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By : Ramakrishna Perumal, Sr. Engineer, Technicas Reunidas
Industry : Power
Keywords :

transformer

Activity:  10 comments  8960 views  last activity : 06 27 2013 14:34:54 +0000
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Basics of Transformer - Q & A (Useful for Freshers)

1) What is a transformer and how does it work?

 

A transformer is an electrical apparatus designed to convert alternating current from one

voltage to another. It can be designed to "step up" or "step down" voltages and works on

the magnetic induction principle. A transformer has no moving parts and is a completely

static solid state device, which insures, under normal operating conditions, a long and

trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire

wound on a laminated steel core. When voltage is introduced to one coil, called the

primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the

secondary or output coil. The change of voltage (or voltage ratio) between the primary

and secondary depends on the turns ratio of the two coils.

 

2) What are taps and when are they used?

 

Taps are provided on some transformers on the high voltage winding to correct for high

or low voltage conditions, and still deliver full rated output voltages at the secondary

terminals. Standard tap arrangements are at two and one-half and five percent of the rated

primary voltage for both high and low voltage conditions. For example, if the transformer

has a 480 volt primary and the available fine voltage is running at 504 volts, the primary

should be connected to the 5% tap above normal in order that the secondary voltage be

maintained at the proper rating. The standard ASA and NEMA designation for taps are

"ANFC" (above normal full capacity) and "BNFC" (below normal full capacity).

 

3) What is the difference between "Insulating", "Isolating", and "Shielded

Winding" transformers?

 

Insulating and isolating transformers are identical. These terms are used to describe the

isolation of the primary and secondary windings, or insulation between the two. A

shielded transformer is designed with a metallic shield between the primary and

secondary windings to attenuate transient noise. This is especially important in critical

applications such as computers, process controllers and many other microprocessor

controlled devices.

 

All two, three and four winding transformers are of the insulating or isolating types. Only

autotransformers, whose primary and secondary are connected to each other electrically,

are not of the insulating or isolating variety.

 

4) Can transformers be operated at voltages other than nameplate voltages?

 

In some cases, transformers can be operated at voltages below the nameplate rated

voltage. In NO case should a transformer be operated at a voltage in excess of its

nameplate rating unless taps are provided for this purpose. When operating below the

rated voltage the KVA capacity is reduced correspondingly. For example, if a 480 volt

primary transformer with a 240 volt secondary is operated at 240 volts, the secondary

voltage is reduced to 120 volts. If the transformer was originally rated 10 KVA, the

reduced rating would be 5 KVA, or in direct proportion to the applied voltage.

 

5) Can 60 Hz transformers be operated at 50 Hz?

 

ACME transformers rated below 1 KVA can be used on 50 Hz service. Transformers 1

KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to the higher

losses and resultant heat rise. Special designs are required for this service. However, any

50 Hz transformer will operate on a 60 Hz service.

 

6) Can transformers be used in parallel?

 

Single phase transformers can be used in parallel only when their impedances and

voltages are equal. If unequal voltages are used, a circulating current exists in the closed

network between the two transformers which will cause excess heating and result in a

shorter life of the transformer. In addition, impedance values of each transformer must be

within 7.5% of each other. For example: Transformer A has an impedance of 4%,

transformer B which is to be parallel to A must have an impedance between the limits of

3.7% and 4.3%. When paralleling three phase transformers the same precautions must be

observed as listed above, plus the angular displacement and phasing between the two

transformers must be identical.

 

7) Can transformers be reverse connected?

