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Electrical System
Instruments and components
Alternating Current
An alternating current (AC) is an electric current
whose direction reverses cyclically, as opposed to
direct current, whose direction remains constant.
The usual waveform of an AC power circuit is a
sine wave, as this results in the most efficient
transmission of energy. However in certain
applications different waveforms are used, such
as triangular or square waves.
• Alternating currents are accompanied (or caused) by
alternating voltages. An AC voltage v can be described
mathematically as a function of time by the following
Vpeak is the peak voltage (unit: volt),
ω is the angular frequency (unit: radians per second)
– The angular frequency is related to the physical frequency, f,
which represents the number of oscillations per second (unit =
hertz), by the equation
t is the time (unit: second).
A sine wave, over one cycle (360°). The dashed line represents the
root mean square (RMS) value at about 0.707
One voltage cycle of a 3-phase electrical system, labeled 0 to 360°
along the time axis. The plotted line represents the variation of
instantaneous voltage (or current) with respect to time. This cycle
will repeat 50 or 60 times per second, depending on the power
system frequency.
A transformer is a device that
transfers electrical energy from
one circuit to another through
inductively coupled electrical
conductors. A changing current in
the first circuit (the primary)
creates a changing magnetic field.
This changing magnetic field
induces a changing voltage in the
second circuit (the secondary).
This effect is called mutual
2-phase transformer
Oil immersed transformer
Domestic distribution
Three-phase distribution
• Show by sample calculation why smaller
cables can be used for long-distance
power transmission when very high
voltages are used
• Transformer area แรงสู ง
• Distance from main line สายส่ ง
• MDB Room ไม่ควรอยูใ่ ต้ดิน แรงต่า มี meter, main
switch, Zone’s DB 0.6*0.9*2.00 m
• LC, PB ,มี Shaft
• Floor to floor (height), Plenum space, Attic