Delta Transformers inc.


You will find in this section a list of the most frequently asked questions.

If none of the answers correspond to your needs, please do not hesitate to communicate with us using the form below:

It is a term for AC frequency in cycles per second.

The solutions are as wide ranging as the problems. So are the prices. We have summarized some solutions and their price ranges in the table below.


Unfiltered Surge Fuses Protect against major surges only. $15.00 and up • Basic protection against lightning strike surges - do not provide clean power.
Filtered Surge Suppressors Spikes, EMI/RFI Surges $60.00 to $300.00 • Small business, home business, home computers, audio and video.
Computer Regulators and Line Voltage Conditioners Spikes, surges, brownouts EMI/RFI $300.00 and up • Office computers, business applications, dedicated lines, labs, test facilities
Isolation Transformers Spikes, surges EMI/RFI $300.00 and up • Business applications, dedicated services, labs, and test equipment.
U.P.S. (Uninterruptable and Power Supply) All Power problems $750.00 and up • Large main frame computers networks.

Unfiltered Surge Fuses are very inexpensive, and may provide damage protection from lightning strikes or other surges, but they do not filter out adverse noise.

Filtered Surge Suppressors are inexpensive solutions to noise suppression and surge protection. The better units inhibit surges above 5000 volts, 200 amps. They should also provide noise filtration of 10dB or more to cover average power disturbances.

Computer Regulators or Line Voltage Conditioners protect equipment from both noise and voltage fluctuations. They are an inexpensive solution, available in both portable or hardwired models. They provide ideal protection in high noise areas where voltage fluctuations exceed the regulating range of the computers power supply.

Super Isolation Transformers provide inexpensive protection against frequency variation or noise related disturbances. This is adequate where voltage fluctuations are not a serious problem. Most high-end computers have built-in voltage regulation, but still require protection from line noise.

U.P.S. Systems are in effect self-contained power centers. They provide backup power for a period of time when utility power is interrupted. Most U.P.S. systems also provide noise filtration and s

A Harmonic is a sinusoidal component of a periodic wave having a frequency that is a multiple of the fundamental frequency. For example, a component whose frequency is twice the fundamental frequency is referred to as the second harmonic, (120 Hz is the 2nd harmonic of 60 Hz).

Not everyone has the same power problem. Finding the most cost-effective solution requires some analysis of your equipment, the power system and the available solutions in the market. The table below lists causes and effects of many common power problems. You, or your electrician can determine the most likely cause of power problems based on knowledge of your location, the kinds of equipment you operate in that location, and the kind of power distribution system in your building. The following table lists the types of Clean Power products available from Delta Transformers to solve your power problems.

Voltage FluctuationsFluctuations exceeding +/-5% of nominal.These can occur from milliseconds to several seconds. These can occur from milliseconds to several seconds. • Heavy loads being added to or removed from the power source Including: utility switching equipment, lightning suppression apparatus, heavy motor start-up loads such as elevators, generators, furnaces and compressors.
• Lighting loads
• Power line faults and fault clearing devices
• Brownouts
• Low voltage to equipment up to 20% below normal. • Loss of Data
• Down Time
• Rerun costs
• Equipment damage
Line Noise
Fluctuations, transients, spikes, dips or bursts of high frequency energy, which can range from 10 to 100 times the line voltage.
• Lightning
• Static discharge
• Power line faults
• Utility switch gear
• Office equipmento Disk drives
• Heavy office cleaning equipment
• Programmable controllers
• Printers and plotters
• Radio transmitters
• Other computers
• Fluorescent lights and dimmers
• Heater controls
• Motors and motor controls
• Power tools and appliances
• Unshielded automobile ignition cables
• Spikes or surges 600 to 6,000 volts from one tenth to 100 millionths of a second duration.
• Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI).
• Output errors
• Character changes
• Skipped program steps
• System crash
• Memory loss
• File loss
• Misregistration
• Circuit damage
• 'Snow' on video equipment and audible noise in audio equipment.
Other Disturbances • Ground not dedicated or isolated
• Shock Hazards
• Electrical safety hazard • Personal safety hazard
• Possible equipment damage

These terms are used to distinguish the winding having the greater voltage rating from that having the lesser in two winding transformers.

K factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. Transformers with a rated K factor of 4, 7, 13, 20 and 30 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 13 should be specified when 100% of the load is non-linear.

It is a high potential dielectric test impressed on the windings to check insulation materials and clearances.

Today, non-linear loads make up a large percentage of all electrical demand. Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. Harmonic currents and voltages produced by single phase, non-linear loads which are connected phase-to-neutral in a three phase four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples - 3rd, 9th, 15th, 21st, etc., phasors displaced by zero degrees). These third order, zero sequence harmonic currents, do not cancel but add up arithmetically on the neutral bus, creating a primary source of excessive neutral current.

It is a dielectric test that determines the BIL capability by applying high frequency, steep wave-front voltage between windings and ground.

Noise is a very broad term that can be applied to a number of AC power line disturbances. Lightening surges or any other sudden changes in load, such as switching motor loads or power factor correcting capacitors can produce voltage spikes and ringing. Phase controlled rectifier loads and arcing devices produce continuous noise unless adequately filtered. Noise sources are either common mode, which appears between both sides of a power line and ground or of transverse mode, which appears from line to line.

It is a standard dielectric test that verifies the integrity of insulating materials and electrical clearances between turns and layers of a transformer winding.

Under voltages and over voltages are caused by faults on power lines, and the subsequent actions of fault clearing devices. Also, by heavy loads, such as machinery start-up and by the slow reaction of power company regulating equipment. Since computer equipment is designed to operate close to nominal voltages, the effects of these voltage variations can cause serious problems. Voltages can drop as much as 20% of nominal. This can result in expensive and time consuming errors, loss of information, downtime, recovery and rerun costs and possible equipment damage.

It is a property that opposes a change in current flow.

Research conducted by both IBM and Bell indicates that most line disturbances to sensitive equipment are line noise and voltage fluctuations.

One form of dirty power usually called a surge can burn out computer, audio, video or nay other electronic circuitry in seconds. A surge is a high voltage pulse riding the normal power wave. Surges will commonly measure 600 to 2500 volts. Even though they occur for only mille-seconds, this is enough time to melt down circuits.

Your computer operates by reading electronic impulses. Dirty power contains a great number of random pulses riding on the normally smooth surface of a power wave. As these random pulses enter the circuits, your computer 'reads' them as data. This can cause a whole range of problems. You may suddenly get garbled numbers or letters in a readout or printout.

You could loose files, skip program steps, have trouble loading programs or have connection problems while on the Internet.

Dirty power is caused by a number of things. Simply put, dirty power is what causes your radio or telephone to 'crackle' during an electrical storm; or what causes 'snow' on your TV when someone is using a power tool, sewing machine or other appliances in your house. This dirty power, or electrical noise, is a nuisance when it appears on your radio, TV or telephone. When it gets into your computer, it can cause serious errors; improper readouts, printing problems, or even damage your computers circuit.

This is a non-ventilated indoor enclosure designed primarily for providing a degree of protection against circulating dust, falling dirt, and dripping non-corrosive liquids. This enclosure is both oil and rust resistant suitable for applications such as oil refineries where oil or other chemical liquids may be prevalent. (Note: not watertight)

This enclosure is the same as the NEMA 4, and is also corrosion resistant. It is ideal for environments such as food processing plants and refineries.

NEMA 4 is a non-ventilated indoor or outdoor enclosure designed primarily to provide a degree of protection against windblown dust and rain, splashing water, hose-directed water, and damage from external ice formation.

It is suitable in areas where exposure to large amounts of water from any direction. (Note: not submersible)

Although similar to the NEMA 3R it also provides added protection against blowing snow and dirt. It is suitable for out-door installations where blowing snow or blowing conductive dust are present.

Similar to the NEMA 3, it is also intended for outdoor use. It provides a greater degree of protection against rain, sleet, falling snow or dirt and damage from external ice formation. It is ideal for any outdoor installation where no blowing snow or blowing conductive dust exists.

