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Federal Pacific Transformer University

Learn the basics of transformer design, construction, application and operation from the Federal Pacific Transformer University.
An optional quiz at the end can test your knowledge of the subject matter presented here. Go directly to a chapter by clicking the chapter link below or scroll down so view all the chapters in order.

Chapter 1 – Why We Use Transformers

Chapter 2 – How Transformers Operate

Chapter 3 – Three-Phase Transformers

Chapter 4 – Typical Transformer Construction

Chapter 5 – Transformer Design Considerations

Chapter 6 – Transformer Applications

Chapter 7 – Special Applications

Take the Federal Pacific Transformer University Knowledge Quiz



Chapter 1 – Why We Use Transformers

Due to the high cost of transmitting electricity at low voltage and high current levels, transformers fulfill a most important role in electrical distribution systems. Utilities distribute electricity over large areas using high voltages, commonly called transmission voltages. Transmission voltages are normally in the 35,000 volt to 50,000 volt range. We know that volts times amps equals watts, and that wires are sized based upon their ability to carry amps. High voltage allows the utility to use small sizes of wire to transmit high levels of power, or watts. You can recognize transmission lines because they are supported by very large steel towers that you see around utility power plants and substations. As this electricity gets closer to its point of use it is converted, through the use of transformers, to a lower voltage normally called distribution voltage. Distribution voltages range from 2,400 to 25,000 volts depending upon the utility. Distribution lines are the ones that feed the pole mount and pad mount transformers located closest to your home or place of business. These transformers convert the distribution voltages to what we call utilization voltages. They are normally below 600 volts and are either single-phase or three-phase and are utilized for operating equipment, including light bulbs and vacuum cleaners in our homes, to motors and elevators where we work. This is the point at which the Dry-Type Distribution Transformer comes into play. It is used to convert the voltage provided by the utility to the voltage we need to operate various equipment.

Chapter 2 – How Transformers Operate (Voltage Transformers)

A Transformer does not generate electrical power, it transfers electrical power. A transformer is a voltage changer. Most transformers are designed to either step voltage up or to step it down, although some are used only to isolate one voltage from another. The transformer works on the principle that energy can be efficiently transferred by magnetic induction from one winding to another winding by a varying magnetic field produced by alternating current . An electrical voltage is induced when there is a relative motion between a wire and a magnetic field. Alternating current (AC) provides the motion required by changing direction which creates a collapsing and expanding magnetic field.

*NOTE:* Direct current (DC) is not transformed, as DC does not vary its magnetic fields.

A transformer usually consists of two insulated windings on a common iron (steel) core: The two windings are linked together with a magnetic circuit which must be common to both windings. The link connecting the two windings in the magnetic circuit is the iron core on which both windings are wound. Iron is an extremely good conductor for magnetic fields. The core is not a solid bar of steel, but is constructed of many layers of thin steel called laminations. One of the windings is designated as the primary and the other winding as the secondary. Since the primary and secondary are wound the on the same iron core, when the primary winding is energized by an AC source, an alternating magnetic field called flux is established in the transformer core. The flux created by the applied voltage on the primary winding induces a voltage on the secondary winding. The primary winding receives the energy and is called the input. The secondary winding is discharges the energy and is called the output.

The primary and secondary windings consist of aluminum or copper conductors wound in coils around an iron core and the number of “turns” in each coil will determine the voltage transformation of the transformer. Each turn of wire in the primary winding has an equal share of the primary voltage. The same is induced in each turn of the secondary. Therefore, any difference in the number of turns in the secondary as compared to the primary will produce a voltage change.


Step-Down Transformers

If there are fewer turns in the secondary winding than in the primary winding, the secondary voltage will be lower than the primary.

Step-Up Transformers

If there are fewer turns in the primary winding than in the secondary winding, the secondary voltage will be higher than the secondary circuit.

NOTE: The primary winding is the winding which receives the energy; it is not always the high-voltage winding.

Common Single-Phase Voltage Combinations

120 x 240 to 120/240; 480 to 120/240; 4160 to 240/480
208 to 120/240; 480 to 120/240; 4160 to 240/480
277 to 120/240; 2400 to 120/240
240 x 480 to 120/240; 2400 to 240/480

The relationship between the number of turns in the secondary and primary is often call the turns ratio (also referred to as the voltage ratio.) It is customary to specify the turns ratio by writing the primary (input) number first.

Example: 30 to 1 is a step-down transformer, whereas a 1 to 30 would be a step-up transformer.

