Refrigeration & Air Conditioning Technology 7th Edition

290 Section 3 Basic Automatic Controls
Notice that the total resistance of 5.46 Ω is lower than the
lowest individual resistance in the circuit. Another way to
look at the resistance calculation is as follows:
1 1 1 1
R R R R
total 1 2 3
1 1 1 1
R 10 20 30
total
1 6 3 2
R 60 60 60
total
1 11
R 60
total
60
R
total
11
R 5. 46
total
To determine the total current draw, use Ohm’s law:
E
I R
120V
546
22 A
We can confirm this value for the total current in the circuit
by determining the current flow through the individual
loads or resistances and then adding them up. The current
flow through the 10-Ω resistance is determined here:
I 10
E R
I 10
120 V 10 Ω 12 A
The current flow through the 20-Ω resistance is determined
here:
I 20
E R
I 20
120 V 20 Ω 6 A
The current flow through the 30-Ω resistance is determined
here:
I 30
E R
I 30
120 V 30 Ω 4 A
By adding these three individual branch currents together,
we get
I total
I 1
I 2
I 3
I total
12 A 6 A 4 A
I total
22 A
12.13 ELECTRICAL POWER
Electrical power (P) is measured in watts. A watt (W) is the
power used when 1 ampere flows with a potential difference
of 1 volt. Therefore, power can be determined by multiplying
the voltage times the amperes flowing in a circuit.
Watts Volts Amperes
or
P E I
The consumer of electrical power pays the electrical
utility company according to the number of kilowatts
(kW) used for a certain time span, usually billed as kilowatt
hours (kWh). A kilowatt is equal to 1000 W. To determine
the power being consumed, divide the number of watts by
1000:
E I
P (inkW)
1000
In the circuit shown in Figure 12.22, the power consumed
can be calculated as follows:
P E I
P 120 V 22 A 2640 W
kW P 1000
kW 2640 W 1000 2.64 kW
12.14 MAGNETISM
Magnetism was briefly discussed earlier in the unit to explain
how electrical generators are able to produce electricity.
Magnets are classified as either permanent or temporary.
Permanent magnets are used in only a few applications that
air-conditioning and refrigeration technicians would work
with, but electromagnets, a type of temporary magnet, are
used in many electrical components of air-conditioning and
refrigeration equipment.
A magnetic field is generated around a wire whenever
an electrical current is flowing through it, Figure 12.23. If
the wire or conductor is formed into a loop, the strength of
the magnetic field will be increased, Figure 12.24, and if the
wire is wound into a coil, a stronger magnetic field will be
created, Figure 12.25. This coil of wire carrying an electrical
current is called a solenoid. This solenoid or electromagnet
will attract or pull an iron bar into the coil, Figure 12.26. If
an iron bar is inserted permanently in the coil, the strength
of the magnetic field will be increased even more.
The magnetic field can be used to generate electricity and
to cause electric motors to operate. The magnetic attraction
CROSS SECTION OF CONDUCTOR
CARRYING CURRENT
MAGNETIC FIELD
AROUND CONDUCTOR
Figure 12.23 This cross section of a wire shows a magnetic field around
the conductor.
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