A ventilated enclosure is constructed to provide circulation of external air through the enclosure to remove excess heat.

A non-ventilated enclosure is constructed to restrict unintentional circulation of external air through the enclosure.

This is a general purpose ventilated enclosure for outdoor use designed primarily to provide a degree of protection against rain, sleet, wind blown snow or dust and damage from external ice formation. It is considered ideal for construction sites, subways etc.

This enclosure is the sprinkler proof version of the NEMA 2.

This is a general-purpose enclosure for indoor use primarily to provide a further degree of protection against limited amounts of falling water (drip proof) and dirt.

This is a general-purpose ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.

Type Tests are required either to qualify a new product or to further certify a production product. The following is a list of type tests performed on Delta Transformers.

Temperature Rise Test

Sound Level Test

Partial Discharge (corona)

Basic Impulse Insulation Level (BIL)

Short-Circuit Test

The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20xC reference temperature. (Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)

The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation.

A normal power frequency such as 60hz is applied to each winding for one minute. These tests are in accordance with table (3) in ANSI C57-12-01.

No-load losses (excitation losses) are the core losses of a transformer that are "excited" at rated voltage and frequency, but which do not supply load. No-load losses include core loss, dielectric loss, and losses in the windings due to exciting current. The transformer is excited at rated voltage with all other windings open circuited. The exciting current and no load loss is then measured.

(Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)

To confirm the voltage ratio of a transformer, the ratio of the number of turns in the high-voltage winding with respect to the number of turns in the low-voltage winding, is measured.

A full capacity tap is one through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise.

On AC circuits, designates the number of times the polarity alternates from positive to negative and back again . . . such as 60 cycles per second. Measured in Hertz.

The magnetic field strength in the core, typically measured in Telsa or Gauss.

A non-rigid connection used to reduce transmission of noise and vibration.

A transformer cooled mechanically to maintain its rated temperature rise, typically using auxiliary fans to accelerate heat dissipation.

"Full Capacity Below Normal." Same as FCAN except that the taps are below rated voltage.


"Full Capacity Above Normal." This designates that a transformer will deliver its rated kVA when connected to a voltage source that is higher than the rated voltage.

A transformer with its coils either encased or cast in an epoxy resin or other encapsulating materials.


Current which flows in any winding used to excite the transformer when all other windings are open-circuited. It is usually expressed in percent of the rated current of a winding in which it is measured.

It is Copper or other conducting material placed between the primary and secondary winding and grounded to reduce electrical interference and to provide additional protection.

The percentage of power transferred from the input of equipment to the output of equipment in Watts. (power out/power in x 100)

A winding consisting of two separate parts which can be connected in series or parallel. Also referred to as dual voltage or series-multiple winding.

A dry type transformer is one in which the transformer core and coils are not immersed in a liquid.

These tests consist of the application of a voltage higher than the rated voltage for a specified time, for the purpose of determining the adequacy against breakdowns of insulating materials and spacing under normal conditions.

A Delta Wye is when the primary is connected in delta and the secondary in Wye when pertaining to a three-phase transformer bank or three-phase transformer.

The delta connection is a standard three phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other.

It is a transformer generally used in control or instrumentation circuits for measuring current.


Losses in watts caused by magnetization of the core and its resistance to magnetic flux when excited or energized at rated voltage and frequency. Also referred to as excitation loss or no-load loss.

It is the electrical grade steel laminations, which carry the magnetic flux.

The constant load which a transformer can carry its rated primary voltage and frequency, without exceeding its specified temperature rise.

An electrical noise or voltage disturbance, that occurs between all of the line leads and the common ground, or between the ground plane and either line or the neutral.

It is the absolute maximum temperature present in the transformer. This number is equal to the sum of the ambient temperature, temperature rise and a variable.T Hot Spot = T ambient + T rise + (10-20) xC.

Turns of electrical grade wire or strip conductor material wound on a form, referred to as a winding.

It is a reduced capacity tap at the midpoint in a winding.

A transformer with coils solidly cast in epoxy resin under vacuum in a mold. Also called cast resin or epoxy cast coil transformers.

Basic impulse level is a means to express the ability of the insulation system to withstand high voltage surges.