Examples of Turn Ratios:


Primary Voltage Secondary Voltage Turns Ratio   Primary Voltage Secondary Voltage Turns Ratio
480 480 1/1   600 120 5/1
480 120 4/1   600 208 2.88/1
480 24 20/1   208 120 1.73/1


Winding Physical Location:
In most transformers the high voltage winding is wound directly over the low voltage winding to create efficient coupling of the two windings.

Note: Other designs may have the high voltage winding wound inside, side-by-side or sandwiched between layers of the low voltage winding to meet special requirements.


As stated previously, the voltage transformation is a function of the turns ratio. It may be desirable to change the ratio in order to get rated output voltage when the incoming voltage is slightly different than the normal voltage. As an example, suppose we have a transformer with a 4 to 1 turns ratio. With 480 volts input, the output would be 120 volts.

The transformer at right has a tap (2) 2-1/2% below normal and one at 5% below normal, it is said to have (2) 2-1/2% full capacity below normal taps (FCBN.) This would give a 5% voltage range. When the transformer has taps above normal as shown, they would be full capacity above normal (FCAN.) For standardization purposes, these taps are in 2-1/2% or 5% steps. The taps are so designed that full capacity output can be obtained when the transformer is set on any of these taps. The universal tap arrangement used on many of our transformers ((2) 2-1/2% FCAN and (4) 2-1/2% FCBN) provides a 15% range of tap voltage adjustments.

Note: Taps are only to be used for steady-state input line variations. They are not designed to provide a constant secondary voltage when the input line is constantly fluctuating.

Series-Multiple Windings (Re-Connectable Transformers)

To make the basic single-phase transformer move versatile, both the primary and secondary windings can be made in two equal parts. The two parts can be reconnected either in a series or in parallel . This provides added versatility as the primary winding can be connected for either 480 volts or 240 volts and the secondary winding can likewise be divided into two equal parts providing either 120 or 240 volts. (note: there will be four leads per winding brought out to the terminal compartment rather than two). Either arrangement will not affect the capacity of the transformer. Secondary windings are rated with a slant such as 120/240 and can be connected in a series for 240V or in a parallel for 120V or 240/120V (for 3-wire operation). Primary windings rated with an X such as 240X480 can operate in series or parallel but are not designed for 3-wire operation. A transformer rated 240X480V primary, 120/240V secondary could be operated in 6 different voltage combinations.

Transformers are designed and cataloged by kVA ratings. Just as horsepower ratings designate the power capacity of an electric motor, a transformer’s kVA rating indicates its maximum power output capacity. The higher the transformers kVA rating for a specific input and output voltage, the larger transformer.

What does kVA mean? K= Abbreviation of the Greek word kilo, meaning ‘times 1000 V= Volts A= Amperes or Amp.

Calculating kVA. There are only two formulas which you will need to know in order to calculate kVA:


Thus, you must select a transformer with a capacity greater than 36.026 kVA.

Alternate Choice: Use the FPT full load current rating chart and the secondary voltage in the FPT catalog to establish the correct kVA.

Connection: Primary in Series, Secondary in series. Connection: Primary in Series, Secondary in series.

Chapter 3 – Three Phase Transformers

Up to this point, we have focused primarily upon single-phase transformers. Single-phase meaning (2) power lines as an input source; therefore, only (1) primary and (1)secondary winding is required to accomplish the voltage transformation. However, most power is distributed in the form of three-phase A.C. Therefore, before proceeding any further you should understand what is meant by three-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. Three-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.

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 three-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. Three-phase power can be transmitted over long distances with smaller conductor size.

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

Delta and Wye Connections

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

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 three-phase transformers, the high voltage terminals are designated with an “H” and the low voltage with an “X.”


Voltage in Delta and Wye Connections

All Federal Pacific three-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 Three-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 3-phase, 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.

This Phase-to-Neutral voltage in a Wye is always equal to the Phase-to-Phase voltage divided by For Example:

Therefore a three-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 transformer is available and you should be familiar with it.

The 240 Volt 3 Phase 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 three=phased system, winding S3 is the workhorse; it has to carry all the 120V lighting and appliance loads plus one-third of all the three-phased 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 three-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 three-phased transformer bank, as shown below.


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 three-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.

Three Phase Loads and Single Phase Loads

If the load is three-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 three-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 three-phase is not practical.


