Parallel Inverter or Centre Tapped Inverter (with Inductive Load)

Parallel Inverter or Centre Tapped Inverter (with Inductive Load)

• Figure A shows the schematic diagram of a 1Φ parallel inverter. SCRs 1 and 2 are the main load carrying SCRs. 
• L and C are the commutating components. Diodes D₁ and D₂ permit the load reactive power to be fed back to the supply and feed back the stored energy on the commutation components to the load. These are called as the feedback diodes. 

Operation Parallel Inverter or Centre Tapped Inverter :

Mode I (t₀ – t₁) :

• In mode I the load voltage is positive and load current is negative. Hence the stored energy is returned back from load to source, via the feedback diodes.
• This mode is similar to mode IV.

Mode II (t₁ – t₂) :

• At instant t₁, the SCR₂is turned on. Neglecting the small drop across the inductance, the supply voltage V appears across the left half of primary winding OA.
• By transformer action, terminal B will be at a potential of 2 V Volts with respect to terminal A.
• Thus the capacitor charges to 2 V Volts with respect to A. The load voltage will be positive and of magnitude V if turns ratio is unity.
• The load current will also be positive and the flow of energy is from the source to the load. (See Figure C).
• At instant t₂, SCR₂, is turned on the voltage on the capacitor is directly applied across SCR , and hence it is commutated due to voltage commutation.

Figure A

Figure B

Mode III (t₂ – t₃) : 

• When SCR₁ is turned off the capacitor will discharge through SCR₂, inductor L and diode D₁ and a portion of transformer winding LA , as shown in Figure D.
• Thus the energy stored in the capacitor will be fed back to the load . 
• During this period, the load voltage will be still positive and greater than V. 
• The load current which earlier was flowing through SCR₁ will now flow through OL and diode D₁ to the negative terminal of dc supply as shown in Figure D.
• This can happen only if diode D₁ is forward biased i.e. if the discharge current of capacitor is greater than I_L.

Figure C Figure D

Mode IV (t₃ – t₄) :

• After some time the discharge current of C will reduce and becomes less than I_L.
• This will reverse bias diode D_1.

Figure E

• The capacitor will no longer discharge through  SCR₂ so but due to the presence of end the current will star flowing through D₂, winding MB and SCR₂, to feed bag the stored energy in L to the load. 
• As the load current which earlier was flowing through this winding OL will now flow from M to through diode D₂, and shown in Figure E and the load reactive energy will be returned to the dc supply. . 
• Since point M is now connected to the negative terminal of DC supply, the load voltage polarity will be reversed and more than V volts. 
• Also the capacitor will be charged in opposite direction and slightly more than twice the supply voltage. 
• The SCR₂ comes out of conduction due to natural communication as soon as all the energy in the inductor L is completely fed back to the load. 

Mode V (t₄ – t₅) :

• At instant t₄  the load current goes to zero indicating that all the reactive energy stored in the load has been fed back to source completely.
• SCR₂ is retriggered at this instant. When SCR₂ starts conducting the load voltage will again become equal SCR₁, is fired to tum off SCR₂, and cycle repeats. 
• Note that the load Voltage will rise above V during modes III and VI and direction of load current.
• If the diodes are connected to points and then the load voltage waveform will be rectangular.
• But with this type of connection the energy trapped in the inductance L will be dissipated in the SCRs and diodes thereby necessiаting derating of the components. 
• Thus the efficiency of the circuits can be improved by connecting the diodes to the tap points (L and M).

Figure F