Two or more single-phase transformers connected together to supply a three-phase load.

It is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits.

The American National Standards Institute Inc. - one of the recognized organizations that specify the standards for transformers.


It is the unit of measurement for electric current flow.

It is the inherent or existing temperature of the atmosphere surrounding a transformer into which its heat is dissipated.

The noise level of the surrounding area, measured in decibels (dB).

It is a transformer that uses "air" as the cooling medium. This term is abbreviated with the ANSI designation AA, indicating open, natural draft ventilated construction.

Polarity and phase-relation tests are made to determine angular displacement and relative phase sequence to facilitate connections in a transformer. Determining polarity is also essential when paralleling or banking two or more transformers.

Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of I2R for use in the winding temperature test, and as base data for future assessment in the field.

In general, distribution transformers can be reverse connected without de-rating the nameplate KVA capacity. However some precautions need to be taken for reverse connection of some smaller transformers. On Delta Transformers under 6kVA three phase and 3kVA single phase, there is a "turns ratio compensation" on the low voltage winding.

When the input voltage, equal to the nameplate rated voltage, is connected to the low voltage winding, the output voltage will be slightly lower than the nameplate rating.

When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. Further, the reverse connected transformer may draw a higher inrush current during energization. Hence the sizing of the line fusing or circuit breaker may be affected.

Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the Delta Transformers "Field Service Guide". Delta Transformers are built to meet the latest ANSI, CSA and UL standards. These standards are outlined in the accompanying table.

A single-phase transformer with 120/240V secondary has two separate 120V secondary windings and is usually connected into a 3-wire system. Care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total load.

Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded.

Transformers rated at 60Hz should not be used on a 50Hz supply due to higher losses and core saturation, and the resultant higher temperature rise. Transformers rated for 50Hz, however, can be operated on a 60Hz supply.

In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments. Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.

Two or three, single-phase transformers can be connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in delta or Wye and the secondary windings can be connected in either a delta or Wye configuration. The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually this type of installation is more expensive than using a single three phase transformer.

Delta Transformers offers a wide range of dry type power transformers, rated from 750kVA up to 25 MVA. They are suitable for commercial, industrial, manufacturing or production process applications. With three phase ratings to 46 kV class and up to 25 MVA, Delta Transformers offers the latest technology and manufacturing processes available today.

Drive isolation transformers are designed to supply power to AC and DC variable speed drives. The harmonics created by SCR type drives requires careful designing to match the rated hp of each drive system. The duty cycle included is approximately one start every 2 hours. The windings are designed for an over-current of 150% for 60 seconds, or 200% for 30 seconds.

Delta Transformers medium voltage transformers are really 5kV class dry type distribution transformers. They are designed primarily for use in stepping down medium voltage power to a lower operating voltage for commercial, institutional or industrial applications.

These units are encapsulated and completely enclosed. The encapsulated design is especially suited for installations in harsh environments where dust, lint, moisture and corrosive contaminants are present. Typical applications include: pulp and paper plants; steel mills; food processing plants; breweries; mines; marine and shipboard installations.

All transformers have operating losses, and heat is the product of these losses. Delta Transformers low temperature rise transformers are designed with reduced 115xC or 80xC full load operating temperature rises. These units decrease total operating losses by 20% and 35% respectively, compared with the standard 150xC rise operating system. Delta Transformers low temperature rise transformers provide greater efficiency under normal operating conditions, and overload capability without harm to their service life or reliability.

There is a growing movement in the electrical industry towards energy efficient products in all sectors including dry type transformers. In addition to the benefits to the environment, energy efficient transformers also can realize substantial savings in operating costs thereby having a direct impact on the initial investment evaluated over a period of time.

The specifications covering energy efficiency in transformers, is the NEMA Standards Publication, TP-1-1996, "Guide for Determining Energy Efficiency for Distribution Transformers". This specification has carefully considered the total owning cost unique for industrial or commercial installations where the load factor is an integral part of the efficiency rating.

Autotransformers are similar to Buck-Boost transformers in that they are also an economical means of adjusting an output voltage. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. Units are designed in either a step-up or step-down application and to meet motor inrush currents.

