THE 3 PHASE POWER
RESOURCE SITE

3 Phase Power Grain Dryer
3 phase power is used today for many applications ranging from the above grain dryer to saw mills, conveyer belt systems, refrigeration and air conditioning, etc.

3 Phase Contents
- 3 Phase Power Home
- 3 Phase Wiring
- 3 Phase Power Calculation
- 3 Phase Power Converter
- 3 Phase Converters
  * Rotary Phase Converters
  * Static Phase Converters
- 3 Phase Generators
- 3 Phase Transformers
  * 3 Phase Isolation Transformers
- 3 Phase Motors
- 3 Phase Electricity
- 3 Phase Power Loads
- 3 Phase Frequency
  * Frequency Converters
- 3 Phase Distribution
- 2 Phase Systems
- 1 Phase Loads on 3 Phase Power
- 3 Phase Loads on 1 Phase Power


3 phase power three phase power three phase electricity three phase generator 3 phase converter, rotary three phase

 

3 Phase Transformers

To Buy 3 Phase Transformers
We Recommend TEMCo Brand.
Call 1-510-490-2187
Web Link:  3 Phase Transformers

3 Phase Electrical Power Transformer

A 3 phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondary winding.

Most power is distributed in the form of three-phase AC. Therefore, before proceeding any further you should understand what is meant by 3 phase power. Basically, the power company generators produce electricity by rotating (3) coils or windings through a magnetic field within the generator . These coils or windings are spaced 120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three (3) lines as in three-phase power. 3 phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage we need and maintain the proper phasing or polarity.

Industrial 3 Phase Transformer 

3 Phase Pole Transformer

 

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.

Three Phase Transformer Windings

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 Transformers

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:  

Three Phase Transformers
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

For additional transformer Information see these resources from TEMCo Transformer:

Transformer Selection Guide • Custom Transformers • Dry-Type Transformers • Auto Transformers • Control Transformers • Step-Up Transformers • Step-Down Transformers • Harmonic Cancellation • Isolation Transformers • K-Factor Rated Transformers • 3-Phase Transformers • European Voltage Transformers • Drive Isolation Transformers • High Voltage Transformers • Epoxy Encapsulated NEMA 4 Transformers • Weatherproof NEMA 3R Transformers • Transformers • AC Transformer • Voltage Transformer • Buy Transformer • New Transformer • Output Transformer • Transformer KVA • Converter Transformer • Line Transformer • Toroidal Transformer • Oil Filled Transformers • Voltage Regulator • Automatic Voltage Regulator • DC Power Supply • PDU • Rack PDU • Power Conditioner • Power Line Conditioner • Load Center • Switchgear • Voltage Converter • Transformer Wiring • Transformer Circuit • Variable Transformer • Pole Transformer • Transformer Pad • VA Transformer • WYE Transformer • Potential Transformer • Transformer Protection • Variac • Transformer Rating • Sunbelt Transformer • Pacific Transformer • Jefferson Transformer • Electric Transformer • Power Transformer • Transformer Sizing • Transformer Rectifier • Center Tap Transformer • Power Distribution • Industrial Transformer • Replacement Transformer • Insulation Transformer • Micron Transformers • Westinghouse Transformer • Power Supply Transformer • Instrument Transformer • Pulse Transformer • Substation Transformer • Furnace Transformer • Pad Mounted Transformers • Transformer Manufacturer • Distribution Transformers • GE Transformers • Step Up Transformers • Step Down Transformers • Buck Boost Transformers • High Voltage Transformers • Isolation Transformer • Single Phase Transformer • Hammond Transformers • Buck Boost Transformers

Continue And Read About Additional 3 Phase Power Details:

- 3 Phase Power Home
- 3 Phase Wiring
- 3 Phase Power Calculation
- 3 Phase Electricity
- 3 Phase Power Loads
- 3 Phase Power Converter
- 3 Phase Converters
  * Rotary Phase Converters
  * Static Phase Converters
- 3 Phase Generators
- 3 Phase Transformers
  * 3 Phase Isolation Transformers
- 3 Phase Motors
- 3 Phase Frequency
  * Frequency Converters
- 3 Phase Distribution
- 2 Phase Systems
- 1 Phase Loads on 3 Phase Power
- 3 Phase Loads on 1 Phase Power

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Smooth 3 Phase Power Wave Form (center)
Because of its smooth wave length properties, 3 phase electrical power is used across the U.S. and throughout the world.



3 Phase Transformer

3 Phase Substation Transformer
(above)
There are a variety of 3 phase transformer types and voltages, depending on your power needs.


3 Phase Power Saw

3 Phase Power Saw (above)
There are a whole variety of power equipment and power tools which require 3 phase power to run.


TEMCo 3 Phase Converter

3 Phase Converter
(above)
Some Types of Rotary Phase Converters will run on 1 phase power and generate 3 phase power to run other 3 phase equipment.


 
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