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M3 > 3.15 Transformers > Flashcards

3.15 Transformers Flashcards

1
Q
A
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2
Q

What is a transformer?

A

An electrical device that changes voltage and current in AC circuits without significant power loss.

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3
Q

What does a step-up transformer do?

A

Increases voltage and decreases current.

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4
Q

What does a step-down transformer do?

A

Decreases voltage and increases current.

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5
Q

What are the main components of a transformer?

A

Two coils and an iron core.

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6
Q

What is the primary winding of a transformer?

A

The input coil.

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7
Q

What is the secondary winding of a transformer?

A

The output coil.

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8
Q

What is the purpose of a laminated iron core?

A

To suppress eddy currents that waste power by heating the core.

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9
Q

How does a transformer operate?

A

It creates a changing magnetic field in the iron core, inducing voltage in the output coil.

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10
Q

What is mutual induction in a transformer?

A

The process of producing an EMF in the secondary coil due to the changing magnetic field from the primary coil.

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11
Q

Does frequency change in a transformer?

A

No, frequency does not change in a transformer.

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12
Q

What is the voltage phase relationship in a transformer?

A

It may be either 0° or 180°, depending on the winding of the secondary relative to the primary.

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13
Q

Why are transformer symbols marked with phasing dots?

A

To indicate the phase relationship between the primary and secondary coils.

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14
Q

Fill in the blank: A transformer transfers energy via a _______.

A

[magnetic circuit].

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15
Q

What is the efficiency range for small power transformers used in electrical equipment?

A

80-90%

Large commercial power line transformers can have efficiencies of over 98%.

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16
Q

What are the two types of total power loss in a transformer?

A
  • Copper loss
  • Iron loss
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17
Q

What causes Copper loss in transformers?

A

Direct current (DC) resistance in the primary and secondary windings.

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18
Q

What are the two subcategories of Iron loss in transformers?

A
  • Hysteresis loss
  • Eddy current loss
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19
Q

What do all transformer losses convert electrical energy into?

A

Undesirable heat energy.

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20
Q

What factors determine the amount of power dissipated as heat in Copper loss?

A
  • Resistance of the wire
  • Square of the current
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21
Q

How can Copper loss be minimized?

A

Using the proper diameter wire.

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22
Q

What type of material is commonly used for the core of a transformer?

A

Ferromagnetic material.

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23
Q

What is the effect of AC on the primary of an iron-core transformer?

A

Generates a fluctuating magnetic field.

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24
Q

What causes eddy currents to flow through the core of a transformer?

A

Induced voltage from the fluctuating magnetic field.

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25
Q

How are transformer coils designed to minimize eddy current loss?

A

They are laminated with thin, insulated layers.

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26
Q

What is hysteresis loss?

A

Energy dissipated as heat when magnetic domains realign with a changing magnetic field.

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27
Q

How often must magnetic domains realign in a transformer operating on 50 Hz AC?

A

100 times a second.

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28
Q

What can be done to minimize hysteresis loss in transformers?

A

Using appropriate core materials.

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29
Q

What happens when an electrical load is connected to the secondary winding of a transformer?

A

A current flows in the secondary winding and out to the load

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30
Q

What is the secondary current in a transformer influenced by?

A

The characteristics of the load

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31
Q

What does the secondary current create in the transformer core?

A

A self-induced secondary magnetic field

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32
Q

What direction does the self-induced secondary magnetic field flow in relation to the main primary field?

A

In the opposite direction

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33
Q

What effect does the opposition of the two magnetic fields have on the overall magnetic strength?

A

It reduces the overall magnetic strength

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34
Q

What happens to the back electromagnetic field (EMF) of the primary winding when the secondary current flows?

A

It reduces

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35
Q

What occurs to the primary current when the back EMF is reduced?

A

It slightly increases

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36
Q

What must a transformer maintain between the primary and secondary magnetic fields to operate correctly?

A

A balance

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37
Q

What is the relationship between the total induced voltage in each winding and the number of turns inside it?

A

Proportional

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38
Q

What is the formula for power output and input in a transformer?