Delta Transformers low voltage general-purpose transformers provide a safe, long lasting, highly reliable power source. They are designed for general lighting and other low voltage applications. They are UL listed and CSA certified.

K-Factor is defined as a ratio between the additional losses created by the harmonics and the eddy losses at the rated 60 Hz. This factor is used to specify the size of the transformer to meet the magnitude of the harmonic load in the circuit. A standard general-purpose transformer does not have the shielding, conductor sizes, core cross-section, or the capacity in the neutral to provide the same service.

K-factor transformers are used as a general-purpose transformer but are designed to withstand the variety of harmonics created in today's office and industrial environments. The expanding use of devices with switch-mode power supplies and rectifier circuits with the subsequent wave distortion requires transformers to withstand the higher harmonics in the neutral conductor in the distribution system.

Delta Transformers shielded distribution transformers provide a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts most noise and transients to the ground path rather than passing them through to the secondary. Applications for shielded transformers are similar to those above, and they are ideal for commercial or electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to 'noise'.

General Purpose distribution transformers are rated for 600 volts and below. They are generally used for supplying appliance, lighting, motorized machine and power loads from electrical distribution systems. They are either ventilated or totally enclosed, and are available in standard ratings from 250VA up to 750kVA.

Single phase Amps = (kVA x 1000)/Volts

Three phase Amps = (kVA x 1000)/Volts x 1.73

Single phase KVA = (Volts x Amps)/1000

Three phase KVA = (Volts x Amps x 1.73)/1000

When a transformer is continually overloaded, excessive heat develops and the insulation system will begin to breakdown. As a result, the life expectancy of the transformer is shortened due to the heat exceeding the rating of the insulation system.

Mylar, Nomex and other high quality insulating materials. Insulation is used to electrically insulate turn-to-turn windings, layer-to-layer windings, primary to secondary windings and ground.

The VA or volt-ampere output rating designates the output that a transformer can deliver for a specified time at its rated secondary voltage and rated frequency, without exceeding its specified temperature rise.

It is a special transformer connection commonly used in grounded transformers. See also grounding transformers.

A standard 3-wire transformer connection with similar ends of the single-phase coils connected. This common point forms the electrical neutral point and may be grounded.

It is the current flowing in a circuit multiplied by the voltage of the circuit. It is an expression of the output rating of a transformer.

The change in secondary voltage that occurs when the load is reduced from rated value to zero, with the value of all other quantities remaining unchanged. Regulation may be expressed in percent (per unit) on the basis of rated secondary voltage at full load.

A vacuum and pressure impregnation process using a resin which, is then oven cured to completely seal and protect the surface of a transformer and provides a strong mechanical bond. This process is standard on all Delta transformer products.

Underwriters Laboratories

It is electrical noise or voltage disturbance that occurs between phase and neutral (between lines), or from spurious signals across the metallic hot line and the neutral conductor.

Normal, routine production tests include: (1) core loss; (2) load loss - winding or copper loss;(3) Impedance; (4) hi-pot - high voltage between windings and ground; (5) induced - double induced two times voltage. Optional special tests include: (a) heat run - temperature testing;(b) Noise tests - sound level measurement; (c) impulse tests - BIL tests: (d) partial discharge.

It is a temporary or brief change in a given parameter. This is typically associated with input voltage or output load parameters.

It is a static electrical device, which, by electromagnetic induction transforms energy at one voltage and current to another voltage and current at the same frequency.

It is the transformer electrical losses, which include no-load losses (core losses) and load losses (winding losses).

It is the increase over ambient temperature of the winding due to energizing and loading the transformer.

It is the maximum temperature that the insulation can continuously withstand. The classes of insulation systems in a transformer are rated as follows:

Class 105xC

Class 150xC

Class 180xC

Class 220xC

A Scott connected three-phase transformer utilizing two primary and two secondary coils called the main and the teaser.

A tap is a connection brought out of a winding at some point between its extremities, usually to permit changing the voltage or current ratio.

