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Electrical Fundementals > Transformers > Flashcards

Transformers Flashcards

1
Q

How many turns does the secondary winding of a 2:1 step-up transformer have?

A

More than primary.

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2
Q

To reduce eddy currents in a transformer you would?

A

Reduce the thickness of laminations in the magnetic core.

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3
Q

A transformer has an input of 400V and a ratio of 2:1. If the transformer is delta/star wound, what will the line voltage output be?

A

346V.

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4
Q

A Transformer with 115V primary voltage and a ratio of 5:1 is supplying a landing light (load 24V 45 amps), what is the current drawn?

A

9 amps.

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5
Q

A Transformer has 4500 secondary turns and 750 primary turns. Its turns ratio is?

A

6:1.

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6
Q

The point of coercivity occurs when the?

A

Magnetic flux is zero even though a magnetizing force is being applied.

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7
Q

The primary winding of a 3-phase transformer?

A

Could be either delta or star wound.

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8
Q

In a transformer, if the rate of change of current increases, the mutual inductance will?

A

Remain the same.

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9
Q

A 4:1 step down transformer draws 115 V and 1 A. The output power will be?

A

28.5 V at 4 A.

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10
Q

The secondary coil of a transformer has 1500 turns and 10 Ω resistance. The primary coil has 1kΩ resistance. How many turns does the primary coil have?

A

15,000.

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11
Q

In a star wound primary transformer, how could you wind the secondary winding?

A

Either delta or star.

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12
Q

A step up transformer has?

A

Two windings.

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13
Q

In transformer core loss is 200 W and copper loss is 220W at no load. What is the core loss at full load?

A

200 W.

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14
Q

The secondary winding of a transformer has an impedance of 10 ohms and 1500 turns. For the primary winding to have an impedance of 1 kilohm, how many turns must it have?

A

15,000.

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15
Q

Eddy currents in a transformer core are reduced by?

A

Decreasing the thickness of the laminates.

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16
Q

Transformer copper loss on full load is 220 Watts. On half load the loss will be?

A

55 Watts.

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17
Q

How can it be determined if a transformer winding has some of its turns shorted together?

A

The transformer will get hot in normal operation.

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18
Q

Eddy currents in a transformer can be reduced by?

A

Laminations.

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19
Q

What is the ratio of turns between the primary coil winding and the secondary coil winding of a transformer designed to triple its input voltage?

A

Primary will have one-third as many turns as its secondary.

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20
Q

A transformer should always have its?

A

Secondary open circuit or on load.

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21
Q

Which of the following would be used to calculate transformer turns ratio?

A

Secondary turns ÷ primary turns.

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22
Q

The voltage in a transformer secondary coil that contains twice as many loops as the primary coil will be?

A

Greater and the amperage greater than in the primary coil.

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23
Q

The area of a hysteresis loop is proportional to the?

A

Energy dissipated in putting the material through a complete cycle of magnetization and demagnetization.

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24
Q

In transformer operation, hysteresis losses are?

A

Energy wasted in reversing core magnetization.

25
Q

What causes a transformer to be noisy?

A

An air gap in the core.

26
Q

A transformer with a 5:1 ratio has a 24V output. What is the input?

A

120 V AC.

27
Q

Copper losses in a transformer are caused by?

A

The resistance in the windings.

28
Q

The copper loss on a single phase transformer is 25 Watts on full load. What will it be on half load?

A

6.25 Watts.

29
Q

A transformer is designed to operate at 60 Hz. It is connected across a 50 Hz supply. The primary windings would?

A

Draw more current.

30
Q

A current transformer, when disconnected from a circuit must be?

A

Short circuited.

31
Q

What is the reflected impedance in the primary of the transformer circuit shown?

A

500 ohms.

32
Q

A transformer rated at a frequency of 200 Hz?

A

May only be used at the rated frequency or above.

33
Q

The dots on the transformer symbol shown are called, and indicate?

A

Phasing dots, point that have the same polarity at the same moment in time.

34
Q

A transformer is built with a material having a?

A

Narrow hysteresis loop.

35
Q

A transformer is built with a material having a

A

narrow hysteresis loop.

The area of the hysteresis loop is the power lost in repeatedly reversing the magnetism.

36
Q

In a circuit which possesses mutual inductance, current is increased to the primary.