A

Power = Volts x Amperes

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39
Q

What is the relationship between the voltage ratio and the turn ratio of a transformer?

A

The voltage ratio is equal to its turn ratio

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40
Q

Fill in the blank: Higher voltage results in _______.

A

Lower current

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41
Q

Fill in the blank: Lower voltage results in _______.

A

Higher current

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42
Q

What does a transformer ratio involve relationships between?

A
  • The number of turns in the primary and secondary windings
  • The voltage across each winding
  • The current through the windings
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43
Q

What condition defines a transformer under no-load?

A

No electrical load connected to its secondary winding

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44
Q

What occurs when the secondary circuit of a transformer is open-circuited?

A

No secondary current flows

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45
Q

What limits the flow of primary current when a transformer is under no-load?

A

Back EMF and primary winding resistance

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46
Q

What must the no-load primary current maintain?

A

Enough magnetic field to produce the required back EMF

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47
Q

What two components make up the no-load primary current?

A
  • An in-phase current that supplies core losses
  • A small current at 90° to the voltage that sets up the magnetic flux
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48
Q

Is the no-load primary current large or small compared to the transformer’s normal full-load current?

A

Very small

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49
Q

What are the losses present in the transformer core that affect the no-load primary current?

A
  • Iron losses
  • A small amount of copper losses
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50
Q

What is the energy component and magnetising component of a single-phase transformer given in the example?

A
  • Energy component: 2 Amps
  • Magnetising component: 5 Amps
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51
Q

What is the power transfer characteristic of an ideal transformer?

A

In an ideal transformer, there is an equal amount of power in the primary and secondary windings, maintaining a power ratio of one (unity).

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52
Q

How does a transformer affect voltage and current?

A

A transformer can step up or step down voltage, but it does not change power; when voltage increases, current decreases and vice-versa.

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53
Q

What is the relationship between primary power and secondary power in a transformer?

A

Primary power equals secondary power.

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54
Q

What happens to power loss when voltage is increased in a transformer?

A

Increasing voltage decreases current, reducing losses by a factor of the square of the voltage increase.

55
Q

What types of losses do transformers experience?

A

Transformers experience copper losses and iron losses.

56
Q

What is the general efficiency range of real transformers at full load?

A

Typical efficiencies range from 94% to 96%.

57
Q

What is the maximum efficiency that high-capacity transformers can achieve?

A

High-capacity transformers can achieve efficiencies as high as 98%.

58
Q

What is the formula for the efficiency of a transformer?

A

Efficiency is equal to the ratio of power output of the secondary winding to the power input of the primary winding.

59
Q

What does the primary power in transformers express as?

A

Volt-amps (VA).

60
Q

What does the secondary power in transformers express as?

A

Watts (W).

61
Q

How can the relationship between the transformer’s input, output, and efficiency be visualized?

A

Using a triangle that superimposes VA, W, and efficiency.

62
Q

What is the purpose of polarity markings in transformers?

A

To denote phase relationships and ensure proper connections to maintain in-phase voltage and current.

63
Q

What is the dot convention in transformers?

A

A marking system that indicates phase relationships by placing a dot next to each corresponding leg of a transformer winding.

64
Q

What do matching dots on transformer windings indicate?

A

They indicate that the instantaneous voltage polarity across the primary winding is the same as that across the secondary winding.

65
Q

What does it mean if the dots on each winding do not match up?

A

There is a 180° phase shift between primary and secondary.

66
Q

How can one reverse the phase relationship in transformers?

A

By swapping the winding connections.

67
Q

Fill in the blank: The efficiency of a transformer is equal to the ratio of the power output of the _______ winding to the power input of the primary winding.

A

secondary

68
Q

What is the relationship between the effective length of a conductor and voltage in a transformer?

A

The effective length of the conductor influences the voltage induced in a coil.

69
Q

How is output voltage calculated in a transformer?

A

Output voltage depends on the number of turns in the primary and secondary coils.

70
Q

If an AC transformer converts 230 V to 24 V with a secondary coil of 100 turns, how many turns must the primary winding have?

A

Calculated using the transformer voltage formula.