 

Dry type Distribution transformers can be reverse connected without a loss of KVA

rating, but there are certain limitations. Transformers rated 1 KVA and larger single

phase, 15 KVA and larger three phase can be reverse connected without any adverse

affects or loss in KVA capacity. The reason for this limitation in KVA size is, the turns

ratio is the same as the voltage ratio. Example: A transformer with a 480 volt input, 240

volt output - can have the output connected to a 240 volt source and thereby become the

primary or input to the transformer, then the original 480 volt primary winding will

become the output or 480 volt secondary. On transformers rated below 1 KVA single

phase there is a turns ratio compensation on the low voltage winding. This means the low

voltage winding has a greater voltage than the nameplate voltage indicates at no load. For

example, a small single phase transformer having a nameplate voltage of 480 volts

primary and 240 volts secondary, would actually have a no load voltage of approximately

250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to

a 240 volt source, then the output voltage would consequently be approximately 460 volts

at no load and approximately 442 volts at full load. As the KVA becomes smaller, the

compensation is greater- resulting in lower output voltages. When one attempts to use

these transformers in reverse the transformer will not be harmed; however, the output

voltage will be lower than is indicated by the nameplate.

 

8) Can a Single Phase Transformer be used on a Three Phase source?

 

Yes. Any single phase transformer can be used on a three phase source by connecting the

primary leads to any two wires of a three phase system, regardless of whether the source

is three phase 3-wire or three phase 4- wire. The transformer output will be single phase.

 

9) Can Transformers develop Three Phase power from a Single Phase source?

 

No. Phase converters or phase shifting devices such as reactors and capacitors are

required to convert single phase power to three phase.

 

10) How do you select transformers?

 

1. Determine primary voltage and frequency.

2. Determine secondary voltage required.

3. Determine the capacity required in volt-amperes.

This is done by multiplying the load current (amperes) by the load voltage (volts)

for single phase. For example: if the load is 40 amperes, such as a motor, and the

secondary voltage is 240 volts, then 240 x 40 equals 9600 VA A 10 KVA (10,000

volt-amperes) transformer is required. ALWAYS SELECT THE

TRANSFORMER LARGER THAN THE ACTUAL LOAD. This is done for

safety purposes and allows for expansion, in case more load is added at a later

date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3)

then divide by 1000.

4. Determine whether taps are required. Taps are usually specified on larger

transformers.

5. Use the selection charts in the Acme catalog.

 

11) What terminations are provided?

 

Primary and Secondary Terminations are provided on ACME Dry Type Transformers as

follows:

No lugs-lead type connection on:

• 0-25 KVA single phase

• 0-15 KVA three phase

• Bus-bar terminations (drilled to NEMA standards)

• 37 1/2-250 KVA single phase

• 25-500 KVA three phase

 

12) Can 60 Hz transformers be used at higher frequencies?

 

Transformers can be used at frequencies above 60 Hz up through 400 Hz with no

limitations provided nameplate voltages are not exceeded. However, 60 Hz transformers

will have less voltage regulation at 400 Hz than 60 Hz.

 

13) What is meant by regulation in a transformer?

 

Voltage regulation in transformers is the difference between the no load voltage and the

full load voltage. This is usually expressed in terms of percentage. For example: A

transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the

regulation would be 5%. ACME dry type distribution transformers generally have

regulation from 2% to 4%, depending on the size and the application for which they are

used.

 

14) What is temperature rise in a transformer?

 

Temperature rise in a transformer is the temperature of the windings and insulation above

the existing ambient or surrounding temperature.

 

15) What is "Class" in insulation?

 

Insulation class was the original method used to distinguish insulating materials operating

at different temperature levels. Letters were used for different designations. Letter

classifications have been replaced by insulation system temperatures in degrees Celsius.

The system temperature is the maximum temperature at the hottest spot in the winding

(coil). These systems are used by Acme Transformer for a large part of the product line.

 

16) Is one insulation system better than another?

 

Not necessarily. It depends on the application and the cost benefit to be realized. Higher

temperature class insulation systems cost more and larger transformers are more

expensive to build. Therefore, the more expensive insulation systems are more likely to

be found in the larger KVA units.

 

All of these insulation systems will normally have the same number of years operating

life. A well designed transformer, observing these temperature limits, will have a life

expectancy of 20-25 years.

 

17) Why should Dry Type Transformers never be over-loaded?

 

Overloading of a transformer results in excessive temperature. This excessive

temperature causes overheating which will result in rapid deterioration of the insulation

and cause complete failure of the transformer coils.

 

18) Are temperature rise and actual surface temperature related?

