Deck 14: Transformers

Question 1

Laminated iron-core or silicon steel) transformers are normally used for:

A) power line 60 Hz) applications.
B) audio frequency applications 20 Hz to 20 kHz).
C) radio frequency applications, above 100 kHz.
D) Both A and B are correct.

Answer 1

Both A and B are correct.

Question 2

Secondary current in a transformer depends on secondary voltage and load resistance values.

Answer 2

True

Question 3

A transformer can be used to step-down decrease) voltage.

Answer 3

True

Question 4

Mutual inductance is defined as the inductance between two coils that are coupled by a magnetic field.

Question 5

A transformer can be used to step-up increase) voltage.

Question 6

Air core or ferrite core transformers are normally used for:

A) radio frequency applications, above 100 kHz.
B) power line 60 Hz) applications.
C) audio frequency applications 20 Hz to 20 kHz).
D) Both A and B are correct.

Question 7

If the primary L1 in Figure has 40 turns and the secondary L2 has 10 turns, what is the reflected resistance into the primary if the load resistance is 120 Ω?

A) 1920 Ω
B) 480 Ω
C) 30 Ω
D) 7.5 Ω

Question 8

When mutual inductance is present, current changing in one circuit causes an induced voltage in the other circuit.

Question 9

If the input voltage Vin) in Figure 14-1 is 120 V rms and the output voltage Vout) is 24 V rms, what is the turns ratio?

A) 1.2:1
B) 120:1
C) 5:1
D) 24:1

Question 10

In a given transformer, 87% of the flux produced in the primary is coupled into the secondary winding, what is the coefficient of coupling?

A) 13
B) 0.13
C) 0.87
D) 87

Question 11

A transformer can pass an AC voltage from the primary into the secondary winding.

Question 12

The two round dots in Figure 14-1 indicate:

A) the two connections are in phase.
B) both marked connections are negative, while the unmarked connections are positive.
C) both marked connections are positive, while the unmarked connections are negative.
D) none of the above.

Question 13

In a step-up transformer, the secondary has more turns of wire than does the primary.

Question 14

Which of the following are common applications for a transformer?

A) to step-up or step-down voltage
B) isolation from the AC power line
C) impedance matching
D) all of the above

Question 15

Even though two coils do not share a common core, they can have some amount of mutual inductance.

Question 16

The product of input voltage and input current in a transformer is equal to the output voltage times the output current, minus any power losses.

Question 17

Voltage and current in the primary of a step-down transformer equals the voltage and current in the secondary.

Question 18

A transformer can be used to step-up increase) Power.

Question 19

A transformer consists of two or more coils, sharing a common core, or placed close together, and are magnetically coupled.

Question 20

A transformer can pass a steady state DC from the primary to the secondary.

Question 21

A transformer that has only one winding is called an) .

A) tapped transformer
B) multiple-winding transformer
C) nonideal transformer
D) autotransformer

Question 22

Figure 14-2
Eddy currents, magnetic flux leakage, winding resistance and hysteresis in a transformer, all contribute to:

A) mutual inductance
B) power loss and heat
C) power transfer from primary to secondary
D) voltage step-up or step-down

Question 23

Which coil would be used as a step-up transformer primary, if coil number 1 has 100 more turns than coil number 2?

A) Coil ratio is too small
B) coil number 2
C) Coil windings must be the same
D) coil number 1

Question 24

With 100 V of primary voltage and a 1:3 turns ratio, the output voltage will be:

A) 33 V
B) 300 V
C) 100 V
D) Not enough information is given.

Question 25

The ability to transfer energy from one coil to another is dependent upon .

A) impedance matching
B) self-inductance
C) mutual inductance
D) coupling

Question 26

A transformer has 430 turns in the primary and 200 turns in the secondary. What is the P-S turns ratio?

A) 2.15:1
B) 1:2.15
C) 1:30
D) 230:1

Question 27

When a transformer secondary lead has been connected to a middle loop that is not an end connector, it is called a:

A) single winding secondary
B) multiple winding secondary
C) center-tapped secondary
D) multiple-tapped secondary

Question 28

A transformer with an air core, ferrite core, or powdered iron core is used for:

A) audio frequency applications
B) power line frequency applications.
C) radio frequency applications
D) all of the above.

Question 29

What transformer-turns relationship will be needed to match an output impedance of 5 kΩ to an 8 Ω speaker?

A) 5:1
B) 8:1
C) 25:1
D) 625:1

Question 30

In a transformer, how will the primary induced voltage be affected, if the secondary current decreases?

A) It will decrease
B) It will increase.
C) It will remain the same
D) It will be canceled.

Question 31

Figure 14-2
Which transformer loss is greatly reduced by laminating the iron core?

A) winding resistance losses
B) Eddy current losses
C) Barkhausen losses
D) radio frequency losses
E) Both A and B are correct

Question 32

Figure 14-2
The transformer shown in Figure 14-2 is called:

A) an isolation transformer
B) an impedance transformer
C) a step-up transformer
D) an autotransformer

Question 33

Increasing the number of turns of wire on the secondary of a transformer will:

A) increase the output voltage
B) have no effect on the output voltage.
C) increase the primary input voltage
D) decrease the output voltage.

Question 34

What is the number of turns in a primary winding, when the voltage applied to a transformer input is 24 Vac with a 225-turn secondary output of 360 V?

A) 3325
B) 15
C) 9.38
D) 5400

Question 35

A voltage is induced in a transformer secondary winding by the action of the:

A) primary magnetic field
B) primary turns ratio
C) secondary magnetic field
D) secondary induced voltage

Question 36

The ratio of the amount of flux linking the secondary coil versus the primary flux is called:

A) the coefficient of accuracy
B) a Weber
C) the coefficient of coupling
D) the turns ratio

Question 37

In a transformer, the primary and secondary coils are magnetically linked, but they are:

A) mechanically connected
B) current connected
C) electrically isolated
D) always in phase

Question 38

With a primary current of 3 A and a 1:3 turns ratio, the output current will be:

A) 3 mA
B) 3 A
C) 1 A
D) 333 A

Question 39

A transformer would never be selected to:

A) step-up or step-down power
B) isolate stages
C) step-up or step-down voltage
D) match impedances

Question 40

A transformer has a 1:6 P-S turns ratio and has a primary current of 12 A. What is the secondary current?

A) 2 A
B) 72 A
C) 12 A
D) 6 A