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MECH1310Chapter14Homework

True/False
Indicate whether the statement is true or false.
 

 1. 
A transformer can be used as an impedance matching device.
 

 2. 
A step-down transformer could have a primary-secondary turns ratio of 4:1.
 

 3. 
Transformer cores are made from laminated iron to reduce losses.
 

 4. 
A transformer with a turns ratio of 1:1 is often used to isolate a load from a source.
 

 5. 
A transformer with a turns ratio of 1:7 is a step down transformer.
 

 6. 
An ideal transformer has no power loss.
 

 7. 
A core material is not always necessary for proper operation of a transformer.
 

 8. 
The impedance matching characteristic for a transformer is needed for a situation where maximum power transfer to the load is desired.
 

Multiple Choice
Identify the choice that best completes the statement or answers the question.
 

 9. 
The hysteresis loss in a transformer is ________.
a.
caused by the resistance of the wire
b.
another name for flux leakage loss
c.
caused by current flowing in the core
d.
caused by rapid reversal of the magnetic field
 

 10. 
Eddy current loss in a transformer is ________.
a.
caused by the resistance of the wire
b.
due to current flowing in the core
c.
another name for flux leakage loss
d.
caused by rapid reversal of the magnetic field
 

 11. 
The phrase "Maximum power is delivered to the load when the load resistance equals the source resistance" is a definition of ________.
a.
Lenz's Law
c.
the maximum power transfer theorem
b.
Kirchhoff's Law
d.
Ohm's Law
 

 12. 
What is the secondary voltage in Figure 14-1 if the turns ratio is 1:3?
mc012-1.jpg
Figure 14-1
a.
360 VRMS
c.
80 VRMS
b.
40 VRMS
d.
240 VRMS
 

 13. 
What is the secondary voltage in Figure 14-1 if the turns ratio is 7:1?
mc013-1.jpg
Figure 14-1
a.
420 VRMS
c.
840 VRMS
b.
8.59 VRMS
d.
17.1 VRMS
 

 14. 
What is the secondary current in Figure 14-1 with a 100 Ù load resistor and 3:1 turns ratio?
mc014-1.jpg
Figure 14-1
a.
400 mARMS
c.
33 mARMS
b.
40 mARMS
d.
330 mARMS
 

 15. 
What is the primary current in Figure 14-1 if the turns ratio equals 1:3 and IS equals 240 mA?
mc015-1.jpg
Figure 14-1
a.
1.2 A
c.
60 mA
b.
120 mA
d.
720 mA
 

 16. 
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?
mc016-1.jpg
Figure 14-1
a.
500 Ù
c.
9000 Ù
b.
1500 Ù
d.
167 Ù
 

 17. 
What is the secondary voltage in Figure 14-1 if the turns ratio is 4.5:1?
mc017-1.jpg
Figure 14-1
a.
4.72 V
c.
5.92 V
b.
26.7 V
d.
540 V
 

 18. 
What is the secondary current in Figure 14-1 with a 50 Ù load resistor and 4:1 turns ratio?
mc018-1.jpg
Figure 14-1
a.
4.8 A
c.
9.6 A
b.
600 mA
d.
1.66 A
 

 19. 
What is the primary current in Figure 14-1 if IS = 40 mA and the turns ratio equals 4:1 ?
mc019-1.jpg
Figure 14-1
a.
10 mA
c.
40 mA
b.
160 mA
d.
4 mA
 

 20. 
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?
mc020-1.jpg
Figure 14-1
a.
4 kÙ
c.
8 kÙ
b.
12 kÙ
d.
16 kÙ
 

 21. 
In Figure 14-1, if there are five times more turns in the primary than in the secondary, then what is the secondary
mc021-1.jpg
Figure 14-1
voltage?
a.
120 V
c.
24 V
b.
600 V
d.
12 V
 

 22. 
What is the secondary voltage in Figure 14-1 if the turns ratio is 9:1?
mc022-1.jpg
Figure 14-1
a.
53.2 V
c.
26.6 V
b.
106 V
d.
13.3 V
 

 23. 
In Figure 14-1, if the primary to secondary turns ratio is changed to 4:1 and a 1 kÙ load resistor is in the secondary, the secondary current equals ________.
mc023-1.jpg
Figure 14-1
a.
480 mA
c.
240 mA
b.
30 mA
d.
120 mA
 

 24. 
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?
mc024-1.jpg
Figure 14-1
a.
40 mA
b.
180 mA
c.
360 mA
d.
cannot compute because the voltage is not given
 

 25. 
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?
mc025-1.jpg
Figure 14-1
a.
1.2 kÙ
c.
5 kÙ
b.
3 kÙ
d.
2.5 kÙ
 

 26. 
In Figure 14-1, if the primary to secondary turns ratio is 7.5:1, the output voltage equals ________.
mc026-1.jpg
Figure 14-1
a.
32 VRMS
c.
9 VRMS
b.
900 VRMS
d.
16 VRMS
 

 27. 
In Figure 14-1, if a load resistor of 75 Ù is placed in the secondary circuit, what is the secondary current?
mc027-1.jpg
Figure 14-1
a.
4.8 A
c.
533 mA
b.
16 A
d.
5.33 A
 