 

No. This can be compared with an ordinary light bulb. The filament temperature of a

light bulb can exceed 2000 degrees, yet the surface temperature of the bulb is low enough

to permit touching with bare hands.

 

19) What is meant by "Impedance" in transformers?

 

Impedance is the current limiting characteristic of a transformer and is expressed in

percentage.

 

20) Why is impedance important?

 

It is used for determining the interrupting capacity of a circuit breaker or fuse employed

to protect the primary of a transformer.

 

Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a

10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60

Hz source. Calculate as follows:

 

Normal Full Load Current =

Nameplate Volt Amps 10,000 VA

____________ ______ = __________ =

Line Volts 480 V

20.8 Amperes

Maximum Short Circuit Amps =

Full Load Amps 20.8 Amps

____________ _ = _________ =

4% 4%

520 Amps

 

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse

required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and

secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is

short circuited (faulted), the following capacities are required:

 

Normal Full Load Current =

Volt Amps 75,000 VA

____________ __ ____________ _________ _____

Square Root 3 x Line Volts Square Root 3 x Line Volts

90 Amps

Maximum Short Circuit Line Current =

Full Load Amps 90 Amps

____________ __ = _______

5% 5%

1,800 Amps

 

The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480

volts.

 

NOTE: The secondary voltage is not used in the calculation. The reason is the primary

circuit of the transformer is the only winding being interrupted.

 

21) Can Single Phase Transformers be used for Three Phase applications?

 

Yes. Three phase transformers are sometimes not readily available whereas single phase

transformers can generally be found in stock. Three single phase transformers can be

used in delta connected primary and wye or delta connected secondary. They should

never be connected wye primary to wye secondary, since this will result in unstable

secondary voltage. The equivalent three phase capacity when properly connected of three

single phase transformers is three times the nameplate rating of each single phase

transformer. For example: Three 10 KVA single phase transformers will accommodate a

30 KVA three phase load.

 

22) Does ACME provide "Zig-Zag" Grounding Transformers?

 

Yes. This system can be used for either grounding or developing a fourth wire from a

three phase neutral. An example would be to change a 480 V - three phase - three wire

system to a 480Y/277 V - three phase - four wire system.

 

23) What color are ACME Dry Type Transformers?

 

ASA 61 (NEMA) light gray is used on all enclosed transformers from .050 to 500 KVA.

 

24) How do you select a transformer to operate in an ambient higher than 40

degrees centigrade?

 

When the ambient exceeds 40 deg. C use the following chart for de-rating standard

transformers.

Maximum

Ambient

Temperature

Maximum

Percentage

of Loading

40 deg. C (104 deg. F) 100%

50 deg. C (122 deg. F) 92%

60 deg. C (140 deg. F) 84%

Instead of ordering custom built transformers to operate in ambients higher than 40 deg.

C, it is more economical to use a standard transformer of a larger KVA rating.

 

25) Can transformers listed in this catalog be reconnected as autotransformers to

increase their KVA rating?

 

Several standard single phase transformers listed in this catalog can be connected as

autotransformers. The KVA capacity will be greatly increased when used as an

autotransformer, in comparison to the nameplate KVA as an insulating transformer.

Examples of autotransformer applications are changing 600 volts to 480 volts in either

single phase or three phase; changing 480 volts to 240 volts single or three phase or vice

versa; or the developing of a fourth wire (neutral) from a 480 volt three phase three wire

system for obtaining 277 volts single phase. This voltage is normally used for operating

fluorescent lamps or similar devices requiring 277 volts. For further details showing

KVA and voltage combinations for various autotransformer connections refer to the

Acme catalog.

 

26) Are ACME transformers shown in this catalog U.L. Listed?

 

All of the transformers, with few exceptions, are listed by Underwriters' Laboratories and

have met their rigorous requirements. We are also prepared to have transformers, which

are not presently listed, submitted for listing to Underwriters' upon the customer's

request. Please contact the factory for details.

 

27) Is CSA certification available for transformers shown in this catalog?

