file 02213
15

Question 30
Are the transformers connecting power plant generators to high-voltage power lines considered
step-up
or
step-down
? Explain your answer.
file 02214
Question 31
In each of these circuits, calculate the amount of load impedance ”seen” by the voltage sources given
the turns ratio of each transformer:
30
Ω
1:1
30
Ω
120 VAC
240 VAC
1:2
Z
load
= ???
Z
load
= ???
Hint: ”impedance” (
Z
) is defined mathematically as the ratio of voltage (
E
) to current (
I
).
file 00659
16

Answers
Answer 1
Only the resistor actually dissipates power. The capacitor only absorbs and releases power, so its ”4
mW” figure does not actually represent power in the same sense as the resistor. To make this sensible, we
must think of all the non-resistive ”powers” as something other than actual work being done over time:
Q = 4 mVAR
P = 3 mW
S = 5 mVA
Correct power triangle
Follow-up question: when making the leap from DC circuit analysis to AC circuit analysis, we needed
to expand on our understanding of ”opposition” from just resistance (
R
) to include reactance (
X
) and
(ultimately) impedance (
Z
).
Comment on how this expansion of terms and quantities is similar when
dealing with ”power” in an AC circuit.
Answer 2
Fluorescent lamp:
I
= 0.674 A ; Actual power = 60 W
Incandescent lamp:
I
= 0.5 A ; Actual power = 60 W
Induction motor:
I
= 0.465 A ; Actual power = 52.0 W
In every load except for the incandescent lamp, more current is drawn from the source than is ”necessary”
for the amount of power actually dissipated by the load.
Answer 3
Power
factor is the ratio between true power (Watts) and apparent power (Volt-Amps), ranging between
0 and 1 inclusive.
Answer 4
”Apparent” power is
apparently
the total circuit power when volts and amps are multiplied together.
”Reactive” power is due to reactive components (
L
and
C
) only, and ”True” power is the only type that
actually accounts for energy leaving the circuit through a load component.
Answer 5
P.F. = 0.872
Answer 6
A
leading
power factor is one created by a predominantly capacitive load, whereas a
lagging
power factor
is one created by a predominantly inductive load.
17

Answer 7
Fluorescent lamp:
P
= 60 W ;
Q
= 54.3 VAR ;
S
= 80.9 VA ;
PF
= 0.74, leading
Incandescent lamp:
P
= 60 W ;
Q
= 0 VAR ;
S
= 60 VA ;
PF
= 1.0
Induction motor:
P
= 52.0 W ;
Q
= 20.4 VAR ;
S
= 55.8 VA ;
PF
= 0.93, lagging
Answer 8
The beer itself is ”true” power (
P
, measured in Watts). Good beer, good. Ideally, we’d like to have
a full mug of beer (true power).
Unfortunately, we also have foam in the mug, representing ”reactive”
power (
Q
, measured in Volt-Amps-Reactive), which does nothing but occupy space in the mug. Bad foam,
bad.
Together, their combined volume constitutes the ”apparent” power in the system (
S
, measured in
Volt-Amps).
Follow-up question: can you think of any potential safety hazards that low power factor may present in
a high-power circuit? We’re talking AC power circuits here, not beer!

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