Note: a similar type of device is called a
Variac
, and it enjoys the same advantages of AC power control
as the variable transformer shown in the question.
file 00252
10

Question 22
How much voltage is there between the following pairs of points in this circuit?
A
B
C
D
E
F
5000 turns
2000 turns
2000 turns
4000 turns
48 VAC
•
Between
C
and
F
:
V
CF
=
•
Between
A
and
B
:
V
AB
=
•
Between
D
and
E
:
V
DE
=
•
Between
D
and
F
:
V
DF
=
file 00667
Question 23
How much voltage is there between the following pairs of points in this circuit?
A
B
C
D
E
F
5000 turns
2000 turns
2000 turns
4000 turns
16 VAC
•
Between
D
and
F
:
V
DF
=
•
Between
A
and
B
:
V
AB
=
•
Between
D
and
E
:
V
DE
=
•
Between
C
and
F
:
V
CF
=
file 00668
11

Question 24
How much AC voltage is there between points
A
and
B
in this circuit?
Line
120 V
277 V
(connected to
277 volt source)
V
Ω
COM
A
V
A
B
file 00666
Question 25
A step-down transformer has a winding turns ratio of 20:1. Calculate the impedance ratio from primary
to secondary.
Also, determine the amount of impedance ”seen” at the primary winding if the secondary
winding is connected to a 90 ohm load.
Impedance ratio =
Z
primary
=
file 02216
Question 26
Calculate the primary winding current (magnitude and phase angle) for this unloaded isolation
transformer, with primary and secondary inductances of 18 Henrys each:
18 H
18 H
120 VAC
60 Hz
Assume the winding inductances are ”pure” (no resistive components).
file 01882
12

Question 27
Calculate the primary winding current (magnitude and phase angle) for this resistively loaded isolation
transformer, with primary and secondary inductances of 18 Henrys each:
18 H
18 H
120 VAC
60 Hz
100
Ω
Also, draw an equivalent schematic diagram (with no transformer in it) illustrating the impedance
”seen” by the AC power source. Assume no winding resistance in either transformer winding, and a magnetic
coupling coefficient between the two windings of exactly 1.
file 01883
Question 28
Calculate the primary winding current (magnitude and phase angle) for this resistively loaded
transformer, with a primary inductance of 18 Henrys and a secondary inductance of 36 Henrys:
18 H
120 VAC
60 Hz
100
Ω
36 H
Also, draw an equivalent schematic diagram (with no transformer in it) illustrating the impedance
”seen” by the AC power source. Assume no winding resistance in either transformer winding, and a magnetic
coupling coefficient between the two windings of exactly 1.
file 01884
Question 29
An unloaded power transformer draws a primary current of 85 mA from its 240 volt, 60 Hz source.
Neglecting any power losses, calculate the inductance of the primary winding. Also, calculate the inductance
of the secondary winding given a step-down voltage ratio of 8:1.
L
primary
=
L
secondary
=
file 02217
13

Question 30
Suppose two wire coils are wound around a common iron core, the ”primary” coil with 100 turns of wire
and the ”secondary” coil with 300 turns of wire:
N
p
= 100
di
dt
= 30 A/s
N
s
= 300
If the inductance of the primary coil is 2 H, what is the inductance of the secondary coil, assuming that
it ”sees” the exact same magnetic circuit as the first coil (same permeability, same cross-sectional area, same
length)?

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