18 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 19
SECTION 5 |
Principle of Operation
5.1 GENERAL
This inverter converts 12 VDC battery voltage to AC voltage with an RMS (Root Mean
Square) value of 120 VAC, 60 Hz RMS.
5.2 PURE SINE WAVE OUTPUT WAVEFORM
The waveform of the AC voltage is a pure Sine Waveform that is same as the waveform
of Grid / Utility power (Supplementary information on pure Sine Waveform and its advan-
tages are discussed in Sections 2.2 to 2.4).
Fig. 5.1 below specifies the characteristics of 120 VAC, 60 Hz pure Sine Waveform. The
instantaneous value and polarity of the voltage varies cyclically with respect to time. For
example, in one cycle in a 120 VAC, 60 Hz system, it slowly rises in the Positive direction
from 0V to a peak Positive value “Vpeak” = + 169.68V, slowly drops to 0V, changes the
polarity to Negative direction and slowly increases in the Negative direction to a peak
Negative value “Vpeak” = - 169.68V and then slowly drops back to 0V. There are 60 such
cycles in 1 sec. Cycles per second is called the “Frequency” and is also termed “Hertz (Hz)”.
The Time Period of 1 Cycle is 16.66 ms.
TIME
0V
Peak Negative Voltage
- V
PEAK
= - 169.68V
V
RMS
= 120 VAC
Peak Positive Voltage
+ V
PEAK
= + 169.68V
Voltage (+)
Voltage (
–)
16.66 ms
Fig. 5.1: 120 VAC, 60 Hz Pure Sine Waveform
5.3 PRINCIPLE OF OPERATION
12/24 VDC to 120 VAC conversion takes place in two stages. In the first stage, 12/24 VDC
of the battery is converted to a high voltage DC using high frequency switching and
Pulse Width Modulation (PWM) technique. In the second stage, the high voltage DC is
converted to 120 VAC, 60 Hz sine wave AC again using PWM technique. This is done by
using a special wave shaping technique where the high voltage DC is switched at a high
frequency and the pulse width of this switching is modulated with respect to a refer-
ence sine wave.
