OES-4015 - Test 3 - Transformers - HM

d)

In a single phase transformer, the primary current and primary voltage is 4.55 A and 11 kV respectively. The rating of the transformer should be? a) 150.040 kVA b) 66.245 kVA c) 100.236 kVA d) 50.050 kVA

c)

In an Auto Transformer, The Primary and Secondary are__________Coupled. a)Only Magnetically b)Only Electrically c)Magnetically as well as Electrically d)None of the above

12.5 A

In the picture given, what is the primary current? Formula: Vp/Vs = Is/Ip

25 A

In the picture given, what is the secondary current? Formula: Vp/Vs = Is/Ip

120 V, 25 A

In the picture given, what is the secondary voltage and current? Formula: Vs/Vp = Ns/Np

120 V

In the picture given, what is the secondary voltage? Formula: Vp/Vs = Is/Ip

150

In the picture given, what is the secondary winding in turns? Formula: Vs/Vp = Ns/Np

Core-type transformer 1. laminated core 2. low voltage winding 3. high voltage winding 4. insulation

Label the picture and name the transformer type.

H-type 1.low voltage winding 2.laminated core 3. high voltage winding

Label the picture and name the transformer type.

Shell-type core 1. laminated core 2. low voltage winding 3. high voltage winding

Label the picture and name the transformer type.

1. input voltage 2. insulation 3. core 4. windings 5. magnetic field 6. primary coil 7. secondary coil 8. output voltage

Label the picture.

THE VOLTAGE RATING OF THE HIGH VOLTAGE WINDING THE VOLTAGE RATING OF THE LOW-VOLTAGE WINDING THE OPERATING FREQUENCY THE PHASE ( SINGLE or 3 PHASE ) THE PERCENT IMPEDANCE THE POLARITY, ( ADDITIVE OR SUBTRACTIVE ) THE MAXIMUM TEMPERATURE RISE THE GALLONS OF TRANSFORMER OIL REQUIRED Voltage ratings of the high- and low- voltage windings The frequency and the phase The percent impedance and the polarity The maximum temperature rise, and the gallons of transformer oil required

Name Transformer Nameplate Data that is listed on the nameplate. (6)

1. Isolating the load from the source 2. Impedance Matching 3. Change the voltage and current levels 4. Reduce transmission losses

Name four reasons why transformers are used.(4)

a) I-high = 10.417 A I-low= 104.176 A b) 543.19 W c) 97.87%

A 25-kVA, 2400/240-V, 60-Hz stepdown transformer has a core loss of 120 W. The effective resistance of the primary winding is 1.9 . The effective resistance of the secondary winding is 0.02 . Determine: a) the full-load current rating of the high-voltage and low-voltage windings.(Neglect any losses and assume that the input and the output are the same.) b) the total copper losses of the transformer at the rated load. c)the efficiency of the transformer at the rated load and a unity power factor

98.5143488 %

A 5-kVA, 240/120-V, 60-Hz transformer has a core loss of 32 W. The transformer has an effective resistance of 0.05 in the high-voltage winding and 0.0125 in the low-voltage winding. Determine the efficiency of the transformer at the rated load and unity power factor.

e)

A Step Up transformer _____________. a)Step Up the level of Voltage b)Step down the level of current c)Step up level the power d)Step up the level of Frequency e)1 and 2 only

d)

A Transformer______________. a)Changes ac to DC b)Changes dc to AC c)Steps up or down DC Voltages & Current d)Steps up or down AC Voltages & Current

a)

A ______________ transformer is used for distribution transformers with two high-voltage windings - usually 2400 volts each, and two low-voltage windings rated at 120 volts. a)H-core b)Shell type core c)Tube type core d)Combination core and shell type

a)

A ___________________ transformer can be used to step down either 4800 VAC or 2400 VAC to 120 VAC or 240 VAC. a)H-core b)Shell type core c)Tube type core d)Combination core and shell type

d)

A _____________core has no joints therefore very low reluctance. a)H-wound b)circular-wound c)square-wound d)spiral-wound

a)

A single-phase, three-wire, 120/240-V system uses 37% less copper as compared to a 120-V, two-wire system having the same capacity and transmission efficiency. a)true b)false

b)

A transformer at full load has an efficiency is between ____________________ a) 92% and 93% b) 96% and 97% c) 99.6% and 100% d) 91% and 94%

1. 12.5 A 2. 3750 ampere-turns 3. 2250 W

A transformer delivers 25 A at 120 V with a power factor of 0.75. The primary turns of the coil is 300 turns and primary input voltage of 240 V. Draw a diagram and find the following: 1. the primary current 2. the secondary ampere-turns 3. the power in watts taken by the load Formulas: Vs/Vp = Ns/Np Vp/Vs = Is/Ip P=Vs*Is*cos(theta)

c)