A transformer in which the low voltage winding (secondary) is connected to the input or power source and the high voltage winding (primary) is connected to the output or load.

It is a transformer where the high voltage winding (primary) is connected to the input or power source and the low voltage winding (secondary) to the output or load.

It is a device that contains components that do not depend on electronic conduction in a vacuum or gas. Semiconductors or the use of otherwise completely static components such as resistors or capacitors performs the electrical function of a solid-state device.

A short circuit condition occurs when an abnormal connection or relatively low impedance, whether made accidentally or intentionally, occurs between to points of different potential in a circuit.

A winding consisting of two or more sections which can be connected for series operation or multiple (parallel) operation. Also referred to as dual voltage or series-parallel.

The transformer winding connected to the load or output side.

Designates the no-load circuit voltage for which the secondary winding (winding on the output side) is designed.

It is a connection for polyphase using two special single-phase transformers. Usually used to change from two-phase to three-phase or three-phase to two-phase.

Saturation is a natural condition in which an increase in current results in a decrease in inductance.

It is a silicon-controlled rectifier.

Usually expressed as the percent change output voltage when the load goes from full load to no load at a given power factor.

It is taps that carry full-rated winding current only, thus reducing available power because of lower output voltage.

A transformer designed to supply AC input to a rectifier to obtain the desired DC output and have the ability to withstand the heating effects caused by rectifier commutation or ripple.

A single winding device with an air or iron core which, produces a specific amount of inductive reactance into a circuit, usually to reduce or control current.

It is the impedance component due to inductance and/or capacitance.

It is a standard test of transformers to determine the ratio of the primary to secondary voltage.

It is a reference to either the primary to secondary winding turns ratio or to the voltage ratio of the transformer.

It is the design characteristics such as primary and secondary voltage, kVA capacity, temperature rise, frequency, etc.

The primary winding is the winding on the energy input (supply) side.

Designates the input circuit voltage for which the primary winding is designed.

Taps added to the primary winding. (See Taps)

It is the relation of watts to volt amps in a circuit.

A transformer generally used in instrumentation circuits for measuring or controlling voltage.

It designates the instantaneous direction of voltages in the primary compared to the secondary.

It is a type of AC electrical circuit; usually single phase 2-wire or 3-wire, or three-phase, 3 or 4 wire.

Single and three phase transformers may be operated in parallel by connecting similarly marked terminals, provided their ratios, voltages, resistances, reactance and ground connections are designed to permit parallel operation. Current and voltage angular displacements are also required to be the same in the case of three phase transformers.

When a transformer is overloaded, excessive heat develops and the insulation system begins to breakdown. Life expectancy of the transformer is decreased due to heat exceeding the rating of the insulation system.

It is the loss in a transformer that is excited at rated voltage and frequency, but without a load connected to the secondary. No-load losses include core loss, dielectric loss, and copper loss in the winding due to exciting current.

National Electrical Manufacturers Association.

Materials or equipment constructed or treated so that it will not be harmed readily by exposure to a moist atmosphere.

National Electric Code

It is a reduced capacity tap midway in a winding. Also referred to as a 'Center tap'. Usually it is in the secondary winding.

They are losses in a transformer, which are incident to load carrying. Load loses include I2R loss in the windings due to load current, stray loss due to stray fluxes in the windings, core clamps, etc., and to circulating currents (if any), in parallel windings.

The load of a transformer is the power in kVA or volt-amperes supplied by the transformer.

Thin sheets of special steel used to make the core of a transformer.

They are easily removable circle of metal in an enclosure that eliminates the need for punching holes for conduit.

"Kilovolt Ampere Rating" designates the output that a transformer can deliver for a specified time at rated secondary voltage and rated frequency without exceeding the specified temperature rise. (1 kVA = 1000 VA, or 1000 volt amperes)

It is a transformer that insulates the primary circuit from the secondary circuit. Also referred to as a two-winding or insulating transformer.

Those materials used to electrically insulate the transformer's windings, turn-to-turn or layer-to-layer, and other assemblies in the transformer such as the core and busswork.

It is an abnormally high transient current, caused by residual flux in the core, which maybe drawn when a transformer is energized.