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Charles Proteus Steinmetz.

Engineering mathematics; a series of lectures delivered at Union college online

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Vr 2 +x 2 Vr 2 +a 2 x 2



(29)



Usually r and XQ are of such magnitude that r consumes
at full load about 1 per cent or less of the generated voltage,
while the reactance voltage of XQ is of the magnitude of from
20 to 50 per cent. Thus r is small compared with XQ, and if
a is not very small, equation (29) can be approximated by



l ( r }

~2W




Then if x = 20r, the following relations exist:
a= 0.2 0.5 1.0 2.0

i = X0.9688 0.995 0.99875 0.99969

That is, the short-circuit current of an alternator is practi-
cally constant independent of the speed, and begins to decrease
only at very low speeds.

131. Exponential functions, logarithms, and trigonometric
functions are the ones frequently met in electrical engineering.

The exponential function is defined by the series,

a . x 2 x 3 x 4 x 5

\ \ \ \

i i i



METHODS OF APPROXIMATION. 197

and, if x is a small quantity, s, the exponential function, may
be approximated by the equation,

s = ls; ........ (32)

or, by the more general equation,

e = los; ....... (33)

and, if a greater accuracy is required, the second term may
be included, thus,



, ....... (34)

and then

/ A/2o2

L- = l as+~ ...... (35)

The logarithm is defined by log z= 1 ; hence, I^OGK<



Resolving r - - into a series, by (10), and then integrating,
1 x

gives

log (lz)=4 f (lT

X 2 T 3 ^4 T 5

= .T-23- T 5J. (36)

L This logarithmic series (36) leads to the approximation,

log(ls)=s; ...... (37)

or, including the second term, it gives

^ log.(ls)=s-.|? 7 ..... (38)

^\

and the more general expression is, respectively,

(as) = log a^l ~ j =log o+log (l j) =log a^,-



198 ENGINEERING MATHEMATICS

and, more accurately,

loge (as)=log ad



o2



(40)



Since logio N = logio eXloge N = 0.4343 log e AT, equations (39)
and (40) may be written thus,




logio(ls) = 0.4343s;

o

logic (os) = logio a 0.4343-



(41)



132. The trigonometric functions are represented by the
infinite series :

x 2 ic 4 x 6



3?



(42)



~IT1Z

which when s is a small quantity, may be approximated by

coss = l and sin s = s; . .
or, they may be represented in closer approximation

- cos s = 1 ;

sins = s|-|r);
or, by the more general expressions,

cos as = 1 and (cos as = 1 ^~

/ a 3 s 3 \
JL sin as = as arid \ sin as = as ( ^- ) .



(43)



(44)






(45)



133. Other functions containing small terms may frequently
be approximated by Taylor's series, or its special case,
MacLaurin's series.

MacLaurin's series is written thus :



*) =/(0) +a/'



...,. (46)



METHODS OF APPROXIMATION. 199

where /', /", /'", etc., are respectively the first, second, third,
etc., differential quotient of/; hence,



........ (47)

f(as)=f(0)+asf(0). \

Taylor's series is written thus,

f(b +x) =f(b) +xf'(b} +p/"(&) +^f'"(V + - - , . (48)

and leads to the approximations :
f(bs)-f(b)rf'(b); 1

/(&a)=/(6)as/'(6). )'

Many of the previously discussed approximations can be
considered as special cases of (47) and (49).

134. As seen in the preceding, convenient equations for the
approximation of expressions containing small terms are
derived from various infinite series, which are summarized
below :



X jl X

n(n-l) n(n])(n-2






X 2 X



X 3 X s



=/(0)
/(6 x) =



())



200



ENGINEERING MATHEMATICS.



The first approximations, derived by neglecting all higher
terms but the first power of the small quantity x = s in these



series, are:



[ + S 2 ];



[4];



loge(ls)= .s:
cos s = 1 ;

sin s = s;



. (51)



f(b8)=f(b)sf'(b);



and, in addition hereto is to be remembered the multiplication
rule,

[sis 2 ]. . . (52)



135. The accuracy of the approximation can be. estimated
by calculating the next term beyond that which is used.
This term is given in brackets in the above equations (50)
and (51).