71
Q

What principle assumes power output equals power input in transformers?

A

Current transformation principle.

72
Q

According to the power formula, what happens to current when voltage decreases in a transformer?

A

The current must increase to maintain the same power output.

73
Q

If the output voltage is double the input voltage, how does the input current relate to the output current?

A

Input current must be double the output current.

74
Q

How many amperes are consumed by a primary winding with 1000 turns if the secondary winding of a transformer with 60 turns consumes 20 A?

A

Calculated using the current transformation formula.

75
Q

What is the purpose of using transformers in communications engineering?

A

To match resistances for maximum power transmission.

76
Q

When does a transformer transmit the most power?

A

When the internal resistance of the generator equals that of the load.

77
Q

What law is used to express the transformation of resistance in transformers?

A

Ohm’s Law.

78
Q

What is the formula for transformation of resistance in a transformer?

A

R_primary = R_secondary * (N_primary / N_secondary)^2.

79
Q

What is impedance matching in transformers?

A

It ensures maximum power transfer when load impedance equals source impedance.

80
Q

What is required to calculate the necessary turns ratio for an impedance matching transformer?

A

The formula for turns ratio involving impedance.

81
Q

True or False: Maximum power is transferred only when the load impedance is less than the internal impedance of the source.

A

False.

82
Q

Fill in the blank: A transformer can match a pre-amplifier of 20,000 ohms input impedance to a moving coil microphone of ______ ohms.

A

200.

83
Q

What happens if the internal resistance of the generator and load resistance differ?

A

The transformer functions as a transmitter to match them.

84
Q

What is the common type of transformer used for three-phase a.c. systems?

A

A single, three limbed transformer

It is more common than using three interconnected single-phase transformers.

85
Q

What is the efficiency percentage of Tight Coupling in a three-phase transformer?

A

99.9% efficient

Tight Coupling improves magnetic linkage efficiency.

86
Q

What are the other types of coupling in a three-phase transformer?

A
  • Loose Coupling
  • Medium Coupling
87
Q

In a three-phase transformer, how are the windings configured?

A

In a star or Y shape

The currents are out of phase by 120°.

88
Q

What are the two types of connection configurations for a three-phase transformer?

A
  • Delta (Δ)
  • Star (Y)
89
Q

What advantage do Star Y connections provide in a three-phase transformer?

A

Opportunity for multiple voltages

90
Q

What is the primary advantage of DeltaΔ connections in a three-phase transformer?

A

Higher level of reliability

91
Q

What is the power rating of a transformer measured in?

A

Volt-amperes (VA), kilo volt-amperes (kVA), or mega volt-amperes (MVA)

92
Q

What is active power in a transformer?

A

Power used to generate real work, measured in kW

93
Q

What is reactive power in a transformer?

A

Power consumed when an inductive load is connected, measured in kVAR

94
Q

What does apparent power combine?

A

Active and reactive power

95
Q

True or False: A transformer can step up power.

A

False

96
Q

What is the formula for calculating the VA rating of a transformer?

A

VA = Voltage x Current

97
Q

What is the maximum safe power loading for a transformer in terms of its VA/kVA rating?

A

Loading above 80% is not recommended

98
Q

What is the formula for calculating the kVA rating of a transformer given voltage and current?

A

kVA = (Voltage x Current) / 1000

99
Q

What is the formula for power factor in an AC circuit?

A

Power Factor = cos(Phase Difference)

100
Q

What does a power factor of unity indicate?

A

There is no phase difference between voltage and current

101
Q

Fill in the blank: A three-phase transformer has ______ windings.

A

six windings

102
Q

How are the primary windings labeled in a three-phase transformer?

A

A, B, C

103
Q

How are the secondary windings labeled in a three-phase transformer?

A

a, b, c

104
Q

What happens to current when a transformer steps up voltage?

A

It steps down current

105
Q

What type of insulation is used on the windings of a three-phase transformer?

A

Resin type such as Kapton

106
Q

What is the purpose of the insulation between the primary and secondary windings?

A

To reduce the possibility of fire

107
Q

What is the phase difference between the currents in a three-phase system?