 28. 
In Figure 14-1, if the secondary current is 50 mA, the primary current is ________.
mc028-1.jpg
Figure 14-1
a.
48.7 mA
c.
150 mA
b.
50 mA
d.
300 mA
 

 29. 
In Figure 14-1, if the secondary current is 50 mA, the reflected resistance seen by the primary equals ________.
mc029-1.jpg
Figure 14-1
a.
14.4 kÙ
c.
2.4 kÙ
b.
800 Ù
d.
7.2 kÙ
 

 30. 
In Figure 14-1, if the primary to secondary turns ratio is 9:1 and a 1 kÙ load resistor is placed in the secondary, the reflected resistance seen by the primary equals ________.
mc030-1.jpg
Figure 14-1
a.
1 kÙ
c.
111 Ù
b.
81 kÙ
d.
162 kÙ
 

 31. 
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.
10 W
c.
0 W
b.
0.5 W
d.
50 W
 

 32. 
When a 1 kÙ 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.
2 kÙ
c.
4 kÙ
b.
1 kÙ
d.
250 Ù
 

 33. 
When a 1 kÙ 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.
500 Ù
c.
1 kÙ
b.
4 kÙ
d.
2 kÙ
 

 34. 
The turns ratio required to match a 50 Ù source to a 200 Ù load is ________.
a.
2:1
c.
4:1
b.
1:2
d.
1:4
 

 35. 
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.
0 V
c.
12 V
b.
3 V
d.
48 V
 

 36. 
A matching transformer's primary needs ________ more turns than its secondary to match a 600 Ù audio signal distribution line to an 8 Ù speaker.
a.
0.115
c.
0.013
b.
8.66
d.
75
 

 37. 
To couple two circuits together with no change in either voltage or current, use ________.
a.
a step-up transformer
c.
a power transformer
b.
an isolation transformer
d.
a step-down transformer
 

 38. 
Transformers ________.
a.
convert a lower current into a higher current
b.
match the impedance of a source to the impedance of a load
c.
convert a higher voltage into a lower voltage
d.
all of these
 

 39. 
Which winding will have the lowest measured resistance in a transformer with four secondary windings?
a.
the 550 V secondary
c.
the 6.3 V secondary
b.
the 5 V secondary
d.
the 12 V secondary
 

 40. 
A step-up transformer will increase ________ and decrease ________.
a.
voltage, impedance
c.
current, impedance
b.
power, current
d.
voltage, power
 

 41. 
When the turns ratio of a transformer is 1:10 and the primary AC voltage in 6 V, then the secondary voltage is ________.
a.
60 V
c.
0.6 V
b.
36 V
d.
6 V
 

 42. 
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 primary
c.
a shorted primary
b.
an open primary
d.
a partially shorted secondary
 

 43. 
To match a 600 Ù audio load to a 4 Ù speaker system, the primary of the impedance matching transformer must have ________ more turns than the secondary.
a.
12.2
b.
3.06
c.
150
d.
The primary should have less turns than the secondary.
 

 44. 
A step-up transformer will increase ________ and decrease ________.
a.
impedance, current
c.
current, impedance
b.
voltage, impedance
d.
current, voltage
 

 45. 
The small dot placed on schematics of transformers indicates:
a.
the phase of the voltages.
c.
the polarity of the voltages.
b.
the direction of the winding.
d.
all of these.
 

 46. 
For a transformer to operate properly, it must:
a.
be connected to a load.
b.
work within its voltage, current and power ratings.
c.
be supplied with the proper AC signal.
d.
all of these.
 

 47. 
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.
 

 48. 
The relationship for the voltage ratio (Vsec/Vpri) in a transformer is:
a.
more than the turns ratio (Nsec/Npri).
c.
less than the turns ratio (Nsec/Npri).
b.
opposite to the turns ratio (Nsec/Npri).
d.
the same as the turns ratio (Nsec/Npri).
 

 49. 
The relationship for the current ration (Isec/Ipri) in a transformer is:
a.
less than the turns ratio (Nsec/Npri).
c.
the same as the turns ratio (Nsec/Npri).
b.
the inverse of the turns ratio (Nsec/Npri).
d.
more than the turns ratio (Nsec/Npri).
 

 50. 
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.
c.
the voltage ratio.
b.
the current ratio.
d.
the power ratio.
 

 51. 
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.
 

 52. 
A center tap transformer has a connection at the midpoint of the secondary winding resulting in:
a.
two equal magnitude secondary voltages.
b.
secondary voltages half that of a non-center tapped transformer with the same turns ratio.
c.
two 180 degree out of phase secondary voltages.
d.
all of these.
 

 53. 
36 V of dc is applied to a windings ratio of 1:2. What is the secondary voltage?
a.
144 V
b.
something slightly less than 72 V since transformers do have some loss
c.
72 V
d.
none of the above
 

 54. 
What type transformer most likely has a 1:1 turns ratio?
a.
isolation
c.
center tapped
b.
power
d.
impedance matching
 

 55. 
An air core transformer would most likely be used for:
a.
isolation.
c.
impedance matching
b.
audio frequencies
d.
radio frequencies.
 



 
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