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.
- REVIEW:
- When used to describe an AC quantity,
the length of a vector represents the amplitude of the wave while the
angle of a vector represents the phase angle of the wave relative to
some other (reference) waveform.
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