Thus, when calculating a series of numerical values by
approximation, for the one value, for which, as seen by the
nature of the problem, the approximation is least close, the
next term is calculated, and if this is less than the permissible
limits of accuracy, the approximation is satisfactory.

For instance, in Example 2 of paragraph 130, the approxi-
mate value of the short-circuit current was found in (30), as



METHODS OF APPROXIMATION. 201

The next term in the parenthesis of equation (30), by the
binomial, would have been H ^ s 2 ; substituting n=\\

s = [ ) , the next becomes -hW ) . The smaller the a, the
\a.r / ' 8 \ax /

less exact is the approximation.

The smallest value of a, considered in paragraph 130, was

a = 0.2. For z = 20r, this gives +^ (~ ~) =0.00146, as the

o \axo/

value of the first neglected term, and in the accuracy of the
result this is of the magnitude of X 0.00146, out of X 0.9688,

XQ XQ

the value given in paragraph 130; that is, the approximation

gives the result correctly within ' OAQA =0.0015 or within one-

u.



sixth of one per cent, which is sufficiently close for all engineer-
ing purposes, and with larger a the values are still closer
approximations.

136. It is interesting to note the different expressions,
which are approximated by (1+s) and by (1 s). Some of
them are given in the following:

l+s= l-s=

1



_J_. J&Ls4^ T '

l+s ;

Mr.

(>-D



<h^**s**





202 ENGINEERING MATHEMATICS.

VI



1



VT=2^'
/T+s





/ 1 +ms
-Vl-(n-wi)8 ;



1




<P=*

\l + (n-



etc.



etc.



^ log, (l-s);



/iTs

l+^A/T Ti




etc.
1+sin s;



1+nsin ;




etc.
1 sins;

1-nsin^;



METHODS OF APPROXIMATION.



203



1 H sin ns]

cos V 2s;
etc.



1 sin ns\

n

cos V2s";
etc.



137. As an example may be considered the reduction to its
simplest form, of the expression:



_i s

-vae cos 2



-a log




a +sz J



then,



3/4 3











= l-alog s (l-j)=l+ S2 ;



204 ENGINEERING MATHEMATICS.

hence,

| L) X 4(l-|s 2 )



l_ i_

4 a 2 a a



T

4 a

138. As further example may be considered the equations
of an alternating-current electric circuit, containing distributed
resistance, inductance, capacity, and shunted conductance, for
instance, a long-distance transmission line or an underground
high-rpotential cable.

Equations of the Transmission Line.

Let I be the distance along the line, from some starting
point; E, the voltage; /, the current at point I, expressed as
vector quantities or general numbers; ZQ-^TQJXQ, the line
impedance per unit length (for instance, per mile); Yn=g jb
= line admittance, shunted, per unit length; then, ro is the
ohmic effective resistance; .TO, the self-inductive reactance;
bo, the condensive susceptance, that is, wattless charging
current divided by volts, and g = energy component of admit-
tance, that is, energy component of charging current, divided
by volts, per unit length, as, per mile.

Considering a line element dl, the voltage, dE, consumed
by the impedance is Z^Idl, and the current, dl, consumed by
the admittance is Y () Edl; hence, the following relations may be
written :



-ro? ......... (2)



METHODS OF APPROXIMATION. 205

Differentiating (1), and substituting (2) therein gives

d 2 F

^ = Z Y E, ....... (3)

and from (1) it follows that,

1 dE
{ = -7- -77 ........ V4)

ZQ di

Equation (3) is integrated by

E = Ae* 1 , ....... (5)

and (5) substituted in (3) gives

=vXn; ...... (6)

hence, from (5) and (4), it follows

/ ^^ ...... ( 7 )

^t-*^ 1 ! ..... (8)



. (9)



Next assume

l = lv, the entire length of line;
Z = l Zo, the total line impedance;
and Y = loYo, the total line admittance;



then, substituting (9) into (7) and (8), the following expressions
are obtained :



(10)



as the voltage and current at the generator end of the line.

