Series Inverter

SERIES INVERTER

Series Inverters are those inverters, in which the commutating elements L and C are connected in series with the load. The value of commutating components is selected such that the circuit becomes under damped. The Two SCRS are used to produce the halves (positive and negative half cycle) in the output. The anode current itself becomes zero in this inverter resulting the SCR turns off automatically therefore this inverter is also called as self commutated or load commutated inverter.

The circuit diagram of the series inverter is shown in the figure A. The SCR (T1) and SCR (T2) are turned on at regular interval in order to achieve desirable output voltage and output frequency. The SCR T2 is kept off at starting condition and polarity of voltage across capacitor is shown in the figure A.

The Operation of the Series Inverter is

Mode 1

i. When SCR T1 Triggered, It will Turned ON and the voltage Vdc directly applies to RLC series circuit.

ii. The Capacitor will charge with the polarity + C - as shown in the figure B.

iii. The voltage across capacitor becomes + Vdc when the load current becomes maximum.

iv. The voltage across capacitor becomes +2 Vdc when the load current becomes zero at point A and the SCR T1 automatically turns off at this point A because the load current becomes zero.

Mode 2

i. During the Period from A to B, the load current remains zero and Voltage across Capacitor becomes to +2 VDC as the SCR T1 turns off during this time period and SCR T2 is already in turned OFF Position

Mode 3

i. Now SCR T2 is turned ON by triggering at point B, the Capacitor will discharge through SCR T2 and R - L circuit as shown in the figure C.

ii. The load current becomes zero after it becomes maximum in the negative direction.

iii. The capacitor discharges from +2 Vdc to - Vdc during this time and SCR T2 turns off automatically at point C due to load current becomes zero.

iv. During Point C to D, the SCR T2 turns off and SCR T1 again turns ON at point D. In this way, this cycle repeat after completing one turn.

v. The Positive AC output voltage half cycle generates due to DC voltage source whereas Negative half cycle generates due to capacitor.

Disadvantages of Series Inverter

i. The load current flows only during positive half cycle from supply source. The ii. ii. DC supply source gets short circuited if SCR T1 and SCR T2 simultaneously turned on.

iii. The rating of commutating components should high because the load current flows through it.

iv. The load voltage waveform gets distorted if the dead zone time or SCR turns on time high.

v. The maximum output frequency of the inverter should be less than the resonant frequency.

vi. The DC supply source is short circuited if the output frequency of the inverter is higher than the resonant frequency.