Chapter 4 – Typical Transformer Construction

Basic Transformer Components

Typical Ventilated Transformer

Typical Encapsulated Non-Ventilated Transformer Cut-Away

FH Motor Drive Federal Pacific Bristol Virginia


Dry-type transformers are enclosed in heavy gauge steel which is degreased, cleaned, phosphatized, primed and finished with ANSI-61 weather -resistant power coat paint. Three types of enclosures are available:

Ventilated transformer enclosures are designed to allow air to flow over the core and coil to assist cooling.

NEMA 3R Ventilated, Outdoor
Outdoor weatherproof operation which meets NEMA 3R enclosure requirements is accomplished by the addition of UL® approved weather shields.


FH Motor Drive Federal Pacific Bristol Virginia

NEMA 1 Ventilated, Indoor

Encapsulated transformers are totally enclosed, non-ventilated and are suitable for indoor or outdoor applications. Construction consists of a core and coil assembly completely encapsulated in a mixture of epoxy resin and sand to provide a rock-hard, durable, air tight, shock-free seal.

NOTE: All enclosures are provided with wiring components where the transformer leads are brought out for ease of customer cable connections.

Buck Boost Federal Pacific Bristol Virginia

Chapter 5 – Design Considerations



The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.


The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. Impedance is expressed as a percent. This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited. The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by the multiplying the reciprocal of the impedance timed the full load current . Thus, if a transformer has 5% impedance, the reciprocal of .05 is 20 and maximum short circuit current is 20 times the full load current.

Insulation System and Temperature Rise

All Federal Pacific FH class transformers are designed with 220 degree C insulation systems. The standard units are rated 150 degree C rise. The insulation system classification represents the maximum temperature permitted in the hottest spot in the winding when operated in a 40 degree C maximum ambient. The hotspot temperature is determined by adding the maximum value for each of the following:

40 degree C maximum ambient
150 degree C maximum average winding rise
30 degree C maximum hot spot in winding
220 degree C ultimate temperature at hot spot

The temperature rise commonly associated with transformers is the temperature of the conductor inside the coil and does not apply to the outside surface. The wiring compartment is ventilated and cooled enough to permit the use of 60 degree C cable for connections. Some customers will specify 220 degree C insulation with 80 degree C or 115 degree C rise to get overloaded capability, better efficiency, and longer life. These transformers are designed to operate with a lower rise per the following example at 80 degree C rise.

40 degree C maximum ambient
80 degree C maximum average winding rise
30 degree C maximum hot spot in winding
30 degree C thermal overload 30%
220 degree C ultimate temperature at hot spot

There may be differences between the voltage ratings of transformers and the rated voltage of some utilization equipment. Some equipment may be rated 230, 460, or 575 volts to allow drop due to impedance of wire, circuit breakers, etc. The respective transformer secondary voltages would be rated 240, 480, and 600 volts which are the system or source voltage . If you are asked to quote on a transfer rated 460V primary, 115/230V secondary would be proper to quote a transformer rated 480V primary, 120/240V secondary.

Basic Impulse Insulation Levels (BIL)

Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. High voltage switches and circuit breakers can also create similar voltage surges when they are opened and closed. Both types of surges have steep wave fronts and can be very damaging to electrical equipment. To minimize the effects of these surges, the electrical system is protected by lighting arresters but they do not completely eliminate the surge from reaching the transformer. The basic impulse level (BIL) of the transformer measures its ability to withstand these surges. All 600 volt and below transformers are related 10 KV BIL. The 2400 and 4160 volt transformers are rated 25 KV BIL.

Transformer Sound/Noise

A Humming is an inherent characteristic of transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformer size.Attention to installation methods can help reduce any objectionable noise. When possible ,locate the transformer in an area where the ambient sound will be equal or greater than the noise of the transformer sound level. Avoid locating units in corners. Make connections with flexible conduits and couplings to prevent transmitting vibration to other equipment. Larger units should be installed on flexible mountings to isolate the transformer from the building structure.

Average Sound Levels In Decibels

IEEE Std. C57.12.01-2015

We know that the air is thinner at higher altitudes which, in turn, reduces its ability to cool the transformer. Therefore, standard dry-type self-cooled transformers are designed to operate with normal temperature rise at heights through 3300 feet above sea level. If the operation is at higher altitudes, the rating should be reduced 0.3% for each 330 feet above 3300 feet.

