SAMLEX AMERICA INC. | 9
Reactive Load: A device or appliance that consists of a combination of resistive, inductive and capacitive
elements (like motor driven tools, refrigeration compressors, microwaves, computers, audio/ video etc.). The
Power Factor of this type of load is <1 e.g. AC motors (PF=0.4 to 0.8), Transformers (PF=0.8), AC to DC Switch
Mode Power Supplies (PF=0.5 to 0.6) etc. These devices require Apparent Power (VA) from the AC power source.
The Apparent Power is a vectorial sum of Active Power (Watts) and Reactive Power (VAR). The AC power source
has to be sized based on the higher Apparent Power (VA) and also based on the Starting Surge Power.
2.2 OUTPUT VOLTAGE WAVEFORMS
TIME
180
160
140
120
100
80
60
40
20
0
20
40
60
80
100
120
140
160
180
Modified Sine
Wave sits at
ZERO for some
time and then
rises or falls
V
PEAK
= 169.68V
Modified Sine Wave
Sine Wave
Pure Sine Wave
crosses Zero Volt
instantaneously
V
PEAK
= 140 to 160V
V
RMS
= 120 VAC
VOL
TS (+)
VOL
TS (–)
16.66 ms
Fig. 2.1: Pure and Modified Sine Waveforms for 120 VAC, 60 Hz
The output waveform of the Samlex PST series inverters is a Pure Sine Wave like the waveform of Utility / Grid
Power. Please see Sine Wave represented in the Fig. 2.1 that also shows Modified Sine Waveform for comparison.
In a Sine Wave, the voltage rises and falls smoothly with a smoothly changing phase angle and also changes
its polarity instantly when it crosses 0 Volts. In a Modified Sine Wave, the voltage rises and falls abruptly, the
phase angle also changes abruptly and it sits at zero V for some time before changing its polarity. Thus, any
device that uses a control circuitry that senses the phase (for voltage / speed control) or instantaneous zero
voltage crossing (for timing control) will not work properly from a voltage that has a Modified Sine Waveform.
Also, as the Modified Sine Wave is a form of Square Wave, it is comprised of multiple Sine Waves of odd
harmonics (multiples) of the fundamental frequency of the Modified Sine Wave. For example, a 60 Hz
Modified Sine Wave will consist of Sine Waves with odd harmonic frequencies of 3rd (180 Hz), 5th (300 Hz),
7th (420 Hz) and so on. The high frequency harmonic content in a Modified Sine Wave produces enhanced
radio interference, higher heating effect in inductive loads like microwaves and motor driven devices like
hand tools, refrigeration / air-conditioning compressors, pumps etc. The higher frequency harmonics also
produce overloading effect in low frequency capacitors due to lowering of their capacitive reactance by
the higher harmonic frequencies. These capacitors are used in ballasts for fluorescent lighting for Power
Factor improvement and in single-phase induction motors as Start and Run Capacitors. Thus, Modified and
Square Wave inverters may shut down due to overload when powering these devices.
SECTION 2 |
General Information