The Inductor
3.13
The amount of current change required in an inductor decides the area-content under
voltage waveform to be applied to it to bring about the change and the time allowed to
bring about it decides the average voltage to be applied.
3.2.4
Instantaneous change in Inductor current
It follows from the last sub–section that the average voltage to be applied to cause a finite amount of
change in inductor current increases to infinite value when we try to accomplish the change in current
in zero time interval. We cannot bring about instantaneous change in inductor current unless we apply
or support such an infinite voltage across the inductor.
Let us say we want to change the current in a 0.5 H inductor from 0 to 2 A by applying a rectangular
pulse voltage from
t
=
0. The voltage area content required is 0.5 H
×
2 A
=
1 V-s. The height of pulse
will depend on the width of pulse. Three cases are shown in Fig. 3.2-2.
When 2.5V pulse lasting for 0.4 s is applied, the inductor current ramps up linearly from 0 to 2A
in 0.4 s with a slope of 5 A/s. When 5V pulse lasting for 0.2 s is applied, the inductor current ramps
up linearly from 0 to 2A in 0.2 s with a slope of 10 A/s. When 10 V pulse lasting for 0.1 s is applied,
the inductor current ramps up linearly from 0 to 2A in 0.1 s with a slope of 20 A/s. We have kept the
area under the voltage waveform at 1V-s in all the three cases. Now consider further shortening of
pulse duration, taking it to near-zero width. The change in inductor current will be 2A if the pulse
height is increased to maintain the area under the voltage waveform at 1V-s. However, the inductor
current waveform will become steeper and steeper until it becomes a
straight-edged waveform as
pulse width
→
0 and pulse height
→
∞
. Notice
that though pulse height
→
∞
as width
→
0, its
area is constrained to remain 1V-s. Such an idealised waveform with zero width, undefined height
and finite area-content of unity is called a
unit impulse function and denoted by the symbol
d
(
t).
Its formal definition is
d
d
( )
( )
t
t
undefined at t
t
t dt
=
∞ < ≤
=
≤ < ∞
=
−
+
−∞
0
0
0
0
0
1
for -
for
and
∞
∞
∫
where the time instant
t
=
0
-
is an instant which is arbitrarily close to
t
=
0 but on its left side and
time instant
t
=
0
+
is an instant which is arbitrarily close to
t
=
0 but on its right side. Thus the interval
[0
-
, 0
+
]
is of infinitesimal width; but
t
=
0 comes in the middle of this interval.
The graphical symbol used for
d
(
t) is shown in Fig. 3.2-2 (b). The height of the arrow-terminated
vertical line representing
d
(
t)
is not the amplitude of the function (amplitude is
undefined at that
point); rather it indicates the area-content of the waveform. The instantaneous
change in inductor
current from 0 to 2A is also shown in the same figure. Now look at the current waveform in the
inductor. It is zero till 0
-
and 2A after 0
+
and a discontinuous jump at
t
=
0. This must be 2 times the
integral of impulse function (1/
L
=
2 in this case). Let us verify this.
d
( )
;
t dt
t
undefined
t
t
t
=
− ∞ < ≤
=
≥
−
+
−∞
∫
0
0
0
1
for
at
for
0
Thiss function is called
unit step function u t
( )
(3.2-4)
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3.14
Single
Element Circuits
This function is defined as a
unit step function and
is denoted by u(
t). Thus, when we apply a
d
(
t) voltage waveform to an inductor of inductance value
L, the current in the inductor jumps up
instantaneously
by 1
/L A.
Unit impulse voltage source will dump 1 V-s of voltage area content into
the inductor instantaneously. Equivalently,
unit impulse voltage source will dump 1Weber-turns
(Wb-T) of flux linkage into the inductor instantaneously. The result will be a change in its current
by 1
/L A.
Applied voltage
Current
10
9
8
7
6
5
4
3
2
1
0.1
0.2
(a)
(b)
0.3
0.4
1
2
δ
(
t
)
Time
Time
0.1
0.2
0.3
0.4
Fig. 3.2-2
Rectangular pulse application and impulse voltage
Current in an inductor cannot change instantaneously unless
an impulse voltage is
applied or supported in the circuit.
The current in an inductor
L
changes instantaneously by 1/
L
A when the circuit applies
or supports a unit impulse voltage across it.
Therefore, if a circuit does not apply or support impulse voltage, the currents in inductors
in that circuit will be continuous functions of time.
Strictly speaking, it is the flux linkage in an inductor that can not be changed instantaneously.
However, in the case of an inductor that is not magnetically coupled to other inductors, this will amount
to what we have stated above since flux linkage in such an inductor is proportional to its current. We
will modify this statement suitably when we take up the study of coupled circuits later in the book.
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