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Table of
Contents. • U.S. Government Printing Office; 1945  618779
Chapter 17 AC CIRCUITS
SOME SURPRISES
You know the fundamental differences between d.c. and a.c. But a.c. has some
special peculiarities all its own. You might say that d.c. plows along like a steady
old battlewagon whereas a.c. cutsup like a frisky P. T.
You may be surprised by the way a.c. acts in some circuits. For example, did
you know that an ac coil can be built with only one ohm of resistance and yet pass
practically no current at 120 volts? And on the other hand, a condenser, which is
made of insulator material, will conduct large alternating currents?
A FEW WHYS
The basic reason for this behavior lies in the ac voltage. Look at figure 170.
It's only a simple sine wave of ac voltage. But that sine wave tells you plenty.
To begin with  it's not a picture of a.c. Don't get the idea that a.c. humps
along like a caterpillar on a wire. It doesn't! Alternating current flows just exactly
the way its voltage pushes. And you know that the push reverses its direction every
so often. That means the current flows first one way and then the other. The sine
wave tells you about this reversal and it also tells you the amount of push at any
instant.
Look again at figure 170. Imagine that this voltage is impressed on a lamp of
100 ohms resistance. At instant 1, there is zero current because, I = 0/100 = 0.
At instant 2, the current is I = 85/100 = 0.85 ampere. And at instant 3 the current
is I = 170/100 = 1.7 amperes, or maximum. notice what was happening between instant
1 and instant 3. The voltage increased from zero to 170 volts. And at the same time,
the current increased from zero to 1. 7 amperes. This is the first outstanding characteristic
of a.c. A.c. IS CONSTANTLY CHANGING IN VALUE.
Figure 170.  AC voltage.
If you wanted to find the current in an ac circuit you'd have to apply Ohm's
law at thousands of instants. But that would be impossible in a practical circuit,
so you use an EFFECTIVE VALUE of ac voltage and ac current. The effective voltage
is equal to the maximum voltage multiplied by 0.707.
Or 
E_{eff.} = 0.707 x E_{max.}
In the example just used, the lamp would have 170 X 0.707 = 120 volts of effective
EMF impressed. And the effective current would be 120/100 = 1.2 amperes.
You're wondering what the term "effective value" means and where it
comes from. It means the amount of a.c. that produces the same heating effect that
a given d.c. produces. Here's the problem: You can't take instantaneous readings
of ac current and voltage for every instant on the sine wave. You've got to have
some value of a.c. that's a true picture of its ability to do work  that corresponds
to dc values. The maximum value is easiest to determine but it won't do because
it's the CORRECT value for only TWO INSTANTS of each cycle.
The heating effect of currents is easily measured and this effect is used to
establish a comparison between a.c. and d.c. It is found that if the maximum value
of the alternating current (I_{max}) is multiplied by 0.707 the result is
the ac current value corresponding to the dc current in .heat producing ability.
For example, 10 amperes of d.c. produces a certain heat; 10 amperes, EFFECTIVE VALUE,
of a.c. produces the same heat. BUT you get this value  10 amperes of a.c.by multiplying
I_{max} by 0.707. In this case, 14.14 amperes (I_{max}) times 0.707
gives you the effective value  10 amperes. Thus a 10 ampere current in a.c. has
a maximum value of 14.14 amperes. But usually this maximum value will cause you
no headaches  ALL AC Meters READ IN EFFECTIVE VALUES.
The point you must understand, and remember, is that, although an ac
meter reads a steady current or voltage, NEITHER the current NOR the VOLTAGE IS
ACTUALLY STEADY. Both go up and down in value according to their sine waves.
The second outstanding characteristic of a.c. is that it CHANGES DIRECTION AT
REGULAR INTERVALS.
Figure 171.  A.c.  d.c. compared.
You noticed in the sine wave of figure 170 that half the time the voltage was
positive and half the time it was negative. Positive and negative indicate direction.
They simply mean that the voltage first pushes in one direction and then in the
other. For example, if you had the ordinary DC circuit shown in figure 171, current
would flow from the negative terminal to the positive terminal  ALL the TIME. But
suppose you impress A.C. on this dc circuit  the current flows from negative to
positive HALF the TIME and from positive to negative HALF the TIME. The lamp is
just as bright on a.c. as d.c. Just as much work is done  just as much power is
consumed  provided the ac effective values equal the dc values.
THREE PURE CIRCUITS
There are three things that limit the flow of current in an ac circuit resistance,
INDUCTIVE REACTANCE, AND CAPACITIVE REACTANCE. That's two more items than you had
in d.c. Remember that resistance ALONE limits current in a dc circuit.