 

Most ACME transformers shown in this catalog are certified by Canadian Standards

Association. They have been designed and tested in accordance with the latest

specifications. Please contact the factory if further details are required.

 

28) What is BIL and how does it apply to transformers listed in this catalog?

 

BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that

consist of the application of a high frequency steep wave front voltage between windings,

and between windings and ground. The Basic Impulse Level of a transformer is a method

of expressing the voltage surge lightning, switching surges, etc.) that a transformer will

tolerate without breakdown. All transformers manufactured in this catalog, 600 volts and

below, will withstand the NEMA standard BIL rating, which is 10 KV. This assures the

user that he will not experience breakdowns when his system is properly protected with

lightning arrestors or similar surge protection devices.

 

29) What is polarity, when associated with a transformer?

 

Polarity is the instantaneous voltage obtained from the primary winding in relation to the

secondary winding. Transformers 600 volts and below are normally connected in additive

polarity - that is, when tested the terminals of the high voltage and low voltage windings

on the left hand side are connected together, refer to diagram below. This leaves one high

voltage and one low voltage terminal unconnected. When the transformer is excited, the

resultant voltage appearing across a voltmeter will be the sum of the high and low voltage

windings. This is useful when connecting single phase transformers in parallel for three

phase operations. Polarity is a term used only with single phase transformers.

 

30) What is exciting current?

 

Exciting current, when used in connection with transformers, is the current or amperes

required for excitation. The exciting current on most fighting and power transformers

varies from approximately 10% on small sizes of about 1 KVA and smaller to

approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of

two components, one of which is a real component and is in the form of losses or referred

to as no load watts; the other is in the form of reactive power and is referred to as KVAR.

 

31) Will a transformer change Three Phase to Single Phase?

 

A transformer will not act as a phase changing device when attempting to change three

phase to single phase. There is no way that a transformer will take three phase in and

deliver single phase out while at the same time presenting a balanced load to the three

phase supply system. There are, however, circuits available to change three phase to two

phase or vice versa using standard dual wound transformers. Please contact the factory

for two phase applications.

 

32) Can air cooled transformers be applied to motor loads?

 

This is an excellent application for air cooled transformers. Even though the inrush or

starting current is five to seven times normal running current the resultant lower voltage

caused by this momentary overloading is actually beneficial in that a cushioning effect on

motor starting is the result. The tables shown in "Steps for Selecting a Transformer"

illustrate some typical requirements for use with motor applications.

 

33) How is an Acme Drive Isolation Transformer (DIT) different than a General

Purpose Transformer?

 

DITs, as the name implies, are designed to be used with motor drives (AC and DC) and

to provide isolation from the service line. They are specifically designed to withstand the

"short circuit-like" duty imposed by the filing of the thyristors. Harmonics generated by

drives create added loads on the transformer. Therefore, it is important that a transformer

of equal or greater KVA to that recommended by the drive manufacturer be installed for

a particular motor application.

 

34) How are transformers sized to operate Three Phase induction type squirrel cage

motors?

 

The minimum transformer KVA rating required to operate a motor is calculated as

follows: Minimum Transformer KVA

Running Load Amperes x 1.73 x Motor Operating Voltage

____________ _________ _________ _________ _________ _____

1000

 

NOTE: If motor is to be started more than once per hour add 20% additional KVA.

Care should be exercised in sizing a transformer for an induction type squirrel cage motor

as when it is started, the lock rotor amperage is approximately 5 to 7 times the running

load amperage. This severe starting overload will result in a drop of the transformer

output voltage. When the voltage is low the torque and the horsepower of the motor will

drop proportionately to the square of the voltage. For example: If the voltage were to

drop to 70% of nominal, then motor horsepower and torque would drop to 70% squared

or 49% of the motor nameplate rating.

The underlying problem is low voltage at the motor terminals. If the ampere rating of the

motor and transformer overcurrent device falls within the motor's 50% RPM draw

requirements, a problem is likely to develop. The overcurrent device may not open under

intermediate motor ampere loading conditions. Overheating of the motor and/or

transformer would occur, possibly causing failure of either component.