Chapter 6 – Transformer Applications

Application of Standard Dry-Type Transformers in an Electrical System

Most utilities will only provide a customer with one service or electrical system. This system may be either single-phase or three-phase. Single-phase installations will normally be 120/240V AC, 3 wire systems. Three-phase installations could be 240 volt, 3 wire, 480 volt 3 wire, 600 volt, 3 wire, 208Y/120 volt, 4 wire , or 480Y/277 volt, 4 wire. These are the most popular installations and their selection can be either on customer preference or availability of the system from the serving utility. With this many choices available, you may wonder why anyone would need a transformer, so let us offer an example. A new industrial plant moves into town and requires an electrical service in their new building. They have a great many motors in use at their company, so they decide it would be more economical to use 480 volt three-phase motors. For this reason they request a 480Y/277 volts three-phase system. This takes care of their motor loads at 480 volts and their office and plant lighting loads at 277 volts. However, to operate their office machinery and incandescent lighting they require 120 volts. They also have some small horsepower motors they want to operate at 208 volts. Since the utility will only provide them with 480Y/277 volt three-phase system, they require a dry-type distribution transformer to provide the 208 and 120 volt loads.

This is the most typical of applications for dry-type distribution transformers. Other applications could be matching the voltage of a motor which does not match your system, isolating a computer or solid state device from system voltage due to voltage drop in an extremely long run of wire. The more important thing is to recognize what transformers can and cannot do . Below is a table of some of those things.

*Note: There are special purpose constant-voltage transformers that can do this.

Selection of a Transformer

When a customer calls you for help in the selection of a transformer these are things you need to know:

1. What is the voltage of this load? The transformer you select must have an output voltage which matches his load voltage. (don’t get confused between system and utilization voltage – See Section V, Paragraph D.)

2. Is the load single-phase or three-phase? Remember the transformer cannot change phases. Three-phase loads must be fed from the three-phase transformer/banks.

3. What is the power requirement for this load? We ultimately need to arrive at a kVA value. If only amps are known, use the full load chart or the following formulas.

Single-phase “kVA” = Volts (load) x Amps (load) / 1000
Three-phase “kVA” = Volts (load) x Amps (load) x 3 /1000
Where 3 = 1.732

4. What is the frequency (hertz or Hz) of the load and line (source?) Remember, transformers cannot change frequency. Generally, all U.S. power companies generate power at 60 Hz. Therefore, the load must also be rated 60 Hz.

5. What is the supply or source voltage? Are primary taps required?

6. Is there a special temperature rise or insulation system requirement ? If not, quote our standard general purpose transformers.

7. Is the transformer to be installed indoors or outdoors? Some transformers, particularly small encapsulated units are rated for indoor or outdoor applications. Others sizes will require the addition of a weather shield for outdoor use.

With the above information you should be able to quickly select a transformer from the catalog.

NOTE: Other considerations which may require special units may include, but are not limited to: copper windings; low temperature rise units; units for applications in ambient temperatures higher than 40 degrees C;units to be used at a high altitude above 3300 feet; special impedances; and many others. If requirements arise that do not fit the description of our standard units, be sure to contact your Federal Pacific representatives for assistance.

Problem: What is the proper transformer for a customer to supply an electric heater rated 100 amps, @ 240 volts, three-phase, 60 Hz? His available supply voltage is 480 volts, three-phase, 60 Hz. The transformer is installed indoors. 150 degree C with standard taps is required.

Solution: We have all of the information required with the exception of the load kVA. We know that:

Three-phase kVA = Volts (load) x Amps (load) x 3 = 240 x 100 x 1.732 = 41.6 kVA.

A 480 volt to 240 volt (Delta-Delta), three-phase, 45 kVA (which is the next standard kVA rating) general purpose transformer is required.

Problem: A single line shows a 25 kVA, single-phase, 60 Hz, 150 degree C rise, 240×480 volt to 120/240 volt transformer fed from a three-phase volt system. Is it correct?

Solution: Yes, the transformer has series-multiple primary windings so connection to 480 volt is acceptable. Remember, that if the transformer is single-phase, the source can be single-phase or three-phase. When the supplies three-phase, any two (2) lines or one (1) line and neutral will be used as shown below. Figure 1 applies for the 480 volt primary voltage in the above problem.


Problem: An industrial plant requests a transformer that can step down 480 volt three-phase to 240 volt three-phase and supply 200 kVA of load at 240 volt three-phase and 5 kVA of load at 120 volt single-phase. What would you quote?