139. If now E and / respectively are the current and
voltage at the step-down end of the line, for Z = 0, by sub-
stituting 1=0 into (7) and (8),



(11)



206



ENGINEERING MATHEMATICS.



Substituting in (10) for the exponential function, the series,

- - ZY ZYVZY Z 2 Y 2 Z 2
i 4.\/ZY H ___ I ____ I - - 1

2 6 24 120



and arranging by (Ai+A 2 ) and (Ai 42), and substituting
here for the expressions (11), gives



Z 2 Y 2



Z 2 F 2



ZY



ZF



(13)



120



When 1=1 , that is, for E and 7 at the generator side, and
E\ and l\ at the step-down side of the line, the sign of the
second term of equation (13) merely reverses.

140. From the foregoing, it follows that, if Z is the total
impedance; F, the total shunted admittance of a transmission
line, *EQ and /o, the voltage and current at one end; EI and (i,
the voltage arid current at the other end of the transmission
line; then,

ZY Z 2 Y 2 } .(. ZY



ZY



ZY Z 2 Y 2



, (14)



where the plus sign applies if E , I is the step-down end,
the minus sign, if E , 7 is the step-up end of the transmission
line.

In practically all cases, the quadratic term can be neglected,
and the equations simplified, thus,



ZI l+- [;



ZY



(15)



and the error made hereby is of the magnitude of less than



Z 2 Y 2



METHODS OF APPROXIMATION. 207

Except in the ease of very long lines, the second term of
the second term can also usually be neglected, which



-,.... (16)

j

zy
and the error made hereby is of the magnitude of less than ~

of the line impedance voltage and line charging current.

141. Example. Assume 200 miles of 60-cycle line, on non-
inductive load of eo = 100,000 volts; and i' = 100 amperes.
The line constants, as taken from tables are Z = 104 140; ohms
and Y= -0.0013; ohms; hence,

ZY= -(0.182 +0.136?);

EI = 100000(1 - 0.091 - 0.068/) + 100(104 - 104/)
= 101400 -20800/, in volts;

1 1 = 100(1 - 0.091 - 0.068/) - 0.0013/ + 100000
= 91 136.8/, in amperes.

zy 0.174X0.0013 0.226
The error is - = = ~ = 0.038.

b b b

Neglecting the second term of EI, z/ = 17,400, the error is
0.038X17400 = 660 volts = 0.6 per cent.

Neglecting the second term of /i, yEo = I'3Q, the error is
0.038 X 130 = 5 amperes = 3 per cent. "

Although the charging current of the line is 130 per cent
of output current, the error in the current is only 3 per cent.

Using the equations (15), which are nearly as simple, brings

2 2y2 Q.226 2
the error down to -^j-= "^. =0.0021, or less than one-quarter

per cent.

Hence, only in extreme cases the equations (14) need to be



used. Their error would be less than -=r = 3.6xlO~ 6 , or one
three-thousandth per cent.






208 ENGINEERING MATHEMATICS.

The accuracy of the preceding approximation can be esti-
mated by considering the physical meaning of Z and Y: Z
is the line impedance; hence Zl the impedance voltage, and

zi

u = - p, the impedance voltage of the line, as fraction of total

voltage; Y is the shunted admittance; hence YE the charging

YE

current, and v=j-, the charging current of the line, as fraction

of total current.

Multiplying gives uv = ZY; that is, the constant ZY is the
product of impedance voltage and charging current, expressed
as fractions of full voltage and full current, respectively. In
any economically feasible power transmission, irrespective of
its length, both of these fractions, and especially the first,
must be relatively small, and their product therefore is a small
quantity, and its higher powers negligible.

In any economically feasible constant potential transmission
line the preceding approximations are therefore permissible.



CHAPTER VI.
EMPIRICAL CURVES.