This condition is more pronounced when one transformer is used to power one motor and

the running amperes of the motor is in the vicinity of the full load ampere rating of the

transforrner. The following precautions should be followed:

 

1. When one transformer is used to operate one motor, the running amperes of the

motor should not exceed 65% of the transformer's full load ampere rating.

2. If several motors are being operated from one transformer, avoid having all

motors start at the same time. If this is impractical, then size the transformer so

that the total running current does not exceed 65% of the transformer's full load

ampere rating.

 

35) Why are Small Distribution Transformers not used for Industrial Control

Applications?

 

Industrial control equipment demands a momentary overload capacity of three to eight

times normal capacity. This is most prevalent in solenoid or magnetic contactor

applications where inrush currents can be three to eight times as high as normal sealed or

holding currents but still maintain normal voltage at this momentary overloaded

condition. Distribution transformers are designed for good regulation up to 100 percent

loading, but their output voltage will drop rapidly on momentary overloads of this type

making them unsuitable for high inrush applications.

 

Industrial control transformers are designed especially for maintaining a high degree of

regulation even at eight times normal load. This results in a larger and generally more

expensive transformer. For a complete listing of ACME industrial control transformers,

refer to Section V in the ACME catalog.

 

36) Can 4-Winding Single Phase Transformer be auto-connected?

 

Yes. There are occasions where 480 volts single phase can be stepped down to 240 volts

single phase by autoconnecting a standard. If connected in this manner, the nameplate

KVA is doubled.

 

37) What about balanced loading on Three Phases?

 

Each phase of a three phase transformer must be considered as a single phase transformer

when determining loading. For example: A 45 KVA three phase transformer with a

208Y/120 volt secondary is to service 4 loads at 120 volts single phase each. These loads

are 10 KVA, 5 KVA, 8 KVA, and 4 KVA.

NOTE: That maximum loading on any phase does not exceed 10 KVA. Each phase has a

15 KVA capacity.

45 KVA = 15 KVA per phase

_____

3 phase

If incorrect method is used, phase B will have an 18 KVA load which is 3 KVA above its

normal capacity of 15 KVA and failure will result even though we only have a total load

of 27 KVA on a 45 KVA transformer.

 

38) What is meant by "Balanced Loading" on Single Phase Transformer

applications?

 

Since most single phase transformers have a secondary voltage of 120/240, they will be

operated as a three wire system. Care must be taken in properly distributing the load as

the transformer secondary consists of 2 separate 120 volt windings. Each 120 volt

winding is rated at one-half the nameplate KVA rating. For example: A 10 KVA

transformer, 120/240 volt secondary is to service an 8 KVA load at 240 volts and two 1

KVA loads at 120 volts each.

 

If the incorrect method is used, winding A will be loaded at 6 KVA, and winding B will

be loaded at 4 KVA. These do total 10 KVA but since each winding is only rated at 5

KVA (1/2 of nameplate rating), we have an overloaded transformer and a certain failure.

 

39) What are typical applications for transformers?

 

transformers should be specified to:

1. Distribute power at high voltage.

2. Eliminate double wiring.

3. Operate 120 volt equipment from power circuits.

4. Insulate circuits/establish separately derived circuits.

5. Provide 3-wire secondary circuits.

6. Buck and Boost (See following questions & answers).

7. Provide electrostatic shielding transient noise protection

 

40)  What about transformers in reverse?

 

Transformers connected in reverse, to proper input voltages, will provide correct

nameplate voltage output, albeit reversed. However, for transformers rated 2 KVA and

below, the output voltage would be less than the nameplate rating, since smaller KVA

transformers have a greater turn’s ratio compensation on their low voltage windings.

CAUTION: When reverse connecting a delta-wye transformer, a wye primary will be

created. Wye primaries may cause problems and are not recommended. If a wye

primary must be used, do not connect the neutral.

 

Any dry type distribution transformers can be reverse connected without a loss of KVA

rating, but there are certain limitations. Transformers rated 1 KVA and larger single

phase, 3 KVA and larger three phases can be reverse connected without any adverse

effects or loss in KVA capacity.

