The
phase-to-ground voltage can be found by dividing the
phase-to-phase voltage by 1.732 (see figure 1-4).
Connecting Single-Phase Transformers into a 3 phase Bank:
If three phase transformation is need and a three phase
transformer of the proper size and turns ratio is not
available, three single phase transformers can be
connected to form a three phase bank. When three
single phase transformers are used to make a three
phase transformer bank, their primary and secondary
windings are connected in a wye or delta connection.
The three transformer windings in figure 1-5 are
labeled H1 and the other end is labeled H2. One end of
each secondary lead is labeled X1 and the other end is
labeled X2.
Figure 1-6 shows three single phase
transformers labeled A, B, and C. The primary leads of
each transformer are labeled H1 and H2 and the
secondary leads are labeled X1 and X2. The schematic
diagram of figure 1-5 will be used to connect
the three single phase transformers into a three phase
wye-delta connection as shown in figure 1-7.
The primary winding will be tied into a wye connection first.
The schematic in figure 1-5 shows, that the
H2 leads of the three primary windings are connected
together, and the H1 lead of each winding is open for
connection to the incoming power line. Notice in figure
1-7 that the H2 leads of the primary windings are
connected together, and the H1 lead of each winding
has been connected to the incoming primary power line.
Figure 1-5 shows that the X1 lead of the
transformer A is connected to the X2 lead of
transformer c. Notice that this same connection has
been made in figure 1-7. The X1 lead of
transformer B is connected to X1, lead of transformer
A, and the X1 lead of transformer B is connected to X2
lead of transformer A, and the X1 lead of transformer
C is connected to X2 lead of transformer B. The load
is connected to the points of the delta connection.
Open Delta Connection:
The open delta transformer connection can be
made with only two transformers instead of three (figure
1-8). This connection is often used when the
amount of three phase power needed is not excessive,
such as a small business. It should be noted that the
output power of an open delta connection is only 87%
of the rated power of the two transformers. For
example, assume two transformers, each having a
capacity of 25 kVA, are connected in an open delta
connection. The total output power of this connection
is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).
Another figure given for this calculation is 58%. This
percentage assumes a closed delta bank containing 3
transformers. If three 25 kVA transformers were
connected to form a closed delta connection, the total
output would be 75 kVA (3 x 25 = 75 kVA). If one of
these transformers were removed and the transformer
bank operated as an open delta connection, the output
power would be reduced to 58% of its original capacity
of 75 kVA. The output capacity of the open delta bank
is 43.5 kVA (75 kVA x .58% = 43.5 kVA).
The voltage and current values of an open delta connection
are computed in the same manner as a standard
delta-delta connection when three transformers are
employed. The voltage and current rules for a delta
connection must be used when determining line and
phase values of voltage current.
Closing a Delta:
When closing a delta system, connections should be checked
for proper polarity before making the final connection
and applying power. If the phase winding of one
transformer is reversed, an extremely high current
will flow when power is applied. Proper phasing can be
checked with a voltmeter at delta opening. If power is
applied to the transformer bank before the delta
connection is closed, the voltmeter should indicate 0
volts. If one phase winding has been reversed,
however, the voltmeter will indicate double the amount
of voltage.
It should be noted that a voltmeter is a high impedance
device. It is not unusual for a voltmeter to indicate
some amount of voltage before the delta is closed,
especially if the primary has been connected as a wye
and the secondary as a delta. When this is the case,
the voltmeter will generally indicate close to the
normal output voltage if the connection is correct and
double the output voltage if the connection is
incorrect.
Overcurrent Protection for the Primary:
Electrical Code Article 450-3(b) states that each transformer
600 volts, nominal or less, shall be protected by an
individual overcurrent device on the primary side,
rated or set at not more than 125% of the rated
primary current of the transformer. Where the primary
current of a transformer is 9
amps or more and 125% of
this current does not correspond to a standard rating
of a fuse or nonadjustable circuit breaker, the next
higher standard rating shall be permitted. Where the
primary current is less than 9 amps,
an overcurrent device rated or set at not more than
167% of the primary current shall be permitted. Where
the primary current is less than 2 amps,
an overcurrent device rated or set at not more than
300% shall be permitted.
Example #1:
What size fuses is needed on the primary side to protect a 3
phase 480v to 208v 112.5 kVA transformer?
* Important when dealing with 3 phase applications always use
1.732 (square root of 3).
To solve: P / I x E
112.5 kVA X 1000 = 112500 VA
112500 VA divided by 831 (480 x 1.732) = 135.4 amps
Since the transformer is more than 9 amps you have to use 125
%.
135.4 X 1.25 = 169 amps.
Answer: 175 amp fuses (the next higher standard, Electrical
Code 240-6).
Example #2:
What size breaker is needed on the primary side to protect a
3 phase 208v to 480v 3kVA transformer?
To solve: P / I x E
3kVA X 1000 = 3000 VA
3000 VA divided by 360 (208 x 1.732) = 8.3 amps
Since the transformer is 9 amps or less you have to use 167%.
8.3 X 1.67 = 13.8 amps
Answer: 15 amp breaker (preferably a 20 amp breaker)
Electrical Code Article 450-3(b)(2) states if a transformer
600 v, nominal, or less, having a an overcurrent
device on the secondary side rated or set at not more
than 125% of the rated secondary current of the
transformer shall not be required to have an
individual overcurrent device on the primary side if
the primary feeder overcurrent device is rated or set
at a current value not more than 250% of the rated
primary current of the transformer.
Overcurrent Protection for the Secondary:
Electrical Code Article 450-3(b)(2) states that a transformer
600 v, nominal, or less, shall be protected by an
individual overcurrent device on the secondary side,
rated or set at not more than 125% of the rated
secondary current of the transformer. Where the
secondary current of a transformer is 9
amps or more and 125% of
this current does not correspond to a standard rating
of a fuse or nonadjustable circuit breaker, the next
higher standard rating shall be permitted. Where the
secondary current is less than 9 amps,
an overcurrent device rated or set at not more than
167% of the secondary current shall be permitted.
Example:
What size breaker is needed on the secondary side to protect
a 3 phase 480v/208v 112.5 kVA transformer?
To solve : P / I x E
112.5 kVA x 1000 = 112500 VA
112500 divided by 360 (208 x 1.732) = 312.5 amps
312.5 X 1.25 = 390.6 amps
Answer: 400 amp breaker
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3 Phase Contents