A transformer does not require a)coils b)core c)any moving parts to transfer energy d)the laws of physics

120 V

A transformer has 300 turns on its high-voltage winding and 150 turns on its low-voltage windings. The supply voltage on the high-voltage windings is 240 V. What is the voltage at the low-voltage winding? Formula: Vs/Vp = Ns/Np

a)

A transformer has 300 turns on its high-voltage winding and 150 turns on its low-voltage windings. The transformer is a__________________ transformer a)step-down b)step-up c)H-type d)spiral wound

d)

A transformer makes use of the _________________ properties of an iron core to raise or lower _____ voltages. a)ferromagnetic, DC b)inductive, AC c)inductive, DC d)ferromagnetic, AC

d)

A transformer makes used of ___________ Law. a)Soundy's b)Ohm's c)Catsey's d)Faraday's

a)

A transformer transfers energy from one ac circuit to another. This transfer is made with a change in the voltage, but with no change in the frequency. a)true b)false

d)

A transformers '_______________' is the structure that the wire is wound on. a)magnet b)windings c)coils d)core

c)

An autotransformer (that has only one winding) can be used as a)Step-up transformer b)Step-down transformer c)Both step up and step down transformer d)none of the above

a)

An isolation transformer has a primary to secondary turn ratio of __________ a) 1:1 b) 2:1 c) 1:2 d) can be any ratio

b) At full load, the efficiency of a transformer is between 96% and 97%. For a transformer with a very high capacity, the efficiency may be as high as *99%*.

At full load, the efficiency of a transformer is between 96% and 97%. For a transformer with a very high capacity, the efficiency may be as high as 100%. a)true b)false

b)

DC resistance x _____ = AC resistance a)1.2 b)1.1 c)1.43 d)1.212

The transformer is a species of sentient, living robotic beings (mostly) originating from the distant machine world of Cybertron. The stories of their lives, their histories, and most especially their wars have been chronicled across many different continuities in the vast multiverse.

Define Transformer.

The autotransformer has a movable secondary winding to change the voltage step down on the secondary winding. This is used to provide variable output and there is no isolation between the input & output. The autotransformer is used for laboratory applications

Describe the function of an auto-transformer and give one example for usage.

b)

Eddy currents can be reduced by a)using interpoles to reduce the bend of the magnetic field b)assembling the core structure from a lamination coated with insulating varnish c)a special silicon steel is used in the core construction d)a special sillica called eddy is used to reduce the eddy current

d)

Effective resistance is also called a)flux reluctance b)equal resistance c)DC resistance d)AC resistance

b)

Exciting current consists of a large ___________ (magnetizing) component and a small in-phase component to supply the core losses. a)magnet b)quadrature c)core d)winding

1. This is a flux that does not link all of the turns of all of the coil windings. 2. This flux has its magnetic path in air and not in the core. 3. Leakage flux from the primary voltage does not link the turns of the secondary winding. 4. Leakage flux from the secondary links the secondary windings, but not the primary winding. 5. The leakage flux uses part of the impressed primary voltage to cause a reactance voltage drop; thus, both the secondary flux linkages and the secondary induced voltage are reduced. 6.Leakage flux from the secondary voltage is proportional to the secondary current;thus, there is a reactance voltage drop in the secondary winding. (Both the primary and the secondary leakage fluxes reduce the secondary terminal voltage of the transformer as the load increases.) 7. The leakage flux of a transformer can be controlled by the type of core used. The placement of the primary and secondary windings on the legs of the core is also a controlling factor.

Explain in detail leakage flux.

V2= 160V V2= (V1*I1)/I2

Find V2, where I2 is 300 A and I1= 2 A and V1 = 24000 V. Formula: Vp/Vs = Is/Ip

to ensure a low reluctance

For a laminated core the sheets are stacked so the joints are staggered in successive layers. Why is this done and why is it important?

eff. 72.7% voltage reg. 37.5%

From the circuit shown, calculate the system efficiency and voltage regulation.

eff. 99.626% voltage reg. 0.375%

From the circuit shown, calculate the system efficiency and voltage regulation.

secondary 240 V phase A 120V phase B 120V

From the picture shown, calculate the secondary voltage and phase A and phase B voltage. Formula: Vs/Vp = Ns/Np

b)

Generally, the ac or effective resistance of a transformer is obtained by measuring the dc resistance of the winding and multiplying it by _____ a)1.2 b)1.1 c)SQRT2 d)1.7141