A. General.

142. The results of observation or tests usually are plotted
in a curve. Such curves, for instance, are given by the core
loss of an electric generator, as function of the voltage; or,
the current in a circuit, as function of the time, etc. When
plotting from numerical observations, the curves are empirical,
and the first and most important problem which has to be
solved to make such curves useful is to find equations for the
same, that is, find a function, y=f(x), which represents the
curve. As long as the equation of the curve is not known its
utility is very limited. While numerical values can be taken
from the plotted curve, no general conclusions can be derived
from it, no general investigations based on it regarding the
conditions of efficiency, output, etc. An illustration hereof is
afforded by the comparison of the electric and the magnetic
circuit. In the electric circuit, the relation between e.m.f. and

g

current is given by Ohm's law, i = , and calculations are uni-
versally and easily made. In the magnetic circuit, however,
the term corresponding to the resistance, the reluctance, is not
a constant, and the relation between m.m.f. and magnetic flux
cannot be expressed by a general law, but only by an empirical
curve, the magnetic characteristic, and as the result, calcula-
tions of magnetic circuits cannot be made as conveniently and
as general in nature as calculations of electric circuits.

If by observation or test a number of corresponding values
of the independent variable x and the dependent variable y are
determined, the problem is to find an equation, y=f(x), which
represents these corresponding values: x\, x 2 , 3 . . . x n , and
2/i, 2/2, 2/3 ... 2/n, approximately, that is, within the errors of
observation.

209



210 ENGINEERING MATHEMATICS.

The mathematical expression which represents an empirical
curve may be a rational equation or an empirical equation.
It is a rational equation if it can be derived theoretically as a
conclusion from some general law of nature, or as an approxima-
tion thereof, but is an empirical equation if no theoretical
reason can be seen for the particular form of the equation.
For instance, when representing the dying out of an electrical
current in an inductive circuit by an exponential function of
time, we have a rational equation: the induced voltage, and
therefore, by Ohm's law, the current, varies proportionally to the
rate of change of the current, that is, its differential quotient,
and as the exponential function has the characteristic of being
proportional to its differential quotient, the exponential function
thus rationally represents the dying out of the current in an
inductive circuit. On the other hand, the relation between the
loss by magnetic hysteresis and the magnetic density: W= ^(B 1 * 6 ,
is an empirical equation since no reason can be seen for this
law of the 1.6th power, except that it agrees with the observa-
tions.

A rational equation, as a deduction from a general law of
nature, applies universally, within the range of the observa-
tions as well as beyond it, while an empirical equation can with
certainty be relied upon only within the range of observation
from which it is derived, and extrapolation beyond this range
becomes increasingly uncertain. A rational equation there-
fore is far preferable to an empirical one. As regards the
accuracy of representing the observations, no material difference
exists between a rational and an empirical equation. An
empirical equation frequently represents the observations with
great accuracy, while inversely a rational equation usually
does not rigidly represent the observations, for the reason that
in nature the conditions on which the rational law is based are
rarely perfectly fulfilled. For instance, the representation of a
decaying current by an exponential function is based on the
assumption that the resistance and the inductance of the circuit
are constant, and capacity absent, and none of these conditions
can ever be perfectly satisfied, and thus a deviation occurs from
the theoretical condition, by what is called " secondary effects."

143. To derive an equation, which represents an empirical
curve, careful consideration should first be given to the physical



EMPIRICAL CURVES. 211

nature of the phenomenon which is to be expressed, since
thereby the number of expressions which may be tried on the
empirical curve is often greatly reduced. Much assistance is
usually given by considering the zero points of the curve and
the points at infinity. For instance, if the observations repre-
sent the core loss of a transformer or electric generator, the
curve must go through the origin, that is, y = for = 0, and
the mathematical expression of the curve y =f(x) can contain
no constant term. Furthermore, in this case, with increasing x,
y must continuously increase, so that for x = oo , y = oo . Again ,
if the observations represent the dying out of a current as
function of the time, it is obvious that for x = 00, y=Q. In
representing the power consumed by a motor when running
without load, as function of the voltage, for x = Q, y cannot be
= 0, but must equal the mechanical friction, and an expression
like y = Ax? cannot represent the observations, but the equation
must contain a constant term.