When you have only one factor ALONE  resistance, inductive reactance, or capacitive
reactance  you have a PURE circuit. Say you have only resistance in an ac circuitthen
it's a PURE resistance CIRCUIT. But pure circuits don't happen very often! In fact
it's almost impossible to get one. However, by studying the action of pure circuits
with anyone of these  resistance, inductive reactance, or capacitive reactance
 you get the best picture of how each one of these things affects current. You'll
have to remember, though, that most PRACTICAL CIRCUITS are combinations of all three.
PURE resistance
This one is easy. Just like a dc circuit, in fact. Figure 172 shows a nearly
pure resistance circuit and the sine waves of current and voltage. The voltage impressed
on this circuit is shown by the solid line. The current flowing is shown by the
dotted line. Just what you'd expect. The current obeys Ohm's law: I = E/R for every
instant. Since the resistance is constant, the current rises and falls with the
voltage.
The sine waves of figure 172 show one very important thing. Voltage and current
are exactly IN PHASE  in time. When the voltage is zero, so is the current. When
the voltage is maximum, the current is maximum. Pure resistance circuits are IN
PHASE circuits.
Figure 172.  Pure resistance.
PURE INDUCTIVE REACTANCE
This one is not so easy, because inductive circuits always contain a voltage
of self induction. That means a coil and probably an iron core. To make as pure
an inductive circuit as possible, you'd wind a many turn coil on a soft iron core
 like figure 173.
In an inductive reactance circuit, this is what happens  the expanding and
contracting flux, set up by the a.c., produces a voltage of selfinduction. In a
pure circuit, this selfinduced voltage E_{si} is just as strong as the
applied voltage E_{a}. But the E_{si} IS not IN PHASE WITH the E_{a}.
Figure 174 shows the first step in understanding a pure inductive circuit.
notice that the E_{a} and E_{si} are 90° put of phase. This
out of phaseness was caused by the expanding and contracting flux. Be sure to note
that this is the FIRST CONDITION. The complete picture is given in figure 177.
Figure 173.  Pure inductive reactance.
Now you have TWO voltages controlling current E_{a} and E_{si}.
The result is a current out of phase with both. In fact, the current's phase is
midway between E_{a} and E_{si}. That makes the current 45° out
of phase with its applied voltage. Since the current reaches its maximum AFTER the
voltage, the current LAGS ITS APPLIED VOLTAGE.
But this is not the WHOLE picture. The current, by its field, produced the E_{si}.
And if the current moves out of phase  lagging, then the E_{si} is forced
further out of phase, as in figure 175. notice that the E_{si} is now opposing
the E_{a} more than half the time. You'll have to look at figure 175 to
see what's going on. During the time labeled 1, the two voltages are opposing each
other  E_{si} is negative and E_{a} is positive. The result is
a lowered current because the voltages that should be pushing current are wearing
themselves out bucking each other. This same condition is true for the time labeled
3. But, during 2 and 4, the two voltages are aiding each other  the current is
pushed in the same direction by both voltages.
Figure 174.  First condition.
Which condition has the upper hand  1 and 3, where the voltages oppose or 2
and 4, where they aid ? Well, which lasts the longest time ? You can see that the
opposing condition lasts longer than the aiding. Therefore, the current IS ACTUALLY
REDUCED BY the OPPOSITION OF the E_{si}.
Figure 175.  Second condition.
Figure 176.  Third condition.
not only is the current reduced  but it's shoved further out of phase. current
is midway between E_{a} and E_{si}, so it must be 671/2° lagging
its E_{a}.
The third and FINAL condition is shown in figure 176. The current, by moving
further out of phase, forces the E_{si} further out of phase. In turn, the
E_{si} forces the current out of phase. And so on. This is like the question, "Which
comes first, the chicken or the egg?" "Which does the forcing out of phase,
the E_{si} or the current?" That's a good question  except you can't
answer it! Each works on the other. current sets up the field that makes E_{si};
and E_{si} always stays 90° away from its current. The E_{si} helps
to push the current, so as E_{si} gets further out of phase, it carries
the current further out of phase. And, as the current gets further out of phase,
it forces the E_{si} still further out of phase because E_{si} is
always 90° from the current.
Figure 177.  Pure inductive reactance circuit.
Where is the end to all this pushing further and further out of phase? When
the E_{si} and E_{a} are 180° out of phase  that's figure 176.
notice that E_{si} and E_{a} are opposing each other ALL the TIME.
And, if they're equal  E_{si} = E_{a}  the total voltage is zero.
Therefore, in a pure inductive reactance circuit, the two voltages  E_{a}
and E_{si}and the current would have the phases shown by figure 177.
Inductive reactance does two things to current  REDUCES the AMOUNT OF current
AND THROWS IT OUT OF PHASE, LAGGING.