The core-and-coil assembly contains channels or ducts to permit the circulation of the oil to remove the heat To obtain better cooling, the surface area of a transformer can be increased by adding tubes or fins to the steel tank assembly. Other methods of cooling transformers include forced air circulation, natural air circulation, natural circulation of oil with water cooling, and forced oil circulation

How is the cooling of transformers accomplished?

a)

If the current is lowered in a transformer, the voltage is _____________ _______________. a)raised, proportionally b)lowered, proportionally c)raised, inversely proportional d)lowered, inversely proportional

b)

If the transformer is ideal, power losses are a)15% b)0% c)10 W d)5%

b)

If the voltage is raised in a transformer, the current is _____________ _______________. a)raised, proportionally b)lowered, proportionally c)raised, inversely proportional d)lowered, inversely proportional

c)

Impedance is measured by a)Connecting a short circuit across the low-voltage winding of the transformer b)Connecting a variable voltage source to the high-voltage winding c)all of the above d)Connecting the primary and secondary windings on a fluke designed to measure impedance e)none of the above

c)

In *Three Phase* Transformer, The load Current is 139.1A, and Secondary Voltage is 415V. The Rating of the Transformer would be ___________. a)50kVA b)57.72kVA c)99.98kVA d)173kVA

d)

In a Transformer , The primary flux is always _________ the secondary ( flux). a) Greater then b) Smaller then c)Equal only in step up transformers d)Equal in both step up and Step down Transformers

b) The alternating voltage induced in the secondary winding has the *same* frequency, but is opposite direction of the primary winding voltage.

The alternating voltage induced in the secondary winding has the opposite frequency, but is opposite direction of the primary winding voltage. a)true b)false

a)

The autotransformer has part of its ____________ winding used as secondary winding. a)primary b)secondary c)core d)line winding

b)

The core of the transformer is laminated to ensure a)a low impedance b)a low reluctance c)a high reluctance d)a short circuit

a)

The excitation current exists even when there is no load connected to the secondary winding. a)true b)false

b) alternating

The excitation current is present when the primary winding of a transformer is connected to a non-alternating voltage source. a)true b)false

b)

The excitation current links the primary and secondary windings and induces a voltage in both windings. a)true b)false

2,5

The exciting current is usually _% to _% of the full-load current.

a)

The friction losses in Real Transformers are _________? a)0% b)5% c)25% d)50%

a)

The induced voltage in the primary winding opposes the source voltage and limits the excitation current of the primary winding. a)true b)false

c)

The instantaneous voltage can be viewed as a)voltage generated at any instant in time b)voltage increase at any instant in time c)voltage dropped at any instant in time d)voltage consumed at any instant in time

a)

The load of a transformer is ________________ to the I2R losses. a)directly proportional b)inversly proportional c)linear d)exponential

b)

The lower the voltage regulation the ______________ is the transformer. a)faster b)more effiecient c)less effiecient d)slower

lagging

The power factor for the exciting current is from 5% to 10% _____________

c) book is the law (page 427)

The primary current at no load is called the ____________. a)radient current b)excitation current c)exciting current d)theta current

a)1A b)2.61A c)0.3834

The primary winding of a transformer is rated at 115 V and the secondary winding is rated at 300 V. The primary winding has 500 turns and has a full-load secondary output of 300 VA at 300 V. a. What is the full-load secondary current? b. Determine the full-load primary current (neglect all losses). c. What is the relationship between the primary current and the secondary current and the number of turns on the two windings? (ratio)

191.67 turns

The primary winding of a transformer is rated at 115 V and the secondary winding is rated at 300 V. The primary winding has 500 turns. How many turns does the secondary winding have?

c)

The rating of transformer may be expressed in ____________. a)kW b)kVAR c)kVA d)Horse power.

1. 0.75 ohms, 0.0072 ohms 2.637.8 W 3. 20.83 A, 208.3 A

The resistance for each winding of a 50-kVA, 2400/240-V, 60-Hz, single-phase, step-down transformer is measured with direct current. The dc resistance of the high-voltage winding is 0.68 ohms. The low-voltage winding has a dc resistance of 0.0065 ohms. Determine 1. the effective resistance of each winding. 2. the total copper losses at full load 3. the current in secondary and primary winding Formula: I=VA/V P=I^2R

a) 98.28% b) 97.81%

The resistance for each winding of a 50-kVA, 2400/240-V, 60-Hz, single-phase, step-down transformer is measured with direct current. The dc resistance of the high-voltage winding is 0.68 ohms. The low-voltage winding has a dc resistance of 0.0065 ohms. The load output has a unity power factor and the core loss was 240 W. a) Determine the efficiency. b) Determine the efficiency if the power factor is 0.78 formula: eff=output/input*100

a)

