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CHARACTERISTICS OF SINUSOIDAL AC POWER
Voltage, current, power factor, types of loads
The voltage waveform of 120 VAC, 60 Hz mains / utility power is like a sine wave. In
a voltage with a sine wave-form, the instantaneous value and polarity of the voltage
varies with respect to time and the wave-form is like a sine wave. In one cycle, it
slowly rises in the positive direction from 0 V to a peak positive value + Vpeak = 170
V, slowly drops to 0 V, changes the polarity to negative direction and slowly increases
in the negative direction to a peak negative value - Vpeak =170 V and then slowly
drops back to 0 V. There are 60 such cycles in 1 sec. Cycles per second is called the
"frequency" and is also termed "Hertz (Hz.). If a linear load is connected to this type
of voltage, the load will draw current which will also have the same sine wave-form.
However, the peak value of the current will depend upon the impedance of the load.
Also, the phase of the sine wave-form of the current drawn by the linear load may
be the same or lead / lag the phase of sine wave-form of the voltage. This phase
difference determines the "Power Factor (mathematically = the cosine of the phase
difference)" of the load. In a resistive type of load (like incandescent lamps, heaters
etc) the sine wave-form of the current drawn by the load has 0 phase difference
with the sine wave-form of the voltage of the AC power source. The Power Factor
of a resistive load is unity (1). The rated output power (in Watts) of the inverters is
normally specified for resistive type of loads that have unity (1) Power Factor. In a
reactive type of load (like electric motor driven loads, fluorescent lights, computers,
audio / video equipment etc), the phase of the sine wave-form of the current drawn
by the load may lead or lag the sine wave-form of the AC voltage source. In this case,
the power factor of reactive loads is lower than unity (1) – generally between 0.8 and
0.6. A reactive load reduces the effective wattage that can be delivered by an AC
power source.
RMS and peak values
As explained above, in a sine wave, the instantaneous values of AC voltage (Volt, V)
and current (Ampere, A) vary with time. Two values are commonly used – Root Mean
Square (RMS) value and peak value. For simplicity, RMS value can be considered as an
average value. Mathematically, Peak Value = 1.414 x RMS value. For example, the 120
VAC, 60 Hz. mains / utility power is the RMS value. The peak value corresponding to
this is = 1.414 x 120 = 170V.
The values of the rated output voltage and current of an AC power source are their
RMS values
AC power – Watts / VA
The power rating of an AC power source is designated in Volt Amperes (VA) or in
Watts (W)
Power in Volt Amperes (VA) = RMS Volts (V) x RMS Amps (A)
Power in Watts = RMS Volts (V) x RMS Amps (A) x Power Factor
NOTE :
The rated power of the inverter in Watts (W) is normally designated for a
linear, resistive type of load that draws linear current at unity (1) power
factor. If the load is linear and reactive type, the rated power of the inverter
in watts will be limited to its normal rated power in watts (W) x Power
Factor. For example, an inverter rated for 1000 W ( at unity power factor)
will be able to deliver only 600 watts to a reactive type of load with a
power factor of 0.6.
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