PRACTICAL INDUCTIVE CIRCUIT
If a pure inductive circuit could be built  and it can't be  the current would
be lagging 90°. Further, the voltage of self induction would exactly cancel the
applied voltage. A pure inductive circuit cannot be built because EVERY CIRCUIT
CONTAINS SOME resistance. Therefore, all practical inductive circuits contain two
factors controlling current  resistance (R) and INDUCTIVE REACTANCE (X_{L}).
Both limit current  in this respect they are alike. And both are measured in ohms.
But resistance tends to keep current IN PHASE. And INDUCTIVE REACTANCE tends to
force current OUT OF PHASE.
Figure 178.  Practical inductive circuit.
A practical inductive circuit  a REAL circuit  contains both inductive reactance
and resistance. Look at figure 178this is a practical circuit. The coil has 12
ohms of resistance (R = 12 Ω) and 12 ohms of inductive reactance (X_{L}
= 12 Ω). The inductive reactance (X_{L}) does just as much to limit
current as the resistance (R). And the X_{L} exerts just as much force to
send the current 90° out of phase as the resistance does to keep it exactly in phase.
Result  the current is half way between 90° out of phase, and exactly in phase
 it is 45° out of phase, lagging. Figure 179 shows the sine waves of current and
voltage for this circuit.
Figure 179.  current and voltage for figure 178.
You can conclude that, in all inductive circuits, the current IS REDUCED AND
LAGS OUT OF PHASE.
PURE CAPACITIVE REACTANCE
This is another one that is not so easy. Because capacitive circuits contain
condensers (capacitors)  and condensers do some strange things.
First, you should know how condensers are built. They're made up of alternate
layers of conductor and dielectric (insulator) materials. Half of the conductor
plates areconnected to one terminal and half to the other terminal. Between every
two conductor plates is a layer of dielectric.
Many materials will serve as conductors and dielectrics in condensers. But waxed
paper is a common dielectric and tin foil is a common conductor. Figure 180 shows
a waxed paper and tin foil condenser. Although this condenser is made of only six
plates, you'll find many condensers having hundreds of plates.
Figure 180.  Simple condenser.
Figure 181 shows a condenser with a.c. impressed across its terminal. The "innards"
are highly magnified so that you can see what happens inside. During the first quarter
of the cycle  that's the first 90°  the condenser is being CHARGED. Voltage is
pushing into the condenser from the left (solid arrows). current is flowing WITH
this voltage (dotted arrows). The electrons of the current pile up on the surface
of the conductor plates. This gives these plates a negative charge. Repulsion occurs
between the conductors negative charge and the electrons in the molecules of the
dielectric. The dielectric electrons strain to get awaythey move just as far from
the conductor's negative charge as they can. This warps the dielectric molecules
out of shape. Instead, of nice symmetrical molecules, they're all lopsided  with
their electroncongested sides AWAY from the negative conductor plate.
notice, in figure 181, how this builds up a negative charge all along one side
of the dielectric plates. Now, compare this to current flow  just about the same,
except that the dielectric has NO FREE ELECTRONS to flow. If the dielectric had
been a conductor, current would flow in the normal way.
So far you've got electrons all piled up along the side of the plates away from
the voltage force. The final act comes when the strained dielectric forces electrons
out of the conductor plates connected to the righthand side. current flows. Electrons
came in on the left sidepiled up on the plates  repelled the electrons of the
dielectric, which in turn repelled the electrons in the plates of the righthand
side. current flows out of the righthand side conductor plates. All this is true
for the first 90° of the cycle, because voltage is increasing. And as long as voltage
is increasing, electrons continue to pile up on the lefthand plates. You can say,
that as long as the voltage is INCREASING current flows across a condenser IN the DIRECTION OF VOLTAGE.
Figure 181.  Condenser action  No. 1.
Exactly at the 90° point of the sine wave, everything stands still. Voltage
is at its maximum. The condenser is charged. The voltage is no longer increasing,
so it can't force any more electrons onto the plates. current stops.
current IS STOPPED  BUT EVERYTHING IS STRAINED. The lefthand plates have too
many electrons. The dielectric's molecules are lopsided, and the righthand plates
have too few electrons. This strained condition is maintained by the maximum voltage
at the 90° point in the cycle.
Figure 182.  Condenser action  No. 2.
Now, see what happens during the next quarter cycle  the second 90°. Figure
182 shows the same condenser, but during the second quarter of a cycle.