 

Typically the output winding is wound first and is therefore closest to the core. When

used as exciting winding a higher inrush current results. In most cases the inrush

current is 10 to 12 times the full load current for 1/10 of a second. When the transformer

is reverse fed the inrush current can be up to 16 times greater. In this case a bigger

breaker with a higher AIC rating must be used to keep the transformer online.

Taps are normally in the primary winding to adjust for varying incoming voltage. If the

transformer is reverse fed, the taps are on the output side and can be used to adjust the

output voltage.

 

41) Can transformers develop three phase power from a single phase source?

 

No. Phase converters or phase shifting devices such as reactors and capacitors are

required to convert single phase power to three phases.

 

42) Are temperature rise and actual surface temperature related?

 

No. This can be compared with an ordinary light bulb. The filament temperature of a

light bulb can exceed 2000 degrees yet the surface temperature of the bulb is low

enough to permit touching with bare hands.

 

43) What is BIL and how does it apply to transformers?

 

BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that

consist of the application of a high frequency steep wave front voltage between

windings, and between windings and ground. The BIL of a transformer is a method of

expressing the voltage surge that a transformer will tolerate without breakdown.

 

44) Can a three phase transformer be loaded as a single phase transformer?

 

Yes, but the load can not exceed the rating per phase and the load must be balanced.

(KVA/3 per phase)

 

For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each

secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each

secondary.

 

45) Why it is not recommend reverse connecting a delta-wye transformer?

 

You can reverse wire the delta-wye, the primary wye is connected without using the

neutral (X0) and it turns into a delta-delta.

 

46) What would happen if you connected the neutral on the wye primaries if reverse

connected? Would this short, overheat?

 

The problem is that you could get a fault current on the neutral which may not trip the

breaker in case of a problem.

 

47) What is an isolating transformer?

 

An isolating transformer has the primary and secondary windings connected

magnetically, but not electrically. Also referred to as an "insulating" transformer.

 

48) What is a non-linear (K-factor) transformer?

 

A transformer that is designed to handle the odd harmonic current loads caused by

much of today's modern office equipment. A non-linear transformer has a K-factor rating

that is an index of its ability to supply harmonic content in its load current while

remaining within its operating temperature limit.

 

49) What is a drive isolation transformer?

 

A drive isolation transformer is designed for use with motor drives. It must isolate the

motor from the line and handle the added loads of the drive-created harmonic current. It

is important to heed the drive manufacturer's recommendations for transformer KVA.

 

50) What is a buck boost transformer?

 

Buck-boost transformers are single-phase isolated distribution transformers having four

windings instead of two. They can be connected as an autotransformer to buck (reduce)

or boost (raise) the line voltage from 5 - 20%. Typical reduced secondary voltages are

12, 16, 24, 32, or 48 volts. Commonly found raised secondary voltages are 208 to 230

or 240 volts.

 

51) What will happen if transformers are operated at non-nameplate voltages?

 

A transformer is designed using specific ratios that relate to the rated KVA, primary

voltage and secondary voltage proportionally. Operating a transformer above or below

the nominally designed primary voltage will reflect a proportional increase or decrease

in secondary output levels. Extreme caution must be observed when overvoltage levels

exist. Excessive input voltage will cause higher core losses, increased noise and

elevated temperatures. Overvoltages for any extended period of time have a significant

effect on insulation breakdown and transformer failures. Transformers can be

specifically designed for extreme voltage conditions if initial specifications state those

requirements.

 

In some cases, transformers can be operated at voltages below the nameplate rated

voltage. In NO case should a transformer be operated in excess of its nameplate rating

unless taps are provided for this purpose. When operating below the rated voltage the

KVA capacity is reduced correspondingly.

 

51) What would be the result of overloading dry type transformers?

 

All transformers are designed to accommodate short periods of overloading. As the

overload becomes excessive and the duration increases, the transformer will

experience a percent loss of life. Prolonged overloading generates excessive heating

which results in insulation deterioration and ultimately transformer failure. Contact

/consult your design engineer to determine loading for your unique application.

