Deck 14: Transformers

Question 1

A step-down transformer could have a primary-secondary turns ratio of 4:1.

Answer 1

True

Question 2

A typical transformer fault would be an open winding.

Answer 2

True

Question 3

If a dc voltage is input to a transformerʹs primary, then an ac voltage is induced in its secondary.

Answer 3

False

Question 4

A transformer with a turns ratio of 1:1 is often used to isolate a load from a source.

Question 5

What is the secondary voltage in Figure 14-1 if the turns ratio is 7:1?

A)
B)
C)
D)

Question 6

The hysteresis loss in a transformer is ________.

A)caused by the resistance of the wire
B)caused by rapid reversal of the magnetic field
C)another name for flux leakage loss
D)caused by current flowing in the core

Question 7

Transformer cores are made from laminated iron to reduce losses.

Question 8

A transformer with a turns ratio of 1:7 is a step down transformer.

Question 9

Eddy current loss in a transformer is ________.

A)due to current flowing in the core
B)caused by rapid reversal of the magnetic field
C)another name for flux leakage loss
D)caused by the resistance of the wire

Question 10

A transformer can be used as an impedance matching device.

Question 11

What is the secondary voltage in Figure 14-1 if the turns ratio is 1:3?

A)
B)
C)
D)

Question 12

An ideal 10:1 transformer with 20 VA of power has less than 2 VA on the output.

Question 13

Autotransformers cannot be used for isolation purposes since there is only one winding.

Question 14

The primary to secondary resistance should be relatively low.

Question 15

Only the number of turns in the primary and secondary of a transformer determines the actual
secondary voltage.

Question 16

The efficiency of all transformers is very low.

Question 17

An ideal transformer has no power loss.

Question 18

The impedance matching characteristic for a transformer is needed for a situation where maximum
power transfer to the load is desired.

Question 19

The phrase ʺMaximum power is delivered to the load when the load resistance equals the source resistanceʺ is a definition of ________.

A)Lenzʹs Law
B)the maximum power transfer theorem
C)Kirchhoffʹs Law
D)Ohmʹs Law

Question 20

A core material is not always necessary for proper operation of a transformer.

Question 21

What is the reflected resistance seen by the primary in Figure 14-1 if the turns ratio is 4:1 and IS is 40 mA?

A)
B)
C)
D)

Question 22

In Figure 14-1, the primary to secondary turns ratio is 1:3 and the secondary current equals 120 mA. What is the primary current?

A)180 mA
B)40 mA
C)360 mA
D)cannot compute because the voltage is not given

Question 23

In Figure 14-1, if the primary to secondary turns ratio is 7.5:1, the output voltage equals ________.

A)
B)
C)
D)

Question 24

When a load resistor is connected across the secondary winding of a transformer with a turns ratio of 2:1, the source ʺseesʺ a reflected load of ________.

A)
B)
C)
D)

Question 25

What is the secondary current in Figure 14-1 with a load resistor and 4:1 turns ratio?

A)600 mA
B)1.66 A
C)9.6 A
D)4.8 A

Question 26

When a 1 load resistor is connected across the secondary winding of a transformer with a turns ratio of 1:2, the source ʺseesʺ a reflected load of ________.

A)
B)
C)
D)

Question 27

What is the secondary current in Figure 14-1 with a load resistor and 3:1 turns ratio?

A)
B)
C)
D)

Question 28

In Figure 14-1, if there are five times more turns in the primary than in the secondary, then what is the secondary voltage?

A)120 V
B)12 V
C)24 V
D)600 V

Question 29

In Figure 14-1, if the primary to secondary turns ratio is changed to 4:1 and a 1 load resistor is in the secondary, the secondary current equals ________.

A)30 mA
B)240 mA
C)120 mA
D)480 mA

Question 30

In Figure 14-1, if a load resistor of is placed in the secondary circuit, what is the secondary current?

A)533 mA
B)16 A
C)4.8 A
D)5.33 A

Question 31

In Figure 14-1, if the primary to secondary turns ratio is 9:1 and a 1 load resistor is placed in the secondary, the reflected resistance seen by the primary equals ________.

A)
B)
C)
D)

Question 32

What is the secondary voltage in Figure 14-1 if the turns ratio is 9:1?

A)26.6 V
B)106 V
C)13.3 V
D)53.2 V

Question 33

In Figure 14-1, if the secondary current is 50 mA, the reflected resistance seen by the primary equals ________.

A)
B)
C)
D)

Question 34

What is the primary current in Figure 14-1 if mA and the turns ratio equals 4:1 ?

A)160 mA
B)4 mA
C)40 mA
D)10 mA

Question 35

What is the reflected resistance seen by the primary in Figure 14-1 if the turns ratio is 3:1 and is 240 mA?

A)
B)
C)
D)

Question 36

If 10 W of power are applied to the primary of an ideal transformer with a turns ratio of 1:5, the power delivered to the secondary load is ________.

A)0.5 W
B)0 W
C)10 W
D)50 W

Question 37

In Figure 14-1, if the secondary current is 50 mA, the primary current is ________.

A)300 mA
B)150 mA
C)48.7 mA
D)50 mA

Question 38

What is the primary current in Figure 14-1 if the turns ratio equals 1:3 and equals 240 mA?

A)1.2 A
B)720 mA
C)120 mA
D)60 mA

Question 39

In Figure 14-1, the primary to secondary turns ratio is changed to 2.5:1 and the secondary current equals 100 mA. What is the reflected resistance seen by the primary?