A

120°

108
Q

What is the principle behind a transformer?

A

A transformer operates on the principle of electromagnetic induction, where a fluctuating electric current in one coil generates a magnetic field that induces a current in a neighboring coil.

109
Q

What are the primary and secondary currents in a transformer?

A

The current in the first coil is called the primary current, and the current in the second coil is called the secondary current.

110
Q

What happens to voltage and current in a step-down transformer?

A

In a step-down transformer, the secondary voltage is smaller than the primary voltage, and the secondary current is larger than the primary current.

111
Q

In a step-up transformer, how do the number of turns relate to voltage and current?

A

In a step-up transformer, the secondary coil has more turns than the primary coil, resulting in a higher secondary voltage and a lower secondary current.

112
Q

What is the formula relating voltage, current, and number of turns in a transformer?

A

The formulas are:
* Vp / Vs = Np / Ns
* Ip / Is = Ns / Np

113
Q

Fill in the blank: The power in an electric current is equal to the current times the _______.

A

[voltage]

114
Q

What is the turns ratio of a transformer with 500 turns in the primary winding and 1000 turns in the secondary winding?

A

The turns ratio is 1:2.

115
Q

True or False: A step-down transformer increases the voltage while decreasing the current.

A

False

116
Q

What is the effect of wrapping coils around a soft iron bar in a transformer?

A

Wrapping coils around a soft iron bar increases the efficiency of electromagnetic induction between the coils.

117
Q

What occurs if the voltage rating of a transformer coil is exceeded?

A

Exceeding the voltage rating can lead to insufficient current limits due to lower reactance, potentially damaging the transformer.

118
Q

In a step-down transformer with 100 turns in the primary and 10 turns in the secondary, what is the relationship between primary and secondary voltage?

A

The secondary voltage is reduced by a factor of 10 compared to the primary voltage.

119
Q

What do the terms ‘step-up’ and ‘step-down’ transformers refer to?

A

‘Step-up’ transformers increase voltage, while ‘step-down’ transformers decrease voltage.

120
Q

What determines whether a transformer is classified as step-up or step-down?

A

The turns ratio between the two coils determines if the transformer is a step-up or step-down transformer.

121
Q

What is the construction of an autotransformer?

A

An autotransformer has only one coil for the input and output sides, with primary and secondary sides sharing a part of the windings.

122
Q

What is the primary function of an autotransformer?

A

To step up or step down voltages.

123
Q

How do the primary and secondary windings of an autotransformer differ from conventional transformers?

A

In an autotransformer, the primary and secondary windings comprise different sections of the same coil, sharing a common section.

124
Q

What is the major advantage of an autotransformer compared to a conventional transformer?

A

Its small design.

125
Q

What is the throughput rating in an autotransformer?

A

The total power output that can be tapped by the wire of the input coil, which is less than the rated power.

126
Q

What is the difference between rated power and throughput rating in transformers?

A

Rated power is the power transmitted by induction, while throughput rating is the total power output.

127
Q

What is a significant disadvantage of an autotransformer?

A

It lacks primary to secondary winding isolation.

128
Q

What happens if the secondary winding of an autotransformer becomes open-circuited?

A

Load current stops flowing through the primary winding, stopping transformer action and applying full primary voltage to the secondary terminals.

129
Q

What is the consequence of a short-circuit condition in the secondary circuit of an autotransformer?

A

The resulting primary current would be much larger than in a double wound transformer, potentially causing damage.

130
Q

What is the function of a variable transformer, also known as a Variac?

A

To vary the output voltage on the secondary side, either through switchable outputs or a slider arm.

131
Q

What is a current transformer (CT)?

A

A special transformer that allows AC currents to be measured without breaking the current-carrying leads.

132
Q

What must always be connected to current transformers to prevent high voltage?

A

A load (ammeter).

133
Q

Fill in the blank: An autotransformer has a _______ for both the input and output sides.

A

single coil

134
Q

True or False: An autotransformer can safely be used for stepping down higher voltages to much lower voltages.

A

False

M3 (19 decks)

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