When the voltage decreases  from 90° to 180° the strain is relieved  the force
maintaining the strain is gradually removed. Every thing returns to normal. And
in returning to normal  here's what takes place. The left side loses its
excessive electrons. These electrons flow through the external circuit to the right
side. Here they fill up the righthand plates. The dielectric no longer has a charge
against it so its molecules spring back to normal symmetrical shapes. The condenser
is DISCHARGED. And look what happens during this discharge. In figure 182, you can
see that the VOLTAGE is in the same direction as in figure 181  from LEFT to RIGHT.
But current is from RIGHT TO LEFT (follow the dotted arrows).
That's right  current IS flowing AGAINST the applied voltage. And the reason
is found in the strained dielectric of the condenser. When that dielectric was being
strained by the INCREASING voltage, it was storing energy (much like an EMF). When
the voltage decreased, the voltage wasn't strong enough to hold the energy in the
condenser. Electrons streamed out  backed by the energy of the strained dielectric.
These electrons make a current AGAINST the VOLTAGE DIRECTION.
Figure 183.  Pure capacitive circuit.
These two facts stand out. The current is in the same direction as voltage,
as long as voltage is increasing. And the current is in the opposite direction to
the voltage, as long as voltage is decreasing. Figure 183 shows you the current
and voltage relationships in a pure capacitive circuit. notice that current LEADS
the voltage by 90°.
Capacitive reactance (X_{C}) does two things to a current. X_{C}
limits current like a resistance and causes current to be out of phase with its
voltage LEADING. X_{C}, like X_{L}, is measured in ohms.
Figure 184.  Practical capacitive circuit.
PRACTICAL CAPACITIVE CIRCUIT
A practical capacitive circuit  a real circuit  is bound to have some resistance.
You can't have any circuit without some resistance. Look at figure 184. The circuit
has a condenser with 12 ohms of X_{C} and 12 ohms of R.
Figure 185.  current and voltage for figure 184.
R and X_{C} are equal. They both reduce current flow and the R tends
to keep current in phase while the X_{C} tends to force it 90° out of phaseleading.
Resultthe current is midway between 90° leading and exactly in phase  it is 45°
out of phase, leading. The current and voltage relationship is shown in figure 185.
ALL THREE TOGETHER
Many circuits are combinations of X_{L}, X_{C}, and R. Ana all
of them  X_{L}, X_{C}, and R  have their own individual effect
on the current. There is a certain method of combining these three items to give
you the IMPEDANCE. Impedance (Z) is the total opposition to the flow of current
in an ac circuit. It corresponds to resistance in a dc circuit.
When you are determining the impedance of an ac circuit, the first step is
to combine the two reactances. They're opposite in action  XL makes current lag
and X_{C} makes current lead. Therefore, when they're combined, the action
of one cancels the action of the other.
If a circuit has 15 ohms of X_{L} and 24 ohms of X_{C}, then
the total reactance (X) is 24  15 = 9 ohms. And the current will LEAD the voltage
because X_{C} is stronger than X_{L}.
If a circuit has 30 ohms of X_{L} and 19 ohms of X_{C}, the
X is 30  19 = 11 ohms. And the current LAGS the voltage because X_{L} is
stronger than X_{C}.
After you have combined X_{L}, and X_{C}, the total reactance
X must be added to the resistance R to get the impedance Z. Here's how you add X
and R 
Z = SQRT (R^{2} + X^{2})
PRACTICE CIRCUIT
Take a practice circuit. The one in figure 186 is a good example. In this drawing
the resistance and reactance values are given. You can find out HOW MUCH current
is flowing, and whether the current is LEADING OR LAGGING.
Figure 186.  Practice circuit.
First, how much total reactance?
X = X_{L } X_{C}.
X = 9  6 = 3 ohms.
Second, how much impedance, Z?
Z = SQRT (R^{2} + X^{2}).
Z = SQRT (16 + 9) = SQRT(25) = 5 ohms.
By Ohm's law (but using Z instead of R for  an ac circuit) you find the current

I = E/Z = 120/5 = 24 amps.
And the current is LAGGING because X_{L} is larger than Xc.
WHERE THEY ARE
You'll find circuits involving X_{L}, X_{C} and R almost everywhere
you find a.c. This is only the beginning. Circuits containing ac induction motors
have a high inductive reactance. This makes the current lag too far behind the voltage.
So condensers are put in the circuit to increase the X_{C} and offset the
X_{L}. Condensers are used in vacuum tube circuits and across switches.
Induction coils are used in radio circuits to choke down current.
If you keep the three actions straight, you can figure out the effect of each
in a circuit. Remember 
All three, X_{L}, X_{C} and R limit current. And the total opposition
to current flow in a.c. is Z. And Z is made up of XL, X_{C}, and R.
R tends to keep current in phase with voltage.
X_{L} tends to make current lag voltage.
X_{C} tends to make current lead voltage.
Chapter 17 Quiz
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