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Basics of Electricity
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Electrical devices for which the
electricity is used could require other voltages. For example, voltage in the UK is 240V AC, but
many lamps for display lighting only require 12V. A transformer is used to change electric power
from one voltage to another.
A basic single-phase transformer consists of two coils. See Figure 1.21. The incoming power
is connected to the primary coil, and the secondary coil supplies the output circuit. The two coils
are not connected physically. Power is transferred from the primary coil to the secondary coil by a
phenomenon called induction. This is described in more detail in a later lesson.
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Figure 1.21. Basic single-phase transformer.
If both coils have the same number of turns, the voltage of the output circuit is the same
as the input voltage.
If, however, the number of turns in the secondary coil is LESS than the number of turns in
the primary coil, the voltage of the output circuit is LOWER than the input voltage. This is called
a step-down transformer.
On the other hand, if the number of turns in the secondary coil is GREATER than the number
of turns in the primary coil, the voltage of the output circuit is HIGHER than the input voltage.
This is called a step-up transformer.
Conductor Current-carrying Capacity
Conductors used to transmit electricity vary in size from about 0.5 to 12mm in diameter.
This is the dimension of bare wire without insulation. See
Figure 1.22.
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Figure 1.22. Typical conductor sizes (Not to scale).
As conductors vary in size, the current-carrying capacity varies; the larger the diameter,
the greater the current-carrying capacity. This can be compared to water pipes; the larger the
pipe, the greater the amount of water that can flow through it.
Current-carrying capacity of conductors also varies with the type of insulation protecting
the conductor.
Figure 1.23 shows three examples of single conductors located in free air.
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Figure 1.23. Current-carrying capacity of single conductor in free air.
Where a number of cables are contained in a common enclosure the current-carrying capacity
will be reduced, due to the risk of the conductors overheating. Sizes and current-carrying
capacities of
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