 

53) What are taps and when are they used?

 

Taps are provided on some transformers on the high voltage winding to correct for high

or low voltage conditions, and still deliver full rated output voltages at the secondary

terminals. Taps are generally set at two and a half and five percent above and below

the rated primary voltage.

 

54) What is the difference between "Insulating", "Isolating", and "Shielded Winding"

transformers?

 

Insulating and isolating transformers are identical. These terms are used to describe the

separation of the primary and secondary windings. A shielded transformer includes a

metallic shield between the primary and secondary windings to attenuate (lessen)

transient noise.

 

55) Can 60 Hz transformers be operated at 50 Hz?

Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service

due to higher losses and resultant heat rise. However, any 50 Hz transformer will

operate on 60 Hz service.

 

A 60 Hz design is physically smaller than a 50 Hz design. DO NOT use 60 Hz rated

transformers on 50 Hz service. Without special designs, higher losses and greater heat

rise will result. Operating 60 Hz transformers at higher frequencies may simply provide

less voltage regulation.

 
10 comments on "Basics of Transformer - Q & A (Useful for Freshers)"
  Commented by  AVINASH GUPTA, B.Tech/B.E. student, SRIT    | 06 27 2013 14:34:54 +0000
very good overview of what transformer is all about?

thanks for giving this article...
  Commented by  Ramakrishna Perumal, Electrical Specialist Engineer,    | 10 20 2012 14:48:52 +0000
Inverter output without filter will have harmonics effect.
  Commented by  Nazmul Islam, B.Sc student, AIUB    | 10 20 2012 08:17:39 +0000
Is it possible to pass the 3 phase SPWM Inverter output through 3 phase .4/11KV distribution transformer without filtering?
  Commented by  Ramakrishna Perumal, Electrical Specialist Engineer,    | 09 06 2012 06:28:10 +0000
It depends on application for 1 phase loads we can use 1 phase transformers and we can also use 3 phase transformer by connecting 1 phase and neutral. For 3 phase loads we have to use 3 phase loads. 3 phase transformers are widely used compare to 1 phase transformers.
  Commented by  tanya sharma, Sourcing engineer, Alstom india ltd    | 09 01 2012 08:09:55 +0000
Sir, There are two type of transforme r, 1) single phase 2) three phase , can you tell which one is more useful and why.
  Commented by  Ramakrishna Perumal, Electrical Specialist Engineer,    | 08 31 2012 12:25:40 +0000
Naveen,
Other disadvantages are Initial Cost will be high and also the cooling of higher capacity transformer will be expensive like may require oil.  
  Commented by  NAVEEN NATH DAS, Electrical Engineer-Utility, NEIGRIHMS    | 08 31 2012 05:57:59 +0000
Sir, could you explain besides more lossess and higher output voltage what are other disadvantages of using higher capacity transformer for lighter loads (in case one has to use 2 MVA transformer in place of earlier 1 MVA). Whether it is advisable for use for a longer period?
  Commented by  Ramakrishna Perumal, Sr. Engineer, Technicas Reunidas    | 11 03 2009 08:54:08 +0000
Mr.Ajay the status quo transformers are called unity ratio transformers.
  Commented by  Ajay Ziz, Deputy Registrar, University of Jammu    | 11 03 2009 06:45:49 +0000
step up & step down , transformers okay ::

but what about status quo transformers ??
  Commented by  Vineet Prakash, Sales/BD Manager, Tata Power    | 11 03 2009 06:21:38 +0000
Nice article Mr.Perumal, it contains all the details of transformers. Thanks for sharing..
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It depends on application for 1 phase loads we can use 1 phase transformers and we can also use 3 phase transformer by connecting 1 phase and neutral. For 3 phase loads we have to use 3 phase loads. 3 phase transformers are widely used compare to 1...
Naveen, Other disadvantages are Initial Cost will be high and also the cooling of higher capacity transformer will be expensive like may require oil.
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