25 June 2017

Parallel Inverter OR Parallel Inverter With Feedback Diodes

  • The power circuit diagram of the single phase parallel inverter is shown in the figure A.  
  • The commutating capacitor C is connected across supply source therefore it is called as parallel inverter. 
  • The turns ratio of half primary winding and secondary winding of the transformer is kept unity. 
  • The SCR T1 and SCR T2 are main SCRs from which load current passes through it.

working of parallel inverter

Mode 1

  • The load current flows through path +Vdc – L – c – a – SCR T1 – Vdc when the SCR T1 is turned on.
  • This will generate flux and resulting emf in the transformer primary winding ac and bc.
  • The charging of capacitor is done up to 2Vdc voltage due to these induced emf. The polarity of capacitor C is done in the figure B.
  • The maximum voltage withstand capability of the SCR T2 is 2Vdc.
  • The induced emf in the transformer secondary is 2Vdc as there is unity turns ratio between half primary and secondary winding.
  • This will result flow of current through load.

Mode 2

  • When the SCR T2 is turned on, the SCR T1 is turned off due to capacitor reverse voltage 2Vdc applied across it.
  • As the SCR T2 is turned on, the load current flows through path +Vdc – L – c – b – SCR T2 – Vdc and discharging of capacitor is done through SCR T2.
  • The charging of capacitor again done with – 2Vdc voltage.
  • The current flows during this interval are in reverse direction as that of when SCR T1 is turned on.
  • The SCR T1 again turned on when it receives gate pulse and SCR T2 is in off condition during this interval.
  • The waveforms of the output voltage become rectangular due to alternately switching of SCR T1 and SCR T2. 

waveform of parallel inverter

Parallel inverter with feedback diodes

  • When load is inductive, the load current becomes out of phase with load voltage and direction of load current reverses with respect to load voltage.
  • The stored energy during this interval feeds back through diodes D1 and D2. The circuit diagram of parallel inverter with feedback diodes is shown in the figure C.
parallel inverter with feedback diodes

  • When SCR T1 is switched on, the current flows through path + Vdc – c – a – SCR T1 – L – Vdc (-). 
  • The flux generated in the transformer primary winding ac and bc due to this current.
  • This will result in 2Vdc voltage induced in the transformer primary winding. The charging of capacitor polarity is shown in the figure D.
  • When SCR T2 is switched on, the SCR T1 is turned off due to reverse voltage of capacitor and capacitor discharges through path C – g – SCR T2 – L – D1 – d – a – f – C.
  • The stored energy of capacitor transfers to load via transformer upper side primary winding. 
  • When SCR T2 is turned on, the current flows through path + Vdc – c – b – SCR T2 – L – Vdc(-) and capacitor again charges with voltage – 2Vdc.
  • The capacitor again discharges through path C – f – SCR T1 – L – D2 – e – b – g – C when SCR T1 is again turned on.
  • This will result in stored energy of capacitor transfer to load via transformer lower side primary winding.

working of parallel inverter with feedback diodes

Advantages

  • Simple forced commutation circuit.
  • Sinusoidal waveform at output is possible by using suitable filter circuit

Disadvantages

  • The parallel inverter is useful only when load is fixed. (The output waveform changes due to change in load)
  • The inverter does not useful for higher power for fixed value of inductor L and capacitor C.
  • The design of inverter is done for fixed voltage.
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