Vectors and AC waveforms
Okay, so how exactly can we represent AC
quantities of voltage or current in the form of a vector?
The length of the vector represents the magnitude (or
amplitude) of the waveform, like this:
The greater the amplitude of the waveform,
the greater the length of its corresponding vector. The
angle of the vector, however, represents the phase shift in
degrees between the waveform in question and another
waveform acting as a "reference" in time. Usually, when the
phase of a waveform in a circuit is expressed, it is
referenced to the power supply voltage waveform (arbitrarily
stated to be "at" 0o). Remember that phase is
always a relative measurement between two waveforms
rather than an absolute property.
The greater the phase shift in degrees
between two waveforms, the greater the angle difference
between the corresponding vectors. Being a relative
measurement, like voltage, phase shift (vector angle) only
has meaning in reference to some standard waveform.
Generally this "reference" waveform is the main AC power
supply voltage in the circuit. If there is more than one AC
voltage source, then one of those sources is arbitrarily
chosen to be the phase reference for all other measurements
in the circuit.
This concept of a reference point is not
unlike that of the "ground" point in a circuit for the
benefit of voltage reference. With a clearly defined point
in the circuit declared to be "ground," it becomes possible
to talk about voltage "on" or "at" single points in a
circuit, being understood that those voltages (always
relative between two points) are referenced to
"ground." Correspondingly, with a clearly defined point of
reference for phase it becomes possible to speak of voltages
and currents in an AC circuit having definite phase angles.
For example, if the current in an AC circuit is described as
"24.3 milliamps at -64 degrees," it means that the current
waveform has an amplitude of 24.3 mA, and it lags 64o
behind the reference waveform, usually assumed to be the
main source voltage waveform.
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