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 is kept unity. 
• The SCR T1 and SCR T2 are main SCRs from which load current passes through it.

Mode 1

• The load current flows through path +V_dc – L – c – a – SCR T1 – V_dc 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 2V_dc 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 2V_dc. 
• The induced emf in the transformer secondary is 2V_dc as there is unity turns ratio between half primary and secondary winding. 
• This will result flow of current through load.

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Mode 2

• When the SCR T2 is turned on, the SCR T1 is turned off due to capacitor reverse voltage 2V_dc applied across it. 
• As the SCR T2 is turned on, the load current flows through path +V_dc – L – c – b – SCR T2 – V_dc and discharging of capacitor is done through SCR T2. 
• The charging of capacitor again done with – 2V_dc 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 waveform of the output voltage become rectangular due to alternately switching of SCR T1 and SCR T2. 

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.

• When SCR T1 is turned on, flow of current through path + V_dc – c – a – SCR T1 – L – V_dc (-).  
• The flux generated in the transformer primary winding ac and bc due to this current. 
• This will result in 2V_dc 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 + V_dc – c – b – SCR T2 – L – V_dc(-) and capacitor again charges with voltage – 2V_dc. 
• 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.

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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