ECE 2210 Homework #21
c
3rd exam
will
include this material
Note: In the following problems, you may assume voltages and currents are RMS unless stated otherwise
or given as a function of time. Transformers are ideal unless stated otherwise.
1.
A load draws
12kVA
at
0.8 pf
, lagging when hooked to
480V
. A capacitance is hooked in parallel with the load
and the power factor is corrected to
0.9
, lagging.
a)
Find the reactive power (
VAR
) of the capacitor.
Draw a phasor diagram as part of the solution.
b) Find the value of the capacitor assuming
f = 60Hz
.
2.
Consider the circuit at right.
The resistor and capacitor together make up the load.
i
(
)
t
R
.
4.6
Ω
v
s
.
.
155.6
V
cos
(
)
.
377
t
a) Find the load impedance of the circuit.
Load
b) Compute the average power dissipated by the load.
C
.
800
µ
F
R
S
.
4
Ω
3.
a) Compute the average power dissipated by the
load (
R
L
and
C
L
taken together).
I
V
S
.
110
V
b)
Compute the power dissipated by the
internal source resistance (
R
S
) in this
circuit.
C
L
.
100
µ
F
rms
R
L
.
24
Ω
ω
.
377
rad
s
Load
4.
Read section 6.3, p 301 in your textbook.
5.
An ideal transformer has
330
turns on the primary winding and
36
turns on the secondary. If the primary is
connected across a
110 V
(rms) generator, what is the rms output voltage?
6.
A transformer has
N
1
= 320
turns and
N
2
= 1000 turns
. If the input voltage is
v(t) = (255 V)cos(
ω
t)
, what rms voltage
is developed across the secondary coil?
7.
A step-up transformer is designed to have an output voltage of
2200 V
(rms) when the primary is connected
across a
240 V
(rms) source.
a) If there are
150
turns on the primary winding, how many turns are required on the secondary?
b)
If a load resistor across the secondary draws a current of
1.2 A
, what is the current in the primary, assuming
ideal conditions?
8.
The primary current of an ideal transformer is
8.5 A
and the primary voltage is
80 V.
1.0 A
is delivered to a load
resistor connected to the secondary. Calculate the voltage across the secondary.
9.
An ideal transformer has a turns ratio (
N = N
1
/N
2
) of
1.5
. It is desired to operate a
200
Ω
resistive load at
150 V
(rms).
a) Find the secondary and primary currents.
b) Find the source voltage (
V
1
).
c) Find the power dissipated in the load resistor and the power delivered to the primary from the source.
d)
Find the impedance the source sees looking into the primary winding by calculating
Z
eq
= N
2
Z
L
and again by
calculating
V
1
/
I
1
.
10.
For the ideal transformer shown in the figure,
find
v
o
(t)
if
v
s
(t)
is
320Vcos(377t)
.
ECE 2210 Homework #21 p1