Solution: A general purpose 300 kVA, 480 volt to 240 volt three-phase transformer with 120 volt lighting tap will work. Remember that the use of the 120 volt lighting tap requires a 30% derating of the nameplate kVA and that the 120 volt loads cannot exceed 5% of the nameplate kVA. (See below)

Derating Calculations: 300 kVA x .30 kVA 300 kVA -90kVA 210kVA (Available capacity for 30 (loads)

120 Volt Lighting Tap Calculations: 300 kVA x .05 = 15 kVA (Available capacity for 120 volt loads)

The following must be known before a transformer can be selected:

kVA: the rating or capacity of the transformer

Phase: Load requirements (single-phase or three-phase) If the load is three-phase, both the supply and the transformer must be three-phase. If the load is single-phase, the supply can be either single or three-phase, but the transformer must be single-phase.

Frequency: usually 60 Hz (Hertz).

Primary Voltage: Designates the load voltage for which the primary winding is designated.

Secondary Voltage: Designates the load voltage for which the secondary winding is designed.

Taps: Adjustment capability for voltage variations.

Location of Installation: Indoor or Outdoor

Other Considerations: Mounting Requirements, Sound Levels, Impedance, Special Applications, K-Rating, Copper Windings, Electrostatic Shields, Temperature Rise, Insulation Class

Chapter 7 – Special Applications

Energy Efficient 80 degree C and 115 degree C Rise Transformer

These types of ventilated transformers incorporate the same features as general purpose transformers with the exception that they handle temperature differently . Specifically, energy efficient transformers utilize a 220 degree C insulation system and are designed for normal operation at a reduced temperature rise either 80 degree C or 115 degree C, instead of the normal 150 degree C rise. This is accomplished by increasing the size of the coil conductors result in a lower operating cost (energy savings) and an improved life expectancy since the 220 degree C insulating is subjected to lower operating temperatures. These units will also have a continuos overload capacity of15% for 115 degree C rise units and 30% for 80 degree C rise units when operate at 150 degree C rise. Again, since the insulation system is rated 220 degree C, this overload operation will not reduce the normal life expectancy of the transformers. Since operating in an overload condition results in higher temperatures and higher losses, the efficiency will decrease under these conditions.

These types of ventilated transformers also incorporate the same features an general purpose transformers. However, electrostatically-shielded transformers can attenuate (reduce) undesirable high-frequency signals. High frequency signals can be thought of as static in a radio. These signals (noise) are commonly produced by certain types of lighting, switching surges, motors and SCR’s which feed this static back into the power lines. The power lines transmit this static to all loads and especially to certain loads that operate sensitive electrical equipment (which can cause some equipment to malfunction.) An electrostatically-shielded transformer incorporates a single turn of foil placed between the primary and secondary windings with one side grounded. The shield will then attenuate (reduce) the noise, thus suppressing its effect upon sensitive loads. (Refer to FPT catalog for more detailed information.)

Typical use of shielded isolation transformers include:

Suppression to transients or noise when traveling from its source to the sensitive load equipment.

Suppression of noise and transients at the point or transients originate, thus preventing them from back feeding from the source to the feeders.

Transforming one voltage level to another.

Isolating one circuit from another.

Dry-Type Transformers and Non-Linear Loads

In general, a transformer’s performance is not affected by the type of loads they serve. However, in recent years, advances in the design of power supplies for various types of office equipment (particularly the personal computer) and SCR drives has presented some unusual problems. Specifically, overheating in standard dry-type transformers, even when ampere measurements indicate the current is within rating. Very basically, the problem appears when the transformer load distorts the sinusoidal current waveform. These distorted (non-linear) current waveforms are said to contain harmonic distortion. (See Note) As transformer’s ability to handle these non-linear/harmonic loads is determined by its K-factor rating. K-factor is a simple numerical rating that can be used to specify transformers for non-linear/harmonic loads. Normally, the customer or consulting engineer will specify K-factor ratings, if necessary, for certain transformers within a distribution system. (Refer to FPT catalog for more information).

Note: Harmonics are multiples of the fundamental frequency (normally 60 hertz). Therefore, the 3rd harmonic equals 60 X 7 or 420 hertz, and so on.

