In this topic, you study Single Phase Inverter – Working, Circuit Diagram & Waveforms.
Single Phase Inverter is an electrical circuit, converts a fixed voltage DC to a fixed (or variable) single phase AC voltage with variable frequency. A single Phase Inverter can be used to control the speed of single-phase motors.
Consider Q, Q, QB and Q as IGBTs. The above Fig. 3.6 (a) shows single phase bridge inverter with RL load. The construct is same as that of single phase bridge inverter with resistive load. And the diodes DI, D2, DB and D4 are the feedback elements.
The above Fig. 3.6 (b) shows the waveforms of single phase bridge inverter with RL load. The operation of the inverter is divided into four intervals (modes) for better understanding.
Interval I (ti — t2) (Mode-Il :
At time t ti the pair of transistors QI and Q2 is turned ON. The transistors act as a closed switch and hence ‘A’ gets connected to positive terminal of the dc source. ‘B’ gets connected to the negative terminal of the input supply or dc source.
Fig. 3.6 (c) : Interval I (tl — t2)
The above Fig. 3.6 (b) shows the flow of current through the load RL during interval I (Mode-I). The value of output voltage is given by,
+V volts
The load current starts increasing exponentially due to the inductive nature of the load. The instantaneous current through QI and Q2 is equal to the instantaneous load current through RL The energy is stored into the inductive load during this interval (i.e. tl — t2).
Interval 11 (t2 – t,) (Mode-Il) :
During this mode, the conducting transistors (QI and Q) are turned OFF at time t2. But the load current does not comes to zero at once due to the inductive nature of load. In order to maintain the flow of current in the same direction there is a self-induced voltage across the load. The magnitude of the voltage is exactly opposite to that in the previous mode. Thus, the output becomes negative equal to ‘—V’ volts. But still the load current continuous to remain in the same direction through DB and D4 as shown in following Fig. 3.6 (d). During this time, the stored energy in the load inductor is returned back to the source through the feedback elements i.e. the diode. And thus the load current starts decreasing exponentially and settles to zero at time, t3, when all the energy stored in the inductive load is returned back to supply. The diodes Dg and D4 are turned OFF at time, t3.
Interval III (t3 — t4) (Mode-III) :
At time t —t3, the transistor Q3 and Q4 are turned ON simultaneously. The output load voltage remains negative equal to (—V) volts but the direction of load current will reverse and its magnitude becomes negative. The current increases in the negative direction and energy is stored in the load. This operation is shown in the following (3.6)
Fig. 3.6 (e) : Interval Ill (t3 – t,)
Interval IV (t4 — t5) (Mode-IV) :
The mode IV operation is same as that in mode I. At instant t4 (to) the transistors QB and are turned OFF. The load inductance tries to maintain the load current in the same direction by inducting a positive load voltage. Due to which the diodes DI and D2 are forward biased, and thus the energy stored in the load inductance is returned back to the input dc supply. The value of load voltage Vo = +V, but the load current remains negative and decreases to zero. The operation in this interval is shown in Fig. 3.6 (f).
At time ts (tl) the load current is finally settled to zero and transistors QI and (22 can be turned ON again.