Voltage, current, power factor, types of loads
The voltage wave form of 230VAC, 50 Hz mains / utility power is like a sinewave. In a
voltage with a sinewave-form, the instantaneous value and polarity of the voltage varies
with respect to time and the wave-form is lijke a sine wave. In one cycle, it slowly rises in
the positive direction from 0 V to a peak positive value +
Vpeak
=330 V, slowly drops to 0V,
changes the polarity to negative direction and slowly increases in the negative direction to
a peak negative value -
Vpeak
=330 V and then slowly drops back to 0 V. There are 50
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 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 and0.6
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-form, the instantaneous value 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 = 1.414xRMS value
. For example, the 230 VAC,
50 Hz. mains / utility power is the RMS value. The peak value corresponding to this is
=1.414 x 230 = 330 V.
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 limitited to its normal rated power in watts (W) x Power Factor.
For example, an inverter rated for 1000W (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.
CHARACTERISTICS OF SINUSOIDAL AC POWER
page 7
