Download Unit 17 – Alternating Current (AC) Circuits Containing Inductance

DELMAR’S Standard Textbook of Electricity - 5th Edition
Unit 17 – Alternating Current (AC) Circuits Containing
Inductance
1) Inductance
a) One of the three primary types of AC loads
i) Capacitance and resistance are the other two
b) All AC circuits contain some amount of inductance do to
the continually changing magnetic field surrounding a
conductor
i) Usually small, and insignificant until a coil becomes
part of the load
(1) Relays, motors, transformers, lighting ballasts,
and chokes
c) Symbolized by the letter d) Characteristics of inductance
i) When magnetic lines of flux cut through a coil, a
voltage is induced in the coil
ii) This induced voltage is always in opposition to the
applied voltage (counter-electromotive force)
iii) The amount of induced voltage is proportional to the
rate of change of current
iv) An inductor opposes a change of current
e) Calculating the induced voltage
i) A 120 VAC source is connected to a coil having 6
ohms of wire resistance
ii) An ammeter connected to the circuit indicates that .8
A of current is flowing
iii) Ohm’s law can be used to determine the amount of
voltage needed to cause .8 A of current to flow in this
circuit
(a) = × Notes
(i) . 8
6Ω = 4.8
(b) Pythagorean’s theorem can be used to
calculate the voltage induced in the coil.
(i) The induced voltage is therefore equal to
the square root of (120 V)2 – (4.8 V)2 or
119.904 V
1. = √120 − 4.8 a. = 119.904
2) Inductive reactance
a) From the example above, it is apparent that induced
voltage can limit current flow in a circuit in a manner
similar to resistance
i) This current limiting property is called reactance and
is symbolized by the letter ii) To differentiate it from the reactance associated with
a capacitor it is labeled (inductive reactance)
iii) is measured in ohms just as resistance is and can
be calculated when the values of inductance and
frequency are known
(1) = 2π
3) Schematic symbols
a) Air-core inductors
b) Iron-core inductors
4) Inductors connected in series
a) Calculate like resistors and resistance in series
i) = 1 + 2 + 3 …
ii) " = # + + $ …
5) Inductors connected in parallel
a) Calculate like resistors and resistance in parallel
b) = #
%
%
%
' ' …
&% &( &)
c) " = #
%
%
%
'
'
…
+& % +& ( +& )
6) Voltage and current relationships in an inductive circuit
a) In a purely inductive circuit, current lags voltage by 90°
(ELI)
b) Demonstrate I and E waveforms over time to prove
7) Power in an inductive circuit
a) A purely inductive circuit dissipates no power except in
the resistance of the wire forming the inductor
b) Voltage and current have the same polarity for half the
time during each cycle and opposite polarity for the other
half of each cycle and thereby cancel out
i) Mathematically:
(1) (+- = + × +) = (−- = + ×– ) = 0
c) Power value for an inductor is reactive power VARsL
8) Reactive power
a) Calculate the same as for watts
i) 0 = × 9) Q (Quality) of an inductor
i) Determined by the ratio of inductive reactance to
resistance
(1) Inductors having a higher ratio of to are
considered to be inductors of higher quality
2
(a) 1 = 3&
ii) An inductor with many turns of larger diameter wire
will have a higher Q than that same inductor with the
same number of turns of small diameter wire