A

the induced EMF in the secondary produces a current in the secondary circuit which opposes the increase in flux due to the increase of current in the primary.

37
Q

A transformer has 500 turns on the primary and 100 turns on the secondary. When connected to a 100 V supply, the output is.

A

20 V.

5:1 step down. 100/5 = 20V.

38
Q

A transformer has 500 turns on the primary and 100 turns on the secondary. The secondary supplies 20V and is connected to a 10 ohm load. The primary current is.

A

0.4 amps.

Secondary current = V/R = 20/10 = 2A. Primary current = 2A/5 = 0.4A.

39
Q

A transformer has 1200 turns primary and 4800 turns secondary. What value of load resistance would be required to give a reflected impedance of 1000 ohms?

A

16 kilohm.

Impedance matching formula, Turns Ratio = √(Impedance Ratio).

40
Q

An autotransformer windings are.

A

connected.

41
Q

What voltage can the neutral line carry in a star transformer?

A

Zero voltage.

The neutral line is grounded.

42
Q

A transformer to supply a 24V load with a 5:1 turns ratio will have what primary supply?

A

120 V AC.

5:1 is step down, so the supply voltage must be 5 * 24 = 120 VAC (AC because transformers don’t work with DC).

43
Q

A transformer has 2000V, 500 turns on the primary and 50 turns on the secondary. What would be the line voltage if the transformer were Delta/Star connected?

A

346V.

It is a 10:1 step down transformer. Phase voltage in secondary = 200V. With star wound secondary, line voltage = 1.73 * 200 = 346V.

44
Q

The line voltage in a Wye wound transformer is.

A

root 3 * phase voltage.

Line voltage = root 3 * phase voltage in a Wye (star) wound transformer.

45
Q

Eddy current losses in a transformer are.

A

magnetic flows induced into the core.

46
Q

A 3-phase star-delta wound transformer gives.

A

30° phase shift.

A star-delta wound transformer gives a 30 degree phase shift.

47
Q

In a 4:1 step up transformer, what would give a reflected impedance of 1Kilohm?

A

16k ohm.

Turns ratio = √(impedance ratio).

48
Q

The input is 115V and is applied across the 300 turn portion of the autotransformer shown. What is the output?

A

460V.

Autotransformer 300:1200 ratio 1:4. Output = 4 * 115V.

49
Q

A near perfect 4:1 step up transformer has its primary connected to 120VAC, if the secondary is connected to a load with 1600 ohms resistance, what is the current in the primary?

A

1.2 amps.

Secondary voltage = 480V, Secondary current = 480/1600 = 0.3A, Primary current = 4 * 0.3 = 1.2A.

50
Q

Flux losses in a transformer can be prevented by.

A

winding the primary on top of the secondary.

51
Q

To supply a 24volt output with a transformer with a 5:1 turns ratio, the input required is.

A

120V.

Assuming this is a step-down transformer (i.e 5:1 means Primary:Secondary).

52
Q

What would you use for the core in a generator current transformer?

A

Air core.

A current transformer uses the generator output cable as its primary, so it must have a hollow (air) core.

53
Q

An aircraft transformer is needed to step up a voltage. What sort would be used?

A

Laminated core.

Laminated core transformers are most efficient.

54
Q

In a 4:1 step-up transformer, 120V is applied to the primary, and the load on the secondary is 1600 Ohms. What is the current in the primary?

A

1.2A.

Voltage in secondary = 4 * 120 = 480V. Current in Secondary = 480/1600 (Ohm’s Law) = 3/10A. Current in Primary = 4 * 3/10A = 12/10A = 1.2A.

55
Q

A 10:1 step up power transformer has 120 V, and 3A flowing through the primary, and 0.285 A flowing through the secondary what is the efficiency?

A

95%.

Power in primary = 120 * 3 = 360W. Power in secondary = 1200 * 0.285 = 342W. 342/360 = 0.95.

56
Q

Aircraft autotransformers are of what type?

A

Laminated core.

57
Q

An autotransformer is.

A

primary and secondary coils on single bar.

58
Q

The windings on a autotransformer are.

A

wound with no magnetic core.

59
Q

How do you reduce Hysteresis loss of a transformer?

A

By laminating the core.

Electrical Fundementals (19 decks)

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