Thus, first, from the nature of the phenomenon, which is
represented by the empirical curve, it is determined

(a) Whether the curve is periodic or non-periodic.

(6) Whether the equation contains constant terms, that is,
for = 0, 2/7^0, and inversely, or whether the curve passes
through the origin: that is, y = for x = Q, or whether it is
hyperbolic; that is, y= oo for x = 0, or a: = 00 for y = Q.

(c) What values the expression reaches for oo. That is,
whether for x = oo, y = oo, or y = Q, and inversely.

(d) Whether the curve continuously increases or decreases, or
reaches maxima and minima.

(e) Whether the law of the curve may change within the
range of the observations, by some phenomenon appearing in
some observations which does not occur in the other. Thus,
for instance, in observations in which the magnetic density
enters, as core loss, excitation curve, etc., frequently the curve
law changes with the beginning of magnetic saturation, and in
this case only the data below magnetic saturation would be used
for deriving the theoretical equations, and the effect of magnetic
saturation treated as secondary phenomenon. Or, for instance,
when studying the excitation current of an induction motor,
that is, the current consumed when running light, at low
voltage the current may increase again with decreasing voltage,



212 ENGINEERING MATHEMATICS.

instead of decreasing, as result of the friction load, when the
voltage is so low that the mechanical friction constitutes an
appreciable part of the motor output. Thus, empirical curves
can be represented by a single equation only when the physical
conditions remain constant within the range of the observations.

From the shape of the curve then frequently, with some
experience, a guess can be made on the probable form of the
equation which may express it. In this connection, therefore,
it is of the greatest assistance to be familiar with the shapes of
the more common forms of curves, by plotting and studying
various forms of equations y=f(x).

By changing the scale in which observations are plotted
the apparent shape of the curve may be modified, and it is
therefore desirable in plotting to use such a scale that the
average slope of the curve is about 45 deg. A much greater or
much lesser slope should be avoided, since it does not show the
character of the curve as well.

B. Non-Periodic Curves.

144. The most common non-periodic curves are the potential
series, the parabolic and hyperbolic curves, and the exponential
and logarithmic curves.

THE POTENTIAL SERIE^.

Theoretically, any set of observations can be represented
exactly by a potential series of any one of the following forms :

; .... (1)
...... (2)



if a sufficiently large number of terms are chosen.

For instance, if n corresponding numerical values of x and y
are given, x\, y\; x%, y 2 \ ... x n , y n , they can be represented



EMPIRICAL CURVES.



213



by the series (1), when choosing as many terms as required to
give n constants a :

By substituting the corresponding values x\, y\] x 2 , 2/2, ...
into equation (5), there are obtained n equations, which de-
termine the n constants a , a\, a 2 , . . . a n _\.

Usually, however, such representation is irrational, and
therefore meaningless and useless.

TABLE I.



e

Too"'


p i=y


-0.5


+ 2x


+ 2.5x2


-1.5s 3


+ 1.5x<


-2*6


+ z


0.4
0.6
0.8


0.63
1.36
2.18


-0.5
-0.5
-0.5


+ 0.8
+ 1.2
+ 1.6


+ 0.4
+0.9
+ 1.6


-0.10
-0.32
-0.77


+ 0.04
+0.19
+ 0.61


- 0.02
- 0.16
- 0.65



+ 0.05
+ 0.26


1.0
1.2
1.4


3.00
3.93
6.22


-0.5
-0.5
-0.5


+ 2.0

+ 2.4
+ 2.8


+ 2.5
+3.6
+4.9


-1.50
-2.59
-4.12


+ 1.50
+ 3.11

+ 5.76


- 2.00
- 4.98
-10.76


+ 1.00
+ 2.89
+ 6.13


1.6


8.59


-0.5


+3.2


+ 6.4


-6.14


+9.83


-20.97


+ 16.78



Let, for instance, the first column of Table I represent the

a

voltage, JQQ = > m hundreds of volts, and the second column

the core loss, Pi = y, in kilowatts, of an 125- volt 100-h.p. direct-
current motor. Since seven sets of observations are given,
they can be represented by a potential series with seven con-
stants, thus,

