Deck 14: Transformers

Question 1

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

A)240 VRMS
B)80 VRMS
C)360 VRMS
D)40 VRMS

Answer 1

360 VRMS

Question 2

The phrase "Maximum power is delivered to the load when the load resistance equals the source resistance" is a definition of .

A)the maximum power transfer theorem
B)Lenz's Law
C)Kirchhoff's Law
D)Ohm's Law

Answer 2

the maximum power transfer theorem

Question 3

An ideal transformer has no power loss.

Answer 3

True

Question 4

A transformer can be used as an impedance matching device.

Question 5

The primary to secondary resistance should be relatively low.

Question 6

A typical transformer fault would be an open winding.

Question 7

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

A)8.59 VRMS
B)840 VRMS
C)17.1 VRMS
D)420 VRMS

Question 8

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

Question 9

The efficiency of all transformers is very low.

Question 10

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

Question 11

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

Question 12

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)caused by current flowing in the core
D)another name for flux leakage loss

Question 13

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

Question 14

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

Question 15

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

Question 16

Eddy current loss in a transformer is .

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

Question 17

Transformer cores are made from laminated iron to reduce losses.

Question 18

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

Question 19

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

Question 20

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

Question 21

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 22

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

A)9000 Ω
B)167 Ω
C)1500 Ω
D)500 Ω

Question 23

if a load resistor of 75 Ω is placed in the secondary circuit, what is the secondary current?

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

Question 24

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

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

Question 25

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

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

Question 26

if there are five times more turns in the primary than in the secondary, then what is the secondary voltage?

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

Question 27

What is the secondary current in Figure 14 -1 with a 100 Ω load resistor and 3:1 turns ratio?

A)40 mARMS
B)400 mARMS
C)33 mARMS
D)330 mARMS

Question 28

if the secondary current is 50 mA, the primary current is .

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

Question 29

if the primary to secondary turns ratio is 7.5:1, the output voltage equals .

A)16 VRMS
B)32 VRMS
C)900 VRMS
D)9 VRMS

Question 30

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)4 kΩ
B)8 kΩ
C)16 kΩ
D)12 kΩ

Question 31

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

A)2 kΩ
B)4 kΩ
C)500 Ω
D)1 kΩ

Question 32

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

A)162 kΩ
B)1 kΩ
C)111 Ω
D)81 kΩ

Question 33

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

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

Question 34

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

A)250 Ω
B)4 kΩ
C)1 kΩ
D)2 kΩ

Question 35

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

Question 36

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)5 kΩ
B)2.5 kΩ
C)1.2 kΩ
D)3 kΩ

Question 37

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

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

Question 38

What is the primary current in Figure 14 -1 if IS = 40 mA and the turns ratio equals 4:1 ?

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

Question 39

if the secondary current is 50 mA, the reflected resistance seen by the primary equals .

A)7.2 kΩ
B)14.4 kΩ
C)800 Ω
D)2.4 kΩ

Question 40

What is the secondary current in Figure 14 -1 with a 50 Ω load resistor and 4:1 turns ratio?

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

Question 41

A step -up transformer will increase and decrease .

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

Question 42

A matching transformer's primary needs audio signal distribution line to an 8 Ω speaker. more turns than its secondary to match a 600 Ω

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

Question 43

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

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

Question 44

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

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

Question 45

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

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

Question 46

The turns ratio required to match a 50 Ω source to a 200 Ω load is .

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

Question 47

To match a 600 Ω audio load to a 4 Ω speaker system, the primary of the impedance matching transformer must have

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

Question 48

The relationship for the impedance matching ratio (RL/Rpri)in a transformer is:

A)opposite to the turns ratio (Nsec/Npri)squared.
B)equal to the turns ratio (Nsec/Npri)squared.
C)equal to the turns ratio (Nsec/Npri).
D)opposite to the turns ratio (Nsec/Npri).

Question 49

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

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

Question 50

A step -up transformer will increase and decrease .

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

Question 51

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

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

Question 52

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

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

Question 53

The relationship for the voltage ratio (Vsec/Vpri)in a transformer is:

A)less than the turns ratio (Nsec/Npri).
B)the same as the turns ratio (Nsec/Npri).
C)opposite to the turns ratio (Nsec/Npri).
D)more than the turns ratio (Nsec/Npri).

Question 54

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

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

Question 55

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 56

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

Question 57

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

A)the same as the turns ratio (Nsec/Npri).
B)less than the turns ratio (Nsec/Npri).
C)more than the turns ratio (Nsec/Npri).
D)the inverse of the turns ratio (Nsec/Npri).

Question 58

For a transformer to operate properly, it must:

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

Question 59

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 60

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

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

Question 61

An autotransformer differs from most transformers in that:

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

Question 62

An air core transformer would most likely be used for:

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

Question 63

If you wanted to monitor a large alternating current without opening the circuit you would likely use a _.

A)current transformer
B)step -down transformer
C)current -sensing inductor
D)any of the above

Question 64

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

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

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

In induction heating, the material is heated due to .

A)the fact that the material acts like an open
B)eddy currents
C)hysteresis
D)high load reactance

Question 67

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

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

Question 68

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

A)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.
B)Standardized wire color codes indicate windings ratio.
C)The turns ratio is the same as the ratio of primary resistance to secondary resistance ratio.
D)either A or C

Question 69

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