SECTION 5 |
Principle of Operation
16
5.1 GENERAL
These inverters convert DC battery voltage to AC voltage with an RMS (Root Mean
Square) value of 230 VAC, 50/60 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 advantages are discussed in Sections 2.2 to 2.4).
Fig. 5.1 below specifies the characteristics of 230 VAC, 50/60 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 230 VAC, 50/60 Hz system, it slowly rises in the
Positive direction from 0V to a peak Positive value “Vpeak” = + 325V, slowly drops to
0V, changes the polarity to Negative direction and slowly increases in the Negative
direction to a peak Negative value “Vpeak” = - 325V and then slowly drops back to
0V. There are 50 such cycles in 1 sec in 50 Hz and 60 in 60 HZ. Cycles per second is
called the “frequency” and is also termed “Hertz (Hz)”. The Time Period of 1 Cycle is
20 ms for 50 Hz and 16.66 ms for 60 Hz.
TIME
0V
Peak Negative Voltage
- V
PEAK
= - 325V
V
RMS
= 230 VAC
Peak Positive Voltage
+ V
PEAK
= + 325V
Voltage (+)
Voltage (-)
16.66 / 20 ms
Fig. 5.1: 230 VAC, 50 / 60 Hz Pure Sine Waveform.
5.3 PRINCIPLE OF OPERATION
The voltage conversion takes place in two stages. In the first stage, the DC voltage
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 230 VAC, 50 / 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 reference sine wave.