, . . +a 6 ;r 6 , .... (6)



and by substituting the observations in (6), and calculating the
constants a from the seven equations derived in this manner,
there is obtained as empirical expression of the core loss of
the motor the equation,



This expression (7), however, while exactly representing
the seven observations, has no physical meaning, as easily
seen by plotting the individual terms. In Fig. 60, y appears



214



ENGINEERING MATHEMATICS.



as the resultant of a number of large positive and negative
terms. Furthermore, if one of the observations is omitted,
and the potential series calculated from the remaining six
values, a series reaching up to x 5 would be the result, thus,

.... (8)



y



16



12




FIG. 60. Terms of Empirical Expression of Excitation Power.

but the constants a in (8) would have entirely different numer-
ical values from those in (7), thus showing that the equation
(7) has no rational meaning.

145. The potential series (1) to (4) thus can be used to
represent an empirical curve only under the following condi-
tions :

1. If the successive coefficients ao, a\, a^ ... decrease in
value so rapidly that within the range of observation the
higher terms become rapidly smaller and appear as mere
secondary terms.



EMPIRICAL CURVES.



215



2. If the successive coefficients a follow a definite law,
indicating a convergent series which represents some other
function, as an exponential, trigonometric, etc.

3. If all the coefficients, a, are very small, with the exception
of a few of them, and only the latter ones thus need to be con-
sidered.

TABLE II.



X


V


i/ 1


1/1


0.4
0.6

0.8


0.89
1.35
1.96


0.88
1.34
1.94


0.01
0.01
0.02


1.0
1.2
1.4


2.72
3.62
4.63


2.70
2.59
4.59


0.02
0.03
0.04


1.6


5.76


5.65


0.11



For instance, let the numbers in column 1 of Table II
represent the speed x of a fan motor, as fraction of the rated
speed, and those in column 2 represent the torque y, that is,
the turning moment of the motor. These values can be
represented by the equation,

i/ = 0.5 +0.02o:+2.5.r 2 -0.3^H-0.015.r 4 -0.02x 5 +0.01.r 6 . (9)

In this case, only the constant term and the terms with
x 2 and x 3 have appreciable values, and the remaining terms
probably are merely the result of errors of observations, that is,
the approximate equation is of the form,



y = ao+d2X 2 + a3X 3 (10)

Using the values of the coefficients from (9), gives

i/-0.5+2.5x 2 -0.3.r 3 . (11)

The numerical values calculated from (11) are given in column
3 of Table II as y f , and the difference between them and the
observations of column 2 are given in column 4, as y\.



216 ENGINEERING MATHEMATICS.

The values of column 4 can now be represented by the same
form of equation, namely,



(12)



in which the constants & , 62, b 3 are calculated by the method
of least squares, as described in paragraph 120 of Chapter IV,
and give

yi= 0.031 -0.093x 2 + 0.076x 3 ..... (13)



Equation (13) added to (11) gives the final approximate
equation of the torque, as,

2/o = 0.531 +2.407.T 2 - 0.224x 3 ..... (14)

The equation (14) probably is the approximation of 1 a
rational equation, since the first term, 0.531, represents the
bearing friction; the second term, 2A07x 2 (which is the largest),
represents the work done by the fan in moving the air, a
resistance proportional to the square of the speed, and the
third term approximates the decrease of the air resistance due
to the churning motion of the air created by the fan.

In general, the potential series is of limited usefulness; it
rarely has a rational meaning and is mainly used, where the


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Online LibraryCharles Proteus SteinmetzEngineering mathematics; a series of lectures delivered at Union college → online text (page 12 of 17)