Basic Series Inverter (Self Commucated Inverter)

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Basic Series Inverter (Self Commucated Inverter)

Circuit diagram of Series Inverter :

  • The series inverter uses a class A type commutation. The commutating components L1, C1 are connected in series to form an underdamped tuned circuit. The basic series inverter is as shown in Figure A.
  • Since the  SCRs turn off themselves this circuit is known as self commutated inverter.
  • Note that both the SCRS are the main load carrying  SCRs.
Basic Series Inverter Configuration
Figure A
Mode I (t0 to t1)
Figure B
Mode II (t1 to t2)
Figure C

Operation of Series Inverter :

  • Refer Figure D for the waveforms of this circuit.
  • At instant t0 SCR1 is turned on. Let the initial voltage capacitor be “ec“with its left plate negative w.r.t right plate and the sinusoided load current starts flowing.
  • The capacitor C1 starts charging in the opposite direction as shown in Figure B.
  • The load current eventually comes to zero at instant t1 when C1 is fully charged and SCR1 comes out of conduction due to natural commutation. This happens because the charging current of the capacitor whích is also flowing through SCR1 reduces to zero.
  • The voltage on the capacitor C1 at instant t1 is greater than V with its left plate positive with respect to its right plate.
  • As there is no discharge path for the capacitor, this voltage will be held constant upto instant t2 where SCR2 is triggered.
  • At instant  t2 SCR2  is turned on (See Figure D) and the load voltage and current both become negative.
  • The capacitor now discharges resonantly through SCR. RL as shown in Figure C. The charging and discharging capacitor currents are sinusoidal due to the underdamped L1C1 resonant circuit.
  • At instant t3 the discharge current goes to zero and SCR2 is turned off again due to natural commutation. The voltage on C1 is equal to e.

Off time :

Voltage and Current Waveforms for Basic Series
Figure D
  • During the time interval between t1 and t2 both the SCRs are in the off state. Load voltage as well as load current are zero. Therefore this interval is known as the OFF TIME of the circuit.
  • The ON time of the inverter output is fixed and the only way to vary its output frequency is to vary the OFF time.
  • Though the circuit is simple in operation and commutation circuit is common for both the SCRS there are few disadvantages also.

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