Analysis Of The Single-Phase Full Bridge Inverter
Analysis of the single phase Full bridge inverter:.
Fig: pole and line voltage o/p of a 1 phase H-bridge inverter
- The single-phase full bridge circuit can be thought of as two half bridge circuits sharing the same dc bus. The full bridge circuit will have two pole-voltages (VAO and VBO), which are similar to the pole voltage VAO of the half bridge circuit.
- Both VAO and VBO of the full bridge circuit are square waves but they will, in general, have some phase difference. The pole voltages staggers in time by ‘t’ seconds. The phase displacement angle ‘Φ’ is defined as :
where, ‘t’ is the time by which the two pole voltages are staggered and ‘T’ is the time period of the square wave pole voltages.
- Taking the phase shift angle ‘Φ’ into account, the pole-B voltage may be written as
- Difference of VAO and VBO gives the line voltage VAB. In full bridge inverter the single phase load is connected between points ‘A’ and ‘B’ and the voltage of interest is the load voltage VAB. Rearranging we have,
- The RMS magnitude of the fundamental componentof load voltage may be written as
- The RMS magnitude of load voltage can be changed from zero to a peak magnitude of 0.9Edc . The peak load voltage magnitude corresponds to Φ = 180 degrees and the load voltage will be zero for Φ = 00. For Φ = 180 degrees, the load voltage waveform is once again square wave of time period T and instantaneous magnitude E.
- As the phase shift angle changes from zero to 1800 the width of voltage pulse in the load voltage waveform increases. Thus the fundamental voltage magnitude is controlled by pulse-width modulation.
- The line voltage distortion due to higher order harmonics for pulse width modulated waveform (except for Φ = 1800) is less than the corresponding distortion in the square wave pole voltage. In fact, for some values of phase shift angle (Φ) many of the harmonic voltage magnitudes will drastically reduce or may even get eliminated from the load voltage. For example, for Φ = 600 the load voltage will be free from 3rd and multiples of third harmonic.