EBK ELECTRICAL ENGINEERING
7th Edition
ISBN: 8220106714201
Author: HAMBLEY
Publisher: YUZU
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Question
Chapter 16, Problem 16.1P
To determine
It is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value. Explain why this is so.
Expert Solution & Answer
Answer to Problem 16.1P
To keep the ratio of voltage to frequency constant, it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.
Explanation of Solution
Given Information:
It is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.
Consider the EMF equation for the induction motor:
where
Therefore, the induced EMF is directly proportional to flux and frequency:
Now, if the frequency is reduced, the magnitude of the induced EMF will also be reduced such that to maintain constant airgap flux.
Consider the applied voltage to the induction motor is V, such that, this applied voltage V is directly proportional to induced EMF E, i.e.,
Therefore, if the frequency is reduced to maintain the constant flux in the air gap, there must be a proportional reduction in the magnitude of the applied voltage.
If the applied voltage is not reduced in proportion to the frequency, it results in excessive motor currents which may cause damage to the motor. Due to this, losses increase, and therefore, the efficiency of the motor is reduced.
So, it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value.
Conclusion:
It can be concluded that it is necessary to reduce the voltage applied to an induction motor as the frequency is reduced from the rated value to maintain constant flux in the airgap.
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Students have asked these similar questions
Create the PLC ladder logic diagram
for the logic gate circuit displayed in
Figure 7-35. The pilot light red (PLTR)
output section has three inputs: PBR,
PBG, and SW. Pushbutton red (PBR)
and pushbutton green (PBG) are inputs
to an XOR logic gate. The output of the
XOR logic gate and the inverted switch
SW) are inputs to a two-input AND
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The two-input AND logic gate output
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PBR
PBG
SW
TSW
PLTR
Figure 7-35. Logic gate circuit for Example 7-3.
PLTW
Goodheart-Willcox Publisher
gate. The temperature switch (TSW) is the other input to the NAND logic gate. The output generated from
the NAND logic gate is labeled pilot light white (PLTW).
Imaginary Axis (seconds)
1
6. Root locus for a closed-loop system with L(s) =
is shown below.
s(s+4)(s+6)
15
10-
0.89
0.95
0.988
0.988
-10
0.95
-15
-25
0.89
20
Root Locus
0.81
0.7
0.56
0.38
0.2
5
10
15
System: sys
Gain: 239
Pole: -0.00417 +4.89
Damping: 0.000854
Overshoot (%): 99.7
Frequency (rad/s): 4.89
System: sys
Gain: 16.9
Pole: -1.57
Damping: 1
Overshoot (%): 0
Frequency (rad/s): 1.57
0.81
0.7
0.56
0.38
0.2
-20
-15
-10
-5
5
10
Real Axis (seconds)
From the values shown in the figure, compute the following.
a) Range of K for which the closed-loop system is stable.
b) Range of K for which the closed-loop step response will not have any overshoot.
Note that when all poles are real, the step response has no overshoot.
c) Smallest possible peak time of the system. Note that peak time is the smallest
when wa is the largest for the dominant pole.
d) Smallest possible settling time of the system. Note that peak time is the smallest
when σ is the largest for the dominant pole.
For a band-rejection filter, the response drops below this half power point at two locations as visualised in Figure 7, we need to find these
frequencies. Let's call the lower frequency-3dB point as fr and the higher frequency -3dB point fH. We can then find out the bandwidth as
f=fHfL, as illustrated in Figure 7.
0dB
Af
-3 dB
Figure 7. Band reject filter response diagram
Considering your AC simulation frequency response and referring to Figure 7, measure the following from your AC simulation. 1% accuracy:
(a) Upper-3db Frequency (fH) =
Hz
(b) Lower-3db Frequency (fL) =
Hz
(c) Bandwidth (Aƒ) =
Hz
(d) Quality Factor (Q) =
Chapter 16 Solutions
EBK ELECTRICAL ENGINEERING
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