A)
B)
C)
D)

Question 40

What is the secondary voltage in Figure 14-1 if the turns ratio is 4.5:1?

A)26.7 V
B)4.72 V
C)540 V
D)5.92 V

Question 41

A step-up transformer will increase ________ and decrease ________.

A)power, current
B)current, impedance
C)voltage, impedance
D)voltage, power

Question 42

The turns ratio required to match a source to a load is ________.

A)4:1
B)1:2
C)1:4
D)2:1

Question 43

The reflected load in a transformer is defined as the actual load as it appears to the source and results from:

A)the voltage ratio.
B)the power ratio.
C)the current ratio.
D)the turns ratio.

Question 44

The turns ratio of a transformer (n) is defined as:

A)the number of turns in the secondary divided by the number of turns in the primary.
B)the number of turns in the primary divided by the number of turns in the secondary.
C)the number of turns in the primary multiplied by the number of turns in the secondary.
D)the number of turns in the secondary multiplied by the number of turns in the primary.

Question 45

What is the problem if a power transformer is delivering a low output voltage even though the primary voltage is correct?

A)a partially shorted secondary
B)a shorted primary
C)an open primary
D)an open secondary

Question 46

To couple two circuits together with no change in either voltage or current, use ________.

A)a step-down transformer
B)a step-up transformer
C)a power transformer
D)an isolation transformer

Question 47

What is the likeliest way to fix a center-tapped transformer which outputs unequal voltages on each half of the secondary?

A)Replace the transformer.
B)Remove some turns from the half with the higher voltage winding.
C)Add some turns to the half with the lower voltage winding.
D)Do not concern yourself; the customer will probably not notice the problem.

Question 48

The relationship for the impedance matching ratio in a transformer is:

A)
B) squared.
C) quared.
D)

Question 49

Transformers ________.

A)match the impedance of a source to the impedance of a load
B)convert a higher voltage into a lower voltage
C)convert a lower current into a higher current
D)all of these

Question 50

A step-up transformer will increase ________ and decrease ________.

A)impedance, current
B)current, voltage
C)voltage, impedance
D)current, impedance

Question 51

If a 12 V battery is connected across the primary of a transformer with a turns ratio of 1:4, then the secondary voltage is ________.

A)3 V
B)0 V
C)12 V
D)48 V

Question 52

The relationship for the voltage ratio in a transformer is:

A)
B)
C)
D)

Question 53

When the turns ratio of a transformer is 1:10 and the primary AC voltage in 6 V, then the secondary voltage is ________.

A)6 V
B)36 V
C)60 V
D)0.6 V

Question 54

For a transformer to operate properly, it must:

A)be supplied with the proper AC signal.
B)work within its voltage, current and power ratings.
C)be connected to a load.
D)all of these.

Question 55

The small dot placed on schematics of transformers indicates:

A)the phase of the voltages.
B)the polarity of the voltages.
C)the direction of the winding.
D)all of these.

Question 56

Which winding will have the lowest measured resistance in a transformer with four secondary windings?

A)the 5 V secondary
B)the 12 V secondary
C)the 6.3 V secondary
D)the 550 V secondary

Question 57

The relationship for the current ration (Isec/Ipri) in a transformer is:

A)
B)
C)
D)

Question 58

Although the input voltage to the transformerʹs primary is correct, the transformer is outputting zero volts. A probable trouble is ________.

A)an open primary
B)a partially shorted primary
C)a shorted primary
D)a partially shorted secondary

Question 59

A matching transformerʹs primary needs ________ more turns than its secondary to match a audio signal distribution line to an speaker.

A)0.013
B)8.66
C)75
D)0.115

Question 60

To match a audio load to a speaker system, the primary of the impedance matching transformer must have ________ more turns than the secondary.

A)150
B)12.2
C)3.06
D)The primary should have less turns than the secondary.

Question 61

What type transformer most likely has a 1:1 turns ratio?

A)power
B)impedance matching
C)center tapped
D)isolation

Question 62

An autotransformer differs from most transformers in that:

A)it does not isolate the primary voltage from the secondary voltage.
B)primary and secondary windings are the same.
C)source current flows through both the windings.
D)all of these.

Question 63

The measured voltage on the secondary of a given transformer is 0 V. Which of the following could have caused this?

A)open secondary
B)open primary
C)shorted load
D)any of the above

Question 64

How can the windings ratio be safely determined for an unmarked transformer?

A)Standardized wire color codes indicate windings ratio.
B)The turns ratio is the same as the ratio of primary resistance to secondary resistance ratio.
C)Apply known low voltage ac to the primary and measure the secondary output. The ratio of the voltages is the same as the windings ratio.
D)either A or C

Question 65

A center tap transformer has a connection at the midpoint of the secondary winding resulting in:

A)two 180 degree out of phase secondary voltages.
B)secondary voltages half that of a non-center tapped transformer with the same turns ratio.
C)two equal magnitude secondary voltages.
D)all of these.

Question 66

36 V of dc is applied to a windings ratio of 1:2. What is the secondary voltage?

A)something slightly less than 72 V since transformers do have some loss
B)144 V
C)72 V
D)none of the above

Question 67

An air core transformer would most likely be used for:

A)isolation.
B)audio frequencies
C)impedance matching
D)radio frequencies.