Inverter is an electronic circuit which converts DC power into AC power. The inverter circuit in which the commutating elements L and C are connected in series with the load to form an under damped circuit is called a series inverter. This circuit is also called load commutated or self-commutated inverter.
This circuit is called load commutated inverter because the load component (L and C) is responsible to turned off the thyristor. It is called self-commutated inverter because in this circuit anode current itself become zero resulting the thyristor turned off.
The operation of series inverter is similar to Class A commutation.
Series inverter is operating at high frequency 200 Hz to 100 KHz. Therefore the size of the commutated component is small. Here, the value of L and C is choose in such a way the R, L & C forma underdamped circuit.
Circuit Diagram of Series Inverter
The Circuit Diagram of Series Inverter is shown in the figure. It consists of two thyristors (TI and T2). The thyristor T1 and T2 are turn on appropriately to get the output voltage of desired frequency. This circuit consist of L and C connected in series with load (R).
Initially we considered that thyristor T2 is turned off and the polarity across capacitor is shown in figure.
Operation of Series inverter
The whole operation is divide into three modes:
Mode-I ( T1 on and T2 off): In this mode we give firing pulse to thyristor T1 so, T1 get turned on and T2 thyristor is turned off initially. So, current flow from supply Vs…..T1……load……back to Vs.
The nature of the load current is alternating due to under damped circuit. So, this time capacitor (C) start charging gradually from -Vs to its max voltage. This time inductor (L) also get charge. When the load current becomes maximum the voltage across capacitor becomes + Vs. When the load current becomes zero at point a the voltage across capacitor becomes +2Vs. Then the load current becomes zero the thyristor T1 automatically turns off at point a.
Mode- II (T1 and T2 both off): This time thyristor T1 turns off because the load current becomes zero from point a to b. In this time duration The thyristor T1 and T2 are turned off and voltage across capacitor becomes equal to +2 Vs.
Mode III (T1 off and T2 on): In this mode we give firing pulse to thyristor T2. So, T2 get turned on. In this time capacitor start discharging its energy from +2Vs to – Vs through thyristor T2 and R – L circuit . Due to capacitor discharging reverse current flow across the load. Now at point C thyristor T2 turns off automatically due to load current becomes zero. The thyristor T2 turns off during point C to D and thyristor T1 again turns on. In this way cycle repeat.
Now, we see in the waveform the time duration ab and cd is called as dead zone.
Application of Series Inverter
Series Inverter is basically used in high frequency applications (200 Hz to 100 KHz) because it generate high frequency sinusoidal waveform.
- It is used in Induction heating.
- For Fluorescent lighting.
- Used in Sonar transmitter.
- Used in Ultrasonic generator.