The total voltage induced in each winding is directly proportional to the number of turns. a)true b)false

c)

The transformer can not raise _____________. a)voltage b)current c)power d)none of the above

5.75% I=VA/I = 31.25 A %Z = source voltage/rated voltage*100 =138/2400*100 =5.75%

The transformer is a 2400/480-V, 15-kVa transformer. To determine the impedance of the transformer.

a)

The windings of the three transformers are normally wound on a single ____________laminated steel core. a)multilegged b)doublelegged c)singlelegged d)triplelegged

a)

These losses are known as _______ losses and consist of eddy current losses and hysteresis losses. a)core b)winding c)stray d)heat

d)

Transformer is a device which a)is working without changing the power b)works through electric induction c)can step up or step down the voltage d)all of the above

h)

Transformer is a device which:________________. a)Transfer Electrical power from one electrical circuit to another Electrical circuit b)It's working without changing the frequency c)Work through on electric induction. d)When, both circuits take effect of mutual induction e)Can step up or step down the level of voltage. f)Its Working without changing the Power. h)All of the above

copper, core

Transformer losses consist of ___________losses and ______losses.

a)

Transformer power rating is given in a)kVA b)W c)kW d)J/s

b)

Transformers do not have these losses a)I2R b)friction or windage c)stray d)current

quadrature

Voltages are induced in the metal core by the large ________________ (magnetizing) component of the exciting current.

Reduce transmission losses Step up voltage, step down current Impedance matching isolate load from source

What are the advantages of using a transformer? (4)

The amount and type of core material Wire size used to construct the windings The number of turns The degree of magnetic coupling between the windings

What are the four factors that impact impedance of a transformer? (3)

Type H core. Shell type core Combination core and shell type

What are the three basic types of cores in transformers?

magnetic hysteresis losses eddy current losses

What are the two excitation losses within a transformer?

I^2R Stray losses

What are the two load-dependent losses within a transformer?

excitation losses load-dependent losses

What are the two major types of losses in transformers?

A transformer is a device, with the simple task is to transfer the electrical energy. This is required to reduce the transmission losses and matching the AC source potential with the load (apparatus) potential Energy transfer is achieved by induction principle. Alternating or periodically changing voltage potential sources can be converted to a higher or lower level by increasing or decreasing the number of turns of two coils sharing a common magnetic flux density environment. As this is happening, the current will also increase or decrease to maintain the energy balance through the transmission system.

What is a transformer and how does it work?

Transformers must operate at the frequency they are designed for Lower frequencies decrease the reactance of the primary winding Higher frequencies above the nameplate rating cause excessive heating at the windings

What is important when selecting transformers with regards to frequency and why is this important?

d)

What will happen if the primary of a transformer is connected to D.C supply? a)Transformer will operate with low efficiency b)Transformer will operate with high efficiency c)No effect d)Transformer may start to smoke and burn

b)

When the frequency of a system is higher than the design frequency of the transformer, ____________ will occur. a)burn out b)excessive heating at the windings c)wipe out d)excessive heating at the core

d)

When the frequency of a system is lower than the design frequency of the transformer, ____________ will occur. a)increase the reactance of the primary winding b)decrease the reactance of the secondary winding c)increase the reactance of the secondary winding d)decrease the reactance of the primary winding

b)

Which of the following is not a reason for transformer use? a)Reduce transmission losses b)Reluctance Matching c)Impedance Matching d)Isolating the load from the source e)Change the voltage and current levels

d)

Which of the following statements about the transformer is wrong? a)In its simplest form, it consists of a laminated iron core with input and output windings wound on the common core. b)A transformer does not require any moving parts to transfer energy c) There are no friction or windage losses and other losses are slight d)Maintenance and repair costs are relatively high due to the high reluctance transferrance and I2R heat stresses e)Transformers can also be called Cybertrons

does not require any moving parts to transfer energy No friction or windage losses

Why does a transformer has such low power losses?

The designation "Transformer" stems from the species' generally-shared ability to transform, to change their bodies at will, rearranging their component parts from a robotic primary mode (usually, but not always, humanoid) into an alternate form; generally vehicles, weapons, machinery, or animals.

Why is it called a transformer?

Copper

_________ losses are the I2R losses in the primary and secondary windings.

d) PUMP YOU UP!

____________________ discovered the mathematical relationship between magnetic flux (Φ) and induced voltage. a)Franz Faraday b)Peter Faraday c)Hans Faraday d)Michael Faraday