Series Inverter

Introduction

• The commutating components L and C are connected in series with the load therefore this inverter is called as series inverter.
• The value of commutating components is selected such that the circuit becomes under damped.

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• 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 power circuit diagram of the series inverter is shown in the figure A.

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• 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 explained as follows.

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Working of Series Inverter

Mode 1

• The voltage V_dc directly applies to RLC series circuit as soon as the SCR T1 is turned on.
• The polarity of capacitor charging is shown in the figure B.
• The nature of the load current is alternating as there is underdamped circuit of the commutating components.

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• The voltage across capacitor becomes + V_dc when the load current becomes maximum.
• The voltage across capacitor becomes +2V_dc when the load current becomes zero at point a ( figure D ).
• The SCR T1 automatically turns off at point a because the load current becomes zero.

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Mode 2

• The load current becomes zero from point a to b as the SCR T1 turns off in this time period.
• The SCR T1 and SCR T2 are turned off in this time duration ( from a to b ) and voltage across capacitor becomes equal to +2 V_dc.

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Mode 3

• The SCR T2 is turned on at point b due to it receives positive capacitor voltage.
• The discharging of capacitor is done through SCR T2 and R – L circuit as shown in the figure C.
• The load current becomes zero after it becomes maximum in the negative direction.

• The capacitor discharges from +2 V_dc to – V_dc during this time and SCR T2 turns off automatically at point C due to load current becomes zero.
• The SCR T2 turns off during point C to D and SCR T1 again turns on.
• This way cycle repeat after it complete one turns.
• The positive AC output voltage half cycle generates due to DC voltage source whereas negative half cycle generates due to capacitor.

Limitations of Series Inverter

• The load current flows only during positive half cycle from supply source.
• The DC supply source gets short circuited if SCR T1 and SCR T2 simultaneously turned on.
• The rating of commutating components should high because the load current flows through it.

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• The load voltage waveform gets distorted if the dead zone time or SCR turns on time high.
• The maximum output frequency of the inverter should be less than the ringing frequency.
• The DC supply source is short circuited if the output frequency of the inverter is higher than the ringing frequency.

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• The maximum current during each high cycle and its time duration depends upon parameter of load this will result in poor regulation of the inverter output.

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Applications

This type of inverter generates sinusoidal waveforms whose output frequency is in the range of 200 Hz to 100 kHz therefore it is applicable for

• Induction heating
• Sonar transmitter
• Fluorescent lighting and
• Ultrasonic generator

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Thank you

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