Single Phase Thyristorised Bridge Inverter with RL Load

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Single Phase Thyristorised Bridge Inverter with RL Load

Q. Single Phase Thyristor Bridge Inverter with RL Load

Q. Single Phase Full Bridge Inverter Explained

Q. Single phase full bridge inverter operation

Q. Single phase full bridge inverter with RL load waveform

The operation of the circuit can be divided into four intervals or modes. The waveforms are as shown in Figure E.

Operation of Single Phase Thyristorised Bridge Inverter with RL Load

Interval I (t1 –  t2) :

  • At instant t1, the pair of thyristors S1 and S2 is turned on. The thyristors are assumed to be ideal switches.
  • Therefore point A gets connected to positive point of de source V through S1 and point B gets connected to negative point of input supply. 
  • The output voltage v0  = + V as shown in Figure A. The load current starts increasing exponentially due to the inductive nature of the load.
  •  The instantaneous current through S1 and S2  is equal to the instantaneous load current. The energy is stored into the inductive load during this interval of operation.
    Equivalent Circuits in Inverter I(t1 - t2)
    Figure A

    Equivalent Circuits in Inverter II (t2 - t3)
    Figure B

Interval II (t2 – t3) :

  • At instant t2 both the thyristors S1 and S2 are turned off. But the load current does not reduce to 0 instantaneously, due to its inductive nature.
  • So in order to maintain the flow of current in the same direction there is a self induced voltage across the load.
  • The polarity of this voltage is exactly opposite to that in the previous mode.
  • Thus output voltage becomes negative equal to – V.
  • But the load current continues to flow in the same direction through D3 and D4 as shown in Figure D.
  • Thus the stored energy in the load inductance is returned back to the source in this mode. The diodes D1 to D4are therefore known as the feedback diodes
  • The load current decreases exponentially and goes to 0 at instant t3 when all the energy stored in the load is returned back to supply D3 and D4 are turned off at t3.

Interval III (t3 – t4) :

  • At instant t3, S3 and S4 and are turned on simultaneously. The load voltage remains negative equal to -V but the direction of load current will reverse and become negative.
  • The current increases exponentially in the negative direction. And the load again stores energy in this mode of operation. This is as shown in Figure C.
Equivalent Circuits in Inverter III (t3 - t4)
Figure C

    Equivalent Circuits in Inverter IV (t4 - t5) or (t0 - t1)
    Figure D

Interval IV (t4 – t5) or ( t0 – t1) :

  • At instant t4 or t0 the thyristors S3 and  S4 are turned off. The load inductance tries to maintain the load current in the same direction, by inducing a positive load voltage.
  • This will forward bias the diodes D1 and D2.The load Conduction stored energy is returned back to the input dc supply. The load voltage v0
     = + V but the load current remains negative and decreases exponentially towards 0. This is as shown in Figure D.
  • At t5 or t1  the load current goes to zero and thyristors S1 and S2 can be turned on again.
  • This will forward bias the diodes D1 and D2. The load Conduction stored energy is returned back to the input dc supply. The load voltage  v0  = + V but the load current remains negative and decreases exponentially towards. This is as shown in Figure D.
  • At ts or t , the load current goes to zero and thyristors 5 and S , can be turned on again.
Conduction period of devices :

  • The conduction period with a very highly inductive load, will be T0 / 4 or 90° for all the thyristors as well as the diodes. (See Figure E).
  • The conduction period of thyristor will increase towards T0 / 2 or 180° with increase in the load power factor. (i.e. as the load becomes more and more resistive).
Voltage and Current Waveforms for Single Phase Bridge Inverter with RL Load
Figure E

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