Type FH Motor Drive Isolation Transformers

Type FH motor drive isolation transformers are designed to meet the requirements of SCR controlled variable speed motor drives. They are specifically constructed to withstand the mechanical forces associated with SCR drive duty cycles and to insolate the line from most SCR generated voltage spikes and transient feedback. Similarly, the two-winding construction also aids in reducing some types of line transients that can cause misfiring of the SCRs. The units are UL Listed and incorporate all the features of the FH transformer line. The transformer can also be supplied core and coil units with UL components recognition. Delta-wye designs are available for all commonly use primary and secondary voltages. All units include primary and secondary voltages. All units include primary taps consisting of one 5% FCAN and one 5% FCBN. (Refer to FPT catalog for more information.) The type FB Insulating and Buck-Boost Transformer has four separate windings, two windings in the primary and two windings in the secondary. The unit is designed for use as an isolating transformer or as auto-transformer. As an auto transformer, the unit can be connected to buck (decrease) or Boost (increase) a supply voltage. When connected in either the Buck or Boost mode, the unit is no longer an isolating transformer but is an autotransformer. Autotransformers are more economical and physically smaller than equivalent two-winding transformers designed to carry the same load. They will perform the same function as two-winding transformers with the exception of insulating or isolating two circuits . Since autotransformers may transmit line disturbances directly, they may be prohibited in some areas by local building codes. Before applying them, care should be taken to assure that they are acceptable according to local code.

Note: Autotransformers are not used in close Delta connections as they introduce a phase shift into the circuit.

As isolating transformers, these units can accommodate a high voltage of 120 x 240 volts or 240 x 480 volts. For the units with two 12 volt secondaries, the low voltage output can be 12 volts, 24 volts, or 3- wire 12/24 volts. Volts. For the units with two 16 volt secondaries, these output voltages can be 16 volts, 32 volts, or 3-wire 16/32 volts. For the units with two 24 volt secondaries, these output voltages can be 24 volts, 48 volts, or 3 wire 24/28 volts. The unit is capacity rated (KVA) as any conventional transformer.

Electrical and electronic equipment is designed to operate on a standard supply voltage. When the supply voltage is constantly too high or too low, (usually greater than (5%), the equipment fails to operate at maximum efficiency. A Buck-Boost transformer is a simple and economical, means of correcting this off-standard voltage up to 20%. A Buck-Boost transformer will NOT, however, stabilize a fluctuating voltage. Buck Boost transformers are suitable for use in a three-phase autotransformer bank in either direction to supply 3-wire loads. They are not suitable for use in a three-phase autotransformer bank to supply a 4-wire unbalanced load when the source is a 3-wire circuit.


To select the proper transformer for Buck-Boost applications, determine:

1. Input Line Voltage – the voltage that you want to buck (decrease) or boost (increase). This can be found by measuring the supply line voltage with a voltmeter.

2. Load Voltage – the voltage at which your equipment is designed to operate. This is listed on the nameplate of the load equipment.

3. Load KVA or Load Amps – you do not need to know both. One or the other is sufficient for selection purposes. This information usually can be found on the nameplate of the equipment you want to operate.

4. Number of Phases – single or three-phase line and load should mach because a transformer is not capable of converting single-phase to three-phase. It is however, a common application to make a single -phase transformer connection from a three-phase supply by use of one leg of the three-phase supply . This particularly true in a Buck-Boost application because the supply must provide the load KVA, not just the nameplate rating of the Buck-Boost transformer.

5. Frequency – the supply line frequency must be the same as the frequency of the equipment to be operated; either 50 or 60 cycles.

Six Step Selection
1. Choose the selection table with the correct number of phases. Tables I, III, and V for single-phase applications and Table II, IV, and VI for three-phase applications. Tables I and II are for 120x 240-12/24 volts: Tables III, IV are for 120 x 240-16/32 volts; and Tables V and VI are for 240 x 480-24/48 volts. (Refer to FPT catalog)

2. Line/Load voltage combinations are listed across the top of the selection table. Select a line/load voltage combination which comes closest to matching your application.

3. Follow the selected column down until you find either the load KVA or load amps of your application. If you do not find the exact value, go on to the next highest rating.

4. Follow across the table to the far left-hand side to find the rated KVA of the transformer you need.

5. Follow the column of your line/load voltage to the bottom to find the connection diagram for this application. Note: Connection diagrams show low voltage and high voltage connection terminals. Either can be input or output depending upon Buck or Boost application.

6. In the case of three-phase loads either two or three single-phase transformers are required as indicated in the “quantity required “ line at the bottom of Table II, IV or VI Select depending on whether a Wye connected bank of three transformers with a neutral is required or weather an open Delta connected bank of two transformers for a Delta connected load will be suitable.

For line/load voltages not listed on the table, use the pair listed on the table that is slightly above your application for reference. Then apply the first formula at the bottom of Table II, IV, or VI to determine “New” output voltage. The new KVA rating can be found using the second formula.

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